no Code 6 without MVHR

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.

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:

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.

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.