Vapour Control Barrier

[Warrentdo]

Moving on from my other previous post - 

I have a warm flat roof made up off -

GRP - 18mm OSB3 - 150mm PIR - VCL - 18mm OSB3 - softwood firings  - 200x47 C24 joists - Plaster board..

I've a few questions on the VCL bit.

Is the VCL just poly sheet? I've no details on the drawing.

It is meant to allow vapour to pass through one way as im not sure Polythene will do this?

Hunting around ive found that 500 gauge / 125mu 0.125mm thk Polythene sheet is ok but im finding it hard to source?

1000ga and 1200ga is readily available but classed as DPM.

Can anyone shed any light on this? 

 

Regards,

 

Warren.

[SteamyTea]

The term Vapour Control Later is a bit misleading.

All it is, is a barrier to water vapour. So is imperious both ways.

The concept is that the inside of the building (in UK) is warmer and has more water, by density, than the external air.

By stopping the flow of this warmer, wetter, air from passing though to the colder, insulated cavity, there is less risk of dew forming.

This reduces the chance of mould forming and causing problems.

 

As you have a GRP roof covering, which is a good vapour barrier, in the outside, is there any ventilation space below?

Edited December 29, 2022 by SteamyTea

[MJNewton]

17 hours ago, Warrentdo said:

Is the VCL just poly sheet?

 

It is meant to allow vapour to pass through one way as im not sure Polythene will do this?

 

It keeps the water vapour that is generated/exists inside the house on the warm side of the insulation so as not to risk forming condensation on the cold inside face of the roof covering (and any other part of the roof construction that sits on the cold side). 

 

 

[Warrentdo]

54 minutes ago, SteamyTea said:

The term Vapour Control Later is a bit misleading.

All it is, is a barrier to water vapour. So is imperious both ways.

The concept is that the inside of the building (in UK) is warmer and has more water, by density, than the external air.

By stopping the flow of this warmer, wetter, air from passing though to the colder, insulated cavity, there is less risk of dew forming.

This reduces the chance of mould forming and causing problems.

 

As you have a GRP roof covering, which is a good vapour barrier, in the outside, is there any ventilation space below?

Hello, only gap is between the joist (between top of plasterboard and underside of 1st OSB.

[SteamyTea]

22 minutes ago, Warrentdo said:

Hello, only gap is between the joist (between top of plasterboard and underside of 1st OSB.

That is where the VCL usually is.

I don't actually think having a VCL in the right place, and then a final vapour proof covering attached will give any problems, as long as there are no leaks.

[Warrentdo]

Currently my VCL is above my OSB. So the vapour will be getting to the bottom side of the OSB and will only get stopped if t gets through the OSB. would have though below the OSB would have been better but may be a struggle stopping the sag. maybe better idea painting something on the underside of the OSB to act as a VCL. that way no screws will ever puncture the VCL unlike what I have now?

Warren.

[MJNewton]

It's perfectly fine where it is - on the warm side of the insulation. 

[steveoelliott]

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.