Green Building Forum - Airtight membrane location Thu, 30 Apr 2026 18:11:55 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304883#Comment_304883 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304883#Comment_304883 Thu, 11 Jan 2024 10:48:07 +0000 GreenApprentice
Been busy renovating our 1960s bungalow and making the floor and walls airtight as we do it. We’ve now completed nearly half the house in terms of living area and need to consider sorting the roof space so we can get some insulation up there. As we’re aware that the airtight tight membrane is to go on the warm side, we have been waiting to be in a position to install that before insulating, but this will require better weather as we’ll need to be doing some of the work from the outside by removing roof tiles (shallow roof space).

As we will be installing MVHR when we’ve finished all the building work, we’ve decided to install recycled-plastic insulation as the first layer (110mm) as we’ll need to be messing about with it to install vents.

The question is, can we put the airtight barrier over this layer of insulation, across the ceiling joists and then pile on other insulation, probably a deep layer of fibreglass or rockwool, the MVHR ducts will be within this layer - we’re going to create a ‘warm area’ around the loft hatch where the MVHR equipment will be housed.

It’s not the end of the world if we have to move the insulation out of the way to install the membrane if this is a complete no-no, but wanted to get a view if this would be ok as it will save time and it will be easier to install over the joists than around them.

Many thanks in advance for any advice]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304886#Comment_304886 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304886#Comment_304886 Thu, 11 Jan 2024 12:18:13 +0000 djh
Why not put your airtight membrane over the roof joists and ceiling first, and then put your insulation over the top? Or use a paint-on airtightness product under the ceiling?]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304891#Comment_304891 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304891#Comment_304891 Thu, 11 Jan 2024 15:24:28 +0000 GreenApprentice We have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation. Due to the amount and type of work involved in that, it will have to wait until warmer weather. The house is very cold so the thinking is to chuck some insulation up there now which will be quick and will help retain some of our heat. Laying the membrane over that will be simpler than removing this insulation and dipping the membrane between the joists before putting the insulation back.
Thanks.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304892#Comment_304892 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304892#Comment_304892 Thu, 11 Jan 2024 18:48:56 +0000 tony Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304895#Comment_304895 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304895#Comment_304895 Fri, 12 Jan 2024 09:57:31 +0000 djh Posted By: GreenApprenticeHi,
We have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation. Due to the amount and type of work involved in that, it will have to wait until warmer weather. The house is very cold so the thinking is to chuck some insulation up there now which will be quick and will help retain some of our heat. Laying the membrane over that will be simpler than removing this insulation and dipping the membrane between the joists before putting the insulation back.
Thanks.I don't think I would compromise the long term plan for the sake of a smaller bill this winter. Either buy some warmer work clothes or turn the heating up. A fan heater in the loft would probably make it comfortable enough to work.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304896#Comment_304896 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304896#Comment_304896 Fri, 12 Jan 2024 10:03:12 +0000 GreenPaddy Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304897#Comment_304897 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304897#Comment_304897 Fri, 12 Jan 2024 10:31:12 +0000 Doubting_Thomas https://www.ubakus.de/en/r-value-calculator/

There's a function that advises on moisture risk. It's the much-criticised Glaser method, so very basic and nowhere near as good as WUFI, but as an initial check for peace of mind it's probably worth your time.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304900#Comment_304900 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304900#Comment_304900 Fri, 12 Jan 2024 13:41:42 +0000 Peter_in_Hungary Is the recycled-plastic insulation loose fill or blanket?
If you put the air tight membrane over the joists etc. then when the MVHR is put in you will need to properly seal the vents. If the membrane is over the joists it can easily be lifted along with the blanket type insulation, the MVHR easily installed without the need for duct penetrations, then the membrane over the ducts and joists then the insulation over that. i.e. no insulation between the joists and then everything is below the membrane except the insulation which is above.
Just a thought.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304902#Comment_304902 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304902#Comment_304902 Fri, 12 Jan 2024 18:19:06 +0000 GreenApprentice
Posted By: Doubting_ThomasIt's been mentioned on here a few times but I highly recommend this website to test construction build-ups (requires free registration to unlock all the features):https://www.ubakus.de/en/r-value-calculator/" rel="nofollow" >https://www.ubakus.de/en/r-value-calculator/

There's a function that advises on moisture risk. It's the much-criticised Glaser method, so very basic and nowhere near as good as WUFI, but as an initial check for peace of mind it's probably worth your time.


I’ll take a look at that link :bigsmile:]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304903#Comment_304903 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304903#Comment_304903 Fri, 12 Jan 2024 18:27:13 +0000 GreenApprentice Posted By: Peter_in_Hungary
Is the recycled-plastic insulation loose fill or blanket?

The one I was looking at is blanket on a roll, wide enough to fit between the joists. Loose fill may be a better option, I haven’t seen that when googling. Will take a look.

I like the idea of putting the ducts under the membrane as it would save sealing ducts out and back into the thermal envelope.

Thanks!]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304905#Comment_304905 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304905#Comment_304905 Fri, 12 Jan 2024 21:49:37 +0000 Mike1 Posted By: GreenApprenticeWe have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation.If you were to use a 'smart' membrane (Pro Clima Intello or similar), rather than the Airguard (which I don't think is), you could probably do this. It's one of the details proposed in a paper from Aarlborg University (but not by Pro Clima, AFAIK) - see p7 of this pdf: https://vbn.aau.dk/files/72606879/Installation_of_vapour_barriers_in_existing_buildings.pdf

It's also possible that the plasterboard could act as the airtight layer, if crack-free and sealed at all junctions.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304906#Comment_304906 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304906#Comment_304906 Sat, 13 Jan 2024 12:02:50 +0000 djh Posted By: Mike1
Posted By: GreenApprenticeWe have bought the DuPont airguard VCL with the plan to put it in the loft on top of the ceiling, over the joists and make fully airtight before adding insulation.
If you were to use a 'smart' membrane (Pro Clima Intello or similar), rather than the Airguard (which I don't think is), you could probably do this. It's one of the details proposed in a paper from Aarlborg University (but not by Pro Clima, AFAIK) - see p7 of this pdfYou mean fig 10? I've been suggesting that GreenApprentice follows the bottom diagram with regards to the membrane but to continue the insulation over the top of the joists so the membrane is everywhere warm. I'm not sure exactly what his current plan is.

edit to add: following my suggestion, it doesn't matter whether the membrane is intelligent because it's warm everywhere.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304909#Comment_304909 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304909#Comment_304909 Sat, 13 Jan 2024 17:07:09 +0000 Mike1 Posted By: djhYou mean fig 10?Yes, the lower diagram in fig 10. Sounds like we're suggesting the same thing then.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304910#Comment_304910 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304910#Comment_304910 Sun, 14 Jan 2024 10:25:42 +0000 GreenApprentice
We’ll also put a breathable membrane on top of the insulation as suggested by Tyvek


Posted By: Peter_in_Hungary
If you put the air tight membrane over the joists etc. then when the MVHR is put in you will need to properly seal the vents. If the membrane is over the joists it can easily be lifted along with the blanket type insulation, the MVHR easily installed without the need for duct penetrations, then the membrane over the ducts and joists then the insulation over that. i.e. no insulation between the joists and then everything is below the membrane except the insulation which is above.
Just a thought.


Thoughts on this - if we put something over the joists to support and protect the membrane (like cardboard?) there would be an air gap above the ceiling that the MVHR ducts sit in, with the airtight membrane above

Would this work?]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304912#Comment_304912 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304912#Comment_304912 Sun, 14 Jan 2024 17:00:17 +0000 Mike1 Posted By: GreenApprenticethe plan was to do the lower diagram, the difference is that the joists are only 7.5cm deep so there will be a lot of insulation above them. Unfortunately I bought the non-smart version of the airguard membrane, but djh is suggesting that so long as there is insulation above that, which there will be, then it will be okYes, with the insulation it will be fine.

Posted By: GreenApprenticeWe’ll also put a breathable membrane on top of the insulation as suggested by Tyvek
Presumably then you have a ventilated roof and the aim is to stop wind washing of the insulation. Which sounds fine, if Tyvek claim suitability for that (I've not read their datasheet).

Posted By: GreenApprenticeThoughts on this - if we put something over the joists to support and protect the membrane (like cardboard?) there would be an air gap above the ceiling that the MVHR ducts sit in, with the airtight membrane above
Two basic options:

1) Keep the airtight membrane against the ceiling plasterboard and cut (and seal) air vents through it, with the ducts running horizontally through (and surrounded by) the insulation. Or
2) create a service void above the insulation to run the ducts through, with the airtight membrane above as you describe. Plywood fixed to the top of the ceiling joist would be more robust than cardboard - 75mm deep joists would just about work with 75mm external diameter MVHR semi-rigid ducts, though you could add strips of ply on the joists to gain extra height where you have ceiling diffusers.

However, IMHO a service void between selected joists that only accommodates MVHR ducts isn't adding a great deal (compared to one that extends across the entire ceiling), so I'd choose the 1st of these options.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304913#Comment_304913 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304913#Comment_304913 Sun, 14 Jan 2024 20:02:38 +0000 djh
I don't know what duct system you're thinking of using, but I can generally recommend semi-rigid, which have a smaller dimension as Mike1 suggests. If you do choose such a system then do think carefully about how you'll manage the ducts around the plenums (a.k.a. distribution boxes). There's an awful lot of pipes to get into a small volume! But that's pretty much their only complication; in all other respects they're much better.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304914#Comment_304914 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304914#Comment_304914 Mon, 15 Jan 2024 11:45:51 +0000 Peter_in_Hungary
Will the air tight membrane be sealed at the edges? (of the membrane strips and the edges of the loft) if so how?]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304942#Comment_304942 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304942#Comment_304942 Tue, 16 Jan 2024 10:40:18 +0000 GreenApprentice
Posted By: Peter_in_Hungary
Will the air tight membrane be sealed at the edges? (of the membrane strips and the edges of the loft) if so how?


Yes, been using tescon elsewhere but happy to consider other products, been looking at the Tyvek alternatives. There’s a lot of ‘loft wood’ to wrap and seal around also especially at the eaves. I like the double-sided butyl tape idea for the gable block walls at each end, but not sure my husband will be happy with having to clamp it

Posted By: Peter_in_Hungary
thick(ish) paper will do that without the expense of tyvek



Like lining paper?

Posted By: djh
I don't know what duct system you're thinking of using, but I can generally recommend semi-rigid, which have a smaller dimension as Mike1 suggests.


That’s what we’re going to be using from BPC]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304954#Comment_304954 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304954#Comment_304954 Tue, 16 Jan 2024 13:50:34 +0000 Peter_in_Hungary Posted By: GreenApprentice
Posted By: Peter_in_Hungary
thick(ish) paper will do that without the expense of tyvek



Like lining paper?

Yes - but something a bit wider than the standard wallpaper roll might be better
On one job I did with 3 x 100mm glass wool I put the paper between the 2nd and 3rd layers figuring that although the last layer would get wind wash it was held much better by the 3rd layer than having nothing to restrain it. (It is a 36M x 6M loft with vents both ends)]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304957#Comment_304957 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=304957#Comment_304957 Tue, 16 Jan 2024 16:47:46 +0000 djh Posted By: GreenApprenticeThat’s what we’re going to be using from BPCWhich system? You might want to start another topic to continue this discussion.]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310337#Comment_310337 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310337#Comment_310337 Wed, 04 Mar 2026 17:19:34 +0000 oliviaaa99 Posted By: GreenApprenticeHi folks,

Been busy renovating our 1960s bungalow and making the floor and walls airtight as we do it. We’ve now completed nearly half the house in terms of living area and need to consider sorting the roof space so we can get some insulation up there. As we’re aware that the airtight tight membrane is to go on the warm side, we have been waiting to be in a position to install that before insulating, but this will require better weather as we’ll need to be doing some of the work from the outside by removing roof tiles (shallow roof space).

As we will be installing MVHR when we’ve finished all the building work, we’ve decided to install recycled-plastic insulation as the first layer (110mm) as we’ll need to be messing about with it to install vents.

The question is, can we put the airtight barrier over this layer of insulation, across the ceiling joists and then pile on other insulation, probably a deep layer of fibreglass or rockwool, the MVHR ducts will be within this layer - we’re going to create a ‘warm area’ around the loft hatch where the MVHR equipment will be housed.

It’s not the end of the world if we have to move the insulation out of the way to install the membrane if this is a complete no-no, but wanted to get a view if this would be ok as it will save time and it will be easier to install over the joists than around them.

Many thanks in advance for any advice

may I ask what you ended up going for in the end? We're in a similar situation (bungalow loft mvhr retrofit and unsure of how to insulate/where the membrane should go!)]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310338#Comment_310338 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310338#Comment_310338 Thu, 05 Mar 2026 09:16:10 +0000 Nick Parsons
It looks like GreenApprentice has not posted since before you joined the Forum, so they may not reply to your post. I'll have a look later and see if I can add anything helpful, and others may dip in too, I hope.

(Having a quick read now I think it's going to be a matter of opinion. I think I could read this twice and answer differently both times! I'll think it through.

Nick]]> Airtight membrane location https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310340#Comment_310340 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18063&Focus=310340#Comment_310340 Thu, 05 Mar 2026 16:49:52 +0000 oliviaaa99 Posted By: Nick ParsonsHi Oliviaaa99,

It looks like GreenApprentice has not posted since before you joined the Forum, so they may not reply to your post. I'll have a look later and see if I can add anything helpful, and others may dip in too, I hope.

(Having a quick read now I think it's going to be a matter of opinion. I think I could read this twice and answer differently both times! I'll think it through.

Nick
Hello Nick, thank you so much for your response, we're feeling totally lost at this stage so any thoughts or opinions would be incredibly helpful! I have started my own post instead if that is more helpful! Thank you so much :)]]>