Screws through a Vapour Control Membrane

Hi ok so:

I have a new loft conversion I have been working on, nearing completion. To maximise headroom, the flat roof section is an unvented, hybrid warm-cold roof, with insulation both above and between the joists.

Externally there is roofing felt, followed by some rigid insulation, then joists with more insulation between, followed by a vapour control membrane, then 6mm plywood and plasterboard.

The vapour control layer was done well, but I had some help getting the ceiling up and I am afraid that now there are screws piercing the vapour control layer. I assume that if they all went into the joists, this would be okay, but I am pretty sure that there are some that missed the joists when the plywood went up and, when the plasterboard went up, it was screwed to the plywood around all of its edges, so again would have missed the joists.

Have I totally screwed up my vapour control layer?

The membrane is Intello Plus, so in theory this will allow moisture out in summer. Also because of the rigid board above the joists, in a sense this is a partial warm roof.

Anyone know their stuff in this area? Everything I read when I started this said you really had to seal everything super well. I don't know how on the case I really need to be at this point, but obviously want it done right.

Cheers all

Jon

Have you got a 50mm ventilated gap between the underside of the felt and the insulation? If not you should, with continuous eaves and ridge vents.

Hi

That is a different system - that is a Cold Roof. As I mentioned in my post, this is an unvented Warm Roof. It is a different system.

If it's a proper warm roof then forget about it, penetrations through the vp cannot be avoided, you're overthinking it.

Yeah that's what I was thinking. (That I was over thinking it). It's a funny thing to be told you have to build a system which is completely sealed, then try to do it, then realise that there are loads of screws in it. Where would the line be between a sealed vp and one that was too damaged? Any idea? I guess if there were lots of lights with no tape around them etc. that would be too much.

If you consider what is going on Dynamically?

The pressure exerted by a screw piercing the ply and or Plasterboard is large, and the pressure exerted on the VB will crush / seal it to the extent that it would be very difficult for water vapour to get past this pinch point and access the roof void, the pressure exerted by a screw is really high.

On the other hand, if you think about it, don't over-think? there would be advise that all underside of the joists would need to have a layer of mastic or similar to prevent molecules of water entering at the screw holes.

Hope I have not over thought the question?

Cheers.

Ken.

Hi Ken

Yes, where the screws go into the joists, no problem, everything will be clamped. But where the screws miss the joists, then they will just pierce the membrane.

And yes, that is an over thinking option - to seal all the screws and the joints from the outside. Bit over the top, but it would maybe give me almost as much peace of mind as taking it all down, taping up all the holes and doing it carefully.

Or as Freddy said, maybe this is just normal for vapour control, and nothing is perfect. I could add, the membrane used is Intello Plus, which is supposed to let moisture out again at certain times of year...

What do you think happens when you use foil backed plasterboard with the foil acting as the vp? Worry ye not.

You should be more worried that you are using a low resistance vapour membrane in a hybrid roof.

Has a condensation risk analysis been done?

The amount of insulation between the joists will determine if there is a risk or not, to much and it will cause problems with condensation.

Some hybrid roofs have had problems with condensation, and you need to bear in mind that the intello plus membranes have lower vapour resistivity than standard polythene further increasing the risk.