Green Building Forum - Wall Build-up for timber frame structure validation

Wall Build-up for timber frame structure validation

Oj0001 — Thu, 27 Aug 2026 11:45:31 +0100

That has all made for very interesting reading, thank you!

How nice it would be to have access to WUFI software to validate different build ups, for all its drawbacks and pessimism, the Glaser Method is very easy to play around with. I don't suppose anybody knows of a UK service that's reasonable for validating a construction?

The more I look at the details I find the idea of moving the OSB to the internal face, keeping it warm and promoting outwards breathability appealing. The only concern I have using EPS or Rockwall EWI in this configuration, is that it wouldn't have full support across the back, but only on studs. Does anybody have experience with this configuration?

Wall Build-up for timber frame structure validation

fostertom — Thu, 27 Aug 2026 13:09:55 +0100

The point of putting the OSB3 vapour-breatheable but airtight sheathing/racking, all edges supported, gapfilling glued and screwed/power nailed, on the outside of the studs, rather inside as is universal timber framing practice, is that in that location it's uninterrupted by joist ends, partitions, service holes etc, all of which have to be carefully sealed around with expensive sticky tapes, which professional airtightness testers know are rarely done perfectly and may start parting within as little as six months. Avoid all that by putting the OSB3 on the outside face of the studwork i.e. in the middle of the insulation sandwich - it's then stupidly easy, foolproof to do by ordinary chippies (see pic). With plasterboard direct on the inside face of the studs (which is OK to puncture for services, so no service void required) and OSB3 on the outside, no problem with restraining the insulation between the studs.

Wall Build-up for timber frame structure validation

fostertom — Thu, 27 Aug 2026 13:14:03 +0100

Inside view of studwork, minimised because sheathed with OSB3 as racking membrane - almost a stressed-skin monocoque, and showing use of 0.7mm galv steel dryliner's straps to support board edges by self-tapping screws, rather than cut n fit timber noggings.

Wall Build-up for timber frame structure validation

Mike1 — Thu, 27 Aug 2026 22:52:46 +0100

In addition to the above, once built you want to avoid vinyl paints, which would block the vapour permeability of the structure, which could lead to problems. That is, choose a paint with an sd-value <= 0.5 m. If you search for something like 'breathable paint' there are plenty of them out there these days.

Wall Build-up for timber frame structure validation

fostertom — Thu, 27 Aug 2026 23:21:56 +0100

That's right. For ever!

Wall Build-up for timber frame structure validation

borpin — Thu, 03 Sep 2026 10:41:51 +0100

In my head (for the house I'll probably never build), I have been toying with a double 90mm CLS stud approach. Outside sealed much as Tom has, but a second stud inside with the studs offset where possible. This inner studd carries the floor above, window openings etc (so reduce cold bridging) and also allows room for european style roller shutters.

It would be a flat roof, cladding of some sort and metal web joists. Probably try to use wool. I used Icyene last time and really liked it for gap sealing. Seen too much about Warmcel slumping to trust it. I'd avoid insulation outside the OSB if I could (might need a bigger internal buildup). I worru about rodent damage to external insulation.

Posted By: Mike1once built you want to avoid vinyl paints

That's an interesting thought.

Wall Build-up for timber frame structure validation

fostertom — Thu, 03 Sep 2026 11:08:29 +0100

Say more about why double studs - why not just the inner set of studs? Not sure how roller shutters affects it.

Insulation outside the OSB is what puts it (and any timber) into the non-condensation zone. Without approx half of the insulation outboard, the OSB is right in the outboard condensation zone.

That's my concern also about timber I-studs, sheathed inside and out and blown full of Warmcel or similar - the outer sheathing and outer 'flange' of the I-joist are right in the condensation zone. But that approach is popular and successful, it seems, so such intermittent condensation may not be problematic. As long as it is intermittent and the re-drying potential is excellent incl I'd a thought a very well-ventilated cladding cavity (which isn't necessary in my scheme).

Wall Build-up for timber frame structure validation

djh — Thu, 03 Sep 2026 12:11:35 +0100

Posted By: fostertomSay more about why double studs

Sounds like a Larsen truss, without the connecting panels between the studs

Wall Build-up for timber frame structure validation

WillInAberdeen — Thu, 03 Sep 2026 12:36:52 +0100

Breathable paint - make sure to budget enough for it!

Just paid nearly £100 for a 5 litre tin of clay based paint, that covered 20m2

Wall Build-up for timber frame structure validation

fostertom — Thu, 03 Sep 2026 13:35:38 +0100

Actually, I hear that standard paints are quite permeable? True? But multiple re-coats over a lifetime?

Posted By: djhSounds like a Larsen truss, without the ...

Yes, that's what it's called, that I was describing, and questioning.

Wall Build-up for timber frame structure validation

WillInAberdeen — Thu, 03 Sep 2026 13:49:16 +0100

Afaics most of the info online is posted by people who sell clay paint or similar! Have you seen different?

The consensus seems to be that vinyl matt or acrylic matt are breathable enough that you cannot rely on them as a VCL, but yet not breathable enough that you can use them in a wood-fibre/lime type of highly breathable construction.

Being water based, they must have enough permeability to allow the water base to dry out. The manufacturers seem very reluctant to share SD values on the data sheets which suggests they're not good.

This leaves me in a dilemma - having spent a lot of time and cash on breathable insulation I'm not risking spoiling it with a non breathable paint - but it would be nice if something else were available a bit cheaper and with certified SD values!

Wall Build-up for timber frame structure validation

fostertom — Thu, 03 Sep 2026 14:51:20 +0100

That's clarifying - thanks Will.

Wall Build-up for timber frame structure validation

djh — Thu, 03 Sep 2026 21:01:11 +0100

Posted By: fostertomActually, I hear that standard paints are quite permeable? True?

When I built, the only one that published an Sd value was Dulux Trade Supermatt so I used that as an undercoat on all the internal walls. I used Earthborn clay paint as an undercaoat on the lime plaster and as a finish coat everywhere. I haven't found an Sd value for the Dulux paint in a quick search today.

Wall Build-up for timber frame structure validation

Mike1 — Thu, 03 Sep 2026 21:41:50 +0100

Posted By: WillInAberdeenBeing water based, they must have enough permeability to allow the water base to dry out. The manufacturers seem very reluctant to share SD values on the data sheets which suggests they're not good.

Watching paint dry: https://www.painternearme.com/color-insights-trends/understanding-science-behind-curing-process

Posted By: WillInAberdeenJust paid nearly £100 for a 5 litre tin of clay based paint, that covered 20m2

That's quite steep. Here in France the prices range from €12.50 (off-white) to €16 per litre, if you buy it in 10L tubs (so £110 to £140 for 10 litres), which is a little more reasonable.

It probably seems worse as regular paints are really cheap in the UK, compared to here. For example B&Q are selling 10 litres of white Dulux Matt for £28; here it will set you back £43 (€55). So I pay around 2 x more for clay, you're paying nearly 4 x more.

There must be some DIY recipes online...

Wall Build-up for timber frame structure validation

Victorianeco — Sat, 05 Sep 2026 21:23:39 +0100

So optimal build up appears to be:

PB
150mm studs infilled as necessary
11mm OSB
100mm EPS
Thin coat render

What if it was masonry?

PB foam fixed
block on flat
150mm EPS
Thin coat render

Sound about right?

Wall Build-up for timber frame structure validation

borpin — Mon, 21 Sep 2026 12:14:44 +0100

Posted By: fostertomSay more about why double studs - why not just the inner set of studs? Not sure how roller shutters affects it.

Insulation outside the OSB is what puts it (and any timber) into the non-condensation zone. Without approx half of the insulation outboard, the OSB is right in the outboard condensation zone.

My concern is the environmental damage to external insulation both from wind/water and rodent/pest. It is like a larsen stud, but tries to avoid the themal bridge over windows. The inner stud need the lintel to carry the floor/roof above, the outer one can largely be left out leaving a recessed section for the shutters.

I also worry about the mounting of external cladding over EWI.

It is just in my mind at the moment as an idea.

Anyone seen theses InSoFast Batt-Brackets in the UK?

Wall Build-up for timber frame structure validation

fostertom — Mon, 21 Sep 2026 13:18:11 +0100

Posted By: borpinMy concern is the environmental damage to external insulation both from wind/water

Not sure how you're tacking that?

Posted By: borpinand rodent/pest

Yes, that's perennial, but not solved by any timber construction - needs stainless mesh/EML or paving slabs.

Posted By: borpinalso worry about the mounting of external cladding over EWI.

And that's suddenly becoming a must, to W S E elevations (and roof slopes) - through-ventilated stand-off cladding, instead of direct-to EPS/woodfibre render, to protect against heat-wave super-heating of external face. Previously, coinciding with exposed rafter-ends/overhanging eaves/verges, I've suspended the cladding from above by the vertical battens (the crosswise ones being attached to that), because it's impossible to ask long-screws to carry that weight by cantilevering out, and not to rely on initial friction.

Wall Build-up for timber frame structure validation

borpin — Mon, 21 Sep 2026 13:43:17 +0100

Posted By: fostertomNot sure how you're tacking that?

No insulation outside the OSB.

Posted By: fostertomYes, that's perennial, but not solved by any timber construction - needs stainless mesh/EML or paving slabs.

But less of a problem and wire mesh over the OSB will prove a significant deterrant and easier to do than mesh over EWI.

Cladding on battens directly on the OSB through to the first stud layer is strong. The second stud layer then reduces the thermal bridges.

As I say, just some vague musings.