UFH on suspended floor or solid floor

[Col2021]

This is going to be in a new extension,  so would I better just to go for a solid floor to give a bigger thermal mass and it would appear to me(as a layman) an easier job with more insulation. 

[Olf]

If you can have a slab, then go for it: no bounce and cheaper to make. And if you have a choice of screed on insulation on slab vs structural slab, always take the latter.

Why do you want higher floor thermal mass in an extension? Floor volume is relatively smaller to the overal volume (driven by at least 3 walls) than in case of the main building, so any difference here will be proportionally smaller.

 

[Col2021]

Extension is doubling the size as the original cottage is 60sq m, extension the same. Extension will be kitchen, dining, living and utility. Having ufh in bathrooms, one will be a suspended floor and the other will be solid floor. Extension wil, also have south facing glass for winter solar gain.

[JohnMo]

I would go solid floor, with plenty of insulation below - 150 to 200mm PIR.

 

Install blinds on windows as evening solar gain will be high in Autumn and Spring as the sun doesn't go that far west before getting very low in the sky.

[jayc89]

41 minutes ago, JohnMo said:

I would go solid floor, with plenty of insulation below - 150 to 200mm PIR.

 

Install blinds on windows as evening solar gain will be high in Autumn and Spring as the sun doesn't go that far west before getting very low in the sky.

 

Perhaps somewhat controversial; I never really got the need for so much PIR.

 

We used 100mm and given our PA still met PH requirement of 0.15. Of course more insulation is better, but the cost of PIR is only going up and the cost to excavate/dispose of an additional 50-100mm soil won't necessarily be cheap either. Is a slab that achieves less than 0.15 really that noticeable?

[JohnMo]

I suppose it depends on you perimeter to area ratio, to what the overall u value becomes.  Our house is long and thin, so needed way more to achieve a good u value.

 

Also we had no disposal costs as everything stayed on site.

[Olf]

18 hours ago, jayc89 said:

Perhaps somewhat controversial; I never really got the need for so much PIR

I think there really are only 2 thresholds: building regs and passive house.

BR condition to meet specific value may result in need to have more than 'standard' (like in JohnMo's case), and for PH to have whatever makes the building 'net zero'.

There might be 3rd condition, cases with UFH and specific floor finishes, where losses downwards are expected to be large if not tackled by extra insutation.

 

But in principle, based on maths explained here, it makes little sense to voluntarily improve above BR requirements, as payback time is too long to justify. Though this reduces quickly: I checked recently and the same board at the same supplier costs the same as when I bough last year, but energy price is now 3x more - and so the payback time reduced 3x.

But that calculation is based on insulation material price only, you rightly mentioned that it may be necessary to dig space for it as well, handling thicker material has also extra associated cost.

[JohnMo]

But the maths is slightly different for the floor with UFH as you have a dominant downwards heat flow because the it has a high delta T than the upwards flow.  Subsoil temp stabilises at approximately 8 to 9 degrees below the floor and the ufh water flow is circa 30 degrees, so the delta T becomes 21 instead of 13 (used in the linked maths.

 

Heat Flow Q = U*A*dT

Heat Loss E_heat = time*average_heat_flow = (days*24hours/day) * U * A * (Average_dT)

For me average dT across 6 month 'heating season' is 21K (i.e. 21 Celsius) --> E_heat = (183days/yr * 24hours/day) * 0.13W/m2K * 1m2 * 21K = 11990Wh/yr = 12kWh/yr per m2.

That is heat energy, to understand what I'll pay, I need to know how much heat costs me. Using an ASHP with a COP of 300% (reasonable) and an electricity rate of £0.3/kWh electricity, I can calculated that heat costs £0.3 / 300% = £0.1/kWh heat.

12kWh * £0.1/kWh = £1.20/yr per m2.

 

0.01 change in U value then the cost difference is (0.01 / 0.13) = 1/13 of £1.20/yr /m so £0.092 per m2.

 

My 192m2 floor has a U value of 0.01, so plugging the figures in gives me a saving of circa £70 every year on downwards heat loss by improving on a u value of 0.13.