I'm in the early stage of planning a flat felt roof extension to contain a
new kitchen/diner as our existing kitchen is too small.
In order to save on space, I'm anxious to keep the wall thickness to a
Is it mandatory (building regs) to make an external wall 300mm approx wide
or can you reduce it substantially.
I have in mind,
INTERIOR-> 12mm foil backed plasterboard--
--75(or100 ?)x50 studding insulated with 75mm foil backed celotex--
--breathable waterproof fabric--
-- vertical roofing battens to give ventilated air space (20mm?)--
--PVC exterior cladding <- EXTERIOR
Providing the construction meets structural and thermal requirements do
you think this type of wall would be a accepted by building control.?
Tom, my kitchen extension is very similar, with 50mm celotex between
the 2 x 3 inch studs, 25mm celotex over. Outside is wood cladding, not
PVC. It was already there, so no idea about building regs for mine.
However, the regs do not stipulate the thickess or materials, just the
properties it must have. One note. A timber extension may not increase
the house value as much as a brick one. I did some modifications to one
wall, and made the wall about 3 times as strong/stiff as a
timber-framed house spec (extra studs etc), and the wall still
resonates when the door is closed. I am happy with it for a single
storey, but it did make me wonder if I could ever live in a US-style
timber-framed house !
I assume that the "25mm celotex over" goes between the plasterboard and the
studs, not outside the studs so to speak?
Outside is wood cladding, not
I used 4"x2" dividing rooms in a previous house but was never aware of any
resonance problems as described.
The proposed extension is below the 15% volume as stipulated in planning
guidelines so I'm hoping to avoid planning applications. Going to see them
at building control this PM.
Cheers and thanks
Can be either. The main thing about timber walls is to ensure that any
timber on the cold side of the wall is ventilated. Search for my
previous post "insulation of external stud wall" as well as various
I think external doors tend to be slammed harder. I remember reading a
post from someone about living in an oak timber-framed house. They said
it could boom like a drum if the front door was slammed. Bricks are
simply stiffer than wood !
I have to say, if you can get the celotex cheap e.g.
then a timber extension can be very cost effective. The foundation slab
can also be more lightweight than for a brick/tile structure.
Actually, I believe it has to go inside. If it goes outside, then
technically, it has to be at least as thick as the insulation between the
studs. (i.e. 50mm outside, 50mm inside, or 75mm outside, 75mm inside).
Why ? This must be the interstitial condensation rules.
I guess any timber that is in danger of getting condensation on it (on
the cold side)
must be ventilated. So putting celotex on the outside should help.
I don't see why not, although you really should have some celotex in front
of the studwork, both to improve u-Value and to avoid cold bridging, which
might result in unsightly mould lines and condensation.
For new build, you can often eliminate the need for ventilation gaps through
careful selection of breathable materials.
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