Insulation and internal walls for summerhouse

18 Oct 2010
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United Kingdom
Our garden contains a summerhouse which looks reasonably solidly built (brick/concerete foundations). Due to age/neglect, the inside walls are looking pretty poor ... they're veneered plywood ... and they've faded in the sun, the varnish is peeling everywhere, and the veneer is coming off in places (mainly under a patch of roof which needed repair ... which is now done).

Anyway, I've been googling and I'm at a loss as the best way to tackle this job. The exterior walls are shiplap and are attached to the stud frame with a vapour barrier between them. While this sounds fine, the vapour barrier looks 20 or 30 years old (a silver foil type thing) and it's sagging in places though it seems to do a relatively good job of keeping rain out.

The walls (which are now mostly taken down) were polystyrene backed plywood (which was veneered) and the ceiling was polystene sheets (with a backing layer of mouse dropping from the looks of things). There's plenty of airflow around the insides as the roof/wall on the back of the summerhouse has no soffit (looks like a deliberate thing ... as if there never was one)

I'd like to insulate and put new walls/ceiling up but I'm unsure of what to do. The reason for insulation is that it has a fireplace and we'd like to use it in winter (in fact, we're likely to use it more in winter than summer).
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You could overboard with foil back plasterboard [drywall], the ply behind would allow easy fixing of stuff to walls.OOps see you have removed the walls , thats a shame. :cry:
It’s a bit more than yer average shed or summerhouse but I’d be wary of using any sort of plasterboard unless there is a damp proof membrane in the floor, it’s regularly heated/ventilated & you can be sure there will be no damp or condensation on the inside. Gypsum based boards don’t like damp at all & if regularly exposed, will warp badly & eventually fall apart. Duplex (foil back) board will provide some degree of protection from damp through the shiplap walls but it’s not ideal; if you can be sure of a dry interior then something like Thermaline Plus
might be better but if there is any chance of damp on the inside, you will get problems with paper lined Gypsum boards. Moisture Resistant might be better but it has no vapour barrier or insulation & you wouldn’t believe the state of the back of some of it I pull out of bath/shower rooms. The vulnerability of Gypsum plasterboard to damp is why most outbuilding manufacturers tend to use timber based internal linings.
Thanks for the info foxhole. Although the walls are down, the ply isn't broken in any way. The construction seemed to be polystyrene nailed to the framework and then the ply glued (with some kind of brown 'gunk' which is now brittle) on top of that. Taking the walls down has broken the polystyrene but not the ply.

Is the best thing to do just to put some more polystyrene up, put ply up and then foil back plasterboard over the top? Or now it is down, take a different approach?

I'm not too disappointed if I created extra work for myself in taking the walls down ... it was useful to do a check of the condition behind the walls and reassure myself that it's not all rotting.
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The vulnerability of Gypsum plasterboard to damp is why most outbuilding manufacturers tend to use timber based internal linings.

So would I be better junking my idea of some kind of plasterboard and just putting some kind of plywood back up? The wood floor seems to be in almost perfect condition (with a bit of renovation) and I'd have though that would have had problems if damp was a big problem. Then again, the outside air can definitely circulate in the walls which, combined with a warm fireplace, makes me slightly worried.
Personally I think a timber based lining would be a better bet unless you can really guarantee low internal ambient moisture. Timber doesn’t mind a little moisture & it’s perfectly natural for the moisture content to change with the seasons, it will breath, expand & contract to accommodate this.

Another alternative could be a waterproof building board but they can be expensive & most don’t have integral insulation
Personally I think a timber based lining would be a better bet unless you can really guarantee low internal ambient moisture.

For insulation, should I go down the same path as before and use sheets of polystyrene or is there a better modern equivalent? Is it simply a case of outer wall->frame->insulation->timber inner wall ... or does the insulation mean I need some barrier/membrane somewhere?
Plain old polystyrene is rather old hat now, several proprietary insulation products are available that give much better insulation values for the same thickness. They vary in price & quality some trade names are Jablite (is probably on of the cheapest), Celotex, Kignspan & some come with an integral vapour barrier. Choice is down to what level of insulation you want to achieve & how much you want to spend really.

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