timber framed buildings and heatwaves

[SimmondsMills]

I am increasingly curious about the future of timber frame (low thermal mass) buildings wrt to overheating.
*Are fenestration and solar shading strategies enough on their own
*can MVHR using earth tubes (if this works) supply adequately cooled ventilation air in heatwaves to low thermal mass structures
*could buildings be lined with thick plasters, or for example 50mm or so of plastered woodwool (wood fibre and cement / magnesite) boards etc to bring adequate thermal mass into the building
* above possibly combined with MVHR cooling etc etc
*a cool 'heatwave retreat' basement?

What does the future hold then for say, timber framed homes, that will have to cope with the predicted heatwaves of the future, and what is the relationship between passive solar design, levels of thermal mass and overheating with respect to timber frame. I know 15% of Germany's PassinHaus(en?) is timber frame, but am not clear on their overheating performance predicted into 2080…?

What the big picture for timber homes?
What a question – sorry!

[Mark Siddall]

Whoa! That’s a big question Andy.

I can make a stab at a few of the points you raise: –

I am reading up on low energy and passive cooling at the moment. The suggestion in the book is that, as far as cooling goes, between 28-32C timber frame is beneficial in that the low thermal mass allows daytime ventilation/ cooling, and due to the low thermal mass the heat gains are dissipated easily in the evening.

NOTE: The suggestion in the book is that if daytime ventilative cooling is used (at temperatures 28C+) in a building with high thermal mass overheating is possible during the evening. This is due to a decrease in evening wind speeds which in turn results in a reduction in the psychological cooling effect which is normally caused by the evaporation of sweat.

At temperatures above 32C direct and/or indirect thermal mass begins to play an increasing part. As I see it for a timber frame house there are three options, all of which you identify: –

1) Design in the thermal mass –
* Concrete floors. With a 100mm concrete thermal mass the first 25mm stores 50% of the heat, the first 50mm 75% of the heat. (Reporting on the introduction of thermal mass in New Zealand, where timberframe is the norm, the Vale's observe that heating bills can be reduced by 40%. I wonder what the impact would be on cooling?)
*phase-change plasterboard is a hi-tech passive solution to thermal mass, a 12mm board acts like 150mm concrete. This means that the other advantages of timber frame can still be exploited (prefabrication, build quality, speed etc.) whilst providing thermal mass.
* 150mm of laminated veneered lumber can offer a surprising quantity of thermal mass 18 wh/m2/C (half that of concrete but more than aerated concrete). This also has the benefit of sequestering large quantities of CO2.
2) MHVR cooling (with a little thermal mass and night purge) https://aecb.net/forum/index.php?topic=716.0
3) Use GSHX aka earth tubes. For more discussion on GHSX see my earlier posting at https://aecb.net/forum/index.php?topic=715.msg2601#new

Personally I don't think the heat wave basement idea will be needed….seems a bit OTT….but I don't have any data to substantiate this…

[Mark Siddall]

FYI. To get a handle on the UK climate my weather data software tells me that the hottest average weather to date has been:

Newcastle: 23.5C
Sheffield: 26.9C
Cambridge: 29.7C
London: 32C (an urban heat island if ever there was one!!!)
….though it does not have last year……

By 2080 the summer temperatures in the North East is expected to rise between 1.5 and 4.5C.
* The low emissions scenario estimates that temperatures will increase by 1.5-2.5C
* The high emissions scenario estimates that temperatures will increase by 3.5-4.5C

So the suggestion is that, in general, up north we will be OK but you guys down south need to think about things more carefully. Some thermal mass coupled or decoupled is likely to be required.
However, if we want to make sure people in the north and south avoid reaching for air conditioning units for those occasional very hot/heat-wave days then thermal mass (and user awareness of how to exploit it!) is likely to be needed also.

Mark

[David Olivier]

Very thick insulation causes a building to behave in a more heavwright manner, so a TF with 400 mm insulation is different to one with 100 mm or 50 mm.

The Dumont House in Saskatchewan, central Canada has no cooling but is all timber-frame with 450 mm cellulose in walls, 600 mm in roof, 300 mm under basement. Small windows too. Reportedly no problems keeping it comfortable since 1992.

I'm sure you can make them comfortable in most of the UK with the above features. Nick Grant's house has fairly small windows and stays fairly cool. But by making the windows small enough to keep the indoor temperatures stable, on the latitude of Canada or southern England you may sacrifice 500-2000 kWh/yr of winter passive solar gains, depending on house size., because you can't use really large south-facing windows.

D.

[Mark Siddall]

If comfort, induced by daytime ventilation, tales of at about 32C then small windows might not be enough to control internal temperatures. Night purging and mass comes into play.

How hot does it get in Saskatchewan? How hot will it get in 2080?

Have there been any studies to model the heavy weight performance of super-insulated timber framed houses? i.e. just how 'heavy' are they?

Mark

P.S. Airtightness will not doubt play a large part in this type of discussion, hopefully the Gold standard should be enough.

[David Olivier]

Central Canada is 2 K hotter than London area in July but it has no heat island effect – this warms London to a higher temp. than the surrounding countryside.

You have to open windows in either case, lightweight or heavy. The difference is that the heavy building may be operable by day in heatwaves with the windows closed, only needing the windows to be opened at night. There are a lot of variables to consider other than the thermal capacity.

D.

[Mark Siddall]

David,
I have been thinking about the house in Canada and the underlying cause of its success during the summer months. Other than the appropriate sizing of the glazing there is another factor at play. This is the decrement delay inherent to the mass of the thermal insulation and its impact upon maintaining suitable internal temperatures during the summer. Foamed insulations are very lightweight, whilst mineral fiber and cellulose are more dense and have a higher specific heat capacity. As a result they have a prolonged decrement delay.

The implication is that, as foamed insulations have a shorter (aka worse) decrement delay, a highly insulated lightweight building built of such materials would conceivably be subject to overheating in the summer whereas a highly insulated lightweight building using another insulation would be perfectly okay. One concern that could arise from this observation is that if SIP systems (a foamed insulation product) begin to take a larger market share of the traditional timberframe market then buildings built using such insulation materials could require replacement, or serious refurbishment, in 2080.

As a consequence knowing the type of insulation used in the Canadian house is vitally important to ensuring that other lightweight buildings, based upon this type of model, perform in the manner that they are intended to.

So, after all that, do you know what type of insulation was used in the Saskatchewan house?

Mark

[Mark Siddall]

David
I should have re-read your post! You say the Saskatchewan house had large quanity of cellulose fibre.

This confirms some information on decrement delay that came my way the other day. The decrement delay of cellulose fibre insulation (in this case Warmcell 500), at a U-value of 0.13 w/m2K, you get a decrement delay of 7.3 hr. Which obviously helps to avoid daytime overheating. A comparative calculation using mineral fibre insulation did not fair so well as it only achieved a 3.7hr decrement delay. The use of 60mm mineral fibre board (Pavatex Isolair) in compliment with the cellulose fibre insulation extends the calculated delay to 11.3hrs (note: U-value falls to 1.1 w/m2K).
All in all this confirms that in timber framed buildings the selection of insulation type has an impact upon not only U-value but other aspects of building performance. Though I have no data to support the claim my gut instinct tells me that foamed insulation will fair no better than mineral fibre, if not worse, much worse.

Mark

[Anonymous]

Mark,
Please note that Pavatex Isolair is not a mineral fibre board. It is a woodfibre insulation board made out of “waste” wood from sawmills in Switzerland.

Summer overheating control is already important and will be one of the most important parts of the design in future – especially if you design light weight construction as timber frames or steel frame are.

Summer overheating is caused by a combination of three reasons:
A) Large internal heat gains from computers, machines, people etc.
B) Sun directly thourgh windows (due poor summer shading)
C) heat passing direct thourgh wall or roof

>>To solve A) you can reduce gains or ventilate to improve, for B) provide better shading or glasing. For C) you can reduce peak heat gain to the room by changing the decrement. With decrement I think about the amount the peak external surface temperature is smoothed out by the structure (decrement factor) and the time that the peak is delayed before it reaches the inside (decrement delay). But overheating protection is always a combination of A, B & C – high decrement is no solution on its one I think. To have some thermal mass at internal surface will help as well.

Decrement is based on calculation which takes in account density (should be high for good decrement), k-value (should be low) and specific heat capacity (should be high). As I come from a woodfibre background I just want to show the benefit of woodfibre insulation to bring thermal mass into timber frame walls e.g. as additional external insulation onto the timber studs; Pavatex Isolair has k-value = 0.047 W/mK, density = 240 kg/m³, specific heat capacity = 2100 J/kgK!
As last figure is based on “kg” this makes the high density of the product so important.
Cellulose is OK for reaching good decrement but its density is only 40 kg/m³. Mineral fibre, PUR, XPS and similar have even less and lower specific heat capacity. Ideal would be to put cellulose between the studs and go on top with the woodfibre as described above. Not mentioned yet other benefits like less thermal bridges etc.

Beat (Swiss in UK)

[Mark Siddall]

Beat,
You're correct, I made something of a typo I'm afraid. I hope I haven't caused confusion, I meant to write wood-fiber. Information on other concerns including incidental gains, thermal bridging and airtightness are to be found on other threads.

Mark

[Nick Grant]

You are obviously just stirring up trouble Adam so I'll see who rises to the bait!

Couple of observations from recent PH conference is that there seems to be plenty of lightweight timber construction in Austria which I'm sure gets hotter than UK. This supports David's comments.

What I have been noticing though on recent holidays is the lovely shutters everyone outside UK has. Good for daytime shading (forget daylight when you are out or enjoy the dappled light of louvres) and great for secure night ventilation.

I'll start a separate thread

[David Olivier]

I haven't seen many shutters in Denmark – masonry houses and cool UK-type summers. I think they appear as a feature when houses overheat otherwise due to:

1. high summer temperatures
2. very low thermal capacity construction methods and/or
3. fairly large windows

Dumont's house shows only that very low thermal capacity construction, combined with very modest window sizes, and very energy-efficient appliances, stays cool in a fairly warm, sunny summer. It doesn't mean that the same house could be built with larger windows.

At yesterday's early adapters' workshop (more are planned for those who missed it). we heard views that well-insulated lightweight dwellings don't overheat in the UK. But possibly all other factors (internal gains, orientation, window areas, etc) were favourable. I know a timber-frame house in Essex which overheats frequently from May to September and photographs of a timber-frame house near Monmouth suggest that it may overheat too.

Thermal capacity is yet another variable for designers to be aware of and the range is huge, from a perhaps 3 kWh/K for a lightweight small house (e.g. a Canadian house) to 50 kWh/K for a heavyweight house (e.g. a German house). Houses built to these extremes will behave totally differently. Other things being equal, if the windows are opened at night and then closed by day to keep in the coolth, the internal temperature in the massive one will be more stable and the internal temperature in the lightweight one will rise more rapidly.

David.

[Author]

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