Insulating floor of room above garage - odd construction

13 Jan 2009
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United Kingdom
Hi Folks,

Moved into a 1930s place last year that has solid walls. CH is on already and it's still bl**dy freezing!

Anyway, one room is above the garage. It has a timber floor on joists which are suspended above a poured concrete slab (garage ceiling). Half of the wall of the room is also in the garage since it has a pitched roof that connects to the house halfway up the external wall (see pic).

The floor in this room is always freezing (not surprising - no insulation).

I was thinking about pulling the floorboards up and insulating between them, but after I thought about it for a while I think it's better to insulate with rigid boards from below because I can go round the corner and get a bit of the external wall too (but still inside the garage).

This will also reduce any cold bridging where the floor meets the external wall (note: plan to have external insulation done over next few years).

Anyway - would peeps agree this is the best approach? And if so, what can I use to stick the boards to the concrete slab from below? Want to avoid mechanical fixings if possible since the slab is a bit sandy and crumbly (don't think the mix was right when it was poured 80 odd years ago).


Garage construction:

Garage slab (bit crumbly):
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I would think you need to look at insulation in the roof and on the walls before going to the expense of insulating the floor. More heat will be lost through those elements than the floor below. As you already have a wood floor above the concrete this will already have given some small benefit in raising the surface temperature so I think there is very little to gain from spending money there.
Roof is already done.

Walls are on the radar longer term (approx £24k cost!)

Thing is, the floor is cold. Expense to insulate it is minimal (few hundred quid?)

I think it's worth doing to have a warmer floor.

Just querying whether best to do as shown in in the pic or pull up floorboards internally and do it that way.
Well as you seem convinced that heat goes down (my physics teacher told me heat rises) then crack on. Either way will cost in time or money and you will not see a ha'pth of difference in room temp or comfort levels, unless there is a howling gale blowing through the underfloor space (and some ventilation will be necessary to avoid other problems).

How are you heating this room? And when you say the roof is done, to what standard is it done? What kind of floor covering do you have? What windows and how big? There are many other factors to consider which you have not mentioned as yet.
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So let me just clarify that the issue where I am seeking advice. The FLOOR is cold.

Since the FLOOR is on top of essentially an outside space (the garage), the FLOOR is cold.

I want to make the FLOOR warmer.

Therefore I want to insulate the FLOOR from the outside.

You are correct in that there are other issues that also need attention (the walls etc), but I can't do everything at once, so I am concentrating on what I can do now, before I can have the walls insulated. So let's put those aside and concentrate on the issue. The FLOOR.

So to go back to your questions.

The room ceiling has 270mm rockwool in the loft installed last year. It's heated by radiator with gas condensing boiler. 1 large window approx 2m x1.5m. Floor covering is nasty laminate soon to be replaced by carpet.

As I said, originally I was intending to insulate between the joists, but it seems better to insulate from below since I can then take the insulation up round the exterior wall which is covered by the garage porch.

and some ventilation will be necessary to avoid other problems

Please elaborate. I can't see any need for increased ventilation.
Hi Nebjamin. Sorry I know your thread is old now but this is exactly the problem I’m having. Did you do the Kingspan insulation and if so did it help?!
Insulation on its own won’t make the floor warm.

you can’t Only rely on glue/foam to adhere pir boards to the ceiling, it will need some mechanical fixings as well.

you could lay pir on the concrete floor and float your floor, if it’s wood/engineered, not if it’s carpet.

if you really want it warm, ufh. :LOL:

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