PassivHaus design specifications explained

Building physics is a relatively new science that’s only recently started to gain ground in the UK construction industry. A lot of builders nod agreeably when you mention “PassivHaus”, but not many know what it means when it comes to actually building your future home to that standard.


PassivHaus takes a “fabric first” approach to create comfortable living spaces. If the fabric of your building (the walls, windows, floor and roof) is well-insulated and airtight, then the requirements to heat it will be lessened. You won’t be paying money to heat up air that escapes from leaky windows or through thin brick walls.

This is the “passive” approach – the home should be able to maintain its heat naturally as much as possible, without “active” heat sources such as electrical fires, stoves and boilers.

A few things to note, though, regarding PassivHaus:

  1. If your home is airtight, you’ll need mechanical ventilation to supply fresh air. PassivHaus design recommends Mechanical Ventilation Heat Recovery (MVHR) units. These take warm, moist air from kitchens and bathrooms, pass it through a heat exchanger, transfer the heat to incoming cold, fresh air from outside and supply that warm, fresh air into your living rooms and bedrooms.
  2. Even with a solar thermal system, you’ll still probably need a condensing boiler to supply your domestic hot water, especially during “peak” periods such as when you have guests over during the holidays.

The Building Physics requirements for PassivHaus

Airtight means no draughts – at all

The house has to be airtight to less than 0.6 Air Changes per Hour (ACH) at 50 Pascals of pressure (10 pascals is similar to the pressure of a light breeze against you. 50 pascals mimics the effect of a strong breeze pushing against your house, forcing air into the gaps in your windows and doors).

An “Air Change” means all the air in your house leaving and being replaced by fresh air, per hour. Obviously this is very fast at 50 Pascals, so it’s almost a “worst case” scenario.

Never any condensation on the windows

The surface temperature of the windows should always be more than 17 Degrees Celsius. This inhibits condensation, which needs a cold surface to form. This temperature can only really be achieved with high performance double or triple-glazed windows.

No summer overheating

Houses designed with lots of south-facing floor-to-ceiling glass windows (what architects call “Solar Passive” design) look great, but can overheat in the summer. Designing your window locations and sizes can mitigate this, as can create shading and using external blinds to prevent the sun from hitting the windows.

Passive Houses are designed so that the risk of the sun overheating the home to a temperature of 25 Degrees Celcius is limited to 10 per cent of the year. In practice, PassivHaus designers aim for overheating to be limited to a maximum of 2 per cent of the year.

Ventilation to get rid of stale, unclean air

A human being needs 30 cubic metres of fresh air per hour to breathe healthily. You could achieve this by opening all of your windows, but in the winter this isn’t practical. Instead, PassivHaus design specifies this limit per person for its MVHR system.

Low energy demands for your eco home

PassivHaus design specifies that, for your home to be truly passive, it should use less than or equal to 15kWh/m² per year to maintain the home at an even 20 Degrees Celcius all year round.

A draughty UK home can use 300kWh/m² per year to stay heated to 20 Degrees Celcius. And you pay for every penny of that heat leaking into the cold outdoors.


Clean energy at the source

Finally, PassivHaus limits the home’s “Primary Energy” demand to 120kWh/m²yr. Primary Energy refers to energy usage at its source; literally the coal, oil or gas at the power station. Getting your home takes up energy in the transfer and travel process, so primary energy is used to better calculate energy saving estimates.


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