Thursday, 22 October 2015

Scotland Shows the Way on UK Building Regulations

New Rules will Boost Deployment of Solar on New Homes

Ideal for New Build - Roof Integrated PV.  Image:  Viridian Solar

This month, new building regulations came into force in Scotland.  Let’s have a look at what they might mean for solar.

The way the building regulations for energy performance of houses work is that the designers have to keep the calculated carbon dioxide emissions associated with heating the house, providing hot water to occupants and running lights and pumps (but not electrical appliances) below a certain set level.  
The calculated carbon dioxide emissions are assessed using a government approved method (SAP2012), which is available in software form.

The level the designer has to keep below is arrived at by calculating the emissions from a house of the same shape, but with energy performance features defined in the regulations, a so-called ‘notional dwelling’.  The thermal insulation performance (U-value ) for the walls, floor, roof and openings is defined for this notional dwelling as well as other features such as values for air-tightness, the type of heating system and other energy saving measures such as use of low energy light fittings.

The reason for taking this approach is that it is not prescriptive.  It allows the building industry to experiment with combinations of measures that achieve the overall goal (carbon emissions) in the best way for them (which almost always means the cheapest way).

The table below shows a few key features of the notional dwelling for the new Scottish regulations and compares them with the same requirements for the previous regulations and also the current regulations in England.

Regulators also worry that cold, leaky homes might be built with low levels of thermal insulation and lots of bolt-on electricity generation, so they also set minimum levels of performance beyond which it is not possible to go.  These are called backstop values.

Comparing the notional values for 2013 with those for 2015, you can see that the thermal insulation has been tightened up somewhat, but not excessively.  You can also see that Scotland 2015 is not significantly better than England 2013 in this regard.

Where the two countries diverge massively is that the notional house in Scotland includes a PV system on the roof for homes heated with gas, LPG or oil, whereas the English regulations include no renewables at all in the notional dwelling.

The Scottish regulations call for the notional house to have a PV system sized as follows:

kWp = smaller of total floor area x 0.01  --- or---   30% of the roof area based on 0.12kWp/m2

If we take an average semi-detached house as an example, with total floor area of 85 m2 over two floors and a roof pitch of 35 degrees, this equates to a total roof area of 49m2

So the solar installation on the notional house would be the smaller of 0.85kWp or 1.8kWp
But solar installations on notional houses don’t help the solar industry.  What does this mean for real houses that are going to be built in Scotland from now on?

A developer in Scotland seeking to build this 85m2 semi could aim for any of the following:

1. Match the insulation levels in the notional values and install a 0.85kWp solar system
2. Exceed the insulation levels in the notional values and have no solar
3. Relax the insulation levels below the notional values and put a larger solar system on

As I discussed in a previous blog, insulation suffers from a diminishing return which means you need to pay for ever more insulation to make the next improvement.

By contrast, solar PV benefits from a falling marginal cost as you increase the size of the installation.  If you’re going to use solar, it’s more cost effective to use a larger system.  

The feedback I am hearing from housebuilders in Scotland is that they are embracing solar as a big part of their strategy for delivering homes to the new regulations.   Schemes we have seen so far indicate that the solar systems will be closer in size to those installed by householders when retrofitting, perhaps in the range of 2-3 kWp.  This is great news for the solar industry and also for the energy bills of people buying these homes.

If only England and Wales would implement such ambitious targets for new homes too.


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