Passivhaus

What is Passivhaus?

Passivhaus is an entirely voluntary building performance standard that anyone setting out to build a low-energy home might be interested in. It’s gaining lots of attention in the UK and is based around the principle of reducing heating demand to a very low level rather than relying on renewables.

The standards far exceed those of the current Building Regulations, and it is popular because there are many different ways to meet the criteria. Designers use a software package to predict the home’s future energy demand.

What is Different About Passivhaus?

It started life as a research project to find out just why so many previous attempts at building low-energy houses failed to deliver what they promised. The lessons learnt from this were then distilled into a set of guidelines about how to get it right. It’s very thorough and exacting to build this way, and numerous studies have shown that it works.

How Does a Passivhaus do This?

The house must be designed from the ground up as a Passivhaus. There is a complex spreadsheet known as the Passivhaus Planning Package (PHPP) into which you feed all the relevant construction details, including:

  • the insulation depths
  • the window sizes and orientations
  • the junction details.

This gives you a predicted space heating demand (expressed in kWh/m²/yr).

You keep adjusting the design until your outcome meets the standard. Passivhaus can be built using almost any construction method. However, there are some universal features, including:

  • massive insulation (average depth 300mm)
  • triple glazing with insulated frames
  • fantastic airtightness levels (must score better than 0.6 air changes per hour)
  • mechanical ventilation with heat recovery.
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  • Uses 90% less energy to heat than an average home. Heating costs around £75 a year
  • Optimises heat from the sun
  • Minimal micro-renewables
  • Mechanical Ventilation with Heat Recovery (MVHR) provides constant fresh air and retain heat inside the house
  • Triple glazing with insulated frame
  • The Tea Cosy Effect (or ‘minimising thermal bridging’). Wrapping building in insulation minimises any breaks or cold spots
  • Super-insulation
  • House retains heat from sun and occupants’ activities
  • First cavity wall Passivhaus in the UK
  • 20 times more airtight than a standard build

Are They Homes Without Heating?

No. What the Passivhaus standard seeks to do is to reduce the space heating requirements to such a low level that you no longer require a conventional heating system.

 

The level is set at 15kWh/m²/yr, and it’s the key Passivhaus target. This means that if you build a 160m² house to Passivhaus standards, you would need just 2,400kWh of energy throughout the year. That’s about a tenth of what a typical British home would use, and about a third of what many so-called eco-homes would consume.

 

But it’s not no heating at all — that would be expressed as 0kWh/m²/yr.

How is a Passivhaus Heated?

 

There isn’t a set method for achieving this and various ways of providing space heating are still emerging. The most popular seems to be adding a small heating element to the ventilation system, turning it into a warm-air heating system. These units are generally rated at no more than 3kW, and they only kick in when the outside temperatures are close to zero. You also have to provide for domestic hot water, so some form of boiler is useful.