Heat Loss Parameter and fabric efficiency - 24

It will be necessary to ensure that the fabric of the building significantly exceeds the standards currently required by Part L of the Building Regulations 2000 (as amended). The requirement will be that the “Heat Loss Parameter” (covering the walls, windows, air tightness and other elements of the building design) is no more than 0.8W/m2K. This standard will mean that space heating requirements are no more than 15kWh/m2 per annum.

Heat Loss Parameter

“Heat Loss Parameter” is a phrase I wasn’t familiar with but it’s embedded in the SAP calculations. It’s defined as the specific heat loss for the whole house divided by the internal floor area. The figure 0.8 is very exacting, equivalent to constructing to Passivhaus standards, which is where the 15kwh/m2 figure comes from. In fact, I have learned from the Treasury that the figure was defined by taking the Passivhaus standard and playing it through the SAP calculations on a notional house. To get down to 0.8, you need to have super-insulated walls, floors and roofs, triple glazing, virtually no unplanned air loss and mechanical ventilation with heat recovery.

Heat and power for space heating, hot water and lighting must be generated either in the home or on the development or through other local community arrangements (including district heat and power) and must be renewable (i.e. non-fossil fuel) energy.

Here it departs from PassivHaus standards and starts to require renewable power input. But what exactly does it mean by other local community arrangements? It is a frustratingly vague phrase.

What about power use over and above space heating, hot water and lights? It goes on:

A zero carbon home is also required to have zero carbon emissions from use of appliances in the homes (on average over a year).

There follows a table which specifies how much additional power you need to generate depending on the overall floor area of the house. Bigger homes, over 150m2 floor area, need to generate 16.54kWh/m2 to cover cooking and appliances. So a 200m2 home, the typical size built by a UK selfbuilder, would need to generate 3308kWh/annum to cover this load.

This additional power must be renewable power produced either within the area of the building and its grounds, elsewhere in the development, or elsewhere as long as the developer has entered into arrangements to ensure that the renewable generation is additional to existing plans.

Now there appears to be an important distinction here. Appliance power, it would seem, can be offset by the developer, whereas power for space heating, hot water and lighting must be site generated, or at least community generated. It’s not exactly clear what the distinction is, or what is meant by entering into an arrangement but the fact that it exists is confirmed by Cl 18 which states - The amount of such additional power can be reduced by any surplus from the arrangements to meet zero carbon on heating, hot water and lighting.

So how does one square this? In what sense can one type of electricity be subject to less stringent requirements than another? It’s something that appears nonsensical. Perhaps I shouldn’t be surprised, because its followed by the silliest clause of all of them.

Qualifying homes will not be permitted to be connected to the gas main. Mains gas is by far and away the most efficient way of burning fossil fuel. So where on earth is the logic in banning it? Especially as there is nothing to stop you using bottled gas for the same purpose? Or of course cooking with grid-delivered electricity, which releases around two and a half times more carbon dioxide than mains gas. In fact, I am pretty certain you could install an electric Aga, just about the most energy hungry household appliance on the planet, and it wouldn’t effect your zero carbon status one jot. I am afraid the Treasury is shooting itself in the foot here.

So how would you go about building a zero carbon house?

First you would need to construct something close to a PassivHaus, a super insulated shell with triple glazing and mechanical ventilation with heat recovery. The PassivHaus rules are quite prescriptive and it’s not clear just how closely you would have to adhere to them.

Then you would have to develop a strategy to provide renewable power to cover your anticipated demand for space heating, hot water, lighting, cooking and electrical appliances. You would need to carry out a heat loss calculation, using the government SAP rating scheme, to work out what this figure would be but you can assume that overall it is likely to be around 50kWh/m2/annum, or 10,000kWh/annum for a 200m2 house, of which perhaps as much as 30% can be offset — although what counts as an acceptable offset isn’t detailed. You could achieve this with:

• 6kW wind turbine, mounted in an appropriately windy spot, cost around £15,000 after grant deductions or

• around 80m2 of photovoltaic solar panels, cost around £30,000 after grants.

Hot water and space heating demand could be met by a biomass or pellet boiler, cost around £10,000 and/or solar hot water panels, (cost around £4,000 but will only deliver part of the load.) But neither of these options produce electricity so presumably there would have to be additional installations to cover electricity demand as well. It’s not exactly clear. Alternatively, heat pumps could be used to reduce the heating demand, but they don’t generate any power either, so it’s hard to see quite how they fit into the picture. One form of micro generation, the CHP plant, can’t be used at all, despite the fact that they do generate electricity, because the current models use mains gas, which is of course banned! Oh the joys of joined-up thinking!

Presumably you would be able to mix and match these technologies so that you could for instance use a heat pump combined with a solar hot water panel to reduce your demand and then fit a smaller PV array to make up the difference and then offset the appliance load. But what isn’t explained is how each individual scheme will be judged: there is a lot of fairly complex carbon accounting going on here and you only have to vary the assumptions slightly to get very different results. Who is to say House A qualifies as zero carbon whilst house B doesn’t? I have no idea.

Finally, what’s the tax break worth? As I understand it, the zero carbon house will be exempt from stamp duty only on its first sale and only up to a maximum of £15,000. If you sell the house for over £500,000, which is where the top rate of 4% stamp duty starts, then you would subtract the £15,000 from the duty normally payable. The beneficiary of this largesse is therefore not the developer but the first purchaser, who pays the stamp duty in the normal course of events, although the developer may gain by being able to sell the zero carbon house for a little more than the market would normally bear, because of the stamp duty holiday. Tax breaks should never be sniffed at, but the reality is that the cost of uprating a normal house to a zero carbon house will invariably be considerably more than the value of the tax break.

One supplementary point worth noting: Tony Verran of HM Treasury, in commenting on first draft of this piece, has indicated that selfbuilders will not be eligible for the tax break because it will only cover the first use of the home for residential purposes. It doesn’t cover bare land and it won’t cover a sale made to a second user, even if it’s technically a first sale.