30 Jun 2010

Part L v the Code for Sustainable Homes

Coming off yesterday's piece on using kWh/m2/a as a metric (measurement) of energy efficiency, I think I have identified a major flaw in the shaping of both Part L and the Code for Sustainable Homes, and I'd appreciate some feedback.

When the Code was launched in 2006, it was much applauded for setting out a roadmap for change. The ultimate goal of zero carbon housing was to be attained in 2016, but there were two intermediate stages set out as well, being 2010 and 2013. The idea was that Part L would be used as the enforcing code to raise the base standard and, indeed, this is how the 2010 changes to Part L have been set out.

I am working from memory here so please feel free to correct me if I am wrong, but the way Part L has been chosen to work is like this. The benchmark date selected was 2002 which was the year of a Part L revision when the way of calculating energy efficiency switched from using elemental approaches to a whole house heat loss calculation. A house built to this standard is said to have a Dwelling Emission Rate (DER) of x.

The next version of Part L (in 2006) then put in place a process which called for the 2002 DER to be improved by a certain percentage, in this instance 20%. So the Target Emission Rate (TER) for 2006 was the 2002 version DER minus 20%. At this point, the Code for Sustainable Homes comes into play and with it a set of targets. Code Level 3 set an improvement of 25% on Part L 2006, and Code Level 4 an improvement of 44%.

So consequently the new version of Part L (coming into effect this October) simply attempts to put this into place. It still refers back to the 2002 model house, but asks that you now build it to a standard that is a 25% improvement on the 20% improvement requested in 2006. This tortured logic is all on display on p11 of the 2010 Part L where you can actually see the calculation set out in two stages. Essentially: "Take the 2002 DER, times (1-0.2) and then times (1-0.25)." Incredible!

Thus Part L 2010 is aligning itself with the energy standards set out in the Code, in this instance Code Level 3. And Code Level 4, which called for a 44% reduction on 2006 levels will doubtless be the basis of the next revision of Part L in 2013.

But (and it's taken me 6 paragraphs to get to the BUT) there seems to be a problem, and its revealed by yesterday's post on the Zero Carbon Hub's metrics. If you stop talking about DERs and TERs (the lingo of Part L) and instead start using kWh/m2/a as a measurement, then we run into a wall identified by the Zero Carbon Hub. You see, I don't think that kWh/m2/a space heating scores for the 2006 house are much more than 60, so a 25% improvement pulls that down to 45 kWh/m2/a for a detached house, which is already slightly below the score (46 kWh/m2/a) which the Zero Carbon Hub are calling for for 2016. And yet we still have another version of Part L to negotiate before then, one which the Code demands a 44% reduction on 2006 homes.

The challenge which the Zero Carbon Hub have identified is to do with our old friend mechanical ventilation with heat recovery (MVHR). They don't think you can build much better than the levels they have set without using MVHR, and they have stated they don't think it's a good idea to make MVHR mandatory in UK homes. Hell, I have more or less agreed with this line in the past and got it in the neck as a result.

Various random thoughts occur.

• Is there really an indoor air quality issue here? Or is it imagined?
• Is there really a point in energy efficient building process when you simply HAVE to use mechanical ventilation, and natural ventilation techniques are no longer adequate?
• If so, where is that point? Could it be expressed in the chosen metric kWh/m2/a?
• If the Part L Target Emission Rates were to be replaced with kWh/m2/a, what would the scores be for the 2002, 2006 and 2010 versions? You can do the SAP calculations easily enough, and then turn this into a kWh/m2/a score, but this calls into question just how accurate SAP is or was?


  1. Sorry, I'm totally confused by this post. Are you suggesting energy use can be further reduced without using MVHR or that if MVHR is required then further energy-use reductions should not be made?

    I suspect a mix of the two; maybe it would be better to separate the two threads out.

  2. Ed,

    I think maybe I am just as confused. I am suggesting that there must be a point at which you can't reduce energy use WITHOUT installing MVHR. I am just wondering where that point is?

  3. The issue here, as ever with energy efficiency, is the diminishing returns available. There clearly has to come a point where it is more cost-effective - in terms of both financial cost and energy cost - and of greater value to move to low and zero carbon energy sources (not necessarily on-site) than to further increase the energy efficiency.

    A housebuilder I work for commissioned some work on this looking at the point where renewables become more cost effective than fabric measures for their Code level 3 and 4 homes. The results in terms of cost effective energy efficiency levels were fairly close to those found by the Zero Carbon Hub.

    This was a purely financial analysis and did not consider any embodied energy or whole life cost issues (which I think would push the balance towards higher levels of energy efficiency) but it seems to suggest that we are indeed approaching the sensible limit of fabric efficiency in new build regulations.

  4. Mark, thanks for the clarification. Yes, it's pretty easy in principle to do some calculations on how cost effective at energy saving a bit more insulation will be vs spending the money on heat-recovery ventilation but a lot harder to know if it makes sense in practice.

  5. Mark BennettJuly 01, 2010

    The point at which MVHR becomes useful is not necessarily the point at which a house loses a certain amount of heat.

    I think it's much more closely linked to the airtightness of the house. If the airtightness of the house is worse than a certain limit (usually 3AC/H seems to be the magic number) then the energy invested in the fans is not recouped by the saving in heat loss from the air, due to the effect of the uncontrolled ventilation.

    Of course, there is some correlation between airtightness and thermal loss, but it is not a direct correlation since there are other ways to lose heat.

    I guess an airtight building with poor insulation could still benefit from MVHR. On the other hand a super-insulated building with very poor airtightness would probably not.

    At the end of the day it is about focusing on the best return for the investment, and you have to tackle airtightness and insulation together to optimise the ROI.

  6. You're assuming that the people writing up the code know what they are doing. From past experience they don't. Usually they are academics with no standing in the real world, doing everything by modelling. And look where modelling took us when it comes to particles in the air. In other words Volcanoes.

    When you model a house you can get everything perfect. Not what the typical British builder does. Especially not a builder working for one of the major housing companies who are pushed to do such things like install two bathrooms a day. To do this kind of work they will cut corner and quality always drops. That's why when you use the term "A Barrett Home" it's usually to describe a tiny, badly designed, crapply built house.

    So basically the code is bollocks and written according to who can do the most lobbying at different points in time. It has nothing to do with making homes better. And all to do with politicians boasting that they are doing the Green thing and that if they didn't then Climate Change would come and flood us just like that little doggy in the cartoon advert.

  7. AnonymousJuly 06, 2010

    Sweden built to 3 ac/h at 50 Pa in 1977. It used mechanical exhaust ventilation.

    Much simpler and cheaper than balanced mvhr

    So did German in its Low Energy Standard

    I think that's all some UK builders are capable of right now.


  8. Good post some very useful information.