What makes a Passive House?

Insulation


A Passive House requires high levels of insulation to reduce heat loss through your floors, walls and roof. If the heat can't escape your house then it will need very little heating.

Though you need more insulation in a Passive House the extra cost incurred is not significant. This is because insulation is not usually an expensive item in construction. While the 2011 Building Regulations require levels of insulation which are very close to the Passive House standard.

There is no preference for what type of insulation is used as long as it is thick enough to reduce heat loss and it is fitted correctly. If there is gaps in the insulation (see below) the heat can escape and your money has been wasted.

Thermal Bridging


A 'Thermal Bridge' or a 'Cold Bridge' is a gap in the insulation which acts like a 'bridge' through which heat can escape. Every time you find the handle of a saucepan getting hot you are experiencing 'thermal bridging'. Heat is always trying to escape and some construction materials such as steel and concrete make excellent thermal bridges. The image below from Viking House depicts some common thermal bridges.

The 2011 Building Regulations request that 'reasonable care' should be taken to minimise thermal bridges. While the Passive House standard demands that you eliminate them altogether. So there is no obligation on you to build your home without thermal bridges.

It should be taken into account however that if you insulate to the Regulations heat loss will simply become concentrated at the thermal bridges. Thus you will have spent all this money on insulation and failed to get the full benefit of it. Also you may have problems with condensation and mould growth at these thermal bridge points. Again the cost of addressing thermal bridges is not a huge cost in construction and many of them can be eliminated with careful planning and attention to detail.

Triple Glazing


Triple Glazing windows are far superior to double glazing in reducing heat loss. The inside surface of triple glazing can be 18 degrees when its -10 degrees outside which also adds enormously to the comfort levels of the house. Glazed areas in the house are no longer unpleasant areas in the house which radiate cold in the winter months. Triple glazing was once one of the most expensive items in reaching the Passive House standard but the cost has reduced significantly over the past few years .

Air Tightness


All the insulation, triple glazing and elimination of thermal bridges will mean little to your heating bills and comfort levels with draughts whistling through the house. This is why the Passive House standard demands high levels of 'Air tightness' where windows and external doors are sealed with special tapes. A good way to think of Air tightness is draught proofing.

The Passive House standard requires that your house be so air tight as to only allow 0.6 air changes per hour. To put this into context the Regulations are closer to 7 air changes per hour. So there is no requirement for you to build an air tight house. There are two facts however that you should consider. The first is that draughts in your home are uncomfortable. The second is that air tightness measures during construction are relatively inexpensive.

Mechanical Ventilation with Heat Recovery


What's the point of making sure your house air tight and draught free and then punching 4 to 6 inch holes in the wall for ventilation. In windy weather your house will be over ventilated and cold and on still days it will be uncomfortable and stuffy. This is why the Passive House requires Mechanical Ventilation to ensure your house is constantly ventilated and healthy. Used and stale air is extracted from your kitchen and bathrooms. It passes through a exchanger in the central unit where its heat is 'recovered' and used to warm the cold fresh air supplied to bedrooms and living areas.

This not only makes your house extremely energy efficient but also a very healthy and comfortable place to be with a constant supply of fresh air. A MVHR system can be expensive and you can on average expect to pay between €4000 and €8000 for one. This cost however can be offset by the far smaller central heating system you will need to install.