Missing vapour control layer in new warm roof

[Ian79]

With apologies for repeating myself from another thread, but wanted to pose the question / seek advice under a relevant heading:

 

We are in process of having a new rubber warm deck with lantern installed on an existing single story extension.

 

The roof is externally complete and waiting for plasterboard ceiling to go on. However, my reading has highlighted the fact that the builders have not installed a vapour control layer as seems to be required as part of this type of roof construction.

 

Should building control officers have flagged this as essential? They had plenty of chance to do so, having inspected at the roof construction stage, and did not raise it….

 

I’m not clear how much of a problem this is - how many future problems we are storing up? For context, it is not a room we will overly heat in winter / live in all the time - but it is south facing with a lantern / plenty of windows….

 

So at this point, I wonder what/ if anything I should do:

 

Should I be insisting that the builders dismantle the roof to install a vapour barrier below the insulation (would be a very big battle i imagine, and very inconvenient for us, as well as them!) ? Or is there any other vapour control /mitigation that could be done from inside, before plasterboard ceiling goes on? Or do we just live with it and assume / hope all ok…!

 

Any views welcome / appreciated.

[JohnMo]

Exactly what is your roof buildup currently?

[Mr Punter]

You can get paints that you can apply which will do the same job.  They can be overpainted.  You need to make sure that spotlights and other penetrations are sealed.

[Ian79]

16 minutes ago, JohnMo said:

Exactly what is your roof buildup currently?

new roof timbers, OSB, foil backed kingspan insulation (untaped, I think!), OSB, rubber membrane.

 

12 minutes ago, Mr Punter said:

You can get paints that you can apply which will do the same job.  They can be overpainted.  You need to make sure that spotlights and other penetrations are sealed.

Interesting! So you mean paint the inside of the plastered ceiling with this, once plasterers are all finished? Guess sealing the spots is for the electrician?!

[Happy Valley]

I would speak to the manufacturers of the rubber warm deck roof for clarification.

 

We have a warm metal roof on the house and a warm rubber bond roof on the garage. Both have at least 100mm Kingspan or similar and both have vapour barriers installed.

[ADLIan]

This form of roof construction should have a VCL on top of the deck/under the insulation. This will be covered in BS 5250, BS 6229, MI's and BBA Certs. Should also have been picked up by BCO as it comes under Appr Doc C. Only real solution is to strip and replace. How is the insulation & membrane fixed down to the deck?

[Ian79]

12 minutes ago, ADLIan said:

This form of roof construction should have a VCL on top of the deck/under the insulation. This will be covered in BS 5250, BS 6229, MI's and BBA Certs. Should also have been picked up by BCO as it comes under Appr Doc C. Only real solution is to strip and replace. How is the insulation & membrane fixed down to the deck?

Ok, thanks for taking time to reply.

 

I am really not sure I can face the big battle with the builders to do that - is that the consensus view that it is absolutely necessary to do so? BC seem happy to sign off as it is (not coming back until whole roof is sealed / room finished/ lantern installed - unless I ask them to)

 

Don’t know how the insulation is fixed to the deck - or even whether it is at all- though deck is pierced with nails in quite a number of places so could be those. I didn’t watch the roofers so closely early on….! Membrane is glued to the upper OSB.

 

 

[Mr Punter]

17 minutes ago, ADLIan said:

This form of roof construction should have a VCL on top of the deck/under the insulation.

 

Why?  I thought the VCL was to stop moist air migrating from inside the building and condensing on the structure and that a paint product on the ceiling could work for this?

[Radian]

There will be a vapor pressure from the room pushing moisture through the OSB. Once it reaches the dew line it will condense and drip back down onto the OSB. Might take 20 years to show as a problem (hence why builders sometimes 'forget' to do it properly).

If the PB ceiling has yet to be fitted you could mitigate by fixing a vapor barrier (plastic sheet) under the joists before the PB goes up. Keep downlighters etc. well sealed.

[Mr Punter]

46 minutes ago, Radian said:

If the PB ceiling has yet to be fitted you could mitigate by fixing a vapor barrier (plastic sheet) under the joists before the PB goes up. Keep downlighters etc. well sealed.

 

I think the paint will do as good a job as the plastic sheet, which will be pierced by multiple screws and may not be properly lapped / taped.

[Radian]

17 minutes ago, Mr Punter said:

 

I think the paint will do as good a job as the plastic sheet, which will be pierced by multiple screws and may not be properly lapped / taped.

Could do both, belt & braces. I hope @Ian79 can report back with what the builders say about this. I'm not totally convinced that BC should have picked up on it. I don't think their schedule of inspection involves looking just before the insulation is laid down on the prepared deck.

[Mr Punter]

29 minutes ago, Radian said:

I'm not totally convinced that BC should have picked up on it.

 

I rarely get a visit more than five times start to finish for new build housing.

[Redbeard]

A common approach is to lay the VCL on top of the bottom OSB deck, before laying the insulation on. A not uncommon approach is not to be too careful where the long screws go, between top deck, bottom deck and (supposedly) into the joists/rafters, meaning that the 'sandwich' is mainly fixed between OSB and OSB, leaving it insufficiently fixed down. I suspect this is why the 'deck is pierced with nails in quite a number of places'.

 

In the circs it may be that a retrofitted polythene VCL may not be fitted too 'tightly', and that in these circs the 'VCL paint'  might be OK. I normally worry about having gypsum (which, in my view, 'sucks' moisture) in a 'sandwich', but in terms of 2 'evils' the VCL paint may be the lesser. It will also allow you to connect wall VCL with ceiling, which can be very tricky if the VCL is above the bottom deck.

[Dave Jones]

On 01/07/2022 at 12:54, Ian79 said:

new roof timbers, OSB, foil backed kingspan insulation (untaped, I think!), OSB, rubber membrane.

 

Interesting! So you mean paint the inside of the plastered ceiling with this, once plasterers are all finished? Guess sealing the spots is for the electrician?!

 

 

the roof has to come off and be redone properly. It will just fill with mould as the moisture cannot get out.

 

The proper make up should be, from inside:

 

OSB board across joists.

adhesive VCL such as alutrix lapped down all the walls. Zero penetrations for lights or cables.

insulation such as kingspan thermataper to avoid having the hassle of firring strips.

waterproof top, edpm etc

 

BCO are mostly kids straight out of a college course with no real world experience and are often limited only to what they can see at the visit.

 

[Ian79]

Thanks all for the comments - much appreciated and learning a lot - though wish I’d had the knowledge to challenge sooner…

 

Clearly we are missing a really important element of our roof and I now understand the theory.  However I guess I’d like to understand real world implications (as opposed to theoretical) - with the best will in the world, I Imagine it is extremely difficult to get a VCl that is 100% joined up to the wall VCl at all locations, with no piercings - and I could believe that a majority of roofs would not achieve this (not wanting to be critical of the construction trade as a whole though, and apologies if being unfair!)  In which case is a compromised (Pierced / not meeting the wall VCl layer) better than no layer at all? Are these roof types routinely seen to deteriorate rapidly (1 year, 5 years, 10 years? More?) due to compromised or total lack of VCl? 

interested in thoughts….

[Radian]

I think it would depend greatly on how the rest of the roof is constructed. For instance, if there are parapets how are they built? How breathable is the makeup of the side elevations? When the sun heats a roof it makes the air in voids expand and then contract again at night. This will tend to repeatedly pump outside air out and in - and this will have a relatively higher moisture content. If it can diffuse through a semi-permeable material like OSB covered with a breathable membrane it will fare better than if fully wrapped in an impermeable skin. This may not describe your exact situation but I think it may still be worth watching:

 

 

[steveoelliott]

With my very limited knowledge, one thing that has always concerned me is that a correctly installed VCL would prevent detection of a leak in the roof membrane. When we had a leak it manifested itself as blown plaster on the wall and water tracking back from the wall across the ceiling. We have a VCL; whether it is installed correctly or not I don't know as the work was performed before we purchased the house.

 

With a VCL I believe a very slight leak may go undetected for sometime which may also cause problems for the roof I would imagine. Of course, that doesn't mean you should omit one but historically they weren't installed AFAIK.

Edited July 5, 2022 by steveoelliott