Wall between garage and utility - how to insulate?

[johnf]

Our utility room was created decades ago by previous owners who flat-roofed the gap between the side of the house and garage and filled in each end with a door. Not surprisingly it's very cold in the winter, so insulating the garage wall seems an obvious improvement to make.
Any advice on how to go about this? I'd prefer to apply the insulation on the 'garage' side of the wall to save space and for ease of working. I was thinking of timber studs with some kind of insulation between, finished with ply sheets on top. Will I need a vapour barrier of some kind? Do I need to leave a cavity or can I fill the void between the studs right up? What sort of insulation would be most suitable? How thick should it be?
Thanks for any advice/suggestions.

 

[noseall]

Foam fix 100mm (foil covered) celotex insulation to the garage side.

Seal all gaps with foil tape.

Fix 25mm x 50mm battens through the inso' into the wall.

Fix plasterboards, tape and skim.

Be aware though, you may be losing more heat via the roof.

 

[^woody^]

It's not worth doing anything in isolation as the floor, other walls and roof will play a big part in the solution - as will whether the room is currently heated or not ..... its no good insulating a cold room

I think that 25mm celotex and plasterboard will be a good insulation/spacing saving compromise for a utility and 50mm will be better if space allows

 

[ajrobb]

Having roofed over the passage between the garage and the house, the garage is now part of the house. I think it may need a fire door.

You call it a utility room, so I assume you have washing machine etc out there. These need to be protected from frost at least. I agree that insulating the garage wall is an easy place to start. You can consider the roof and doors at a later date.

As you suggested, I'd batten out the wall in the garage with polythene between the wall and battens. Have battens at the top and bottom of the wall and vertical battens at 400mm centres. Fill between the battens (no cavity) with cheap mineral wool (loft insulation) held at the top. Cover over with 12.5mm plasterboard.

Plasterboard is cheap and acts as a fire barrier. Plywood is like lining the garage with kindling. If the garage gets damp, you might use moisture resistant plasterboard. Don't take the plasterboard down to the floor where is might get wet.

If you use 2x4 timber for battens, the wall insulation will be as good as modern houses but the roof and doors will lose many times more heat. 2x2s will give insulation as good as cavity wall insulation in older houses.

Thick loft insulation can be got cheap and will compress into smaller cavities. The insulation is governed by the size of the cavity rather than the nominal thickness of the mineral wool.

A more expensive solution would be Kingspan K17 dot'n'dabbed to the garage wall with a single row of screws half way up.

 

[johnf]

Thanks for the replies, all. I should have mentioned there is a low-wattage electric heater in the room that keeps it above freezing according to the thermometer, though last winter it was down to about 5C for extended periods. I understand that insulating one wall won't be enough by itself to make the room substantially warmer. I plan to tackle it in stages as time and money allows.
Sounds like plasterboard is a must. Ajrobb: regarding the need for a fire door, there is a personal door from the garage into the utility - it is solid timber, with what looks like thin asbestos sheet nailed over it and the doorframe on the garage side. Does that sound adequate from a safety point of view? I don't fancy doing anything to that at all, other than sticking some draughtproofing strip around the edges.