insulation below underfloor heating

[Fran Smitherman]

I introduced myself about a month ago under the heading New Summerhouse.  I had some interesting exchanges on solar panels to power underfloor heating in the summerhouse.  I have now rejected this option in favour of a north-facing sloping roof (thus getting 3 metres of height at the south-facing patio-door-lined side) with no solar panels, and powering the underfllor heating from the mains.

 

I've been to B&Q and looked at the sheets of underfloor-heating insulation sold there, which seem to be pink poystyrene 1 cm thick with a grey coating to each face.  Is there a more eco-friendly alternative, perhaps on the lines of the shredded-paper insulation available for lofts.  Perhaps not, as anything underfloor will get walked on/compacted and lose insulting capabilities.

 

Fran   

[Jeremy Harris]

I'm afraid you'll need a great deal more than 10mm of insulation underneath UFH if you don't want to waste most of the heat downwards.  The bare minimum is around 100mm of something like PIR, preferably more (we have 300mm of EPS under our UFH heated floor).

 

As a quick and dirty estimate, if your summerhouse has a floor area of 20m² (just a guess) and it needed about 1000 W of heat (another complete guess) into it to keep it at a comfortable temperature, then with just 10mm of underfloor insulation around 600 W to 700 W would be wasted heating the ground underneath (this rough estimate ignores the thermal resistance of the flooring itself).

 

Ideally you want to aim to reduce the heat loss downwards so that it's no more than about 10% of the total, i.e. you draw (and pay for) 1100 W from the mains supply and the heating delivers around 1000 W into the summerhouse (just guestimate numbers).

[ProDave]

This sounds like a retro fit UFH system. I assume this is a new building, so should you not be designing in sufficient floor insulation and incorporating the UFH as you build it, not looking for a cheap retro fit system?

[Fran Smitherman]

This is indeed a new build and, yes, I would like to design in sufficient floor insulation.  

 

The idea so far is to make a concrete plinth 3 metres by 6 metres with the top surface 10 cm above ground level, then a waterproof barrier (plastic sheeting), then double skin breezeblock walls on 3 sides (west, north, and east) and 4.5 metres of patio doors on the south side, then insulation (J S Harris mentions 30cm of EPS) directly onto the waterproof barrier plastic sheeting, then 5 packs of B&Q underfloor heating pads (each 4 metres x 0.5 metre), then tiling.  In this scenario the plastic sheeting is under the breezeblock walls but the insulation abuts the breezeblock wall and the heating pads abut the north wall but have a gap at each side (unless we can buy heating pads which are 4.5 metres x 0.5 metre).  

 

Do I need to think again?

 

Fran