Single storey extension.(wall thickness)

[Tom]

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 minimum. 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

[sm_jamieson]

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 ! Simon.

[Christian McArdle]

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.

Christian.

[Tom]

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 Tom

[sm_jamieson]

studs, not outside the studs so to speak? 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 others.

resonance problems as described. 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.

a timber extension can be very cost effective. The foundation slab can also be more lightweight than for a brick/tile structure.

Simon.

[Christian McArdle]

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).

Christian.

[sm_jamieson]

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.

[Christian McArdle]

It is, at least, how I understand them. I may be wrong.

Christian.

[<me9]

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