Friday, 17 June 2011

Passive? Or massive assive?

So, this is a passive house, but what is a passive house, and who really cares? Aren't houses all passive? I mean, they don't run around do they! And if this is passive, why does it have an active ventilation system? Pumping air in and out twenty four hours a day doesn't sound very passive!
And why do people say 無暖房住宅 (mudanbou jutaku - literally no-heating house)?

The Passive House, or Passivhaus if you prefer the German name, is a standard based on the idea that, if a house has sufficiently low thermal losses, then you don't need a central heating system. In the long-term, any extra initial cost will be saved a few times over in lower heating bills. In fact, if there is no need to make a central heating system, there may be no extra initial cost. 

Hence mudanbou jutaku. The problem with this term is that it doesn't really mean there is no heating, just that there is no central heating, so you don't need a radiator in every room. It's possible to add a little heat to the ventilation system, so the air coming in is a degree or two warmer. The window manufacturer suggested that if we were cold we could just switch on a 100 watt bulb for a few minutes and the room would be warm enough.

Japanese building is at a stage where there has never been a radiator in each room, and central heating is something that is new and seen a desirable thing to put in your house. At the same time it seems that cutting-edge European building is trying to get away from central heating.

I can't help feeling that I'm paying over the odds for this in Japan, where a lot of the building concepts are alien, building materials are sourced from local cartels, energy standards are lax and voluntary, people who can do the necessary insulation and draft-proofing work are few, far between and charge a premium.

So what is the Passive House standard?

Low thermal losses means three things: 
* high insulation, which will stop heat being conducted and convected away from the walls, windows and doors
* zealous draft-proofing, which means that, in the winter, warm air is not going to be lost
* a heat exchanger on the ventilation system. Thermal efficiency without suffocation!

Thermal gains are also important, so in the winter as much of the winter sun should get in through the windows as possible, which is known as passive solar design. The sun is higher in the summer, so careful placing of fixed shading, and the judicious use of movable shading can stop the house getting too hot when the outside temperature is above the comfort zone around 20 degrees centigrade. Energy efficient appliances within the house are also important, otherwise the house will get too hot in the summer.

The standard states three things:
1. The energy loss from the house should be under 15 kWh/m2; 15 kilowatt hours of energy per square metre of floor space per year. 
2. The total primary energy use of the house should be under 120 kWh/m²a. This is referring to the original fossil fuel, so if the house uses electricity, you need to multiply the electricity consumption by 2.7 to account for inefficiencies in the power stations and getting the electricity from them to your house.
3. The house should leak less than 60% of its volume of air each hour. This sounds like a lot, but houses in Japan generally leak about ten times this, and old houses in the UK are worse, although leaky houses are a good idea when a coal fire is burning in each room!

They also recommend:
a heating load less than 10W/m²
windows with U value less than 0.8 W/m²K, (although see here for localisation).
a ventilation system which recovers over 75% of the outgoing heat
thermal bridge-free construction

See more on Wikipedia and at Passive House US.