Vapour Barrier? Drylining?

[atlasfever]

Hi All,

Hoping for some feedback:

As I understand, for a single skin brick wall insulation, you can use a rigid insulation bonded to a plasterboard. And, I have read that you need a vapour barrier that is bonded between the insulation and plasterboard.

It was suggested that I use GTEC 27mm XP, which does not have this 'official' vapour barrier. This is about half the cost of supply, than GTEC K, which definitely has a vapour barrier. I called up GTEC, they said that you can only use the XP if you're insulating a cavity wall, but if you're insulating a skin brick wall, you need the 2nd one. Technically, the rigid foam does provide some moisture resistance, 9MnSG, but the better one is about 100 MnSG. I have no idea what this means, only that the folks at GTEC said that the natural moisture resistance of the foam is not enough. (FYI: British Gyproc makes the same, except it's "Thermal Board Plus w/o vapour" and "super" with vapour barrier.

I feel that I need to do what's suitable in the long term, not what's cheap. But, I have a habit of doing the 150% level of perfection, when maybe you just need like 85% if you know what I mean. So, any feedback on this? My gut tells me to pay up and do the proper one, with vapour barrier (and by gut, I mean a thick document from english heritage for instance, a document from the council office and conversations with insulation providers

Thanks.

 

[Zebra]

I'm no expert, but looked into this myself a while back, and will share what I can remember. A vapour barrier is to stop warm moist air from inside the room finding its way through the insulation and condensing there. Warm air can carry more moisture than cold air, which is why when it hits a cold surface, it dumps all its water there. The thing that you are trying to avoid, in having a vapour barrier, is interstitial condensation, where condensation forms within the structure of the wall. With a single skin brick wall (is it 4.5 inch or 9 inch?) this is more of a risk than with a cavity wall, because a single skin wall is cold.
Suppliers of insulation are often happy to run some calculations through a computer programme which works out whether there is a risk of interstitial condensation with any particular wall build-up. It also works out the U-values that can be achieved with any particular wall build-up.
One thing to be aware of is that vapour barriers are only as good as their the person who installs them. If there are gaps where the moist air can get around them, or if they are punctured by electricians cutting holes for sockets etc, then their effectiveness is much reduced. They have to be sealed at the edges where they meet other wall structures, and there are special tapes sold for the purpose.
Also, a vapour barrier is, as I understand it, normally totally vapour impermeable. There are other types which let a certain controlled amount of moisture through - these are often called "vapour control layers", and where the outer layers of the wall are breathable, then this is acceptable. What you don't want is the inner layers of the wall more breathable than the outer layers, because then moisture can get into the wall from inside and condense in your brickwork without being able to get out again.
There may be cheaper alternatives. What about using foil-backed rigid foam insulation and taping all the joins, and then using normal plasterboard on top? The foil layer provides a vapour barrier itself, though whether this will be good enough to meet your English Heritage requirements I don't know.
Sorry for the ramble, have no idea if this helps. Hopefully someone more helpful will be along soon.
Z
PS I'm with you on always going for the 150% option, you don't want things to come back and bite you later!