I have insulated my garage conversion (attached, single block, with roller door) with celotex aluminium backed sheeting on all walls followed by 12mm OSB, the ceiling is plasterboarded & 6" `space blanket ` .loft insulation on top of that.During the recent freezing snap it has kept the temp` around 8 – 10 deg C when the outside was down to – 2,
I have now added two electric oil filled radiators bought from my local supermarket at a cost of £19.90 ea, one placed at ea. end of the garage (17ft long), I switch them on approx 1 hr before I intend going in there & boy do they work! its usually up to approx` 18 – 20 deg.. very cosy.. when I get in. The good thing about these are that they are thermostatically controlled(as most are) so switch off when desired temp is reached, or I turn them down to suit.
I have used space heaters during my working life & although good for localised heating they will make you drowsy(CO) which is not good when you are working on machinery, therefore you will need a free flow of fresh air, or at least decent ventilation, so in some respects conversely defeats the object? so this method really needs consideration.. plus the cost of propane/diesel.