What is the best underlay to use for Underfloor Heating

[revelation]

Hi all

 

We have underfloor heating fitted between joists on our FF and Loft.  We have laid 18mm Caber boards over this, and before putting on the engineered wood flooring we were looking for the best underlay to use in this situation to help with heat transfer. 

 

 

 

 

[SteamyTea]

This is an interesting problem as usually we are looking to thermally isolate elements, not join them together.

Underlay also needs to deaden sound and allow some 'bounce' underfoot.

So I thought I would look at two common materials, rubber and cellular rubber.

These have thermal conductivities of 0.13 and 0.045 W.m^-1.K^-1 respectively.

Now assuming that the underlay is only 6 mm thick, then:

 

Rubber 0.006 [m] / 0.13 [ W.m^-1.K^-1]  = 0.0462 [R = m².K.W^-1]

1 / 0.0462 [R] = 21.65 [U = W.m^-2.K^-1]

 

Cellular rubber 0.006 [m] / 0.045 [ W.m^-1.K^-1]  = 0.133 [R = m².K.W^-1]

1 / 0.133 [R] = 7.52 [U = W.m^-2.K^-1]

 

So cellular rubber will be 3 times more insulating.

Edited November 29, 2021 by SteamyTea

[Olf]

0.13/0.045 = 2.88 - yep, one is 3 times more insulating

 

Not sure how flat the floor finish is, my understanding has always been that the purpose of the underlay is supposed to mask all the imperfections of the subfloor first and then add impact sound insulation.

The product I'm looking at is https://www.interfloor.com/app/uploads/2016/08/Heatflow-Wood-Laminate-DURALAY-TS-3.pdf . Although R = 0.035 m² K/W, at 3mm thickness only it gives overall U=28.6 W/m²K. 3mm thickness requires decent floor finish though.

 

[revelation]

11 hours ago, SteamyTea said:

This is an interesting problem as usually we are looking to thermally isolate elements, not join them together.

Underlay also needs to deaden sound and allow some 'bounce' underfoot.

So I thought I would look at two common materials, rubber and cellular rubber.

These have thermal conductivities of 0.13 and 0.045 W.m^-1.K^-1 respectively.

Now assuming that the underlay is only 6 mm thick, then:

 

Rubber 0.006 [m] / 0.13 [ W.m^-1.K^-1]  = 0.0462 [R = m².K.W^-1]

1 / 0.0462 [R] = 21.65 [U = W.m^-2.K^-1]

 

Cellular rubber 0.006 [m] / 0.045 [ W.m^-1.K^-1]  = 0.133 [R = m².K.W^-1]

1 / 0.133 [R] = 7.52 [U = W.m^-2.K^-1]

 

So cellular rubber will be 3 times more insulating.

 

I appreciate the detail.  Our floors are reasonably flat as we had new joists out in and the caber above.  

 

The products I was looking at are 

 

https://www.carpet-underlay-shop.co.uk/products/thermo-pro-x-underfloor-heating-underlay-for-wood-or-laminate

 

& https://www.everbuild.co.uk/products/adhesives/floor-adhesives/sika-silent-layer-mat/   this comes in 3mm and 5mm.

 

 

 

[SteamyTea]

18 minutes ago, revelation said:

https://www.everbuild.co.uk/products/adhesives/floor-adhesives/sika-silent-layer-mat/   this comes in 3mm and 5mm.

They do not show the thermal conductivity, but it could be estimated from the data sheet.

It has a density of 30-33 kg.m³ and is made from polyurethane.

18 minutes ago, revelation said:

https://www.carpet-underlay-shop.co.uk/products/thermo-pro-x-underfloor-heating-underlay-for-wood-or-laminate

This one has a Tog rating of 0.3

A Tog (comes from the word for clothing, Toga)1 TOG = 0.1 m².K.W^-2.

So at 0.0018m thick

0.0018 [m] / 0.9 [ W.m^-1.K^-1]  = 0.002 [R = m².K.W^-1]

1 / 0.002 [R] = 500 [U = W.m^-2.K^-1]

It should really be called a thermal conductor, not an insulator, but it is very thin.

Edited November 29, 2021 by SteamyTea

[Olf]

Nah, why overcomplicate again?

Stated 0.3 tog => R = 0.03 m²K/W

U=1/0.03=33 W/m²K

Clearly the winner, the trick used of course is thickness of next to nothing - but if the floor surface permits, that is the way to go.

[revelation]

I think that is the one I will go with, we want it to conduct heat through easily rather than insulate.  However we would like it to provide some sound proofing too.