What is a Passive House? - 04

A Passive house is a building in which indoor air temperatures of a minimum of 18°C are maintained year round without the need for heating appliances.

That’s right, you don't need neither radiators nor wood or gas burners nor air conditioners to live in a comfortable, dry and well ventilated house. This isn't a case of wishful thinking but rather of applied physics. All you need is a maximum heat load of 10W/m². Each new building can be a Passive House.

Is this an experimental concept?

No. There are currently more than 8,000 Passive House buildings operating in Europe (most of them functioning under more extreme temperatures, than you will find in Ireland), the oldest one of them up and running since 1991. The Passive House concept is proven both scientifically and practically to work well.

Is a Passive House also a Zero Energy House?

No, it is not. You still need a minimal amount of external energy to provide a comfortable and healthy indoor climate in a Passive House, although in some regions in Europe that amount would be next to zero. You certainly can go the extra mile, and turn a Passive House into a Zero Energy House. But this very last part of the process is the most expensive. A Passive House tries to balance economy and ecology, and therefore stops short of the last measures. You can easily cover the remaining need for heating and cooling energy with renewables, though, but the applied technology can be expensive.

Why is it called Passive House?

A Passive House tries to provide a comfortable and healthy indoor climate without the need to use active heating or cooling appliances. It heats and cools itself. There are machines to facilitate this purpose, namely an efficient ventilation system (not air-conditioner!), but I consider a fresh air ventilation system a vital ingredient of a healthy and comfortable home anyway. A good ventilation system with efficient heat recovery will actually save energy.

Do I need to have a South facing site to build a Passive House?

No, there are plenty of examples where a Passive House works perfectly without being aligned to the south. It’s a bit harder though, but definitely possible.

Do I need thermal mass?

No. Thermal mass is a helps in the retention of solar energy. Nice to have, but not unchangeable.

Can I open the windows in a Passive House?

Sure you can! Only – you don’t need to do it anymore. But you definitely can do it, if you want. That’s freedom of choice, isn’t it?

Are there specific shapes needed for a Passive House?

No. You might very well build a Passive House that looks like a 19th century villa. You can have it look traditional or modern. They come in all sizes, too.

What about summer?

The good news: insulation works both ways on opaque components like walls and roof. The “solar” design you chose to catch the sun in winter however could be a mayor problem in summer. One more reason why you shouldn’t watch window sizes, because thereby you also magnify solar gain. But there is a solution: keep the sun out in summer simply by using movable shades, like shutters. Always put them on the outside of your windows. It would be nice to automate closing and opening in dependence of solar radiation, too. That could be done quite easily with a small photovoltaic sensor and solar driven motor.

The ventilation system can, in combination with a mostly passive ground heat exchanger, work as a dehumidifier and also cool down indoor air a bit. Again: a big step towards comfort with only a small amount of energy!

How does it work?

It works by minimizing heat losses and maximizing passive heat gains. To minimize heat losses you first and foremost need lots of insulation. By insulation I mean materials with a thermal conductivity ≤ 0,1 W/(m K). This could be glasswool, polystyrene, foam glass, fibreboards, straw bales or others – in a word: low density materials with many enclosed air cavities. Neither rammed earth, nor clay, nor loam, nor concrete would do, since all of those don’t have noteworthy insulation values. All the windows, too, need to be very good insulated. And don’t exaggerate window size.

Think of your home as a present: you need to wrap it all around neatly. And don’t forget the corners!

Next you need an airtight barrier interior of the insulation layer. That could be made of any material that is and stays airtight. Building papers, polyethylene foil, plaster etc.. Just don’t forget joints and connections. They need to be airtight, too.

You need to ventilate your home. But you don’t need to loose indoor warmth doing it. Heat recovery is the solution. It only works well in airtight houses, though.

Now that you’ve cut the losses, maximize the gains. Catch as much winter sun as possible – but avoid overheating at the same time. Try to align the house as south facing as you can, and provide shades on the outside of the building.

You also receive gains from appliances and occupants. But it isn’t very wise to maximize gains here. In the case of appliances: you wouldn’t want to waste the energy you save on heating to run inefficient appliances (inefficient appliances only generate minimum heat). Heating problems solved, think about warm water supply. You can easily get it without burning fossil fuel or fossil generated electricity. Solar water panels are readily available to do the job without sending monthly bills. Solar panels may be integrated into the heat recovery system. Even a clothes dryer can be part of the ensemble. It is very well possible to build a Passive House in Ireland!