Wet Underfloor Heating between joists

[ATO]

Looking at doing this to a suspended joist floor. The boards with pre cut grooves are very expensive, whats stopping me from using a hot knife/gun and cutting grooves myself in standard insulation board? It seems like it would save about 70% of the cost!

 

[Mr Chibs]

Nothing other than they will be sat in foam without the silver backing... but you could get a roll of aluminium duct tape to sort that.

What thickness of insulation are you using?

 

[ATO]

Haven't decided yet but it'll be 125-150mm

 

[SandyLyon]

The aluminium plate / foil needs to be in full contact (no gaps) with the flooring otherwise it won’t work efficiently.

 

[ATO]

Would the foil face of the insulation board plus foil tape in the groove be sufficient for that?

 

[Mr Chibs]

The foil covering on our boards, is merely a brown paper with foil covering, so foil tape won’t be hugely different.

Are you screeding on top of this lot?

 

[ATO]

The foil covering on our boards, is merely a brown paper with foil covering, so foil tape won’t be hugely different.

Are you screeding on top of this lot?

No, boards on top of the joists.

 

[Mr Chibs]

Sorry I meant what will be on top of the joists/insulation?

 

[SandyLyon]

Needs to be done properly otherwise it will not perform as wanted, the idea of the aluminium spreader plates is to distribute the heat from the plastic pipe over a larger area, this plate needs to be in contact with the floorboards / chipboard.


Aluminium plates will perform better than foil but will cost more, finished flooring and building insulation will also affect performance, there is plenty of information available on the net.

There are different types of spreader plates, some can be fitted from below to existing floor others from above if installing new floor.

 

[Madrab]

No, boards on top of the joists.

You need some type of spreader plate or other to distribute the heat effectively, especially if it's not getting screed.

There are solutions that use styrene boards with thicker (200 Micron) ally tape and close pipe centres that does distribute the heat quite effectively though that does need to be in direct contact with either the floor finish or a thermal mass. The other alternative is that the foil covered foam boards are used and then a gap is left above it and filled with a dry sand and cement biscuit mix, that acts as a conductor and then works as a thermal mass spreading the heat.

I don't think just cutting foam insulation boards would work very well.

 

[ATO]

So If I cut grooves in the insulation boards to fit these ally plates for example then I should be good to go? Or I suppose they can just be laid on top without grooves cutting out.

JG Speedfit UFH Spreader Plate - JGUFHSP400SINGLE | Trading Depot

 

[Madrab]

Yes or fill the gap below the plate with normal insulation and the just sit the plates on top of the joists. Check the distance between your joists as spreader plates are sized for specific C2C dimensions and the joists may need supports added.