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
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)?
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:
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
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 !!
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.
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.
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