Green Building Forum - Timber-frame house and EWI? Mon, 25 May 2026 20:27:10 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146526#Comment_146526 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146526#Comment_146526 Sun, 20 May 2012 20:56:27 +0100 borpin Posted By: TimberI am not saying there will 100% be problems with your proposed system, but it does fly in the face of convention. Oh I like doing that :bigsmile: Too often I feel things are done just 'because'.

I don't see the need for the VCL as the PIR (bonded together and taped) should do the same job. How is the moisture laden air (needed for condensation to form) going to migrate, and from where to where? The Icynene is carefully selected as an open cell insulation allowing any moisture to migrate in the warm direction. The condensation Risk Analysis I did (which could be wrong mind you) showed no risk. Weak points are between the TF frames to ensure a good seal.

If it is not detrimental to the structure, then it is simple, quick and very effective.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146539#Comment_146539 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146539#Comment_146539 Sun, 20 May 2012 22:07:38 +0100 Ed Davies Posted By: Ed DaviesAh, so you were fibbing; the VCL is on the inside. Fine.
Posted By: borpinNo the celotex (foil faced PIR is outside the frame.

The frame's irrelevant. The key point is that what you're regarding as the VCL is on the inside of (enough of) the insulation.

I can't imagine how you're going to bond and tape the inside of the PIR properly with the frame in the way in places. Do you have a cunning plan? Is that what you mean by “Weak points are between the TF frames to ensure a good seal”?

By the way, my plan is much as Timber says, inside-to-out: rafters / (ply|OSB|T&G) decking / taped & sealed VCL / (PIR|PUR) / OSB sarking / (steel|oduline|whatever) / PV|ST]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146544#Comment_146544 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146544#Comment_146544 Mon, 21 May 2012 07:03:34 +0100 borpin Posted By: Ed DaviesThe frame's irrelevant. The key point is that what you're regarding as the VCL is on the inside of (enough of) the insulation. The frame is not irrelevant. The VCL is there to prevent moisture forming in an area that would be detrimental to the structure.

Your buildup has OSB on the outside, mine doesn't. This is my point - really examine *why* you have the VCL - what is it actually doing? Why do you need the inner OSB? Racking? Why the external OSB? why not just counter battens? Where would the moist air come from and go to and where would it meet a cold surface? Your VCL is stopping moist air movement to where exactly - the inside of the PIR (as it can't go through it except in very small quantities even if you don't bond the PIR and tape it) So this air reaches the inside of the PIR, will the moisture condense? unlikely but remember, if it does it would condense on the inside of the VCL as that is the same temperature.

My point is we seem to blindly follow the VCL dogma. In a first gen TF house (Studs, OSB and filled with rockwool) it was very necessary (in theory) as any moist air _would_ condense on the cold OSB. The construction of TF (and structure in general) has changed yet our thoughts on a VCL seem not to have.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146550#Comment_146550 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146550#Comment_146550 Mon, 21 May 2012 07:52:26 +0100 DarylP
I apologise if I misled you, I wasn't trying to fib:cry:; I was trying to illustrate the need to do without a 'conventional' VCL, as the inner (and outer) foil faces of the Celotex forms the VCL.
The outermost part of the TF is still the OSB sheathing, the outside of this is adjacent to 100m PIR, so this keeps the OSB and everything inside of this at near room temps/humidity, even in Dec and Jan according to the JPA Dewpoint analysis.

BC didn't seem to have an issue with it, and like I say the owners should be in soon....

Cheers..:smile:]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146568#Comment_146568 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146568#Comment_146568 Mon, 21 May 2012 10:17:58 +0100 Viking House http://www.viking-house.ie/downloads/Tsong79.pdf
http://www.viking-house.ie/downloads/Spokane.pdf]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146571#Comment_146571 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146571#Comment_146571 Mon, 21 May 2012 10:46:23 +0100 Ed Davies Posted By: borpinThe frame is not irrelevant. The VCL is there to prevent moisture forming in an area that would be detrimental to the structure.
Indeed, you really don't want moisture and rot on the structure. But you don't want it anywhere else, either; the VCL is there to prevent significant quantities of water vapour getting to any cooler parts of the building.

Your buildup has OSB on the outside, mine doesn't.

Yes. It's a bit distracting in that I'm actually taking about my roof as I don't plan to have walls (A-frame). The outside OSB is the sarking board. Agree that for a wall in Scotland counter-battens would be fine but I think they get sniffy about those in roofs in windy places.

This is my point - really examine *why* you have the VCL - what is it actually doing?

I don't see how to seal the joins in PIR foil facing reliably when placing the sheets up against some other structure, be it a timber or steel frame or some sort of decking. It seems to me that laying some sheets, stapling it down and taping the joins so the staple holes don't leak is much simpler and more reliable.

Why do you need the inner OSB? Racking?

Yes, racking I imagine (to be determined). Also in my case it will be the finished wall/roof surface hence the probable use of T&G for at least part of the wall/roof just to look nice.

Why the external OSB? why not just counter battens? Where would the moist air come from and go to and where would it meet a cold surface? Your VCL is stopping moist air movement to where exactly - the inside of the PIR (as it can't go through it except in very small quantities even if you don't bond the PIR and tape it)

I'd have thought massive amounts of moisture (relatively speaking and over a long period of time) would migrate through the gaps between sheets.

So this air reaches the inside of the PIR, will the moisture condense? unlikely but remember, if it does it would condense on the inside of the VCL as that is the same temperature.

But the outside of the PIR is at much lower temperature. If the vapour gets all the way to the outside or even to half way out (e.g., with two layers of 130 mm PIR appropriately staggered) it'll condense.

My point is we seem to blindly follow the VCL dogma. In a first gen TF house (Studs, OSB and filled with rockwool) it was very necessary (in theory) as any moist air _would_ condense on the cold OSB. The construction of TF (and structure in general) has changed yet our thoughts on a VCL seem not to have.

You're right: it's the outer OSB layer's lack of permeability which is the problem. In many ways I'd rather do a pure “breathing” roof (as per Neil May) but cost and concern about fixing through such a thick layer of insulation put me off.

Still, I'm mostly interested to know how you can make the inside surface of the PIR really vapour tight when it's assembled against a frame.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146572#Comment_146572 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146572#Comment_146572 Mon, 21 May 2012 10:52:49 +0100 Viking House Posted By: Ed Davies Still, I'm mostly interested to know how you can make the inside surface of the PIR really vapour tight when it's assembled against a frame. The Canadians who built some houses in Ireland first put on a 50mm sheet of PUR, taped, sealed and foamed all the joints and then put on the second layer of PUR and taped and sealed that!
The links I posted above suggest that the amount of moisture diffusing through an airtight wall is pretty minute!]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146577#Comment_146577 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146577#Comment_146577 Mon, 21 May 2012 13:16:54 +0100 Ed Davies Posted By: Viking HouseThese 2 studies may be of interest!
http://www.viking-house.ie/downloads/Tsong79.pdf
http://www.viking-house.ie/downloads/Spokane.pdf
Thanks - read the whole of the first and the abstract and conclusions of the second.

However, they're hardly discussing high levels of airtightness. In particular, the airtightness on the outside of the insulation is practically non-existent with near-air/vapour-open cellulose and mineral wool and poorly installed U-F with major shrinkage and cracking problems. They mention the use of felt as a moisture barrier but don't indicate how well sealed it typically was. I get the impression it wasn't that tight, though. This means that any water which does condense will have ample opportunity to dry out pretty rapidly.

This is supported by their observation of large numbers of water leaks of various types (through the cladding or from pipes) not causing much damage.

I think it would be courageous to extrapolate those findings to a wall or roof which has an airtight barrier on the outside, even if that barrier is supposedly vapour permeable. Also, I can't see how it helps with the case of PIR sheets which are pretty impermeable, I think, but have the potential for airpaths between the sheets if they're not joined properly.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146578#Comment_146578 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146578#Comment_146578 Mon, 21 May 2012 13:18:23 +0100 Ed Davies Posted By: Viking HouseThe Canadians who built some houses in Ireland first put on a 50mm sheet of PUR, taped, sealed and foamed all the joints and then put on the second layer of PUR and taped and sealed that!
Clever, but is that really a huge saving vs putting a sheet in first?]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146590#Comment_146590 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146590#Comment_146590 Mon, 21 May 2012 15:27:09 +0100 Ed Davies Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146592#Comment_146592 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146592#Comment_146592 Mon, 21 May 2012 15:30:32 +0100 DamonHD
Rgds

Damon]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146614#Comment_146614 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146614#Comment_146614 Mon, 21 May 2012 20:08:40 +0100 Cerisy
Basically it is a "breathing"wall construction as the house will be air-tight (using the OSB sheathing on the frame as the air-tight layer - either internally or externally depending on how we source the frame) and the ventilation system keeping the humidity of the interior to acceptable levels.

Is the advice in this thread moving away from the breathable construction? Thanks, Jonathan]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146616#Comment_146616 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146616#Comment_146616 Mon, 21 May 2012 20:16:16 +0100 borpin Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146622#Comment_146622 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146622#Comment_146622 Mon, 21 May 2012 20:54:16 +0100 Timber
You refer to a vapour membrane on the outside of the frame and insulation. This is a LOW moisture vapour resistance breather membrane. On the INSIDE of the wall will be a HIGH vapour resistance Vapour Control Layer (VCL).

PLEASE make sure you get this the correct way round.

For your wall construction, please IGNORE this thread!!

If you need further clarrification please do say. I would be happy to do you a drawing, to make sure there is no doubt!

In addition, I would recommend the use of a service void on the inside of the wall (inboard of the vcl) as this will allow you to achieve a much better air tightness performance.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146630#Comment_146630 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146630#Comment_146630 Mon, 21 May 2012 22:21:37 +0100 Cerisy
Borpin - the last condensation risk analysis was probably at college quite a few years ago (nearly 37 years!). Rusty would be an understatement! I could increase the external insulation from 50 to 75mm - still issues to be resolved!

Timber - I appreciate the that the external membrane is low moisture resistance such as Tyvek House Wrap. If I have the OSB sheathing on the inside of the frame do you think I can fix the VCL to that to make achieving the required air tightness easier? I can then fit the 120mm internal semi-rigid insulation and then the 50 x 75mm stud wall for services and plasterboard - filling this internal wall with insulation around the services. Thinking of the sequence of works as well as the ease of access for the air tightness sealing.]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146635#Comment_146635 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146635#Comment_146635 Mon, 21 May 2012 23:19:27 +0100 Ed Davies Posted By: CerisyBasically it is a "breathing"wall construction...
...do you think I can fix the VCL to that...

Have we a terminology problem? Isn't a wall with a (working) VCL, by definition, not a “breathing” one?]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146655#Comment_146655 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146655#Comment_146655 Tue, 22 May 2012 09:19:21 +0100 davidfreeborough Posted By: CerisyIf I have the OSB sheathing on the inside of the frame do you think I can fix the VCL to that to make achieving the required air tightness easier? I can then fit the 120mm internal semi-rigid insulation and then the 50 x 75mm stud wall for services and plasterboard - filling this internal wall with insulation around the services. Thinking of the sequence of works as well as the ease of access for the air tightness sealing.What's your U vlaue target? Have I understood your build-up correctly?

Weatherboarding;
Ventilated drainage cavity;
Breather membrane;
50mm semi-rigid mineral wool between battens;
150mm frame fully-filled with mineral wool;
OSB racking board;
Vapour control membrane;
120mm semi-rigid mineral wool;
75mm service cavity fully-filled with mineral wool.

If so the vapour control layer would be half way through the insulation & at risk of collecting condensation. It would be safer to fit it on top of the internal semi-rigid insulation securing it with the battens forming the service cavity. Alternatively, move the internal insulation to outside the frame.

David]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146660#Comment_146660 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146660#Comment_146660 Tue, 22 May 2012 10:28:45 +0100 Cerisy Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146667#Comment_146667 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146667#Comment_146667 Tue, 22 May 2012 11:39:38 +0100 davidfreeborough
How rigid is your semi-rigid insulation? Is it rigid enough for you to fix the service space battens through it to the OSB/frame? If so, then you just need to start at the top & make sure that every other batten lines up with a lap in the VCL membrane.

David]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146723#Comment_146723 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146723#Comment_146723 Wed, 23 May 2012 09:19:31 +0100 Timber
As others have said, putting the VCL in the middle of the wall is a bit risky really, and may cause condensation.

What you are constructing, essentiall, is a twin stud wall construction.

How far along the process are you? Could you do something like this....

Cladding
Cavity
Breather membrane
OSB (sealed at joints to provide an outer air tightness layer
Load bearing timber studwork filled with insulation
Cavity filled with insulation
Second non load bearing studwork wall filled with insulation
High vapour resistance VCL membrane laped, taped and sealed to provide inner air tightness layer
25-50mm battens forming service void (also filled with insulation)
Plasterboard

That sort of gets you what you are after, but with a slightly more conventional set of layers and possitions of materials.

Ideally - you would construct the wall the other way round like this.

Cladding
Cavity
Breather membrane
OSB (sealed for outer air tightness)
Non load bearing timber frame (filled with blown in insulation)
cavity (filled with blown in insulation)
VCL
OSB
Load bearing timber frame (services between studs and insulation between)
Plasterboard

The second wall means that the thermal bridges at junctions are reduced, as well as keeping the structural frame on the inside of the insulation, and subject to internal air.

How far along with the project are you????]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146725#Comment_146725 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146725#Comment_146725 Wed, 23 May 2012 09:54:31 +0100 Cerisy
My choice of insulation is tempered by the need to make a very small budget stretch a long way - nothing new there! I had originally looked to fit Pavatherm externally but the cost and fixing issues steered me towards semi-rigid Rockwool - a material I have used many thimes before! I'm happy to take on new skills, but at our age I do need to get this completed within a reasonable time frame and I know that the services, for example, are going take a while to get right, so ease of construction on the walls was important!]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146728#Comment_146728 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146728#Comment_146728 Wed, 23 May 2012 10:18:41 +0100 Timber
For ease, the first of the two alternatives is better (easier to construct).]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146742#Comment_146742 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146742#Comment_146742 Wed, 23 May 2012 13:45:11 +0100 djh Posted By: Timberthe first of the two alternatives
Idle curiosity, but why does the first alternative only need one set of OSB, whilst the second alternative needs two?]]> Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146792#Comment_146792 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=146792#Comment_146792 Wed, 23 May 2012 21:15:40 +0100 Timber Timber-frame house and EWI? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=147063#Comment_147063 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6797&Focus=147063#Comment_147063 Sun, 27 May 2012 20:56:27 +0100 Viking House