Garage walls and roof are up, now it's time to tackle the floor. I had planned to insulate the garage floor, walls, roof and install an insulated garage door but am meeting with considerable resistance regarding this. I know I can easily add insulation to roof and walls later, door will be of the insulated type, however, but I have no idea about the floor. The plan at the moment is to level, then to add 50-100 mm hardcore then 100mm concrete. This will leave the garage floor at least 100 mm below the house floor.

Should I insulate now by putting in sth like floormate under the concrete slab or insulate later by putting sth over the concrete thus bringing the garage floor to the same height as the house? (Adjusting the driveway to accommodate either height is not a problem.)

Both my husband and the builder have been adamant that insulating underneath the concrete slab is a waste of energy as I'd be sinking all the heat into the concrete floor, not the room. Is this true?

The garage is 3m wide by 6 m long, adjacent to the extension, sharing one fully insulated side wall with the extension and the back wall with a fully insulated utility room. Second side wall is uninsulated single walled brick, front wall the same but mainly garage door. Garage will serve as hobby/fitness room eventually, maybe even as a garage.

I'd be grateful for any advice on this either way - and if you have converted a garage before and insulated its floor, I'd appreciate any information on the amount of money/time/hassle that involves.

I would put the insulation under the slab (making sure there are upstands around the edge to limit the thermal bridging to the walls). The slab will not act like a heat sink - it will act as thermal mass inside the garage, gradually becoming the average temperature of the room and will help to even out day/night temperatures.

Insulating above the slab will work also, but you will lose the thermal mass of the floor to help balance the room temperature. If you have large potential for passive solar gain (i.e. large south facing windows) then this could lead to overheating. Of course, you have a big door to open if this becomes a big problem....

I think in the UK there are regulations about floor heights in garages vs attached houses. I think the rule is the garage floor must be 100mm lower than the house floor, just in case of leakage of volatile chemicals (e.g. petrol). If you go for insulating later and still want to retain the potential to use it as a garage then bear this in mind when setting your floor levels.

Posted By: davidfreeborough Regarding the height of the finished floor: where the garage is attached to the house, it was common to have the garage floor 150mm below the house floor. I'd heard this was in case of petrol spills, the fumes would not enter the house. I don't know if this was a regulation or just common practice.
David

I believe it's a regulation and to stop a petrol leak flowing under the door.

If you do decide to put the insulation on top of the slab and have any intention of using it as a garage then you will need to reinforce the screed over the insulation. As David has said, the loads from a vehicle need thinking about and a normal screed would not be enough.

Thanks for all the input, definitely going for insulation under the slab, probably more than 50 mm.

aa44: the load problem you mentioned is the deciding factor in putting the insulation under the slab - this way we can market the garage as a garage if we ever sell the house - whatever we decide to use it for in the meantime.

neelpeel: still researching thickness needed for weight of a car, I get your point about the thermal mass but as the use of this room is still up in the air at the moment I will probably play it safe and go for a thicker slab to allow for the car.