[Tom Foster]

From http://www.linkedin.com/groups/OSB-is-not-airtight-recent-2163729.S.60928033 :
Seamus O'Loughlin: “By the way; If you change the airtightness level in a PHPP calculation from 0.6ACH to 2ACH, the energy demand of the house barely changes, so there's no ecological payback for all the work, products and tapes required to get the airtightness down to PH levels except to get a piece of paper and a badge on the house.”

Ann-Marie Fallon: “I just tested your concept in our EnerPHit … The biggest impact it has is on heat load, as you have to design in a heating system that copes with infiltration if the boundary isnt there to optimise the heat balance thats bigger than at the design air tightness of 0.6 say. It also has to do with Thermal comfort parameters ISO EN 7730 to ensure alternating areas of hot and cold surface and ambient temperatures are minimised in a PH. Having more infiltration in such a well insulated home such as 2 ach would have an impact on the occupant comfort.”

Anyone understand that reply?

[Tom Foster]

No, masonry can be very problematic – esp with substantial IWI – and steady moisture-leakage outward wd exacerbate that. Masonry may be much more (or much less) tolerant, but can still do with checking in WUFI.

In particular, providing a rain-sponge (masonry cavity wall outer leaf) as outer facing to substantial insulation seems far from robust , reliable or traditionally proven.

I'm still aware that this thread started about heat loss, and I went and diverted it to the (in some ways contrary) question of moisture risk.

[Tom Foster]

treatment of thermal mass … I'm writing a book which may help rectify this.

I'll look forward to that!

North America has differences from northern Europe. Many well-known cities in the NE USA are 10-15 degrees nearer the equator than English cities. Blue skies are more common in winter than they are in N W Europe.

According to WUFI data, Seattle (cold year) seems to be not unlike Oostende, which in turn is a slightly tough climate applicable to southern England. Main difference is Seattle's wind is from SSE, Oostende's from SW.

Southern Scandinavian weather may similarly do for N Britain.

No other European or US/Canadian weather (e.g. not Vancouver) in WUFI comes close. I note this because meanie UK and Irish met offices won't release 'free' climate data for issue with WUFI, unlike all of Europe, US, Canada, and rest of world.

[Tom Foster]

Try me! when you're ready. Can discuss options/variants with you, see how to reduce risks, if any.

[Tom Foster]

I'll read that, thanks Dave. Here's the Fraunhofer take on it.

Currently on WUFI forum, Fraunhofer guys are defending whole-building blower-door test result (or what you expect yr building design to achieve) as gd basis for setting the moisture-loss air leakage rate (i.e. the weak dispersed air leaks, discounting the strong major-crack air leaks) that should be input into a WUFI simulation. I'm saying that cd be way off.

Say you've really taken steps to design-out the weak all-over leaks, for an air barrier that is durably and persistently airtight (in my case, OSB bubble-glued and screwed 'jointless tea-cosy' all over the outside of the stud frame – studs and rafters). Then the blower door test will reflect only window leaks and/or other major leaks.

Running WUFI on such a wall construction, can assume the joints really are airtight, whatever the blower-door test result. Then the data you want is not the blower-door result, but the air permeance of the OSB proper (not its joints). So I'm saying, start testing materials for air permeability, e.g. they're testing Smartply right now. But they don't get the point! I'd sooner rely on Fraunhofer tests, rather than e.g. http://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/98109.htm

[Tom Foster]

To be clear, in the off-topic red herring I offered, was talking about internal air infiltration, or leakage, outward thro the fabric.

Of course infiltration in either direction affects the ventilation portion of heat loss proportional to the bulk air flow, driven by both stack effect (height of building) and wind profile.

However, as far as moisture risk is concerned, different picture. Inward infiltration generally causes no problem. Nor do large outward leaks, surprisingly, because there's enough energy flow in the airflow, to warm up the crack's surrounding surfaces, so there's little condensation. It's the small, all-over leaks that can cause more interstitial condensation than traditional diffusion. And as wind is reversible, causing as much inward as outward flow, its effects are self-cancelling, so wind profile is disregarded. Stack effect, causing steady 24/7 outflow, is the driving force for condensation risk. It's a calculation still in early days of development, so far guided by a bit of empirical research and a lot of value-judgement.

Though this is different from the subject of your post, if you want to “have a WUFI analysis done”, I'd be glad to be commissioned to explore your case with you, and improve your safety margins, if that looks necessary.

[Tom Foster]

This is a typical WUFI simulation scenario, and it's shown to be a generally robust solution.

Yes the outer parts of the insulation get more or less wet, depending on the liquid water absorbtivity of the outer skin material (or treated surface) and on local driving rain (wind direction/strength/duration + rain strength/duration) but it usually dries out seasonally. However under extreme conditions/absorbtivity and/or poor seasonal drying, WUFI may show that the moisture accumulates year by year; then something has to be changed.

It better be a non-organic, non rotting insulation, as it spends so much of its time above 20%-by-weight moisture content (whether vapour or liquid). Rot won't happen till 27% sustained, unless encouraged by adjacent established rot; 20% is the rot-warning safety point.

[Tom Foster]

bulk air leakage, how is bulk defined?

Any air movement.

model humidity as found in a house with MHRV, or is the water vapour coming in from outside?

There's so many surprising turnarounds, that such research shows, and is embodied in WUFI. One of them is that MHRV, or not, is irrelevant to internal humidity! MHRV is just a means of constantly importing outside air, which means that inside water vapour concentration even more faithfully follows outside (tho RH is modified by being warmed).

All four of the alternative common standards for deciding internal humidity for calc purposes, which can be chosen amongst in WUFI – EN13788, EN15026, ASHRAE 160, and applying a decrement-and-delay sine curve, all more or less say that inside humidity follows outside, with lag and decrement depending on ventilation rate vs internal water vapour generation rate.

So the answer is 'overwhelmingly from outside', the only question being with what delay the outside overwhelms the inside humidity! And the result is that internal RH levels are 'normally' much higher than we've been used to assuming, or futilely striving for.

[Tom Foster]

That is v surprising. Another point about air leakage is that Fraunhofer research is showing that moisture transported by bulk air leakage into and thro walls etc, has a massive effect on the moisture situation within those walls – puts diffusion in the shade. Therefore, minimise such moisture transport by minimising air movement through.

Also blower-door testing says nothing about stack effect, which in reality is what drives air leakage, far more than wind pressure. That's to say, for a given blower-door test result, a 3 storey house will have 3x the air leakage, in practice, of a bungallow.

[Tom Foster]

Sounds like you have something to prove – that you're more right. That makes you turn down the offer of an interesting discussion that could bring understanding. You said '23hrs' as if it's an important virtue – I'd like to know how that's a good number. Any idea?

[Tom Foster]

23hrs seems a non-useful lag, re-delivering yesterday's heat at same time today. 8-12hrs is useful, re-delivering daytime heat in the evening.

To me, simple massiveness added to the walls, floor, partitions etc of a near-PH building is ineffective, being bi-directional i.e. window-collected solar heat In daytime, Out by same route reversed evening – activates only the near-100mm of the mass at best.

Mass has to be used more knowingly, so heat is input to one face of the mass and travels uni-directional through to the interior. That is the case with wall-collected solar heat, but is defeated by super-insulation, so is hard to see how ICF walls would make any difference either way; U-value alone would count.

[Tom Foster]

This is more complicated than general, reliable practice. EWI is dead easy – no specialists reqd. 2x60mm is a bit skinny – aim for 200 or even 250. Enlist the regional tech rep of your chosen EWI product supplier – the SW Parex rep for example is wonderfully helpful.

Wd be an extreme situation, to have to parge first. Don't know what yr wall is made of, but if suitable, 200 'platinum' EPS can be fixed simply with e.g. Parex cementitious adhesive, dot n dab and/or strips of. If not sound, then mechanical fixings also reqd – the rep will advise. He'll encourage you to buy the EPS (1200x600x200 blocks – 'aged') direct from insul wholesaler, SGI, Encon etc – shop around. The blocks shd be foamed together at the joints, as you go – so no need to lap 2 layers. Rasp the surface/joints flush. Then EWI supplier's thin base coat, with glassfibre mesh pressed into it, then v thin acrylic finish in a wide range of textures and colours. The rendering will need an experienced plasterer but the rest anyone can do, with care and attention to detail. Round windows, edges etc are another story.

Easy – and getting cheaper all the time.

[Tom Foster]

Wow that's a rich vein of things to read Mark, which I wasn't aware of. Off to Dublin Thurs for a WUFI refresher, returning to catch Therm in Bristol on Fri!

[Tom Foster]

For a start, MF performance degrades where it's touching something – it needs to 'see' an air gap, however small. So 2 sets of battens, crosswise to ea other, are necessary, each of them deeper than the full (not the half) thickness of the MF – because the MF weaves under/over both sets of battens so gets pushed from side to side of the cavity.

When the MF industry was still fighting to get theoretical and practical recognition of MF's effective but unconventional mode of functioning, stand-off brackets were under development, to do away with counterbattening (which degrades performance by too much contact). These, a bit like non-conductive wall ties of the type that supports insulation, would skewer (or clamp) the MF to hang free and flat within the cavity, instead of weaving about, so reducing the necessary cavity depth. Plasterboard or whatever would be fixed to pads on the ends of the brackets.

But the MF industry accepted defeat by the vested interests of the conventional insulation industry, so now they half-heartedly market worst-of-all-worlds solutions where under-rated MF is teamed with the most expensive and un-eco kinds of conventional insulation boards. That's what you've been specified – and the MF manufacturers don't even bother to stipulate the double battening.

[Tom Foster]

Discussion currently in http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=11107&page=1#Item_13 – incl using generic FEA freeware http://lisafea.com .

FEA models in time-steps, and so needs materials' density and HSC info as well as conductance; whereas Therm knows nothing of time and just gives the steady-state (or eventual equilibriated time-step) situation.

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