In colder climates like the UK the houses still have central heating but the heating requirement is minimised as much as possible. A typical heating demand would be 15 Kwh/m2 per year – about a quarter of what is permissible under SAP 2012. Some passivhaus flats in London have heating bills of less than £100 a year.
How is it done? Fit as much insulation as possible into the building. Be prepared to allow more room in the building design for insulation in the floor and the roof. It’s easy to insulate these without making the house any bigger. Use timber frame walls to get rid of the wasted space that is used by the inner blocks of a standard brick and block wall. That’s an extra layer of blocks that could have been an extra layer of insulation. The inner layer of blocks will also absorb heat, heat that should stay in the house. Into all these huge spaces you’ve created pack as much insulation as you can, good quality stuff, PIR board or even foil or vacuum insulation. But that on its own won’t do it. You need to understand all the other things that cause heat loss from the building. You need to think about air tightness and all the little gaps through which cold air could get in. Extra insulation can be added around window frames, airtight tape can be used and membranes put in the floor and roof. Together these reduce your air permeability to values of less than Q = 1. It’s also necessary to design your glazing to make the most of available sunshine. Make sure that the south facing side of the building is provided with a lot of glazing as this is where the greatest amount of sun is. Use the best quality glazing to maximise solar heating – either triple glazing or advanced solar double glazing. You need to have a low U value (insulating effect) and a high G value (solar heating effect.)
A word of warning. You have now made the building so airtight that people won’t be able to breathe. MVHR (mechanical ventilation with heat recovery) is essential in a passivhaus. The warm air from the kitchens and bathrooms comes into the MVHR and warms up the cold air coming from outside. This warm air is pumped all over the house and the central heating system should turn off because it is too warm. For this to work the house needs to be really airtight. A passivhaus is so airtight that MVHR is like a free energy machine.
What do I think? Passivhaus is excellent but goes beyond some people’s budgets. PIR and foil insulation are expensive and mineral wool and fibreglass are cheap. To use the best insulation in sufficient quantities would be costly. Triple glazing and advanced solar double glazing are also very expensive and so is MVHR. If builders need to spend whole days lining every thermal junction with airtight tape and leave service voids in their timber frame walls it all adds to labour costs. Brick/block cavity walls are much cheaper and remain the standard throughout much of the industry.
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