Vapour barriers and dry-lining

[electricianid]

I am confused about the recommendation of a vapour barrier between the plasterboard (or equivalent) and the insulation in a typical dry line system or come to that in a ceiling. I understand that this is to prevent warm humid air condensing in the cooler insulation layer.
My queries are,
1) If there is a layer of gypsum plaster applied to the PB perhaps with a few coats of semi gloss paint, will this not act as a vapour barrier alone?
2) If insulated PB is used (plasterbaoard laminated with say 30mm of insulation) then obviously it is not possible to include a vapour barrier in the recommended location, what then?
3) If PB is fixed onto a stud frame insulated with Celotex why would a vapour barrier be necessary, Celotex seems to be pretty waterproof (the foam part and the foil) so if any condensation were to occur between the insulation and PB even to the point of saturation surley it would just evaporate back into the warm room (path of least resistance)?
4) How would one go about fitting electrical boxes which may be 40mm deep, 12mm in the PB leaving 28mm to be buried in the insulation whilst maintaining the vapour barrier, ditto for downlighters in the ceiling?
5) On removing the 35+yr old ceiling in my house the fibre glass insulation and timber were completley dry (to touch) with no evidence of decay, despite there not being a vapour barrier, there would have been lots of humidity in this time as the only heating was gas bottle fires.Any ideas as to why this was not subjest to condensation?
6) Would it be acceptable to somehow enclose the pieces of insulation in a waterproof layer instead?
All of the intended drylining/ceilings will include a vented air gap between the stud wall/insulation and the external solid brick wall, studs could be of either timber or metal.
Apologies if I'm just being dumb, any opinions welcome.

 

[Johneds]

Building regulations doesn't so much say that you must include a vapour check - what it really says is that you must determine the risk and include one where the risk is high.

A plasterboard wall with a thick layer of mineral wool insulation and an impermeable outer roof membrane would be the ideal conditions and the risk would be high - very high. A plasterboard wall with a layer of closed cell foam insulation and an outer breather membrane would allow only a small amount of vapour to pass and the risk would be much reduced.

Vapour will also permeate joints between plasterboard sheets. So using foil backed board will stop some vapour but not all. It is also true that even a vapour check does not stop all vapour transmission - that is why the term vapour check is used rather than barrier. (although I must admit I am in the habit of using barrier myself)

So in answer to your questions;

1. A couple of layers of gloss would stop some vapour, but it's impossible to say how much and if it would be enough? Decorative layers are also unreliable.

2. Insulated PB is in itself a vapour check. There is still some vapour transmission via the joints but this is generally considered small and only a small risk. Standard wallboard with separate foam insulation and vapour check in between would be a better solution though.

3. With standard wall board on insulation between studs you have the added factors of more joints between the insulation and cold bridging across the studs. This is a situation where a vapour check would be advisable.

4. Electrical boxes can be sealed with vapour check adhesive tape or various other products produced for the purpose.

5. Old ceilings often suffer less from condensation because old roof constructions usually have much higher natural ventilation. Long term condensation in still quite common in this situation though. It causes the insulation to flatten and shows as a darkening of the surface on the cold side.

6. I don't think enclosing the insulation in waterproof layers is of any benefit. Ventilation of the cold side is an accepted method of dealing with condensation but the gap (cavity) must comply with minimum sizes and this must be vented to the open air - also via minimum vent sizes.

The bit I don't get is why not just include a vapour check? It's cheap, easy to fit and cuts out the risk completely. Reducing air loss through the building fabric also has the benefit of reducing heat loss.

 

[chuckey]

Intersticular condensation is long term problem. The most common failures is in flat roof consruction. With no vapour proof membrane, the plaster board all ways breaths some hot wet air, as this travels up through the insulation, the water changes from its vapour state into the wet stuff and the rot sets in. I have spoken to somme builders and stripping flat roofs to replace the joists is not uncommom. This is why the buillding regs specify ventilation over the insulation.
Even with the membrane the rot will happen as the vertical ends of the walls will breath wet air. Its just a matter of will it take 5 or 100 years before the roof rots out.
To return to your walls, imagine that you are tanking the building but more so because the molecules of water vapour are more slippery then great big lumps of water wiith their surface tension. They also move in all directions, water tends just to runs downwards.
As far as I can see there is no way to provide a vapourproof transition between 2.5mm running behind a plastic membrane to a 13a outlet mounted on the front side of the membrane. The only way would be to have a larger box mounted behind the membrane with an open front with flanges on it. The cable is taken into the box via a silicone sealed gland on its side. The membrane is now fitted and "glued right across the front of the box using more silicone. The plasterboard is now fitted, a hole cut in it and the outlet fitted so it is inside the box. What a palaver!!!! You cannot even use a surface fitted box, because its fixing screws will penetrate the Membrane and cause air leaks!
Frank

 

[electricianid]

Thanks for the concise replies, I will use a vapour check since it's cheap and easy as you say Johneds, but I am cursed with a need to understand what I am doing hence the lengthy q's. I am however, skeptical as to whether it will completley cut out the risk of condensation though.
On a related topic does anyone know what the regs are regarding pipe and cable runs in the brick/insulation void behind the aforementioned stud wall? I am sure that it's a no-no in the 'cavity' of a twin leaf cavity wall structure, but in a drylined solid wall building...?

 

[Penners]

Intersticular condensation is long term problem.

Only if you wear nylon underpants.