Insulating an outbuilding

[ClareG1964]

Hi all, during this enforced period of staying at home I am looking to make our garden outbuilding/studio/workshop more habitable during colder months. It's a substantial and solid building, tongue and groove exterior walls, slate gable roof, plasterboard interior walls fitted to studwork. The roof space is well insulated with rockwool. The walls, however, are not currently insulated (apart from very thin slabs of polystyrene in the cavities, which I'm assuming are not very effective). I am looking for ideas on how best to improve the insulation in the walls. Currently they comprise (from the outside in): timber tongue and groove, sarking-type membrane (presumably waterproof), cavity, plasterboard.
Ideas I've had so far...
(1) Fix insulated plasterboard to the existing plasterboard (quick & easy, expensive..?)
(2) Remove existing plasterboard and (a) fill cavities with rockwool, cover with vapour barrier, replace plasterboard (bit more labour intensive but relatively cheap?) - or (b) use Celotex instead of the rockwool.
Any thoughts on these two ideas - or any other better ideas?
I'll be putting a heater in there too.
Thanks!

 

[noseall]

A lot depends upon the depth of your wallet, the requirements of occupancy and the comfort level.

It seems like you have a grasp of the differing types of improvements, so your can answer your own question - i.e. do I want to occupy this building for long periods of time, do I want to heat it, do I want to keep sensitive items in there etc, etc.

 

[ClareG1964]

Thanks Noseall! "Yes" is probably the answer to all those questions... I guess I really wanted to check that I wasn't missing something, ie. another solution that I hadn't thought of. I may have a grasp, but not much experience! Also, definitely not a deep wallet, so cost is an issue. Just wondered whether anyone else has done a similar project and has some good insights.
Cheers.

 

[^woody^]

As you're going to be holed up for a long time, you have plenty time to do a proper job with the best insulation - which would be take the plasterboard off, and fit celotex within the frame, then barrier or tape joints and re-board and skim.

 

[ClareG1964]

Is it OK to just tape the Celotex joints, instead of covering the whole lot with a vapour barrier?
And do I need to leave a ventilation gap behind the Celotex? (I think the sarking membrane stuff inside the timber exterior is probably meant to be waterproof so probably not breathable?)
Thanks!

 

[noseall]

Is it OK to just tape the Celotex joints, instead of covering the whole lot with a vapour barrier?
And do I need to leave a ventilation gap behind the Celotex? (I think the sarking membrane stuff inside the timber exterior is probably meant to be waterproof so probably not breathable?)
Thanks!

Ventilation gap on the cold side of the Celotex - yes.

Fit Celotex between studs and foil tape. Fit another layer of Celotex across the studs and foil tape. Fix duplex plasterboards. VB x 3

 

[ClareG1964]

OK, thanks. Sounds like a lot of Celotex! And it's quite a big building.... Don't think I can afford to do it that way, to be honest. Might have to use rockwool... In which case, what is the best way to ensure the ventilation gap behind the rockwool? Maybe I'm overthinking all this, as usual - after all it's just a glorified shed... But it would be good to be able to use it in winter (or to banish my teenage son to it in winter...)

 

[^woody^]

If you leave a ventilation gap, then you must also form ventilation openings along the top and bottom of the external cladding, and provide rain screening and insect mesh. And you will find this very difficult to do retrospectively.

A ventilation gap is not required with what is essentially a breathable frame already.

The provision of a vapour check layer (which includes taped joints of foil faced insulation) is sufficient to prevent moisture getting into the frame from internal air.

 

[ClareG1964]

Thanks Woody. Am now inclining towards rockwool. It may not be as effective as celotex but it's got to be better than the current thin sheets of polystyrene, I'm guessing.

 

[noseall]

If you leave a ventilation gap, then you must also form ventilation openings

Call me a cynic, but that wood cladding doesn't look all that airtight to me.

 

[JohnD]

the cladding is shiplap. It sheds most rain, but the joints open up in dry weather, and driving rain will penetrate.

For an excellent job, you can have a layer of roofing membrane (modern replacement for tiling felt) behind the cladding. It will prevent rain, dirt and insects entering, reduce draughts, but breathes enough to keep the cavity dry.

BTW I see yours is on sloping ground. If the shiplap on the other side is within about a foot of the ground, or in contact with vegetation, it will get damp and rot. Luckily you only need to replace the bottom two boards when that happens. Mine are spaced off the ground with brickwork but the bottom few boards were treated very very generously with spirit based wood preserver (not the same thing as stain).

Good guttering is essential.

 

[ClareG1964]

Thanks JohnD. Actually behind the cladding there IS a layer of roofing membrane (the sarking type membrane I mentioned in the opening post) which completely keeps out rain, etc. I had assumed it was not breathable. But if it is, then that's good - no need for a ventilation gap?

Yes, sloping ground, but all well above ground level fortunately (apart from the left hand side in the pic - the boards did rot because I had allowed some junk to be stored up against that wall for far too long - but I replaced them and they are well treated with preserver).

The building is pretty sturdy. It was built as a bookbinder's studio, and has been up for about 30 years at least, no sign of rot anywhere else yet! The pic was taken about 15 years ago, since then I have re-treated the exterior.

 

[noseall]

Actually behind the cladding there IS a layer of roofing membrane

Yes, I thought when you wrote this...

sarking-type membrane

I thought you were referring to breathable membrane. Underslaters felt is not breathable, so an air gap may be pointless without additional vents.

 

[JohnD]

show us a photo of the stuff you have. 30 years old I expect it will be felt.

you can wangle the joints open with extra battens.

 

[ClareG1964]

I don't think I can take a photo of it - as it is inside the walls! I only saw it the once, when I was replacing the rotten boards mentioned in the previous post. And I can't really remember it very well. But might go out there now just to check and see if I can see any at all.... I seem to remember I did have to replace a bit of it so there might be some scrap lying about. I will return...