Mon, 05 Oct 2026 23:27:37 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309206#Comment_309206 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309206#Comment_309206 Thu, 11 Sep 2025 14:20:10 +0100 pete-s
The top floor of the house, in the attic, has stud walls separating the rooms and stairwell from the rest of the (unheated) roof space. These walls are original lath and plaster over wooden studs. The roof is clay tiles with a felt underlay.

My plan is to fill the space between the studs with flexible woodfibre insulation and then fix rigid woodfibre over the whole lot to help increase the insulation and hopefully limit any cold bridging.

In this scenario do I need to include a VCL and where would it go in the build-up?

I've attached a top-down plan of the wall in case my description above isn't clear!

Thanks in advance!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309210#Comment_309210 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309210#Comment_309210 Thu, 11 Sep 2025 21:11:48 +0100 Julio https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309211#Comment_309211 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309211#Comment_309211 Thu, 11 Sep 2025 21:16:07 +0100 Nick Parsons
If you are 'renovating' more than 50% of the thermal element (in the room) you will need BC approval, which means you also have to meet the U value target, or prove why you cannot. The sloping ceilings above need a U value of 0.16W/m2K, while the wall only 'needs' 0.3. I try to get as close as I can to 0.16 (or even better) on all surfaces. You'll only do it once!

Edit: Sorry, you asked where the VCL would be. Well, as above, (a) you may not need one and (b) if you do, it's immediately behind the plasterboard/final finish.

Aha! Julio posted while I was typing!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309279#Comment_309279 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309279#Comment_309279 Tue, 16 Sep 2025 16:29:39 +0100 pete-s
I absolutely agree with the mantra of getting the best U value you can. As Julio suggests access is a little tight so I only really want to have to do this once!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309284#Comment_309284 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309284#Comment_309284 Tue, 16 Sep 2025 17:16:14 +0100 Peter_in_Hungary Posted By: JulioVCL wants to go on the warm side or near the warm side of the insulation ideally. What is access like, tight I imagine.
Yup, the VCL needs to go on the warm side of the insulation.
Perhaps the easiest way would be to put the VCL on the inside of the lath and plaster and then plasterboard over.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309286#Comment_309286 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309286#Comment_309286 Tue, 16 Sep 2025 20:44:30 +0100 djh Posted By: Peter_in_HungaryPerhaps the easiest way would be to put the VCL on the inside of the lath and plaster and then plasterboard over.As long as thought is given to whatever wiring or other services are wanted in that surface.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309289#Comment_309289 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309289#Comment_309289 Thu, 18 Sep 2025 07:22:59 +0100 revor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309290#Comment_309290 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309290#Comment_309290 Thu, 18 Sep 2025 07:51:16 +0100 fostertom
It used to be marketed as a 'vapour retarder' but there's no such thing, so they dropped that. The thing about vapour barriers is the smallest single break in it is nearly as if there's no barrier at all - the vapour whistles through a tiny gap as if it was wide open. Think how quickly a bucket of water empties, with just a tiny hole, then imagine the water 1000x or something less viscous (as is water in vapour form) - it'd then all drain through in a second.

With vapour barriers, they need to be perfect or don't bother. So with WUFI I look for solutions that are fine with no VCL at all, and save all that faff.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309291#Comment_309291 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309291#Comment_309291 Thu, 18 Sep 2025 08:15:16 +0100 GreenPaddy With the vapour open materials noted on the the OP's drawings, if the loft void is ventilated, which is likely in a 1920's house, then I'd be confident NOT to use a VCL. But as is often mentioned on GBF, suppliers will often do a WUFI calc for you, to confirm interstitial condensation likelihood.
Air tighten of course around edges/interfaces with plasterable tapes etc.
The more challenging insulation zone is the inclined ceiling (in terms of depth available and maintaining the ventilation void to the felt, from eaves to ridge.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309294#Comment_309294 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309294#Comment_309294 Thu, 18 Sep 2025 11:50:54 +0100 Peter_in_Hungary Posted By: GreenPaddyWith the vapour open materials noted on the the OP's drawings, if the loft void is ventilated, which is likely in a 1920's house, then I'd be confident NOT to use a VCL.
Would you therefore also say that with a conventual roof / attic that is well ventilated with a plasterboard ceiling and 30 cm glass wool above the PB a VCL is also not required?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309296#Comment_309296 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309296#Comment_309296 Thu, 18 Sep 2025 11:58:44 +0100 djh Posted By: Peter_in_Hungaryplasterboard ceiling Plasterboard can have many different properties from vapour-open to vapour-blocking, so I think you need to specify more precisely what you mean.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309297#Comment_309297 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309297#Comment_309297 Thu, 18 Sep 2025 12:29:41 +0100 Peter_in_Hungary https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309303#Comment_309303 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309303#Comment_309303 Thu, 18 Sep 2025 19:26:15 +0100 GreenPaddy https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309304#Comment_309304 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18347&Focus=309304#Comment_309304 Thu, 18 Sep 2025 20:34:28 +0100 djh