insulation of external stud wall

I an involved in maintenance on a small timber extension on a standard brick-built house. The extension is a stud wall approx 13cm thick with plasterboard on the inside and plywood on the outside, and currently has no insulation or vapour layer at all. This needs to be improved and have insulation added etc. What insulation should I use (at a reasonably price). At B+Q, all I could see was polystyrene (fire retardant), and mineral-wool style products Knauf DIY (unspecified R-value) and Knauf Crown. What would be recommended ? Would some type of insulating board be of benefit to stop thermal bridging by the studs ? Is a breather membrane needed on the outside (behind the plywood)? TIA, Simon.
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Hi Simon
The insulation you describe is not really ideal for this application. Expanded polystyrene board would do the job but the quilt is really a no-no. A rigid polyurethane foam board with foil facing on both sides such as Kingspan Kooltherm K7 or Celotex RR would be much better. This will provide vapour protection as well as better thermal insulation. You can order it at any builder's merchant, or if you have a specialist insulation firm near you like Sheffield Insulations, you can get it direct. I've never seen it in the DIY sheds. It's very easy to cut and fit using a bread knife or fine hand saw.
You don't say from which side you are working but I would recommend you remove the plasterboard rather than the ply. To achieve a U-value of 3.5 W/mēK (current Building Regs requirement) you will probably need about 75mm thickness of board and it would be best to have 50mm board fitted tightly between the studs and 25mm board over the face of them under the plasterboard, to counteract cold bridging. The joints of the insulation should be carefully taped with aluminium duct tape before fixing the plasterboard. Alternatively you can use a plasterboard with 25mm foam already bonded to it, but for a small quantity this is probably impractical.
There is no need for a breathable membrane inside the ply but what you MUST have is a vapour barrier on the inside of the insulation. This prevents vapour condensing on the cold side, which would rot the timbers. The foil facing to the insulation board will provide this. It would also be very wise to introduce some through ventilation to the voids between the studs on the outside of the insulation if at all possible.
Here's the link to the Kingspan info: http://www.insulation.kingspan.com/newdiv/k7.htm
Peter
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Thanks Peter. I am sort of working from both sides ! The "external" wall is actually between a kitchen and lean-to conservatory. Sometimes (in the sun), the external is the "warm" side, but when it's cold and worst for condensation, the outside is indeed the cold side. The wall needs to be brought up to spec for thermal isolation of house/conservatory. I cannot really add an anti thermal bridging layer on the plasterboard side, since there a cabinets etc up to the wall. Can I add this to the outside ? If so, how does this affect the vapour control etc ? I would certainly affect the through ventilation, since you wouldn't want cold air behind the bridging board.
I know ... lots of questions, I want to get this right !! Cheers, Simon.
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Simon wrote...

OK - I get the picture. There is no problem about having the 50mm insulation flush with the outside (plywood) face of the studwork with 25mm insulation over the studs on the outside face. I imagined replacing the outside plywood 50mm further outwards would create more problems with roof, gutters etc than doing it on the inside, but if you can remove and refix the plywood easily then it means you won't have to do any damage inside at all.
The principle I explained before is exactly the same, only this time there will be a void between the plasterboard and the insulation and, being on the inside of the insulation, this will be more or less at internal room temperature all the time. Therefore there is much reduced risk of condensation within the void, which means you need not worry about a vapour barrier internally. However, to be on the safe side I would recommend giving the whole wall surfaces 2 good coats of an oil-based paint (undercoat will do) then emulsioning over this if you prefer. If it's a kitchen or bathroom I think it would be a good idea to have a decent extract fan with automatic humidistat control.
Assuming there is no problem about the wall being 50mm thicker on the outside, you should fix the 25mm insulation overall the studs to provide the thermal bridging and then tape the joints as I said. Over this you should fix ex 38 x 25 preserved softwood roofing battens vertically nailed through the insulation into the studs. This creates a 25mm gap behind the plywood, which needs to be ventilated as I said before. Then the plywood can be refixed on the battens.
If this is still a problem to achieve come back and we'll think of a different way, but it will mean you can't overcome the cold bridging situation.
HTH Peter
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Thanks for all the info. Actually 25mm on the inside would be OK - the cabinets are not actually against the wall, there is a blanking plate which could be reduced in size. So I have info for both external and internal cold bridging, I can take my pick! I have a further query - how is finishing around doors and windows usually done if 25mm board is fixed in front of the stud wall ? On edges around doors etc, there would be the edge of a thermal board to conceal etc. Presumably this would be hidden behind window ledges, door frame lining etc. Simon.
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