Windows to meet the AECB Energy Standards

[Andy Mitchell]

Here is a very quickly prepared briefing paper I wrote in relation to requirements for supply of windows to meet the AECB Energy Standards.  The intention is to give a broad overview from a manufacturer's point of view and start discussion in this forum.  I hope to stimulate discussion, and that others more qualified will respond/ amend/ provide more technical input.

THE AECB DRAFT ENERGY STANDARDS AND FENESTRATION

The implications of the voluntary introduction of the AECB Energy Standards with regard to sourcing suitable windows in the UK market.

BACKGROUND
The changes to Part L (Part J in Scotland) of the Building Regulations in 2002 have had a very significant impact on the performance of standard windows in the UK.  A combination of stricter thermal performance requirements, and application of the Regulations, for the first time, to retrofitting of windows has meant all manufacturers have had to comply with these standards (generally a whole window U value of 2.0 w/m2k).  This has been achieved largely by the introduction of hard coat low emissivity double glazing units (predominantly Pilkington K glass) with minimum air filled cavity of 16 mm into conventional timber or uPVC frames.

The proposed changes to Part L in 2005 do not significantly change the standard of windows which will be required in the UK market.

UK AVAILABILITY OF WINDOWS IN ADVANCE OF PART L
Because many other northern European countries, notably all of Scandinavia, Germany, Austria and Switzerland, as well as Canada, have had both mandatory building codes well in advance of those pertaining in the UK, and often additional voluntary advanced codes, there has been market demand for, and therefore supply of, windows offering higher performance for many years. 

During at least the last 3 decades, prior to 2002, therefore, anyone wishing to specify higher performance windows has tended to specify an imported advanced window.  A few specialist UK manufacturers, my own company included, have offered advanced windows in the UK but this supply was very limited.

The impact on the UK industry of the 2002 Part L revision was to force all manufacturers to produce a thermally improved window.  Although this resulted primarily in a surge in demand for hard coat low-E units, manufacturers of more advanced products have also reported significant increases in demand.  This perhaps indicates that the focus on the new thermal window requirements led to increased interest in moving beyond the minimum standard.  For the timber industry many manufacturers were forced to either abandon timber frame production or develop and expand this to justify the increased investment in machinery able to manufacture the new profiles required to carry larger glazing units.  This has acted as a stimulus to development, and on the timber side there has been some tendency for a limited number of manufacturers to also move to offering options in advance of Part L.

STANDARDS, WHAT STANDARDS?
The current Part L predominantly requires a window to demonstrate an overall U value of no more than 2.0 w/m2k. 

he manufacturer’s published value must be based on the GGF Standard Window configuration which is currently 1230 X 1480 mm, with a centre mullion, one fixed light and one opening light.

The published value is at present acceptable if reached from one of the following methods:
1. Calculation to BS En 10077 pt 1
2. Calculation to En 10077 pt 2
3. Physical ‘hotbox’ test by an approved testing facility
Method 1 is low cost, while method 2 calculation is relatively complex and expensive.  Physical testing is probably the most expensive method.  Depending on the method used a variation of U value up to 0.2 can be expected.  Physical testing will deliver the most accurate result, and normally the lowest one.  Thus a window U value calculated under BS En 10077 pt 1 at 1.6 w/m2k might produce a result of 1.4 w/m2k when physically tested.

For imported windows, the variability in published value may be even more significant.  First, the window configuration will be rather different from the UK standard.  Some Scandinavian countries, for example, have worked on a window of 1200 x 1200 mm for some years.  Second, the parameters and standards for modelling or testing vary.  For example, the Canadian basis for calculating U values is much stricter, assuming 21C inside, -18C outside (in comparison with 20C inside and 0C outside), higher wind speeds than European calculations and a window of only 600 x 1200 mm.

This discussion is really to give a context for published U values, and a preamble to discussion of the values required by the draft AECB Energy Standards.

TECHNOLOGIES BEYOND PART L
The simplest method of moving beyond the current requirements of Part L is simply to substitute a more advanced glazing for the hard coat low emissivity glazing currently required to meet the current standard.

Advanced ‘soft coat’ low emissivity glazing systems can significantly improve the performance of sealed units.  These units are more demanding to fabricate because the improved coatings are, as they sound, soft , and therefore vulnerable to damage.  They require specialist methods of fabrication, including handling methods and removal of coatings where bonding to spacers is to take place. 

Soft coat low-E systems are generally combined with Argon gas fill  (hard coat can also be gas filled).  The best double glazed sealed units available on the market currently deliver a centre-pane U value of 1.1 w/m2k. 

Combining 1.1 w/m2k double units with mainstream timber or uPVC  frames can be expected to deliver an overall window U value of circa 1.5 – 1.6 w/m2k (depending on method used – see above). 

An additional small improvement on this can be achieved by the introduction of insulated spacer bars to replace the aluminium bars which currently dominate the UK market.  The best of these could be expected to give a reduction of overall U value of around 0.1 w/m2k.  This technology is widely available in northern Europe and Canada, but has had little or no impact as yet in the UK.  An additional advantage of this technology is the almost complete elimination of condensation on the glazing units.

To further improve performance, triple sealed units can dramatically improve the centre-pane U value.  Soft coat low-E units with argon fill can give centre-pane U values of 0.6 w/m2k (0.5 w/m2k with the more expensive krypton fill).  These units can now be obtained UK-manufactured, although availability is severely limited.  Combining these units, with ‘warm edge’ spacers into conventional timber or uPVC windows (where rebates are large enough to take them) can result in a window with U value of circa 1.0 w/m2k

Further improvements beyond 1.0 w/m2k can be achieved by one (or both) of the following:
1. multiple layer low-E glazing, generally achieved by the introduction of further low-E coated thin films;
2. improving the U value of the frames.

BEYOND PASSIVE U VALUES
The design of ultra-efficient buildings generally tends to include incorporation of passive solar design, to minimise the requirement for additional heating.

Although beyond this report, it is possible for designers to work with the ‘energy balance’, or effective U values, on different aspects of a building to optimise the solar contribution to any heating requirement.

The main factors to be considered in optimising solar gain should be:
1. frame profile – low profile frames will increase the solar gain available;
2. light/solar transmission characteristics of the low-E glazing system selected (these vary significantly);
3. aspect of window – for example, triple glazed units with reduced solar gain may be selected for more northerly aspects, while double glazed units with higher solar gain characteristics may be selected for southerly aspects.

AECB SILVER STANDARD
The Silver Standard requires an overall window U value of 1.5 w/m2k. 

It is clear from the above discussion that good conventional timber (or uPVC) frames with the best soft coat low-E double units, with warm-edge spacer technology will generally meet this standard.  A number of UK manufacturers either already offer this or would be likely to be willing to offer this if requested, now that these units are becoming widely available.

It seems reasonable therefore to conclude that sourcing suitable UK-manufactured windows to meet this Standard should present no problem.

AECB GOLD STANDARD
The Gold Standard requires an overall window U value of 0.8 w/m2k.

To the best of my knowledge, no UK manufacturer offers a window which approaches this value.  I am also not aware of a UK company currently importing a window which meets this standard.

Windows meeting this standard are available in Austria, Germany, Norway, Finland, Switzerland, Canada (not necessarily an exhaustive list).  Frame technologies include insulated pultruded fibreglass and insulated aluminium clad timber.  Glazing technologies include the advanced options discussed above.

I am aware of at least one building in the UK incorporating windows meeting this standard. 

Should there start to be incipient demand, with the voluntary introduction of the Gold Standard, it is likely the market would develop initially in the following ways:
1. a few specialist or existing import companies starting to import suitable windows;
2. one (or more) specialist UK manufacturers exploring the introduction of a UK manufactured product meeting the standard.

A MANUFACTURER’S EXPERIENCE AND PROSPECTS
Green Building Store’s Ecoplus System range of high performance timber windows has been on the UK market since 1995.  Manufactured in Yorkshire, the range has always incorporated one of the best soft coat low-E glasses on the market (Iplus from Interpane Ltd).  The published U value for this range is 1.6 w/m2k.

The company is relaunching the range in February 2005.  The new range will incorporate one of the best warm-edge glazing spacers on the market, and also introduce a triple glazed version (with centre pane U value of 0.5 w/m2k).  Overall U values will be:
Double glazed 1.5 w/m2k
Triple glazed 1.0 w/m2k
(see attached calculations)

The new range will therefore meet the requirements of the AECB Silver Standard with no modification.

The company does not currently offer a window that can meet the 0.8 w/m2k standard required by the AECB Gold Standard.  The best we could offer would be our triple glazed option, achieving 1.0 w/m2k.

The possibilities for our company to offer a Gold Standard window would be as follows:
1. import from Canada, Germany or Scandinavia – this is an option we have already considered briefly, and would, as a market leader, be willing to explore further in the light of the potential introduction of the Gold Standard;
2. develop a suitable design for manufacturing a window ourselves (we have been approached with a design which could potentially meet the standard  but have not seen any commercial potential to date).

Development costs to produce a suitable window in the UK would be relatively high.  Because of the very low demand in the early stages of the introduction of the Gold Standard, it is unlikely that any manufacturer would find it commercially viable to develop such a window, without some form of external funding.  It is possible that such funding could be potentially available from appropriate energy related Quangos or NGOs.

Green Building Store is a specialist supplier of energy-efficient and environmentally sensitive building products throughout the UK.

Chris Herring is currently Director of Marketing and Development.

[David Olivier]

Chris

Congratulations you're the winner of the competition for the longest posting on this site! I hope to reply without approaching that record.

I've never actually specified windows with a U-value any higher than 1.5 W/m2K – even in the early 1980s this was straightforward using standard Swedish or Norwegian windows – and I've never had clients who wanted anything other than wood or (for non-domestic) metal-clad wood. However, more recently I've come across people doing barn conversions or refurbishing listed houses who want oak, not softwood windows, so a local joiner must make the windows and with difficulty get warm edge sealed units made in the UK.

There are a few importers of windows which meet the Gold Standard if one takes advantage of its allowance of slight juggling of U-values (so long as the total building heat loss is not increased) and one importer meets it without any difficulty. However, I, the rest of the design team and the client/owner of a building in Kent are deterred – probably fatally – by the prices being asked for the first UK order and two of the importers can't meet the requirements which now apply to non-domestic glazed doors; e.g., unobstructed door width, in other words, only one supplier is in the running.

So as far as I can see we need UK-made product(s) or agents who can import windows to order (no stocks) from Scandinavia without adjusting the price skyward as the shipment crosses the North Sea.

As you know, and have indeed seen, my own house windows were made in Canada (fibreglass frames, U-value <0.8 when calculated the European way). Unlike the above situation the price delivered to the UK was very reasonable (even including 7% import duty which is non-refundable).

I suspect other domestic projects could utilise these windows in future. Non-domestic would be more difficult given the rules which now apply especially to doors.

Regards

David.

[Anonymous]

Got any links for the Canadian ones David?

[David Olivier]

Tahir

Their web site is http://www.thermotechwindows.com. There are at least two other co.s in Canada, and possibly four, making f/g windows. You could try Duxton Windows in Winnipeg for starters.

It all began with a Canadian govt. super-windows research program in the 80s, which considered all issues related to window energy efficiency. Now, if we'd had that in the UK.

David.

[David Olivier]

You might like to consult the catalogues of any Norwegian wood window manufacturer'; e.g. NorDan. The sections and details of their wood windows are designed to repel the Norwegian weather which is probably worse than Manchester's! You may wish to make further allowances for any minor differences between oak and softwood – incidentally oak isn't such a good insulator as pine or fir – but anyway good luck to you.

David.

[Nick Grant]

Yes good luck! Oak is lovely but a pig to make windows out of. Leaded lights good to take up the movement!!!

Chris Herring's company and others have had success with laminated Oak and good detailing and I have seen chunky fixed lights in barns that seemed to work. However have seen some expensive mistakes in oak (and softwood) by very skilled joiners.

If you are master joiners and spend time on design and timber selection I dare say its possible but if you just want to save money I'd probably get mass produced softwood windows (sorry Chris), modify them, paint with OS Country Colour and detail well to protect them in the wall. A made a window once, just trad casement as replacement, never again!

If you are talking conservation building in an old house then thats another matter and I don't know.

[David Olivier]

In Silver the U-value of *opaque* doors is specified; glazed doors can be same as windows. The definition of the Std. is being re-drafted to be clearer, to stipulate how to take account of the thermal bridge between window (or door) and wall and to specify how much tradeoff between U-values is permissible; please watch this space.

25 mm polystyrene plus a few other layers gives you U = c.1.0. 80% of North American houses since 1980 have had external doors with U-values of c.0.6; i.e., polyurethane foam-filled steel. Bob Lowe, an AECB member who spoke at the last Conf. has one in his house which he bought at Toronto's version of B&Q for £70 and brought it over (sorry, I digress.)

David.

[Anonymous]

Chris,
Thanks very much for all that, I now feel like I should be able to make some informed decisions once my various joinery quotes come in the post…then it's off to the bank with my balaclava and gun!

[David Olivier]

I've sometimes found in the last 10 years that triple-glazed Scandinavian windows with a low U-value cost less than UK windows which were only just able to meet the Regs at the time. In one case, the U-value of the UK windows was 3.0 W/m2K (but they were good-quality) and good-quality Swedish windows with a U-value of 1.3 W/m2K cost less.

In the case of windows able to meet the Gold Standard, there are none yet made in the UK so one has a choice of German, Danish (but made in Germany and re-badged) Austrian, Norwegian or Canadian manufacturers.

David.

[Anonymous]

David
I've looked at your link to the canadian manufacturer
but it seems like a long way for a window to travel
got any links to German manufacturers for reasons previously given
thanks Jim

[Anonymous]

Lots of useful information
thanks

Jim

[Mark Siddall]

Chis,
Agreed some more technical comentary on the building science front is required. There are a whole range of system combinations available with various U-values and SHGCs (some of the SHGC's look reasonable i.e. over 50%).

If such windows can work for Passive House designs isn't is possible (or at least conseavable) to get a UK, or even some form of pan-European, license (excluding Belgium or something) from Southwall Technologies? (it would seem from their blurb that they own the patent)

If things look reasonably favourable perhaps the thing to do would be to agree a license provisionally the Southwall pending reciept of Passive House approval. If the system passes you have deal, if not no harm done.

If a UK owned company could start to develop cheap high performance windows using this technology we could even begin to think about exporting them back to Germany. Given all the talk about Zero Carbon houses recently these windows should be selling like hot cakes within a couple of years.

I almost wish I was in manufuacturing……do I smell the wiff of Natural Capitalism? ;- )

[Mark Siddall]

David,
In your postings your use “AFAIK” quite regularly its obviously an abbreviated term…..what does it stand for?

Cheers,
Mark

[SimmondsMills]

pretty sure it's 'As Far As I Know'….

[David Olivier]

Yes

D

[Mark Siddall]

A question for Chris:

It would seem that one of the stumbling blocks in developing advanced window systems in the UK is that they represent a significantly over specification compared to the UK norm. So the question is how can the market be stimulated to move incrementally towards improved windows systems? I've had an idea that I would like to think could work, but I have no about whether it is truly a realistic proposal. This is where you kick in.

The following explains me thought process to date:
The difference between any new UK energy standards and those of mainland Europe it that we have the luxury of knowing exactly how the standards will need to develop and which technologies will help to achieve those goals.
Let me explain my thought process: –
On mainland Europe aka Germany low energy standards have developed incrementally through successive years of research and development. They have learned the hard way. We don;t have to.
Due to the hard work of the Passive House Institute we know in order to achieve windows with a U-Value of 0.8 that the technology required is achieved by insulating the frames and triple glazing the windows. My question is can the technology be dissected in some form in order to broaden its appeal?

In my fanciful theory what could be done is, rather than relying upon argon filled units to satisfy B.Regs or the Silver standard, insulated frames could be used instead.
If the frame is suitably engineered then in could be designed to accept both double and triple glazed units (e.g. a suitable packer would be used on the warm side). The framing system can then be used to satisfy a range of standards. Having appeal to a much broader range of specification standards it has vastly increased economic viability, and a vast amount of future proofing to boot. All in all this manufacturer would be so far ahead of the game that they could stand to make a pretty penny.
In effect we now have the beginnings of a Gold standard window, all we need now is the expensive glazed units.

The areas that my theory needs more detailed input:
1) Can this frame be manufactured in such a manner as to be competitive, or cheaper than, argon filled windows? I don't have a clue.
2) Can this frame really be used as a substitution for argon or even low E? Again not my area of expertise. (It may that its suited to windows of a certain size but not others.)

Anyway, its just a thought.

Mark

[Mark Siddall]

I'm in danger of repeating myself….but here goes: –
The hope was to do a thermal performance/cost slight-of-hand in such a way as to achieve the same overall U-value for the same cost but in an alternative and innovative manner(as far as the UK market is concerned).

As for my comments on expensive glazing; I meant that triple glazing costs more than double (increased materials, energy, labor etc)….but there is no reason why an insulated frame could not be used to improve, or achieve, the performance of both. In essence by making a one-frame-suits-all the supply demand profile is changed and the extra over costs of the frame are minimised as a consequence.

Mark

[Mark Siddall]

I have had a few more thoughts about this idealised window. Perhaps it can do more than offer and insulated frame and optional double or triple glazing.
I have heard of uPVC window manufacturer that supplies a window in two sections, a sub-frame and a preglazed window unit. The advantage for the spec. housebuilding market is that it saves time during construction and at it is a two part assembly the preglazed windows can be kept out of harms way until a suitable stage. A modification of this system could be adopted with the new eco-window. Such a window would allow the same advantages as the above and have the following features:
1) DPC and airtightness membranes would be pre-installed with the sub-frame
2) I have seen PH details that show and extended frame that apparently helps to resolve thermal bridging details. see http://www.baudetails.info/ for details. Consequently the sub-frame could be made available in two sizes PH/Gold and Silver. Obviously the window/sub-frame interface would need gaskets etc for airtightness.
3) If the sub-frame can also be used as pre-insulated cavity closer then the construction tolerances can be reduced thus helping to avoid the 10mm potential thermal bridge that occurs at the interface between the wall and the window. (On reflection this could be why the PH window on the Baudetails site has the extended frame i.e. In this example I have merely thought of a slightly different approach to the solution noted in item 2.)

[Mark Siddall]

I am reading Factor 4 by Lovins et al at the moment. There is mention of Passive House standard. Apparently, the insulation on the frame, if taken 3cm across the face of the triple glazing (i.e. beyond the traditional frame), it can eliminate the cold bridging almost entirely thus moving the standard large step closer to zero heating.
This forming of an improved extended frame (see item 2 below), and the appropriate detailing of the interfaces between window and wall (i.e. 10mm construction tolerance joint that can lead to thermal bridging), could in terms of whole life cost (saved fuel bills) help to make the cost of the window all the more affordable.

Mark

[Mark Siddall]

Chris,
I agree, all design decisions involve trade off in an attempt to achieve an optomised design this applies not only to technical performance but architecturally speaking also (architecturally keeping the frame to a minimum is generally the most desirable i.e. the pursuit of most architects.) To return to the technical; I take your point about installation psi values and window psi values.

Window psi values
When I read what Lovins had to say it conjured an image in my mind of the windows isotherms. Seen in elevation the coldest extent being at the glazing edge, the warmest in the centre pane. By extending the insulation across the face of the glazing by 3cm the cold edge was reduced thus reducing the psi value and leading to a greater proportion of the pane achieving the U-value associated with the centre pane. (All of which you have no doubt explored in far more detail that I am likely to, given that I just want to be sure that the glazing achieves the desired standard, which would be determined by whether or not it has PH Certification, or similar approved.)

With regard to my January 30, 2007 posting; Lovins only mentions the insulation/ frame detail in passing, so I can’t really contribute any more detail. Your comment regarding the change in spacer technology could indeed be an alternative technical solution that has gained popular acceptance, sadly I can’t comment i.e. my mental picture of the isotherms could be the same in both cases, though if the two strategies were combined, thermally speaking, it would be beneficial (though perhaps to costly). I recognise that increasing the area of exposed glazing is particularly beneficial for increasing solar heat gain of the window as a consequence the minimum cover over the face of the glass is desirable. If the spacer bar alone does the trick so be it.

Installation psi values
This detail is the one that I, as an architect, will have the most dealings with, and is therefore my greatest concern. As you highlight there are two options 1) to have an over sized frame 2) to have a frame that is as shallow as possible.

1) Traditional construction tolerances dictate that a 10mm joint is left around the perimeter of the frame so as to allow installation. This joint represents a considerable thermal bridge. Given that the UK has such shoddy workmanship even with good site inspection one can not be sure that this joint will be packed with insulation.
Now you note that it’s due to the type of window systems that the Germans tend to use that they tend to extend the insulation across the face of the frame (thus minimising the under performance of the window), this may well be, but I can see another benefit, one I believe to be pertinent to the UK. My concern is making a buildable detail that will result in minimal thermal bridging.
By adopting a wider frame the traditional UK installation detail can abandoned and with it the cold bridge that can arise from the 10mm joint perimeter joint. This is achieved by extending the wall insulation beyond the face of the frame, thus moving the construction joint behind the line of the wall insulation. The frame can then be tightly butt jointed to the back of the insulation.
The impact of the detail is such that, yes, a greater proportion of the façade has a lower U-value than optimal, but by having eliminated the thermal bridge of the 10mm construction joint, you have actually increased the overall thermal performance. The psi value could be tweaked minimised by adjusting the location of the window within the sectional depth of the wall thus increasing the cover of insulation across the concealed proportion of the frame. (Hope this make sense and has not turned into gobbledygook.)

To summarise: The German detail of bedding the frame within the insulation has the architectural benefit of reducing the apparent frame depth whilst also minimising the thermal bridge that can arise from construction tolerances. Based upon this line of analysis I believe that designing a shallow frame will not assist with the mitigation of the thermal bridge due to the fact that the 10mm construction joint remains unaddressed/subject to failures in on-site quality control. In effect what has happened is that the window designer/manufacturer has unfortunately passed the buck for installation psi-value to the architect, thus leaving it to the architect/contractor to design/build their way out of what it now a very tight corner.

2) Given the above issues raised relating to the 10mm construction joint in item 1 the only solution that I can perceive in creating a shallow frame would be to create a two part frame (think Thermabate and window designed as a complete package). This two-part frame would be engineered in such a manner that the first frame would be installed so as to form a casement that the wall insulation can butt up against (similar the ply box used in the Autonomous House or Peter Warm’s details in BFF a while back.) Being engineered to very tight tolerances the 10mm construction joint can now be mitigated, thus avoiding the thermal bridge.
In effect this detail bares many similarities to the deep frame option but will allow the contractor to keep the glazing out of harms way for a longer duration whilst construction in being undertaken on site.

NOTE: This is all intuition, no psi calcs have been performed.

I hope this offers a little more clarity. If not let me know and I'll try again. ;- )

Mark

[David Olivier]

I'd agree that the German windows seem to pay no attention to passive solar. By contrast some Canadian windows are designed very much with passive solar in mind.

The glazing of a high-performance triple or 2+1 glazed window gains more heat in January than it loses (on a S wall). By contrast, the opaque area of such a window is a total energy loss.

I think it's wrong to focus on U-value almost to the exclusion of building overall energy performance. But the Passive House Standard comes close to doing this.

For a given size window, I don't see how the German approach can ever match the Canadian approach of using the slimmest possible frame and sash. In the ideal building, one wants all glass and fully-insulated wall, and no opaque frame (with its high U-value).

The only advantage of 120 mm high frames and sashes over 50 mm high frames and sashes is that they permit the glazing unit to be larger, e.g. ?1500×2000 mm instead of ?1200×1200 mm. Other things being equal, larger windows have a lower U-value and higher g-value. However, there are some ways around that problem too.

David.

[Mark Siddall]

Oh, by the way I’ve had a thought about the perimeter detail. Consideration should be given to the replacement of the window. Overlapping the insulation across the face of the window frame will a require that the window be installed from the inside (allowing the aesthetic of the frame to be diminished). When the window is to be replaced damage to the window reveals and cill may occur.
Using the two piece frame noted in item 2 could avoid this, however there will be no guarantee that the manufacturer will not have changed the profile/engineering of the sub-frame nor that the manufacturer will still exist.
Though I have no answer to these issues they should be considered when developing the design of the window.

NOTE: In Scotland they use an overlap detail to offer additional weather protection only rather than weatherproofed insulation it is often traditional masonry. This rises a few thoughts 1) what size of frame is typical to Scotland (by having an overlap detail are the frame larger as I believe they would need to be?) 2) Is the Scottish overlap sufficient to address the thermal bridging/construction tolerance issues that arise from the PH/Gold standard? 3) How do they replace windows in Scotland? (This would presumably be assistance in developing a strategy for the replacement of windows as raised in the above posting.)

Mark

[Mark Siddall]

Chris,
Recently I’ve been thinking that windows should be appropriately design for orientation as a consequence this has brought me back to the 3cm insulation overlap across the glazing detail.

At the beginning of this thread you noted that the best double glazed units can offer a centre pane U-value of 1.1 w/m2k and an overall window U-value of circa 1.5 – 1.6 w/m2k. If the Fiest/Lovins detail holds true then using the 3cm insulation overlap then the thermal bridging could be avoided and the overall U-value can drop to 1.1 w/m2k. This represents a substantial improvement (at what cost?).

On PH/Gold houses this the avoidance of this thermal bridge will also be advantageous on the north, east and west facades where the losses can exceed the gains. If the overall U-value of 0.8 w/m2k can be dropped to 0.6 w/m2k then larger windows could be achieved allowing reduced heat loss and better daylight.

This detail was considered on a project that Lovin’s had some involvement with the “Hanover House” in the US (I’m not sure whether it was employed or not). See links
http://www.buildinggreen.com/elists/furnacefree.cfm
http://www.energysmiths.com/clients/hanoverhouse.php

The secret to the success of allowing this detail to be developed commercially would surely be ensuring that it can be used in both double and triple glazed windows.

Mark

[David Olivier]

Mark

It's not so easy to remove the thermal bridging which is caused by the frame. The area of wall where the external insulation overlaps over the front of the window frame doesn't have the same low U-value as the rest of the wall.

AFAIK the Hanover House which I wrote about 10 yrs ago had Canadian fibreglass windows same as my house.

Someone needs to do the modelling in PHPP. I started but have too much else to do.

David.

[Mark Siddall]

David,
I recognise that we are into the realm of subtle trade offs here, and as you say its into the realms of the PHPP and the like. I am mindful of the fact that, in terms of carbon emissions, it's not just about saving energy from heating (and subsequent trade offs within the building envelope) but it's also about reducing the need for artificial lighting. The subsequent need therefore is to set all the systems off against one another in order to optimise the whole. Hence the posting……these were my driving my thoughts at least.

Mark

[David Olivier]

Mark

It's very important actually. Optimising the fenestration for fewer, larger windows (near the limits of a manufacturer's technology) may reduce the heat demand of a new house by more than putting an extra 50 mm of insulation on the walls or roof. Also it can do this for negative cost – not added cost. My hunch which is unproven is that conservatories (people usually put radiators in them, using more energy per m2 than the rest of the house) would be less common if modern houses had (a) larger windows and (b) more generous floor areas.

David.

[Mark Siddall]

David,
Leaving the finer technical issues to the windows designers, it sounds as though we have consensus that the holistic approach considering thermal bridging, window psi-values and installation psi-values alike, as well as the desire to maximize daylighting in order to minimize carbon emissions is, in principle, sound.

The issue then is whether the 30mm insulated cover strip detail does, as suggested by Lovins, and as apparently undertaken by Fiest, iron out the windows remaining isotherms, and as a consequence ensure that the entire glazed area achieves a 0.6 w/m2k U-value. Whether this detail actually succeeds in its intent, or is simply an alternative to the super-spacer, is indeed one for the window designers/manufactures.

Mark

[David Olivier]

Mark

If you can get hold of the paper by me and Dr Bob Lowe in the 1995 CIBSE Conference Proceedings, we suggested such a holistic approach and also put forward the concept of effective window U-value (including the thermal bridges due to the installation.)

David.

[Mark Siddall]

David,
Were they published? Are they online? (Had a quick Google and had no success.) Are they likely to be in a library or do I have to hunt them down via an astute services engineer?

Mark

[David Olivier]

Mark

Sorry – 1995 was the paper age and CIBSE hasn't digitised its past publications at all.

David.

[Mark Siddall]

What a bummer, Oh well life goes on.

M

[Mark Siddall]

Currently reading “A Handbook on Low-energy Buildings and District-Energy Systems” by L.D. Danny Harvey. A fascinating and detailed 700 page book on the subject (seeks to pull together all the latest research across the field), though at £150! you’ll probably want to get it from the library (or get a “cheap” second hand copy).

Anyway a section of the book examines the embodied energy of window gasses. It turns out that the embodied energy from refining Krypton gas is staggeringly high; the EE for Argon on the other hand is much much lower. The way the author presents it is that the only way that Krypton can really ‘recover’ its embodied energy is by virtue of displacement i.e. the fact that it could allow you eliminate the need for perimeter heating, however, the author also notes that if, in a given climate, Argon filled windows can allow this displacement to occur then, it is argued, you can not legitimately claim the EE offset savings when specifying Krypton i.e. if the lower embodied energy option works this is by right the ethically appropriate option.

For Double Glazed units the book suggests that once you have a U-value of about <1.3 perimeter heating is not generally required. NOTE: the determinate factor for the U-value requirement is the minimum predicted external temperature, so please check details before seeking to design out a perimeter heating i.e. don't rely upon this thread! (Thought refer to the book for details if you want.)

Triple Glazed units can be argon filled can achieve 0.6w/m2k so, for the nominal improvement in U-value, krypton would not seem to be the most sustainable (i.e. low EE) inert gas.

(Xenon is has an even higher embodied energy that Krypton.)

Mark

P.S. For any glazing techies out there, the author, when discussing windows, makes regular reference to a number of books most predominantly “Windows in Buildings: Thermal, Acoustic, Visual, and Solar Performance” by Muneer. Could be worth a trip to the library.

[David Olivier]

Mark

PHI notes that to avoid perimeter heating in say Germany or Denmark the window U-value must be <0.8.

A paper to the 10th PH Conf noted that in Nice or Marseilles this could be safely raised to 1.2.

IMO some people are ignoring historical data to maintain that the UK and Ireland are almost a branch of the Cote d'Azur! They're neglecting the design temperature from about 60 years of weather records and basing their assertion on our average winter temperature in recent years. The first temperature can properly be described as climate, the second temperature is better described as weather.

In my view a house (or office) in southern England with windows U=1.3, walls U = 0.25 and air permeability c 2.5 needs a radiator or UFH in the room to avoid a sense of discomfort in cold spells for the last five years (still not as low as the design temp. of about -4 degC).

I think though that in designing low-energy buildings it's a weakness to treat the UK as a single climate zone. Countries or states such as France, Germany, California & Sweden are sub-divided based on regional design temperature, insolation, etc and different standards are set. The UK climate varies more from north to south than the German climate and some sites in the far SW have a mean January temperature 4 degrees K higher than Oxford for instance (let alone Aberdeen). At present the only properly-validated UK weather data in PHPP is for Manchester, whose climate is fairly representative of the UK average but isn't very close to the extremes.

David.

[Mark Siddall]

David,
My primary observation was highlight the need to avoid the use of Krypton where possible, due to high EE, in favour of using Argon based solutions. In this light all other matters were secondary observations.
The Canadian data gleaned from the book referred to above examined the potential for perimeter-free heating and external temperatures (from -30C or so up to more “hospitable” winter temperatures.) So as to try and be a little on the conservative side I read from the graph presented in the book the -10C external temp.

For safeties I would tend to agree that one should err on the safe side and wait till you achieve Gold/PH standards before discounting the perimeter heating.

Mark

[David Olivier]

Mark

Sorry to go off-topic!

If the krypton comes from fractionally distilling liquid air, in the course of making liquid nitrogen and other gases, I can't see why the EE should be dramatically more than for argon. It's certainly scarcer though and this might affect the price; 1% of the atmosphere is Ar.

Bear in mind that spectroscopic-grade Kr would probably have a much higher EE than impure Kr. 90% Kr is good enough for filling windows and is the grade normally used.

Sounds an interesting book though. Where do you manage to find the time?!

David.

[Mark Siddall]

It may sound a little sad but researching sustainable low-energy architecture is something of a hobby, it also keeps the inspiration/aspiration alive (not always easy in when you have commercial clients).

As for Krypton, your comments/understanding is beyond my awareness of gasses etc. all I can go off is what I have read.

Mark

[Mark Siddall]

Just to round out the debate a little more; the book mentioned earlier studies a 1.1 x 1.1m window with a number of gas infills (Heating Degree Days 3000).

Argon: 12kj EE saves 1227mj over 20yrs : total saving 1215mj (factor 10 saving)
Krypton: 508mj saves 2000mj over 20yrs : total saving 1452mj (factor 4 saving)
Xenon: 2.86gj saves 4.5gj over 20yrs : total saving 1.64gj (factor 1.5 saving)

Krypton vs Argon = 237mj additional saving over 20 years.

This suggests that there is a fine line in the energy savings between krypton and argon, and given that the saved EE from removing perimeter heating that could be incurred by both Argon or Krypton, means that whilst the energy benefits are there, the cost of these marginal savings may be prohibitive compared to more economic energy saving alternatives.

More insulation anyone?

Mark

[David Olivier]

Optimum width for Kr differs from Ar. In temperate climates, is usually 12 mm instead of 16 mm.

U-value difference could be >0.1 W/m2K. Assuming it's 0.1, saving on window given

= 1.2 m2 x 0.1 W/m2K x 90,000 Kh/yr = 11 kWh/yr.

= 20 x 11 = 220 kWh over 20 years.

Not 237 MJ which is 65 kWh.

David.

[Nick Grant]

As I think has already been said, the desired U value isnt determined by kWh/y since we are down to the level of 20p/year saving per window.

There was a paper at the PH conf explaining how the prescribed figure was obtained. I've lent out my proceedings but David might be able to sumarise.

Nick

[David Olivier]

I was taking the number of degree-hours, 90,000 deg-hrs, which one gets using PHPP and Manchester weather data. Using degree-hours as units leads to an annual heat consumption measured in kilowatt-hours – usually the units one wants it in. 3000 deg days per annum is 72,000 deg-hrs, which sounds much too low as a UK average.

AFAIK condensation on high-performance DG is usually a sign that relative humidity is much too high, although this is very common in the UK. A RH of 30-50% is usually considered healthy. 70% is considered unhealthy and is very likely to lead to mould growth on cold areas, such as thermal bridges. Mould is very bad for health.

David.

[Nick Grant]

We have had slight condensation at bottom edge of DG units (argon low e, semi warm spacer) with about 50% RH, eg when -5°C outside. Air probably about 18°C, typically in the morning.

Not been bad this year as hasn't been cold.

Nick

[Mark Siddall]

David,
Glad you clarified that. (I thought that you'd used the data that I'd referenced from the book as a guide.)

Mark

[Mark Siddall]

I emailed Danny Harvey with a few questions about the graph (Fig. 3.30) in his book Low-Energy Buildings and District-Energy Systems (the one referred to above, see https://aecb.net/forum/index.php?topic=36.msg3624#msg3624 and subsequent posts) The graph that suggests the relationship between external temperature, U-value and the possibility of disposing of perimeter heating.

Danny offered the best answers that he could and kindly put me in contact with Geoff McDonell of OMICRON Consulting, Canada (who provided Danny with the data referred to in the book.) Attached below is an edited copy of my queries and the responses from both parties.

I can also clarify that the graph was originally developed for office environments, though I believe it could work equally well for residential. The suggestion made by the graph is that it is possible to remove perimeter heating is due to the cold down draft problem being negated, this does not mean that a heating system is not required, rather it permits greater user satisfaction/comfort.

Addressing this concern does create a certain amount of design freedom as
1) low temperature underfloor heating can be introduced without the same risk of causing thermal discomfort
…passing thoughts are that poor design (not avoiding glazing that can cause down drafts) could resultant occupant dissatisfaction and lead to the use of electric fan heaters!

2) Shorter service runs are possible as rads don’t need to be placed on window walls (or external walls for that matter.) Any material/labour cost savings help to offset the additional cost of the glazing system. Also this helps to avoid the unnecessary heat losses caused by placing a rad against the external wall (a hard to quantify energy saving).

Mark

[David Olivier]

The limit of 0.8 W/m2K is governed by window height and design temp but it's more practical to standardise on
1 one window for climates like the UK / Ireland /Belgium / Germany,
2 one for Norway / Sweden / Finland / Russia / Baltic states which are colder or have lower design temps.
3 one for southern France, Spain, Italy which are warmer. Nor-Dan's double-glazed super window with U = 1.2 suffices for Nice according to a paper I read.

The Isles of Scilly and the tip of Cornwall have similar design temps. to southern Europe (but less insolation). PHPP comes with 50 sets of weather data for modelling buildings in Germany; until we have 50 sets of UK weather data it's hard to model buildings accurately in the far SW or far NE.

I agree about within limits widening the SU; I found 10 yrs ago that this slightly reduced the whole window U-value although the effect is minimal on large windows. Something I haven't checked is whether warm edge SUs should be buried in the frame or sash; Thermotech don't do this & I presume it dates back to the 1990s when German Passivhaus windows still used metal spacers. It makes the frame or sash bulkier which is unacceptable in passive solar buildings.

David.

[Mark Siddall]

I have read that the glazing rebate should be 20-30mm even with reinforced polycarbonate spacers.

Heat loss is reduced when the glass is set deeper into the frame (i.e. 25-30mm.) Heat losses through the window can be reduced by 8% with these measures.

As has been acknowledged at other points in time, this approach does conflict with exploiting solar gains.

Given that all heat losses are to be considered in PH/Gold, with heat losses 8% higher than may otherwise be the case (from a window!) you will have to improve the wall fabric to compensate. How much are you really winning by cutting the frame down to expose a little more glass within the structural opening? You could argue that as the windows are bespoke for the site the architect should just design the window opening 20mm larger (if the gains are that critical/finely balanced.)

Also if the window is set within the insulation plane (to minimise psi value) the window edge/frame will be in shadow for a good proportion of time. As a consequence the solar gains that a minimised frame is perusing will no doubt be lost anyway……just a thought.

Mark

[Mark Siddall]

For those that do German… I don't ;- (

http://www.passiv.de/04_pub/Literatur/HiWin/HiWin_An-TB-A.pdf

For those that don't there are some nice pics/graphs etc.

Mark

[Mark Siddall]

Chris,
I can only tell you what I've read. Not being fully versed in psi value calculation procedures I can't offer any real direction beyond that. The info in from the IEA so I would expect that it is reliable.
Shading issues: winter low angle sun also means shorter days, shorter days mean the sun tracks across the facade more quickly and thus any shadowing from window reveals will proportionately increase depending upon the depth of the reveal. (So goes the logic.) Whether one trades off against the other (gains vs heat loss) is anybodies guess. I haven't heard of or seen a study for this. So it's just conjecture.

Mark

[David Olivier]

Insulation versus insolation seems very important for the UK. The UK and Ireland have the “luxury” of a longer, but milder heating season and a cooler summer than Germany. Given a high thermal capacity structure (this is not guaranteed though, especially in Scotland), there's scope for using very large south windows, without intolerable overheating. The Germans haven't it seems to me paid as much attention to solar gain as the Canadians. Using PHPP, small variations in frame fraction or in the south-facing window area are often seen to have more effect on a building's space heating energy consumption than variations of +/- 25 mm in the insulation thickness.

BTW, how does Superspacer compare to Thermix or Swisspacer?

David.

[Mark Siddall]

Passive solar performance of a glazing systems:

The rule of thumb for calculating the passive solar performance of a glazing system is ‘g’ value multiplied by a factor ‘s’ should be equal or greater than the glass U-value.

g * s => Ug

If this rule is fulfilled then unshaded south facing glazing should provide a positive energy balance during the winter.

Regarding glazing, does anybody know what the ‘s’ factor is for the UK?
(I’d be very grateful of you could also quote the source of this data.)

Also:
Can anybody confirm the emissivity for low-E coatings (both hard and soft).

Thanks,
Mark

[Mark Siddall]

As a clue to the answer, which I hope someone has, in Central Europe the 's' factor is 1.6 w/m2k (the rule of thumb and the Central European 's' factor were noted by in an IEA document by W. Feist of the PHI).

I'm not certain what the 's' factor is though it seems sensible to assume that it is some form of solar factor (though the units seem a little unusual.)

Mark

[Nick Grant]

The German for confectionary is Süßigkeiten so I'd guess it's a fudge factor..

[Mark Siddall]

For the glaziers amongst us some Canadian stuff the may be of interest: –

Condensation Between Panes of Double Windows
irc.nrc-cnrc.gc.ca/pubs/cbd/cbd005_e.html

Window Air Leakage
irc.web-t.cisti.nrc.ca/cbd/cbd025e.html

Mark

[Tom Foster]

Time for another 3G price/availability survey. Meanwhile, I'd like advice …

In http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=527&page=3, 21 Dec 07 I published 3 best quotes received – £300-330/m2 delivered (which seemed remarkably gd value) – for pine Scandinavian-style 3G windows, 4-12-4-12-4 – Uw (whole-window U-value) typically 1.0 to 1.1 – so these were not the new breed of Passivhaus-grade super-3Gs.

15mths on (from quote invite time) the game's moved on and we're now looking at
a) those PH-grade pine 3Gs (a far bigger selection of them now available, than were for the 'old' 3Gs); also possibly
b) alu-wood ditto i.e. pine frame as before but all external surfaces formed of aluminium extrusions.

Can anyone suggest %age price-level difference, for each of a) and b), compared with the 'old' 3Gs? Note that £300-330/m2 was probably a low base for comparison – £375/m2 might have been more generally available; plus there may have been price cuts since then – or maybe not, as they say materials continue to rise while labour falls.

[Author]

Posts