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?