Getting to grips with the new Part L

If you want to get a handle on how to get your yet-to-be-built house through the new edition of Part L, the thermal building regs for England & Wales, you could do worse than drop into a website called PlayThe Regs. I found it a bit buggy but then it just may not like Macs. I tried on two browsers, Safari and Explorer, and they didn’t like it at all, but I eventually I got it running in Opera.

Playthe Regs permits you to twiddle around with different values for 13 variables and it calculates the effect this twiddling will have on your Dwelling Emission Rate. In effect, it’s a cut down version of the SAP calculations that underpin Part L.

Now what’s all this about a Dwelling Emission Rate (DER)? For the first time, the regs are looking directly at carbon dioxide emissions and the DER is a value based on kilograms of CO2 emitted by each square meter internal floor area per annum.

The process now looks like this. Instead of saying you have to have U values of x etc, etc, you start with a model house which meets the old Part L (2002 version). If it was heated by a non-condensing gas-fired boiler (which it probably would have been), it will have had a DER of around 32 kg/m2. What you now have to do is to improve this emission rate by 20%, i.e. 24kg/m2. This 20%-off figure becomes your Target Emission Rate or TER. This is where the variable twiddling comes in. For instance, just by switching to a condensing boiler, the emission rate drops by nearly 4 kg/m2: that’s half the job done in one fell swoop.

Then you have to adjust other variables:
• Improve the glazing U values from 2.0 to 1.8 (you have to anyway) saves 0.5kg/m2
• Bring down the wall U value from 0.35 to 0.27 saves 1.2 kg/m2
• Improving the airtightness reading (from 10 changes per hour down to 7) saves 0.5kg/m2
• Attending to thermal bridging details saves 2kg/m2

Total all these together with a condensing boiler and you easily get your DER down below 24kg/m2. That’s considered to be a PASS.

There is, however, no guidance within Part L on how you go about improving air tightness or thermal bridging. You can find this in supplementary documents accessed via the Energy Savings Trust website. What’s actually there are two 10-year-old publications emanating from the BRE’s former energy wonk department, BRECSU. It’s all good stuff, but leaning heavily on Good Practice Guides from the 1990s doesn’t seem to be that well thought out. What is planned, in due course, is some super-duper new website with robust details that you will be able to use in your designs. But this lies some time in the future. At the moment, there is just an embarrassing absence of guidance.

If you are a curious type, you will use PlayThe Regs to do some twiddlling to see what other results can be achieved and what the effect of each variable might be. I don’t want to spoil your fun but there are one or two anomalies I wish to comment on. Two apparently quite similar heat pump based technologies have widely differing outcomes.

Mechanical ventilation with heat recovery (MVHR)
The default setting is for the house to have no whole house ventilation system. There are two other options, both whole house systems, one with and one without heat recovery. Both options appear to make emissions worse.
• Mechanical ventilation without heat recovery adds 5kg/m2
• Mechanical ventilation with heat recovery adds 2kg/m2

Just why MVHR should make matters so much worse isn’t explained. According to these calcs, you would do better to avoid all whole house ventilation systems and build a really leaky structure than to build a near airtight structure with an MVHR air handling unit. Intuitively, this feels crazy.

Ground source heat pumps (GSHP)
In stark contrast to MVHR, ground source heat pumps sail through the SAP calcs. In fact, installing GSHP gives you a pass even if you set every other variable to score as badly as possible.

This is a bizarre outcome. It’s brought about in good part because Part L has fudged the fuel factors and is far kinder to electric heating than it should be, in terms of carbon emissions. Presumably, some pretty effective lobbying went on by the electric heating industry in the run up to the launch of Part L 2006, but it’s had this strange effect of making GSHP a shoe-in if you want to get a house through the regs. It really shouldn’t be, but that is one of the problems of using dumb algorhythms like SAP calculations to decide how a house should be built, instead of employing common sense.