Wednesday, 20 May 2015

Why are there hardly any Passivhaus homes in Cambridge?

Would you like to have heating bills of only a couple of hundred quid a year? You can - if you buy a Passivhaus home. This energy standard is so impressive, homes are heated almost entirely with just the energy from the people and appliances running in them. You don't need more than just a couple of small radiators for backup, so you can use that wall space for other things. Norwich has just announced £300 million funding for Passivhaus starting with 237 homes to be built this year. Passivhaus is proven technology in this country - so why isn't it being done in Cambridge?

The energy group discussed this at our last meeting in May. First, Margaret Reynolds explained about the standard. The fundamental requirement is that the building is designed to use no more than 15 kWh/m2/year for heating. This is about a sixth the normal for new homes. It means a 100m2 property - a wee bit bigger than average - using electricity for heating would cost no more than £210/year for heating. Passivhaus homes generally use electricity because there isn't enough for a gas boiler to do. If the electricity is renewable that means they are very low carbon too.

Passivhaus standard is achieved by careful design and high quality construction standards with particular attention to:
  • High standard insulation for walls, windows, roof and even the floor, with no thermal bridges where the heat can leak out by bypassing the insulation
  • Very good air tightness
  • Mechanical ventilation with heat recovery, giving you fresh air in every room, but with the minimum of heat loss.
There is also a slightly less stringent standard for Passivhaus retrofit (EnerPHit) and there are examples in this country but they are much more rare. It is a huge job to make an existing house properly airtight and to insulate it all the way round without leaving any thermal bridges. For example, you often need to raise the roof a bit to leave room for insulating round the eaves properly! EnerPHit isn't usually financially viable, but some of the measures may be. For example, external wall insulation, Passivhaus standard triple glazed windows, loft insulation and as much air tightness as possible will get you some of the way.

So, back to the original question - why isn't anyone building Passivhaus homes here in Cambridge? Some of the new developments are being built to reasonably high standards. For example, all homes on the North West Cambridge site are being built to Code for Sustainable Homes level 5. That is, or was, a government standard which covers carbon emissions, water use and runoff, waste management, biodiversity, sound insulation and pollution. It doesn't require particular standards for energy use directly and you can offset carbon emissions from energy use by generating renewable energy. So this standard isn't quite as stringent for energy use though it covers lots of other areas. In any case, our government has scrapped it and is introducing a simpler 'zero carbon homes' standard instead. This isn't fully defined yet but it will come into effect next year. It looks like this will also have provision for carbon offsetting through renewable energy generation or 'allowable solutions'. Also small sites will be exempt (though exactly what that means is not yet defined either).

Our local council does not have the powers to require that new homes and other buildings are built to Passivhaus standard. Developers could choose to use it, and there is evidence from other parts of the UK that Passivhaus homes are in high demand. However, in Cambridge the housing market is so over-heated that there isn't any price premium for energy efficiency. (A lot of homes are bought by foreign investors and empty most of the year anyway).

Developers may not be interested but self builders are. We know of one home already and we understand this will be taking part in OpenEcoHomes in September this year so you will be able to visit it then. We know that the K1co-housing development are considering it too. Passivhaus certification is quite expensive to get simply because of the extra documentation required, but you can use Passivhaus planning tools and technology to get a home that is at least in the same league. If you are interested in this, contact Margaret and she will point you in the right direction.

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