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Really helpful post, thanks a lot. Have now found Ali Dymock's channel and subscribed, almost thought it was @TW 's garden room when browsing Ali's video series. I had thought with hot air rising doing the roof could be a good move, plus the extra work that the windows would require has probably made my mind up. At least it's water tight going into winter, that's the main thing. Good point also regarding how long it will take to pay off the saving you get from a full insulation job, with a portable heater it should be a massive improvement.
jambon-
Don’t write-off the cheaper options that do a pretty good job as well.
Although traditional stuff like rockwool is the go-to, you may find some cheaper synthetics are just as useful until you go spendy.
For example my shed has had that Thermawrap bubble-wrap foil stuff for a few years now and it’s actually excellent. Once the heat is in, it’s in. In the summer the space stays cool as well.
If I were doing a garage, I would gladly staplegun some rolls of the Thermawrap and see how it goes. Doesn’t hurt and doesn’t cost the earth, also super quick.
Edit: in the shed I did walls and roof then covered the walls with ply board. For the garage I mean, staple Thermawrap in the roof - you won’t see it anyway.
pdlouche
Those Ali Dimock (sp?) videos on YouTube are great for this sort of info.
Iirc insulating the roof is the most efficient as hot air rises, but the walls are the largest surface area - so actually make more difference. However, that's obviously more expensive as you'll use more materials...and you're starting with brick anyway.
Also consider your reason(s) for insulating. If it's to have the space warmer when you're in there, the truth is that in that size space a quick blast from an electric heater will probably warm it enough. And while it may be relatively expensive on electricity, it'll take a long time to pay off the saving you get from a full insulation job.
My equivalent has a reinforced concrete roof, so that's comparatively OK, and I find it manageable. As yours are tiles, I'd be tempted to just do the roof. Also cause if you do the walls you've got to do the windows too.