Suspended Timber Ground Floor U-values

[Leigh Caller]

I've been preparing U-value calculations for the above floor type in accordance with ISO 6946 and 13370, but this particular building design does not feature the perimeter walls usually associated with this construction. In other words, the void below the floor is completely open and varies in height from 0.7 to about 2.7m. Would I be correct in using the exposed floor method here in lieu of EN 13370 where the floor void height exceeds, say 0.5m and is fully exposed? BR443 does not quote a maximum void height or maximum ventilation opening area for suspended floors, but ISO 13370 states that where the depth of the underfloor space exceeds 0.5m, then the method analogous to that for basements should be used to obtain the thermal transmittance (U-value) of the ground. 

At present I am calculating the thermal transmittance of the floor deck (Uf) using RSe = 0.04 and excluding any insulating effect from the ground or void below the floor. Does this sound sensible?

[Nick Grant]

Hi Leigh

Unless I have misunderstood it does sound like this is to ambient. I think the Rse is 0.17 according to AECB Carbonlite Building Science course manual but if you have insulation in the floor then the Rse won't have much effect on the U value result.

Cheers

Nick

[Leigh Caller]

Thanks Nick, the floor is to ambient, but BR443 suggests Rse of 0.04mēK/W for floors exposed on the underside. I think I will take a conservative approach and use this. As you say at U-values approaching 0.1W/mē.K external surface resistance is making very little difference to the result as the insulation dominates.

I believe 0.17 applies to downwards heat flow where the external space is sheltered (as I understand from the Carbonlite Science for Passivhaus material and wider reading).

Cheers.