Insulating floor above garage - joists on slab

13 Jan 2009
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United Kingdom
Hi all,

I know that there are loads of questions about floor insulation, but this is a slightly different scenario, so apologies if any repetition.

I have a bedroom above a garage. The floor sits on joists(?) which are bedded in mortar onto a concrete slab which forms the garage ceiling. There is a gap of about 50-60mm between the slab and the underside of the floorboards (see pic).

I want to insulate this floor and planning to put Celotex or Kingspan between the joists snug against the slab and underside of floorboards. I have also considered insulating from underneath, but this isn't ideal as there's not much headroom in the garage as it is and don't want to lose much more.

The question is whether there are any issues regarding ventilation of the timber joists. At the moment the void is not ventilated, so I don't think there will be any issues. Just interested in what other ppl think.


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Interesting floor spec'??

There is no need to vent. There won't be any air in any case as it will be filled with insulation.
Be alert for the possibility of there being asbestos sheets somewhere between the garage and the room above.
Thanks for the replies folks. And for the tip on the asbestos B-A-S. Think it's just concrete but will keep an eye out.
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I did this about a year ago and had no issues.

It makes a world of difference to the room above though - a lot cosier through winter :D
I've just had the whole house wrapped in 60mm phenolic boards. Amazing difference. Condensation in the bathroom practically eliminated.

Now take pleasure watching the exhaust from my neighbour's boiler while mine remains off! :)

Loft: check. Walls: check. Floors are next. Good to hear it makes a difference, certainly gets chilly in there in bare feet.
We had the joist space between our garage and floor above in our townhouse filled with 'Warmcel' which is recycled newspaper fluff which was blown in via 100mm holes. Ours was done from underneath as the floor is suspended timber with a plasterboard ceiling to the garage.

Made a terrific difference to the comfort levels of the room above.

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