Home office in garage - damp proof, insulate & ventilate

[Holy1Kenobi]

OK, so the birth of my first child has meant that my office has been turned into a nursery and I am to be banished to the only space available - the detached garage!

It's as cold and damp as you would expect for a single skinned brick garage in the UK, but essentially water tight. The advantage is there is plenty of space - I can easily sacrifice a 3m x 2m footprint, and still have room for my workshop and a car.

I plan to construct a timber 'box' in one corner, up against the window (double glazed, rescued from a previous house refurb). I want this to be free standing, and completely free from support from the fabric of the garage as I expect I might have to do some repairs to the garage roof in the next 2 years or so.

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• Holy1Kenobi
• 25 Jan 2013
• 1

I will use celotex type insulation boards in the roof, walls and floor of the structure between the studs, clad on the outside with 12mm OSB, 18mm WBP ply on the floor and plastered on the inside.

My question is how do I ensure sufficient damp proofing and ventilation between the walls (brick) and floor (concrete, no DPC) of the garage that will be butting up against this wooden 'box'? My thoughts at the moment run to wrapping the outside of the structure that are against the walls in vapour barrier, and laying a thick polythene sheet under the structure. However, I am aware that a certain amount of ventilation around a structure like this might be a good idea to prevent decay to the brickwork etc...?

Any help would be appreciated!

 

[oakeycokey]

What you're doing here is essentially creating an insulated dry-lining system, combined with some insulated studwork.

First step would be to prevent any damp from getting into the structure - I would slap a load of thin (20mm or so) Celotex against the walls, fixing them with hammer-in fixings. This will ensure that the timber wall structure doesn't touch the wall and can't transfer cold (and associated risks of condensation and mould) into the interior space. As for the floor, lay the joists onto plywood or slate packers so that the joists are not in permanent contact with the floor (if using plywood, add a square of DPC underneath any timber to concrete interfaces). This method ensures that there's a bit of airflow underneath the floor to keep things dry as well.

Insulation. It's all about careful fitting, and making sure that there are no gaps between the joists/studs and the edges of the insulation boards that will inevitably cause thermal bridging and condensation risks (we prefer to use a Rockwool or something as you can compress this nicely between the studs and it's relatively breathable, however you need more of it to achieve the same U values). If you're using a Celotex, get some of the 100mm wide silver tape and tape the inside of the insulation to the studs (also, if you've been a bit sloppy with the saw, fill any gaps with some expanding foam). For the floor, pin some 2x1 batten to the lower part of the inside of the joists and drop the insulation in, the 2x1 will keep the insulation in place.

Internal boarding and VCL. Adding a VCL is always a good idea on the internal face, usually just behind the plasterboard. This can be simple non-breathable polythene sheeting if you're going down the Celotex route, or a posh breathable membrane like Proclima Intello if you're planning a breathable construction. A staplegun will do the trick with this, but use wide-crown staples as the narrow crown ones will punch right through the membrane. I would go for a tape and jointed plasterboard interior as it's a fair sight easier and quicker than a skim. Also if you're planning a home office, it might be worth considering creating a 'services void' of 2x1 roofing batten, mounted vertically onto the inside of the studwork and horizontally across the sole plate and headers so that you have a bit of flexibility for cabling etc (or, as it's a garage you might be tempted to run all electrics in conduit externally and punch the cabling through as appropriate).

Externally, the OSB structure will be fine. To make sure things are toasty and warm make sure that you get the door draft-stripped too. Roofing - go get a sheet of EPDM membrane from Rubbaseal or a local roofing merchant. It might also be worth putting in a decent sized hit and miss vent somewhere in the walls because you're creating a very lightweight structure that will be prone to overheating in the summer!

That is all.