Is T8 Poroton only significantly better than T12 in heating costs?

Hello everyone,

we are converting a barn into a residential house. The old walls remain standing and inside, a new structure is basically being built. The wall construction looks as follows:

30 cm solid clinker brick
4 cm air gap
36 cm T12 Poroton
1.5 cm lime-cement plaster

Results in a U-value of 0.27 W/m²K

For comparison:

30 cm solid clinker brick
4 cm air gap
36 cm T8 Poroton
1.5 cm lime-cement plaster

Results in a U-value of 0.19 W/m²K

Our energy consultant has calculated heating costs of €930 for a building area of 240 m² and heating with an air heat pump using T12. For T8, it should be €890. However, it should be noted that he mistakenly calculated the electricity costs at 7 cents per kWh. Currently, it is rather at least 18 cents for us.

Is it possible that the better bricks only have a minimal effect on the heating costs?

Best regards, Tobias

Hi Tobias,

the U-values might be roughly correct at first, a pure T8 wall at 36.5 cm has 0.21 and a pure T12 wall at 36.5 cm has 0.30. I am indeed surprised that the 30 cm clinker plus air only brings so little, but when you see that 30 cm solid clinker has a U-value of 1.85 (!!!!!), that might fit.

But I would question the final result again:

A difference in wall insulation of about 1/3 is supposed to make only 5% difference in heating costs? Then the house must either consist almost entirely of windows or you have a really badly insulated roof, so that the wall hardly matters.

And I also find the overall calculation odd:
About 900 euros heating costs at 7 cents/kWh (not very trustworthy either that he makes such a mistake here) amounts to just under 13,000 kWh electricity consumption. With an annual performance factor of 3 for an air-water heat pump, you get a heating load of 39,000 kWh, that’s 160 kWh/sqm per year ?!? For a house with a U-value of 0.19 ?!? no, something is not right there!

Did he calculate with open windows?

For comparison: We currently live in 135 sqm living space plus 60 sqm heated basement and come to about 9,000–10,000 kWh heating energy that we consume per year with a wall structure with also U 0.19.

He should explain the calculation to you again calmly. That would also mean that you have to expect about 2,600 euros heating costs per year.
That would be more than my mother pays in her 250 sqm palace from 1975!

It may be that the calculation is correct, for whatever reason, but at first glance I would be very suspicious.

Best regards,

Andreas

I think the consultant calculates the kWh heat with 7C, which fits, whether gas or heat pump, at least approximately

I think the consultant calculates the kWh heat with 7C, which is reasonable, whether gas or heat pump, at least approximately

I had thought about something like that too. But then you end up with an annual heating energy consumption of only 13,000 kWh. I find that very optimistic for a 240 sqm house with only average insulation.

As I already wrote, I think the OP should have the calculation explained to him very thoroughly...

Regards,

Andreas

Since it is at least the Energy Saving Ordinance, the consumption is approximately appropriate.

Yes, that can very well be the case. Assume a heat loss of 20% through the masonry. That gives you 0.2 (20%) * 0.3 (30 poorer insulation) = 0.06 --> 6% energy savings. €930 * 0.94 -> €874 7 cents per kWh heating energy is okay - that is the gas price. Annual performance factor for the air-water heat pump of 3 means 0.238 / 3 = 8 cents per kWh heat... so that fits as well. Do you mean 240 m² floor area or exterior wall area? At 240 m² floor area, 13 kWh is ambitious... but not impossible. Post your energy saving ordinance documents, then many things would be clearer.