The concept of insulating the extension and existing ground floor with 300mm sheets of insulation sounds simple enough, but actually doing it is not necessarily quite so straightforward..
To prevent air pockets between the boards, they were placed at least 6mm (but no more than 15mm) apart so that expanding foam could be squirted into the joints. The foam caused the boards to lift up as it expanded, so next job was to place blocks on the top to keep the boards in position.
It might have been easier to use 2 layers of 150mm board instead of the 300mm boards, but any dirt between the first and second layer of insulation could introduce another set of air pockets. Maybe interlocking insulation boards would have been more user-friendly and give a better end result.
The up-stands and down-stands of insulation are not without their difficulties either. Making a neat job with no gaps is important, and requires some care to keep the insulation layer continuous and tight up against the wall.
Up-stands need to be stuck down (with foam or tape) before the concrete is poured - or you get a cold bridge where concrete fines flow into the gap. Our builder suggested that it might be easier to place longer up-stands and place the main horizontal layer of insulation inside them - a good idea, but the insulation was already on site, and not in the right combination of sizes to fit that approach.
These are the things you only know when you've tried it. The main thing is that the insulation is placed as carefully as possible to avoid gaps, and foamed or taped before concrete is poured on top. It isn't rocket science, but it can be a pain.
Most builders will think this super-insulation lark is completely crackers - it is very over the top by comparison with current building regulations. However, if we want a house that needs virtually no space heating, these details matter.
