One of the things which PassivHaus has brought us is a new way of measuring energy efficiency. It's being called a performance metric and it's expressed in kilowatt-hours per square metre per annum. Or kWh/m2/a. It's a much clearer way of presenting the issues at hand but it's not without its problems - of which more in a minute.
The subject is dealt with at some length in the document produced by the Zero Carbon Hub late last year. It looked at using another metric, kg/co2/m2/a, but rejected it because of the confusion relating to the different carbon factors in various fuels - i.e. if you selected to heat with biomass, you could virtually ignore energy efficiency. It also looked at using the so-called Heat Loss Parameter which is a sort of amalgamation of all the U values surrounding the building, but felt it was too little understood and no one outside the UK has ever heard of it. And it alighted on kWh/m2/a because it was the one pushed by the PassivHaus standard and it has some sort of meaning for ordinary mortals. If you can just figure out how many bloody square metres there are that you are trying to heat, then it gives a good indication of what the fuel usage will be.
Now the Zero Carbon Hub has come in for a fair bit of stick from darker greens than me for setting a target which doesn't look so very hard to beat, compared with the Passivhaus standard. These comparable metrics look like this:
• PassivHaus standard: no more than 15 kWh/m2/a
• Zero Carbon Hub suggestion: no more than 39 kWh/m2/a for flats and mid terraces, no more than 46 kWh/m2/a for detached houses, semis and end terraces.
By way of comparison, the last time I carried out this exercise was in producing the 2008 edition of the Housebuilders Bible, when I put my benchmark house through its paces. I configured the house in different times and the performance metrics looked like this:
• 1975 (pre-insulation) standard: 270 kWh/m2/a
• 2006 (as built) standard: 60 kWh/m2/a
Interesting that the 2006 standard really isn't that far short of what the Zero Carbon Hub are suggesting for 2016. Which, I guess, is why they have come in for so much flack. But, to their credit, they have chosen a metric which makes it much easier to compare one standard with another. Would that Part L would do this as well, instead of creating maximum confusion with its over-complex DER and TER calculations.
And what of the problems? Well none of these metrics is perfect and a metric based on kWh/m2/a can be abused. You might think that a kWh is a kWh, but are we talking about heating demand or consumption? The definition clearly states that it is demand that is being measured, so the efficiency of the delivery method shouldn't be taken into account. Nevertheless, expect one of two heat pump suppliers to misuse the metric to suggest that they can heat draughty old piles for much less than the Zero Carbon standard.
Another old chestnut is playing around with the footprint that you are measuring. We still don't have a standardised way of measuring floor areas, so there are advantages in exaggerating the floor area. This factor was graphically illustrated on the PassivHaus study tour to Hanover that I went on with the BRE in 2007, where Carsten Grobe, the architect/selfbuilder we visited, admitted that he had chosen to include his basement within the heated envelope because it made the floor area grow by 30%, and therefore made the task of meeting the 15kWh/m2/a metric that much easier.
There will be confusion, also, about what's included in the kWh measurements. Does it include provision for hot water? No. It's too dependent on other factors and has nothing to do with the built fabric. Does it include anything for cooling? Sort of. As far as I can see, it only takes the cooling effects of thermal mass and natural ventilation into account, and assumes that no active cooling gets used. Lighting? No. Ventilation? No. That's because they are not insisting on mechanical ventilation for the 2016 standard. Really, it's just space heating.
Finally, there may be a danger that consumers will take the metric as a guarantee, and we may find that if they use more energy than this, they start complaining. Heaven help us! The idea of the metric is that this amount of energy will be capable of keeping the building warm and comfortable throughout the year, but the actual performance is still very much dependent on how the house is lived in, and experience of Passive Houses to date reveals that the best performing homes use about a quarter of the energy of the worst, and that the metric score lies somewhere in the middle. It can in reality be very hard to distinguish space heating demand from hot water, and so it's far from straightforward to tell just how successful a scheme has been just by analysing fuel bills.