Review/criticism of low profile underfloor heating plan.

[Joshua Graham]

Hi,

 

As is attached, the image shows a crosssection of the layers in the underfloor heating system I am planning. I have got a company that is giving me a good deal on routing ply for use as an low profile under floor heating system. The system brings the floor up by 30mm; 6mm cement fiber board, 18mm structural ply and 6mm xps insulation. it will be in a living room and kitchen area and is a wet system. I want to know, If anyone sees any immediate issues with it as shown in the diagram, my doubt I have is if the xps will be rigid enough to support such a system but the reason I was thinking this, is to bring it up to 30mm as it brings the floor exactly level with the rest of the house and a little more insulation never hurt, though it is sandwiching the subfloor because a vapor control layer, so maybe it would hurt if water got in... Please let me know if you would do a system like this any differently, I already have the ply, pipes, spreader plates as surplus from a pervious job, so I am planning to work them in no matter what.

[JohnMo]

Be prepared for high flow temps and quite high downwards heat loss, you need double your proposed insulation. That level of insulation is fine for radiators, but not fine for UFH.

[Joshua Graham]

5 minutes ago, JohnMo said:

Be prepared for high flow temps and quite high downwards heat loss, you need double your proposed insulation. That level of insulation is fine for radiators, but not fine for UFH.

I should have mentioned, there is also mineral wool below the pir, to the remaining depth of the joist. In terms of high flow rates, that is the whole purpose of using 100mm centers, the pipes should run at a lower temp should they not? what would your recommendation be?

Edited October 11, 2023 by Joshua Graham

[JohnMo]

Aim for 0.15 U value or ideally better, to manage downward heat loss.

 

Ultimately centres and flow temp depend on how many W/m2 you need to get out of the floor? Have you completed heat loss calculations?

[Joshua Graham]

2 minutes ago, JohnMo said:

Aim for 0.15 U value or ideally better, to manage downward heat loss.

 

Ultimately centres and flow temp depend on how many W/m2 you need to get out of the floor? Have you completed heat loss calculations?

Yes, we did do it about a year ago, which is where the 100mm pipe center came from. But much has changed since then, the property is not airtight, but it is certainly tighter than most and all insulative measures have been carried out. It may be worth another calculation. Do you see any issues with the structure proposed in the diagram (beyond insulation), would this all be functional from a structural perspective?