Forum Replies Created
- AuthorPosts
I have 2 x 2000 litre ex-orange juice barrels (4 tonnes of rainwater) up the slope, providing 4-6m head to leaky pipes in 6.7 x 3m polytunnel. Lasts about a week in dry summer! Ordinary leaky pipes need more pressure than that, so am looking for lo pressure replacements, and tripling the storage.
In that case, WUFI study looks like a gd idea.
True in theory – but what conditions could that be, that internal RH is lower than the wall, except v short lived maybe?
Dave, wet dashing isn't an example of 'different permeability for liquid water and the vapour' – unless v strong cement-based, it's permeable or highly permeable to both vapour and liquid. It doesn't repel liquid, but instead relies on easy drying out.
With thick IWI that's no longer viable because no internal heat gets into the wall to help the vapour to move out, only seasonal solar gain vs driving rain, so moisture equilibrium rises and rises. That's not such a problem with a freestanding wall, quite apart from both surfaces being exposed to evaporation – but not nice when its inner surface is in contact with internal insulation etc.
28 February 2014 at 2:35 pm in reply to: Passionate junior Green Architect looking for professional FRIENDS!! #39109I'd also get into Linkedin, incl Linkedin Groups, do more on this forum https://aecb.net/forum and also http://www.greenbuildingforum.co.uk .
Are we really happy with coatings that claim to repel liquid water but are vapour permeable? I've been suspicious, avoiding – but some authorities seem to say it's OK – and absolutely crucial, according to WUFI, when considering significant IWI inside masonry walls that are too 'precious' to insulate properly (aka EWI).
In WUFI, brickwork for example that is highly repellent to driving rain, absorbs little and so can survive the paucity of re-drying potential that significant IWI creates. Whereas only slightly absorbent masonry is quickly in trouble on that score, leading to steadily accumulating water content year by year, often on a scale of decades.
Repellent treatment is therefore a serious 'desirable' and reportedly can be successful and trouble free. But has to be the right, hi-tech brand(s), as there's a great deal of rubbish, harmful product around.
Anyone got experience, incl brand names?
Does Meteonorm cover Ghana? It generates climate files for any location, interpolating/extrapolating from a large array of weather stations. Christian from WUFI/Fraunhofer says these are good enough except for missing driving rain data, which is important in WUFI but prob not in PHPP (or in Ghana!)
If only a) the big housebuilders and b) ordinary builders wd just bite the bullet and allow a re-think to happen, of how to build eco, they'd find, miraculously that it wd be a lot easier and cheaper.
Instead they cling tenaciously to minimised incremental modification of the old familiar ways, which naturally struggle to scrape through even watered down Bldg Regs uprates, and that only by creeping elaboration of the old favourites, adding more and more expensive and tricky specialist products to the dogs dinner.
Consider the 20s to 60s 11″ cavity wall – two 4″ masonry skins, one plastered, and 2″ cavity with dirt-cheap ties, on 2'-6″ deep 6″ conc strip found. Nothing cd be simpler.
Builders think they're still building good old cheap reliable cavity walls even tho they now have crumbly lightweight inner skin, 6″ cavities filled or part-filled with risk-prone insulation, hefty stainless ties fitted with insulation spacers, expensive dry lining, on 3'-4″ deep 8″ founds – plus more elaborations.
It's a different, elaborate animal, but is considered 'easy' tho in fact it's merely 'brainless'.
Eco builders even push the hopeless concept to 12″ filled cavities, hugely expensive imported ties, wide wide founds which increase effective footprint and excavation/disposal costs – and all they achieve is an extremely expensive way of applying purely decorative wallpaper to what could be a re-thought way of building.That re-thought way might be single (inboard) skin masonry, which rarely has to be 215, mostly 140, or even 100, with struct calcs, on found that can be even narrower than the 60s cavity wall. EWI'd externally, as thick as you like, any facing you like, boarded, tiled, rendered, brick-slipped or patent fake-render brickwork. EWI makes insulation continuity easier at all points, can be taken right down to found possibly eliminating need for underfloor insulation.
This kind of re-thought eco-building is actually even easier than trad 60s cavity wall. And so it goes, with a bit of re-thinking.
Go for it!
Have you read Kate de Selincourt's article 'The cost of building passive' in Passive House Plus mag UK issue 3? At one point she suggests that with the incentive to simplicity of the envelope, a PH can cost no more that a standard spec house, laden as it is with frills – and figures to support that.
Techniques like EWI can result in a shell that's even simpler to build than a 1960s cavity wall, let alone the elaborated dogs dinner that cavity wall has become in trying to scrape thro Bldg Regs! WUFI study can reveal simplified (but unfamiliar, re-thought) safe ways of building stud, too.
this was first pointed out in Canada or the USA almost 40 years ago!
I repeated it in an AECB mag. article in about 2001 and did a few simple calcs. to illustrate that air movement transported more water than diffusion
Well it's sure arrived here now If I have anything to do with it!
Mark, are you saying that even a big localised crack like that would be a condensation threat, not just the smaller all-over ones like staple holes and overall dispersed OSB air permeance?
Fraunhofer research identifies the latter as condensation threat but not so much the big leaks (the opposite for thermal leaks of course).
Didn't realise I hadn't posted the Fraunhofer state-of-the-art (rudimentary!) paper on the subject http://www.brikbase.org/sites/default/files/best3_kuenzel.pdf and also Dave Howarth's one above. The answer is a black-box calc that comes out of a WUFI study! in which the only parameters are blower-door test result grade 1 2 or 3, and building's internal height.
They say they've calibrated the model against actual measurements but I have grave doubts about using the blower-door test result as criterion. It tells nothing about whether the leakage is the condensation-dangerous all-over kind, or is concentrated in a few condensation-harmless major window leaks, say, or wherever in between.
If great care has been taken to eliminate those window leaks, and also say edge-joint leaks between OSB boards used as air barrier, then the data you need is not blower-door test result, but air-permeance of OSB. This http://www.linkedin.com/groups/OSB-is-not-airtight-recent-2163729.S.60928033 and http://www.mendeley.com/download/public/35879/3686773611/9b3c4e723d2042867cf783960db3ebd04e66c472/dl.pdf report the present uncertainty of the latter, so I wish Fraunhofer wd start testing for that as well.
It's on the cusp – about to take off.
I realise that my reason for diverting the thread onto moisture etc, was that tho increased a/c rate apparently makes little difference to heat demand, it is exactly what causes great interstitial condensation risk, because of bulk-transport of moisture into cold parts (as distinct from and in addition to less significant diffusion of moisture).
It's the slow, steady, all-over leakage that is biggest problem for condensation e.g. thro staple holes in membranes, OSB permeance etc. Concentrated larger leaks cause little condensation problem because they have enough oomph warm up their passage walls.
This kind of leakage is driven 24/7 by stack-height buoyancy, insignificantly by reversible intermittent wind. So, for given blower-door test result, you need to worry about this effect 3x as much in a 3-storey building, as in a bungalow.
- AuthorPosts
