Ideal floor insulation thickness

[kingkevbo]

Hi,

I'm currently renovating an old house in the Czech Republic, planning to install underfloor heating. The old floors have been dug up down to the soil, and there has been a new damp proof course put in the walls, ready to be joined up to a damp proof membrane in the floor.

I'm trying to decide how much soil to dig away, so I need to know how thick the insulation boards should be ideally. Given I can calculate thickness of hardcore, concrete, recommended screed thickness over the underfloor hearing pipes, and final floor covering etc, I can adjust the final floor height with the thickness of the insulation.

I understand that the PIR boards are the way to go, and seem to be manufactured anywhere between 20 and 240mm thick. There must be a point of diminishing returns where the thickness is concerned, and I was just wondering from builders with experience, what they would normally fit when it's known that there will be underfloor heating pipes placed on top.

Thanks.

 

[^woody^]

For PIR, 80mm is UK standard, but for Passive Haus standard that would be 200-300mm.

It's not worth going greater than 100mm unless you are insulating elsewhere too, or if the purchase cost difference is not that great.

 

[endecotp]

If there’s plenty of depth available you also have the option of polystyrene, which is much cheaper than PIR but needs about twice as much.

 

[kingkevbo]

OK, thanks for the replies. I don't have a much depth as I like, unless I dig down more. I could but I'm lazy and I'd rather use the more expensive PIR than dig more. Any savings may end up being cancelled out by the cost of disposing of the old soil anyway.

Another reason I want to use PIR, is that I have read that it's dense enough that you can clip underfloor heating pipes directly to it, rather than use the plastic trays that you can put the pipes in, which I guess would save some money too.

I think 80-100mm sounds just about right with the depth I currently have.