For a while, I have suspected that the thermal efficiency requirements for Code 6 would almost certainly require MVHR. But I was always dimly aware that I hadn’t actually done the numbers and so couldn’t be sure. Now I am: no MVHR means no Code 6.
To reach Code 6, you have to achieve a SAP heat loss parameter (HLP) of 0.8W/m2K. The HLP is the total fabric and ventilation losses divided by the dwelling area.
To see how hard this target would be to hit, I put together numbers for five dwelling types, shown in the table below. A “mid level” flat is one with flats above, below, and on either side. A “top floor” flat is the same but with no flat above. The other types should be self explanatory.
|
Mid level flat |
Top floor flat |
Terraced house |
Semi |
Detached |
|
| Total area m2 |
50.0 |
50.0 |
75.0 |
75.0 |
75.0 |
| Gross wall m2 |
38.2 |
38.2 |
48.0 |
104.3 |
160.5 |
| Net wall m2 |
25.7 |
25.7 |
29.3 |
85.5 |
141.8 |
| E window m2 |
6.3 |
6.3 |
9.4 |
9.4 |
9.4 |
| W window m2 |
6.3 |
6.3 |
9.4 |
9.4 |
9.4 |
| Roof m2 |
0.0 |
50.0 |
37.5 |
37.5 |
37.5 |
| Floor m2 |
0.0 |
0.0 |
37.5 |
37.5 |
37.5 |
I then played with the U-values and infiltration rates to try and get down to an HLP of 0.8 without resorting to MVHR. I ratcheted the U-values and air tightness right down and managed to succeed only in the case of the mid-floor flat:
|
Name |
Mid flat |
Top flat |
Terrace |
Semi |
Detached |
|
Pressure testing results: q50/20 (m3/m2) |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Number of sides on which sheltered |
3 |
3 |
2 |
2 |
2 |
|
MVHR |
FALSE |
FALSE |
FALSE |
FALSE |
FALSE |
|
Number of fans and passive vents |
2 |
2 |
3 |
3 |
3 |
|
Doors U-value (W/m2.K) |
0.90 |
0.90 |
0.90 |
0.90 |
0.90 |
|
Windows U-value (W/m2.K) |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
|
ground floor U-value (W/m2.K) |
0.14 |
0.14 |
0.14 |
0.14 |
0.14 |
|
Walls U-value (W/m2.K) |
0.16 |
0.16 |
0.16 |
0.16 |
0.16 |
|
Roof U-value (W/m2.K) |
0.14 |
0.14 |
0.14 |
0.14 |
0.14 |
|
HLP |
0.78 |
0.93 |
0.90 |
1.01 |
1.12 |
Just look at what I had to do just to get this far. The pressure testing results are 2m3 per m2 of fabric area at 50 Pa. Last year, at the pressure testing of a prototype house I worked on, we only just managed to hit this figure and it was the best result the tester from the BRE had ever seen. How are constructions from your more run-of-the-mill contractors likely to perform?
In addition, the window U-values are down to 0.9 W/m2K. That’s triple glazed low-E with some fancy gas filling and thermally broken frames. Very very expensive.
The wall U-values are down to 0.16. In a masonry cavity wall, that would mean 300mm of rock wool. I’ve also assumed no thermal bridges. Having been through this process once with an architect I can confidently say that doing away with thermal bridges altogether takes a huge amount of time and attention to detail.
So all that work and only the mid floor flat made the target. In fact only the mid floor flat even got close.
What happens when you add MVHR? Now all the units meet the requirement apart from the detached house, which, amazingly, still has some work to do.
|
Name |
Mid flat |
Top flat |
Terrace |
Semi |
Detached |
|
Pressure testing results: q50/20 (m3/m2) |
0.1 |
0.1 |
0.1 |
0.1 |
0.1 |
|
Number of sides on which sheltered |
3 |
3 |
2 |
2 |
2 |
|
MVHR |
TRUE |
TRUE |
TRUE |
TRUE |
TRUE |
|
Number of fans and passive vents |
0 |
0 |
0 |
0 |
0 |
|
Doors U-value (W/m2.K) |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
|
windows U-value (W/m2.K) |
0.90 |
0.90 |
0.90 |
0.90 |
0.90 |
|
ground floor U-value (W/m2.K) |
0.14 |
0.14 |
0.14 |
0.14 |
0.14 |
|
walls U-value (W/m2.K) |
0.16 |
0.16 |
0.16 |
0.16 |
0.16 |
|
roof U-value (W/m2.K) |
0.14 |
0.14 |
0.14 |
0.14 |
0.14 |
|
HLP |
0.50 |
0.67 |
0.64 |
0.76 |
0.89 |
And none of this considers how people will actually use the houses (or whether they’ll want to!). As Mark Brinkley pointed out a couple of weeks ago, this type of Passiv Haus living isn’t for everyone. It requires a thorough knowledge of how your heating and ventilation systems work and willingness to take an active role in managing these systems.
In order to achieve these results in practice, you’ve got to learn how to drive your house: shut windows, clean air filters regularly, and stay vigilant for damp, stuffiness, or any other signs of a mechanical fault. Fine if you’ve volunteered, but not likely to work if the occupants are unwilling or feel they’ve had this kind of living foisted on them.
In my view, the HLP requirement for Code 6 is a dead end. As soon as the implications become clear to house builders and the general public, it’s likely to get chucked out the hermetically sealed window. And considering there are much much better ways to spend the money, that’s no bad thing.
carbon limited 
Thanks for doing the maths. It’s what I expected. Interesting that the detached house doesn’t get there even with MVHR. Just how are they going to get built? Or is this part of a wider agenda to keep building nothing but flats?
Mark
[…] Mels post .. and Phils post for excellent round ups and for good technical comment take a look at CarbonLimimited from […]
Great Round up there my son.
I also think it is a dead end. Particularly where one might be looking at biomass for the main heating source. Why recover heat at 0.422KgCO2 when the fuel source is 0.025Kg.
I appreciate the idea of Passiv-Haus, and the notion of tunnelling through the cost barrier to remove for example a central heating source. But I dont believe in reality that the tunnel goes anywhere. Meeting these standards just requires more money and is not in my opinion money well spent.
There is of course a much more detailed debate to be had about this, but given that I am about to go and get a cold beer in Sevilla and further my understanding of the subjuntive in Spanish, it can wait. It is possible too, that the above may not be compatible in 34 degree heat. hasta luego….
Useful round up. This was the understanding we’d come to but hadn’t set it all down with the unit type permutations.
I was wondering if you’d had a look at the new EST guides with recommended routes to achieving zero carbon. Its not clear if they have noticed that they have run out of roof space for PV/SHW pretty rapidly. Pretty strong support for the offsite argument.
Regards
Tom
http://www.energysavingtrust.org.uk/uploa ds/documents/housingbuildings/CE292%20Ene rgy%20efficiency%20and%20the%20cfsh%20Lev els%205&6_May%202008.pdf
Surely detatched houses will be likely to be larger than this area with proportionally less external wall area? This makes quite a difference in the calculations. Otherwise yes why are we building detatched houses which are energy in-efficient?
Also Im hoping you are reducing the wall area to take account of the window area and any sloping roof?
@Robert:
I think detached houses by their nature are less energy (and materials) efficient than semis or terraced housing.
Regarding the floor area, you’re probably right that a typical detached house is bigger than I’ve allowed for here. However, I think the comparison is still valid and it’s unlikely that the slight change in surface area to floor area ratio will make much difference. In fact, I’ve assumed a box here, without any architectural features apart from windows, so this is a best case in terms of surface area to floor ratio.
Yes, wall area takes account of windows. That’s the net and gross figures in the table. I didn’t allow for a sloping roof, but you could argue that most houses are insulated between joists anyway rather than warm roof construction.
[…] standard, based on Passiv Haus. This is a big point of contention as it will almost certainly force all new homes to have MVHR. This would be very bad on several levels and I’ll come back to this point in a future […]
Or just use passive heat recovery ventilation – not as thermally efficient heat recovery, but no electrical load either.
Walls, roof and floor u-values of 0.1 are far from impossible. Windows and doors can get a fair bit lower too. But only worth upgrading windows and doors on fully detached houses.
What is bad about heat recovery ventilation? Letting hot air flow out of buildings in cold weather is a complete waste.