everyone’s talking about Passivhaus
We’re not aiming for Passivhaus certification, so we are not referring to our project as a Passivhaus, or more specifically an EnerPHit house – a slightly relaxed standard for retrofit. But the term has become familiar currency and ‘low energy refurbishment’ doesn’t quite cut it. We are adopting many of the Passivhaus principles (as described below).
overview of a Passivhaus
The term Passivhaus is an advanced German Standard for low energy construction for buildings that provide excellent health and comfort conditions: both cool in summer and warm in winter in exchange for very little energy usage. A very high standard of construction quality is needed to build and deliver a successful Passivhaus if it is to meet exacting energy criteria for the long term.
Passivhauses are provided with high efficient whole house ventilation units which, if designed, installed, operated and maintained correctly, should provide for good indoor air quality by removing pollutants and maintaining good humidity levels. Of course windows can be opened for summer and purge ventilation and, with the right degree of control, the whole house ventilation unit can be switched off during these times. In winter, the heat is extracted from the exhaust air and returned back into the house through a heat recovery unit, resulting in further savings for heating.
the Passivhaus Standard
The Passivhaus EnerPHit Standard requires:
- a maximum space heating and cooling demand of less than 25 kWh/m2 year or a maximum heating and cooling load of 10W/m2.
- a maximum total primary energy demand of 120 kWh/m2/year.
- an air change rate of no more than 1.0 air changes per hour @ 50 Pa
how does our house compare?
As bought and after using the house fairly frugally for a year (our thermostats were set at 18° C throughout the heating season), our first year’s energy consumption was:
- space heating was 143kWh/m²/year
- primary energy was 247kWh/m²/year
- air change rate was 13.9 @ 50Pa ( equivalent to 17.4 m³/h/m²@50Pa – this is the value used in Building Regulations)
So, we have quite a challenge, reducing space heating by approx 85% and primary energy by approx 50%! Note that we are comparing the measured energy use, not (potentially higher) predicted energy which the PHPP model would use.
Why not go for Passivhaus/EnerPHit certification?
You could ask why buildings can only claim to be passive when the PassivHaus Institut
says they are? Does that mean that everything else is a failure? Not at all. In our view, thermal improvements should be made to the absolute best possible standards as opportunities arise. These decisions should be based upon economic and practicable limits. For example, we are replacing our windows with triple-glazed units with a U-value of 0.9 W/m².k. Compare this to standard double-glazed units that have, since 2006, required the minimum performance standard of 2.2 W/m².k (we suspect our existing windows range from between 2.8 and 4.0W/m².k.), and it is easy to see that we are going way beyond the minimum requirement. For Pssivhaus Institut certification, we would need to purchase certified triple-glazed windows with a U-value of ≤ 0.8 W/m².k. However, after fairly extensive review of window suppliers (and many quotes), we came across units with a value of 0.9 W/m².k. These work out around £10K less than the equivalent Passivhaus certified units. The payback for the difference between the two specifications of 0.1 W/m².k. would never be realised and therefore the Passivhaus, in this instance was not economical.