Insulating a garage (sort of!)

SteelyC

Hi

First time in this section of Boards, looks like there are lots of knowledgeable people here so was hoping I could get a bit of advice.

I have an attached garage which I’m planning to insulate and floor. It’s 4.5m x 2.5m.
Not looking to make it habitable, as at the moment we won’t be removing the steel garage door to put a wall and window in, and won’t be putting heating in. It’s going to be a storage room so the aim of insulating it is mainly to limit the amount of heat loss from the house through it.

The external wall is 9-inch cavity block, the steel door is currently boxed in with a plywood stud wall with about 100mm fibreglass in it. Floor is concrete. Ceiling is a concrete slab.

Current thinking is....

Floor: Floating floor with DPM, then 100mm Xtratherm boards, then slip layer, then 9mm or 12mm OSB or plywood, then laminate click board floor. Should we build a wooden frame subfloor, or will the board straight onto the insulation work ok? I’ve read blogs etc with both approaches. There is a slight slope to the floor but I’m not fussed about that, as I said, not planning a habitable room.

Walls: Warmboard (38mm insulation + 12mm plasterboard) on the external wall, on 2x1 battens. Just 12mm plasterboard on battens on the internal walls.

Ceiling: Not sure! Plan would be uninsulated plasterboard (can’t afford to lose too much height), attached either on battens or straight to the concrete ceiling using mushroom fittings but I haven’t really got my head around this bit yet!

Any advice on any of that would be appreciated! Trying to do it as cheap as possible, but not so cheap that it’s a waste of money (or end up causing us trouble).

Thanks

TheBoyConor

Ah now, is the plan for it to eventually be a habitable room at some future time?

If you are putting down OSB on the insulation you need to be sure that the floor is very even with no irregularities of slope, otherwise the OSB will be cocked and won't lie properly.

You would also have to lay a double layer of OSB. If you lay just one layer, the edges of the board will deflect under load and dig into the insulation. A double layer with fully staggered joints prevents this and makes the floor stiffer overall.

unless the garage door is effectively insulated and made airtight, heat loss will be massive. Those doors do allow gales to blow through.

If you are not insulating the roof slab, then the rest of it is kind of pointless. That is a massive heat loss route. If you have no headroom to loose, then you could insulate on top of the slab and add a membrane, but detailing to prevent cold bridging is going to be very tricky.

SteelyC

Thanks for that, confirms some of the things I had been thinking about.

No plan to fully convert it any time soon - don’t have the money and don’t need the space at the moment anyway. I know that if we do convert it some time down the line we would be ripping out whatever we do now and starting again, hence the desire to keep it cheap as the moment.

Two layers of OSB is a good shout, had been wondering about that alright.

Re whether it’s worth doing at all without insulating the ceiling and with the door still in place (it does have some degree of insulation with the Rockwool) - my thinking is that the garage is already there and sucking heat, so anything we do is better than nothing!

TheBoyConor

Just on the door, the rockwool isn't worth a damn if the door is allowing in whipping drafts that are howling around in behind the stud wall.

Without looking after airtightness, insulation is a waste of time. Especially if drafts are blowing around inside wherever the insulation is located.

GreeBo

Why batten the walls? I'd just mechanically fix the warmboard directly to them.
Can you still get at the inside of the steel door? Even taping it would significantly help with draughts (and allow the insulation to actually do something!)

Calahonda52

OP:
If this is the mission objective:
so the aim of insulating it is mainly to limit the amount of heat loss from the house through it.
Then work on the relevant wall only.
The concrete roof is a massive thermal bridge, the floor to a lesser extent, as are the two walls touching the house?
What is the house wall construction..

The 2" gap is a waste just stick the slab directly on the wall.

How airtight is the access door to the house?

If it's just the cold when you open the door into the garage from the house you could build a draught porch in the garage, very cheaply as it does not need to be water proof, and use it as a decompression chamber.

If the house wall is a cavity wall then the porch is the way I would go

Please do the work once and right and don't contemplate doing it now to rip out later: its a foolish idea which wastes money and also has a serious impact on the environment producing non- recyclable waste.

TheBoyConor

Well said Ch52.
I too would be insulatin the dividing wall if I was just keeping the house warm. However, I suspect that this garage job is a sort of half prelude to making the garage habitable in the future. The thing with that is you either do it right or don't bother, becasue half doing the job is a waste of time, money and materials. If OP wants to make it habitable in the future, then the work will have to be Part L compliant for that part of the building, so a half done job with insulation here or there won't cut it and will have to be all pulled out again.

policarp

If height is a problem with the ceiling why not insulate from the outside?

SteelyC

Apologies, hadn’t realised there were more responses here.

Thanks all for the advice, have taken it on board and figuring out a plan with some adjustments.

Have already sealed up the steel door so drafts there aren’t a big issue, and the door from the garage into the house is decent as well, luckily.

I don’t have any plans to make it habitable, so this is definitely about just it being a heat sink for the house, but cant afford for it to be a money sink, hence not going all-out on it. My thinking is that the fact that it exists In the first place means it is a heat sink, so anything I do is an improvement! I’ve got a batch of second hand but good condition kingspan from a friend’s house demolition so that’s going in somewhere, and I’m pricing up other bits and pieces now.

Thanks again!

TheBoyConor

It has been said here already - if the garage is an unheated space then you would be far better off just insulating and draughtproofing the wall and door dividing the house and garage.

It will be far easier, far cheaper, take less time and will have a much bigger impact on heat loss from the habitable areas of the house.

If you have no need to keep the garage warm then what is the point insulating it and having half parisitically heated by heat seeping through from the house? It makes no sense.