Posted On09 May 2019

What questions should I ask when buying an eco/green/low energy home?

From many years of painful conversations with people who have “bought a pup” I bring you this simple list which I hope will empower you to get what you think you are getting.

I have to declare my bias: I moved to Passivhaus after many years designing the thermal envelope and building services for low energy buildings. In the early years there was a great deal of “greenwash” and we came up with some simple ways round it, but were wowed by the Passivhaus standard and the detail that went into the calculations, and more importantly, the monitored evidence. So don’t be surprised if I’m pushing Passivhaus as the complete package – I know it works.

That said, there are other people working away to give you a comfortable home without costing the earth. But there are still some people who, how shall we say, aren’t quite on the ball as regards the energy performance of their offering. How do you tell who is OK? Simple, 3 questions:

  1. Has it got a Mechanical Ventilation System with heat recovery (MVHR)? Any low energy home will have this, so if it hasn’t, walk away. There are some other ways of dealing with good ventilation with low energy use, but I haven’t yet seen anything that is as robust as MVHR units.
    Does the system use flexible or plastic rectangular ductwork? Walk –they leak like sieves. Spiral wound or semi rigid ductwork is OK
    Can you see the Design and Commissioning documents? We have seen so many badly designed MVHR units I want to scream. Commissioning documents show the air flow through each terminal, as set up, and form the basis of performance. No documents, walk!

  2. What is the airtightness of the building. If it hasn’t been tested, walk away, as they obviously haven’t got it. Current building regulations have many get-outs so contractors can declare that they “tested one the same 3 years ago”. If you do have a test result you can grade it as follows:
    Over 10 m3/m2/h at 50 Pa test pressure – impossible as this is the Bregs limit if its tested, so if it is over 10, someone will have pretended to have never tested it and they will have moved onto another house/flat on the same site.
    Between 10 and 5 m3/m2/h @50 test pressure. About the mean for UK new Housing. Walk.
    Between 5 and 3 m3/m2/h. Bad but better than the average.
    Between 3 and 1 m3/m2/h. Not bad. Could do better. e. Below 1 m3/m2/h, hey, near Passivhaus territory. Congratulations!

  3. Well if they have done the above, they have cracked Passivhaus, so they might as well get it certified- have they? Beware of contractors claiming “Passivhaus principles” which might be fine, but without the evidence in the first two questions I wouldn’t go near them, life is too short. If it is a certified Passivhaus, it will answer all the above. (just to be clear, I am a certifier)

Notice I haven’t even looked at insulation yet, the above are usually all that is needed. Good luck. See here a simple pictorial of the above points.