What is Passivhaus?
Passivhaus, developed in Germany in the 1990s, is a quality assured standard and methodology for low energy building - which can help to create buildings that use around 75% less energy than standard practice for a UK newbuild. However, it should be noted that passivhaus is entirely voluntary; though it is gaining a lot of attention in the UK as it is based around the principle of reducing heating demand. The standards of passive house far exceed those of the current building regulations, and it is so popular because there are many different ways in which to meet the criteria.
What’s different about Passivhaus?
This concept began life as a research project, aimed at finding out why so many of the previous attempts at building low-energy housing failed to deliver what they promised. The lessons that were learnt from this were then further distilled into a set of guidelines about how to go about getting it right. It is very thorough and exacting to build in the passivhaus style, but numerous studies have shown that it works.
How is Passivhaus achieved?
The house must be designed from the ground up as a Passivhaus. There is a somewhat complex spreadsheet known as the Passivhaus Planning Package (PHPP), into which you need to feed in all of the relevant construction details, including:
The junction details
The insulation depths
The window sizes and orientations
This will then give you a predicted space heating demand which is expressed in kWh/m2/yr.
You then simply have to keep adjusting your design until the outcome meets the standard criteria.
Key Features of Passivhaus
Optimisation of passive solar gain
Total energy demand for space heating and cooling is less than 15kWh/m2/yr treated floor area
Windows: energy efficient glazing and frames should have U-values not exceeding 0.80 W/m2K
Super insulation: exterior insulation of passivhaus building to achieve a U-value or less than 0.15 W/m2K
Total primary energy use for all appliances, domestic hot water and space heating and cooling equals less than 120kWh/m2/yr
Ventilation: over 80% of heat recovery from ventilation exhaust air, using an air-to-air heat exchanger
Stringent levels of airtightness: air leakage through unsealed joints must be less than 0.6 air changes per hour @ 50 Pa (equivalent of an air permeability of less than 1m3/m2/h @50 Pa)
Simple compact shape; minimising the external surface of the building compared to the internal floor area
Minimal thermal bridging
Proven Effectiveness of Passivhaus
The first houses that were built to Passivhaus standards were completed in 1991, and since then, over 50,000 houses, schools, offices etc have been constructed to the passivhaus standard in Europe, the USA and many other parts of the world. There has been extensive monitoring of Passivhaus buildings, which has shown conclusively that they can deliver what they promise. The design has clear specifications that the heating load should not be more than 15kWh/m2/yr, and the data confirms this - of course, there is an acceptable range, depending on lifestyle, but the studies have consistently shown that the median value is always at the design figure.
What are the drawbacks of a Passivhaus?
Passivhaus is an idea that works better for some than it does for others. If you want to build yourself a Passivhaus to live in, you need to know how to ‘drive’ it, for example:
When to open the windows
When to change filters
When to boost the ventilation system
This isn’t actually that difficult to wrap your head around, it’s just like learning a new car’s climate control system. You do have to start thinking of your home as a machine for living in; you can’t just start knocking it about and adding in new bits here and there.
This might be the biggest drawback of Passivhaus - it’s not readily adaptable. Of course, you can do what you like to your Passivhaus, but it can cease to be one if you mess about with it or sleep with the windows open. Building a Passivhaus is a lot of trouble to go to if you’re not going to respect it.