Home Office over Detached Garage

12 Jan 2004
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United Kingdom
Realised I'd not posted a thread on this. Completed it back in 2018, thinking I would use occasionally, but was a god send during the working from home in the pandemic, and still very pleased with it. Cost was under £1k IIRC.

Essentially I moved into a house with a detached double garage, with a decent roof space, with reinforced floor, planned to do something with the space for a while but eventually got round to it.

The first jobs for me were to insulate under the floor boards with rockwool, and got a roofer to install an extra window, I picked up a second hand velux window off ebay for £50, this was actually the same age/ model as the existing one (just a bit wider). Then I installed some batons for knee walls, I made them 700m thinking that was a reasonable height, later realised 600m would make a lot more sense with Plasterboard widths!

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Next up came the insultation, I used 100m Celotex, with 130mm rafters this allowed 30mm airflow under the tiles, think modern standards are a little thicker, but when the rest of the house was built 100mm was the regs! The end walls simply had rockwool.
Next up was the plasterboard, the sides and small ceiling were not too bad, but the sloped areas were a pig to get in place. It was at this point I realised my mistake with the 700mm kneed boards, but managed to use the pieces OK on the end walls.

Now the fun started with the plastering. Having never done this before it was a steep (and messy) learning curve. It was all butt joints, and no tapered boards!
Then there was a lot of debate over the stairs and how to fit a door. Ended up using this neat door arrangement with a pneumatic piston like on a car boot (I found a site that made custom pistons, so spec'ed that up based on my door ) to support the weight of the door, (which was insulted) and filling around the top of the steps. Used the left over 2x4 to make the banisters around the hole for safety. Carpet and underlay were picked up on marketplace, for £50. Then it was a case of fitting smoke detectors (upstairs and downstairs), fire extinguishers etc and a lick of paint!


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Today its in full use, was lucky to get some low height drawers and cupboards which work well with the knee boards and an oil filled radiator. Used it once a week and then lockdown hit, so it was in full time, been very impressed with it, even the other year when it was -10c the temperature inside was fine, and in the summer (when the tiles are too hot to touch) the inside has remained cool.
Biggest problem I had was getting the internet in the room, all that metal in the cellotex, thick walls and the distance meant the signal was crap. I picked up a Netflix extender, still rubbish, but thankfully it could be used as an access point but had to wire a ethernet cable from the house.

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Great job!

I have a very similar garage with room above that I'd like to convert.

What did you do about insulation beneath the floor? Did you use the same Celotex? If so how did you attach it?

I have a very similar garage with room above

So do I, and I packed between the joists with mineral wool. Heat loss through a floor is nil by convection, about nil by radiation, very little by conduction, can be a lot by draughts. Mineral wool can be packed tightly into irregulr gaps without precision cutting, and will block it all. Especially round the edges of the room where drauhts are worse, so full-fill here.

I also sealed all possible cracks, e.g. round the edges of the room, and around pipework and other holes, using pink fireproof expanding foam.

I don't use celotex indoors because mineral wool is non-flammable and does not emit poisonous fumes in a fire. It also happens to be cheaper and less work to fit.

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So do I, and I packed between the joists with mineral wool.
View attachment 258768

That's interesting - thanks!

I should add that our room is already partially converted, and boards/carpet already down in the room above, but the joists are exposed beneath. In all likelihood, a more skilled person than me will need to be called in for the job :)
lining the ceiling downstairs will muffle sound and draughts, and add fire resstamce, which is important if it is a garge containing fuel, oils, white spirit etc
having done some research it looks like spray foam insulation could be a good solution - applied from beneath, so no need to lift the floor above, and will get into all the nooks and crannies.

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