Location of VCL - within insulation

24 Sep 2009
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United Kingdom
Hi All
Designing a wall type for an extension im doing, currently build up looks like this:

Outside to In

Oak Cladding (laid vertical)
Timber Battens (50mm) (25mm vert, 25mm horz)
Breather Membrane
9mm OSB
140 Stud filled with wool insulation
50mm PIR
25mm batten (and services void)
12.5mm plasterboard

I have purposefully left the VCL out of this list as I'm trying to work out the best place for it.

It seems pretty standard if I wanted to place the VCL on the inside of the PIR (ie between PIR and batten, but I'd like to explore the option of placing it outside the PIR.

Can I staple the VCL to the 140 timber frame, then place the PIR over this. Fixing the VCL to the frame will be easier and the PIR will offer protection to the VCL.

Anybody think this will create a moisture problem or built this detail themselves?

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I'm not an expert, but here goes!
Simply put, interstitial condensation can form in a material where there is a temperature differential between the two sides.
The VCL reduces the amount of moisture that can enter the material, and this minimises what will condense out within it.
Having the VCL behind the PIR may cause interstitial condensation in the PIR and dampness in the battens.
To an extent the problem may be limited in your case - most PIR is foil backed and (especially if the joints were foil taped) would create a primary VCL anyway.
It seems pretty standard if I wanted to place the VCL on the inside of the PIR

However, there may possibly be a good reason why this is standard practice! ;)

I hope that makes sense :)
Best place for vapour check layer?

You need to understand what a vapour check is meant to do, then the place for it becomes clear.
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Quite comfortable I understand what the VCL is for.

Question wasn't really about an individual component and its purpose, rather whether the movement of one element within a 'system' would create any other issues. A modern wall has obvious performance criteria to meet but also needs to provide 'in operation' functionality (enabling items to fixed, holding switched and sockets, protecting the system parts etc etc), I am trying to maximise performance and functionality of my particular wall type.

As its getting close to BC submission time I thought I'd find the time to do the calcs (hence the post at 1am) so, in case anyone else in the future has the same question the build up I ended up with is:

12.5mm plasterboard
25mm batten and air void
60mm PIR (Celotex GA4000)
1000 gauge VCL
140 timber frame filled with 140mm Knauf Earthwool Frame Therm
9mm OSB
Tyvek Housewrap
Timber Cladding (ignored in calcs)

This provides a U value of 0.16 with the calcs passing for surface humidity or interstitial condensation.

The reason I am attracted by this setup is the VCL is arguably more protected, the 25mm void can accept switch and socket back boxes without interfering with the VCL and the number of punctures into the VCL during construction will also be reduced. It will also mean people putting holes in the plasterboard in the future can be done with no concern about the fabric/system of the building.

Same performance, easy to build and better functionality!

Would be interested in hearing from anyone else who has built a similar system before.
Just been looking into some more details of PIR, and the likelihood of interstitial condensation forming within it, is very low (until a fair few years of out-gassing have passed).
However, the joints between the panels are still a weakness for condensation.
Foil taping and properly filling the joints, as well as having the VCL, may be a good 'belt and braces' approach?
Agree Random & noseall, good bit of taping to the PIR will also be prudent for a belt and braces approach.
This provides a U value of 0.16 with the calcs passing for surface humidity or interstitial condensation.
Lol. Passes the calcs. :rolleyes:

We see that every single year when the new crop of graduates are released into the real world.

BTW, what happens to the moisture, the higher local humidity trapped in the void?

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