Can SBR be used as a vapour barrier?

[dave_104]

Hi
I believe I need to put a vapour barrier above/within a small suspended ceiling I've made. The area will be used for kitchen things and I'm expecting I need to protect the main roof joists above from the condensation of cooking etc.

Trying to thread in a plastic membrane is going to be really tricky, especially when it comes to installing all the recessed downlights, taping up cable entries, and getting around other obstacles, etc.. I guess I could possibly leave the membrane really baggy and put loft lids above the lights to push the membrane away, but then I thought of plan-B.... It would seem a lot easier if I coated the back side of the ceiling plasterboard (there's just 3 sheets) with a heavy coating of SBR, and then use sealed downlights, and possibly add a little SBR to the jointing compound so we have a continuous layer. SBR can act as a vapour barrier so I read, would this seem like a mad idea ?

Thanks
Dave

 

[Ivor Windybottom]

No!

 

[robinbanks]

Warm deck roof?

Liquid vapour barriers exist though..

Before digging into this post too much further I'd like to see a sketch of the cross section you have

 

[dave_104]

100% correct there - it is a warm deck flat roof.
I know we can get DPM in liquid form (i.e. Blackjack etc), and as this is going to be in a protected space then I'd be surprised if there isn't liquid VB too, although it's not the traditional method used.

 

[Deluks]

What's the condition of the old felt?

You could use foil backed plasterboard or a continuous polythene sheet, sold as 'vapour barrier'

Steve Roofer on YouTube has a video about sealing round downlights, you box them in with a foiled taped box.

 

[Ivor Windybottom]

I don't understand what the issue is. Just attach some polythene sheet to that OSB, then plasterboard over it.

Put the polythene up with anything - double-sided tape even. It doesn't matter what, the plasterboard will hold it up later.

Recessed lights are always an issue... for insulation, condensation and fire regulations. It's why many have moved away from them, especially as LED fittings can be very thin nowadays. But if you really want them then they can be dealt with.

I don't see what's unusual about your room that doesn't apply to absolutely every other.

 

[ToolboxTalks]

SBR can help reduce vapour transmission, but it’s not really a substitute for a proper vapour control layer. It’s difficult to get a completely even, continuous coating, any small gap could let moisture through and cause condensation issues.

If fitting a plastic membrane is too tricky, you could look at foil-backed (vapour-check) plasterboard instead, and use sealed downlights or loft caps to keep the barrier intact. Much easier and more reliable in the long run.

 

[Ivor Windybottom]

foil-backed (vapour-check) plasterboard instead

Then you'll get fed up of everyone complaining their phone doesn't work in there.

We rented a house with a loft conversion that had been done entirely with the stuff. Rubbish mobile reception in the floors below, and almost no wifi coverage up there.

 

[dave_104]

Hi all
Thanks for your replies. I think I'll use foil backed plasterboards, foil tape over the timbers where they join, and foam up the edges. And also bring a plastic membrane across the plywood section and up the front with that being all connected with foil tape too.
The good thing is that I only needed to pay just £4 for next day delivery from Wickes of the 3 foil plasterboards which is a bargain. I was expecting 10x that for delivery!
I just need to work out how to fit and seal the downlights, I think the Youtube vid refers to making a box out of the cut square if it were insulation, and I couldn't figure how to do that with PB, so I reckon I could use the loft lids I've got, reach over the top and silicone them in place before I fit the next PB, and then get a foam nozzle up through the light fitting cutouts to seal off their cable entry points. Or I could also silicone them to the PB's and let it dry before lifting the boards up into place.

 

[Ivor Windybottom]

Prioritise fire safety over insulation and condensation concerns.

Check you're not creating a fire risk around the potentially warm light fittings.

 

[dave_104]

I'll be using these as I seem to have loads of them in my shed -

 

[Ivor Windybottom]

You'll be fine. Airtight is the ideal, in reality you do your best. 95% insulated is still lots better than not bothering to try. You'd find all sorts of holes and gaps in most "professional" jobs.

 

[dave_104]

Thank you

 

[robinbanks]

Just don't have downlights; they're **** (speaking as someone who has nearly 100 of them at home, I do regret making them the ubiquitous light fitting)

There are far more interesting, high performance, surface mounted light fittings to be had these days