Extension ceiling fabric choice by builder - Is this a breather membrane?

[andeebee]

Hi all, Would just like some reassurance over a building fabric usd in my renovation / extension.

I had originally asked that I wanted to fit a breather membrane like Pro Clima Intello fabric into the extension ceiling space between the living area and the warm roof, the builder replied saying he had some material that was expensive, so I felt reassured that this would be suitable.

My extension has had a green poly looking material fitted for some weeks now, which doesnt look like the Intello fabric up close, it looks like poly.

Can anyone tell from this photo what has been fitted and whether it is suitable to be fitted in the ceiling?

I found a similar looking material listed on ebay as a 'vapour control barrier' in the same green colour, but was to be used under wooden flooring.

 

I have given up trying to get the place airtight now as there are so many holes, but I dont want any problems due to the wrong material being used.

[Nickfromwales]

Defo supposed to be a VCL. Breather would be on the external surfaces. With correct detailing, it could become the AVCL, but a lot of questions would need answering to assure you of this, plus a lot more attention to detailing.

 

Are you having MVHR? I can't recall.

[Russell griffiths]

That looks like the cheapest polythene you can get. 
if it’s a warm roof shouldn’t the vcl already have gone in under the insulation. 

is this the same builder that didn’t seal up all the roof area that you had to redo. 
and the same one that built the timberframe 

I questioned a section of your timberframe saying it was wrong but you didn’t reply to that bit. 
 

if it is then you need a new builder. 

Edited yesterday at 10:36 by Russell griffiths

[Nestor]

On my warm flat roof I fitted the VCL after the Rockwool in between the joists, then OSB, Batten, Plasterboard.

I fitted 40mm Pavatherm above the joists.

[Nickfromwales]

Horse and the cart seem a bit confused here tbh. 
 

@andeebee, do you have a pic of the architects section for this roof build up?

[Redbeard]

14 hours ago, andeebee said:

 

I have given up trying to get the place airtight now as there are so many holes, but I dont want any problems due to the wrong material being used.

 

While the extension is incomplete it is the best time to chase and resolve the leaks. They will bug the hell out of you in future if you don't. I agree with @Russell griffiths that the membrane on the sloping soffits looks like standard thin poly VCL. There's a small chance it could be something else but that green is suspiciously indicative of 'Bargain Basement VCL'. I'd like Intello too.

 

Please do not give up on leak-finding! I assume the walls are leaky too? But even if they are not, as far as the roof goes you want (a) the right membrane and (b)someone  to make sure it is as tight as a tight thing from an air-tightness POV.

 

If the reason you have given up with a/t is the difficulty of finding the holes have you thought of making a basic 'blower door' (old desk fan or car radiator fan in a lump of ply) to tape into an opening? If you de-pressurise the room you're checking then, certainly in this weather, you will feel cold air rushing in. Arm yourself with good-quality a/t tape and you're off. I am sure someone(s?) on here has posted pics of such a home-made fan. Found it: 1st post on p.4 of this: 

 

 

Good luck. Do not give up, and please do not let them begin to plasterboard till you are happy.

Edited yesterday at 13:05 by Redbeard
@Sparrowhawk's fan

[Iceverge]

I'd would prefer nothing there than that layer of poly.

 

It will be far too resistant to vapour to allow any drying back into the room and your roof rafters will get damp. 

 

Intello or some other low sd or variable sd membrane or nothing 

 

What's being constructed is a recipe for rotting your timbers. 

[andeebee]

21 hours ago, Nickfromwales said:

Horse and the cart seem a bit confused here tbh. 
 

@andeebee, do you have a pic of the architects section for this roof build up?

We have removed the superfoil element. It was added as an additional U value factor. I opted to increase the thickness of the PIR on top to 150mm.

I guess had the superfoil remained it would have provided the same impermeable layer that the poly is now providing...

[andeebee]

On 25/01/2026 at 00:42, Nickfromwales said:

Defo supposed to be a VCL. Breather would be on the external surfaces. With correct detailing, it could become the AVCL, but a lot of questions would need answering to assure you of this, plus a lot more attention to detailing.

 

Are you having MVHR? I can't recall.

Yes, MVHR will be fitted throughout building

[andeebee]

Windows arrive this week

This is an element I can control as a. Its a square hole and b. the windows will (hopefully) fit

I taped the centre of the timber perimeter of the window aperture over the weekend with Illbruck tape and have some compriband ready for the external seal layer and illbruck airtight foam for the internal sealing

I will then tape the external bulding fabric to the window reveal onto the pre applied tape

The roof lantern is also being fitted this week. I plan to line the internal reveal around the lantern with intello and tape that to whatever I decide on the with the ceiling fabric