Insulating garage roof bathroom above

16 Oct 2007
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United Kingdom
Hi Folks

My house has an en suite bathroom above the garage. There's rockwool between the joists and 2 layers of plasterboard for the fire reg's. The bathroom gets cold in the winter so I want to improve the insulation.

I was thinking of using 150mm thick celotex/kingspan between the joists, with either 2 layers of plasterboard or a layer of fireproof.

Do you need to leave a air gap between the floor above and the insulation? Or can it go flush to the floor?

Many thanks
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Fitting insulation between the joists is good but it will not deal with any cold bridge transfer through the joists.

A typical detail would be say to fit 150mm rockwool between joists with a 50mm layer of celotex going across the the joists prior to double boarding or fire-boarding.
Given the likely area of the joists and the insulating effect of 150 mm ( ?) of wood ( R 1.2 = same as 45 mm fibre-wool ), the thermal bridging effect is going to be limited.

If you want to improve the insulation, fill the space between the joists and if you want to make it as good as possible then add a sheet of insulation-board below the joists, but I think the extra benefit will be rather small.

Don't leave any gap, fill all space with insulation.
Thanks for the reply's. I had wondered about the joists transferring cold to the floor. There is also a vertical panel behind the sink which runs the width of the garage which will need similar treatment.
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You need to remember that cold does not pass through anything, but the heat does pass outwards! Insulate the room not the garage

Your celotex should be directly under the floorboards, and any air gap or extra quilt below this

Timber is a good insulator so the joist wont be making the floor feel cold. By the time you have your carpet down, and the fact that heat rises, I don't think that thermal bridging would be a significant issue in this case

While messing about in the floor, seal any joists into walls and then seal the ceiling afterwards. Draughts via air gaps could mess things up

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