Warm Flat Roof - Ceiling Membrane

[hankmarvin]

Good afternoon,

Quick question.

My roof construction of new extension is Joists > Deck > 100mm foil taped insulation board > deck > epdm.

Do I need to fit a membrane between bottom of joists and plasterboard?


Thank you

 

[catlad]

I would recommend using foil back plasterboard.

 

[steveroofer]

I take it that "foil taped insulation" means you are using insulation that has foil on both sides and you are taping the bottom junctions to form a continuous vapor barrier.
If this is so then you don't need to have any other membrane below.
Althow the manufacturers will say you can use the foil as the vapour barrier in practise it is not a good idea.
The first problem is how do you tape all the junctions at the bottom!
The second problem is how to you seal the edges and stop any escape of monster from below from rising at these critical points.
The third problem is how do you fix the
insulation down. If you screw through you punched the foil, if you glue you need to find a glue that both manufacturers agree will work and at the moment both parties are passing the buck.
Fourth and last problem is the insulation has foil on both sides so any moister that passes through the first then could get stuck by the second.
It's just not good practice to add extra barriers that are not needed.
A quote from a research paper
"When is a vapour barrier not a vapour barrier when there is a better one"

 

[hankmarvin]

Well the roof insulation is all on top, it's a warm roof, so I taped the top side.

But I'm talking about in the room, at the bottom of joists.

I didn't think I needed anything below as it's a warm flat roof but could there be a big enough temp difference each side of plasterboard.

Screwfix do a roll of vapour barrier for £18, same as foil backed plasterboard I guess but obviously I don't want to do the process if it's not needed on a warm roof.

 

[steveroofer]

In warm roof construction the vapour barrier goes on the underside of the insulation only. You should have no other vapour barrier anywhere.
The vapour barrier that Screwfix do is for walls not for roofs and does not reach the correct specification for a flat roof.
Unfortunately the best vapour control system for a flat roof is expensive is a tar based sticky back material with a foil over the top. If you look at some of my videos you will see its the only one we use it cost around £8 m².

The good news for you is that we know that most warm flat roofs are laid incorrectly and still work. The warm flats roofs is a good robust method and takes a lot of bad workmanship for it to go wrong.
We know that even if the incorrect vapour barrier or insulation is used they typically work. (I estimate that over 95% of warm roofs are installed incorrectly)
The key factors are

1. Avoid air movement
2. Avoid thermal bridging
3. Use the correct continues vapour barrier at the underside of the insulation ( if this is fixed correctly you won't have any air movement up into the insulation)
4. Use the correct insulation so that you end up with a roof that has only one vapour control layer ( what I mean is tissue faced insulation)
5. Pay attention to details so fill all joints between the insulation with expanding foam.
6. Extend the new warm roof down to the next thermal element of the property. (I show this on my video New warm roof)

 

[hankmarvin]

Ahh well that's ok then, I put dpm on top of my first deck followed by the insulation and then another deck and epdm.

I shall carry on with just plasterboard

Cheers