OSB VCL and foams vs natural materials & moisture open wall

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Posted (edited)

Context- retrofitting & extending a 60s cavity wall house to EnerPHit standard.  Our (PHI certified) architect is advocating moisture open approach and has specified:

  • Retrofit cavity =  Plasterboard | void | OSB | Blocks | cavity filled w/ bonded EPS bead | Brickwork | 200mm Pavatherm | Render
  • New-wall = Plasterboard | void | OSB | 300mm I-beam with Warmcell | 80mm Pavatherm | Render


I'm generally pessimistic about the airtightness prospects on cavity walls (a tiny crack on the inside can leak through the entire cavity and out through another tiny crack in the outside, of even down and through the foundations and under the garage slab we saw in one PH tour!)

A local builder experienced in EnerPHit (around 10 certified installs) has recommend we totally ditch moisture open design and fill the cavity with closed cell Icynene to in effect completely convert it to a solid wall (and also allows some considerable cost savings on other natural material selections).  I've been forced into doing my own fair share of Googling to discover there's still lots of conflicting advice and I'm still no wiser on the "right" answer, but overall feel keeping with moisture open path is my preference.  However some vapour permeability questions linger that I'd love some gathered wisdom / opinions / wild-guesses on:

  1. Smartply Pro-passive OSB datasheet variously states it is a VCL and a vapour barrier. Does this mean we're already well down the road of a "moisture closed" build up anyway? (Or, moisture "half-closed" to coin a phrase?)
  2. If the answer to the above is "yes", does this mean there's really no additional downside in using closed-cell Icynene anyway?
  3. Are bonded EPS beads good enough anyway at inhibiting air movement (as suggested here) so I should just forget about foam fill?


Edited by joth

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only comment i would make at this stage is  that from my investigations your taking about £30 persqm for closed cell foam when done to the inside of a bulding( before you closed wall with PB) at 100mm thickness 

,and the cost to go down to 50mm was not that much cheaper once they are doing it.

 but to fill  a cavity -apart from not being able t be certain its all filled with no voids ,they will have to be drilling walls every meter or so both horizontally and vertically 

before they even start .

so i can,t see it being that cheap?

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15 hours ago, joth said:

fostertom says

"vapour resistance at this (or any) point in the wall buildup is BAD." ... "VCLs are bad news" etc

Good old Tom ;)

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21 minutes ago, SteamyTea said:

Good old Tom ;)

This is the question from 2014.

I'm struggling to get my head round the physics (is that the right word?) of a timber frame construction, so you will have to bare with me on this one ---- so looking on a typica timber frame wall build up,

1 - plaster board layer
2 - a vapour impermeable layer (this stops the water vapour inside the house being pushed, pulled or otherwise drawn into the wall construction and hence stops condensation within the wall.
3- the timber frame and insulation
4 - wall sheathing
5 - a wind proof breathable layer (this is to allow any moisture within the wall to exit into the big wide world).
6 - a rain shield layer of some sort, cladding, bricks or block (rendered).

So do we need a sheathing layer and the wind proof breathable layer? What exactly are we trying to achieve?


These are the full answers from Tom Foster.

1 - yes
2 - leave it out altogether - although still mainstream, recent research, WUFI modelling and bitter experience shows that vapour resistance at this (or any) point in the wall buildup is BAD. It's a myth that water vapour originating from inside is the problem in walls - walls can't help containing enormous amounts of moisture, hopefully safely in vapour form, not condensing to liquid at any point - and 95% of that originates from outside and ebbs and flows diurnally and seasonally. Don't try to block it - the drying out needs to happen both outward and inward. An inboard VCL halves the drying potential of the wall, which may never dry out, but increase its water content year by year. VCLs used to work, still just do for mere Bldgs Regs levels of insulation, but accidentally IMO, as an air barrier rather than as a vapour barrier as everyone imagined. Now, with v high levels of insulation, VCLs are bad news.
3 - yes - but only vapour permeable insulation i.e. not closed cell PIR, PUR etc.
4 - yes
4a - Now you need more vapour permeable insulation, right across the external face including the studs, which on the system you describe are a straight-through uninsulated thermal bridge. External insulation will almost entirely kill that bridge. You need 250-300mm of insulation; unless you have studs that deep, it can't all go between the studs; part must be in additional layer outside the sheathing.
5 - yes
6 - but masonry will require an outboard foundation, so vastly increasing your foundation dig, muck-away costs, lots more concrete, greater effective 'footprint'. Cladding will need to be hung from the eaves.

Instead of both 5 and 6, patent thin-coat render direct on the external insulation is by far the cheapest.

You need a layer that's impermeable to air (but very permeable to vapour) at some point in the sandwich - doesn't matter where.

In addition a windproof layer outboard, if your outboard insulation is fluffy or has an 'open' surface - EPS is a little bit 'open' so prob needs that; PIR/PUR are not 'open' so don't need that.

This windproof layer is usually also the 'breather felt' layer, laid close fitting at well lapped joints. Don't trust sticky tapes anywhere. The breather felt is well enough water proof to catch any water that may get thro the rain shield. It must be very impermeable to vapour; it won't be very air-impermeable but good enough to keep blasts of wind off the insulation face.

Don't take my word for all this unless paying me a consultancy fee - check it to your own satisfaction.

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