14 Nov 2011

Does Passiv mean Massive?

Ken Neal makes some interesting points on my last but one post.

• I do prefer to design passive houses rather than a PassivHaus, which are really active houses with all the kit and controls required. I do use and agree with all the standards, especially the airtightness but prefer to use passive stack ventilation. The additional heating load is about 1kW on a reasonably sized house which can be met, in rural areas where most of my designs are, by using a small wood stove.

Is he right? I don't think so. The Passivhaus only really requires one bit of kit - the mechanical ventilation system (MVHR). Sure, this kit requires controls, but they are not overly complex. Think Off — On — Boost. That's pretty much it, although there may be some element of timing involved as well. But nothing more complicated than a conventional heating system. And if you do choose to use natural ventilation instead of MVHR, you have the problem of not being able to circulate the warm air around the house, so that your wood stove wouldn't be heating the other rooms in the house. Natural ventilation requires significant input on the air intake side and that can realistically only mean trickle vents, which immediately blows a hole in your airtightness strategy. You can see why Passivhaus and natural ventilation don't really go hand in hand.

• I also prefer thermally massive houses, where you can get several days carry over of heat, to typical PassivHaus lightweight structures.

Passivhaus is agnostic on this point. There is no presumption in favour of lightweight or heavyweight structures. You can have whatever you want, so long as the thermal sums add up.

• I like a house which just sits there and does its thing on its own with very little control or active input.

Well, that is pretty much what a Passivhaus is aiming to do. But to get there, you need MVHR. Natural ventilation just doesn't cut it.

• Regarding embodied energy in materials, this only becomes significant if you are designing for a short life, say sixty years. In our energy constrained future we won't be able to build quantities of new houses, or alter significantly existing ones, because the energy to do so will be in short supply or too expensive or both. If you look at the age of some of our present housing there is no reason to suppose that a well built passive house won't last for three or four hundred years. So over that lifespan embodies energy is a very small proportion of the energy used in a building.

Basically, I agree with Ken on this point. But it does rather contradict his first point about the additional 1kW heating load placed upon a house without MVHR. Because that 1kW load will mount up over the lifetime of a house. After about twenty years, it will equate to the embodied energy of a lightweight house. After 40 or 50 years it will equal that of a heavy, masonry house. Those kilowatts, they all add up.

Mike Jones also makes some interesting comments on the same post.

• I plan to self-build to Passivhaus standard but I'd like a simple efficient MHRV system that I can service myself so I do not have expensive maintenance costs. Are the Passivhaus recommended MHRV systems easy to maintain by the householder or is maintenance a factor to be added to the building running costs? Anyone know?

I don't know enough about MVHR to answer that for sure. One of the issues to be resolved is that whilst there are dozens of MVHR systems on the market, very few are "Passivhaus certified." And whether these need professional servicing, I have no idea.


  1. "In our energy constrained future we won't be able to build quantities of new houses, or alter significantly existing ones, because the energy to do so will be in short supply or too expensive or both."

    I don't understand this - does he have a crystal ball? Who is to say we won't have infinite free energy in the future.

  2. Agree with your analysis Mark but do need to correct your comment about MVHR being able to move heat around the house. Temperatures around a Passivhaus are very even but this is because of the low heat loss and lack of cold ventilation air.

    Heating power that can be conveyed by air = 0.33 Wh/m3.K x temperature difference x m3/h. So if warm room is 2C warmer than cool room and we are moving say 60m3/h then heating power = 40W

    Mind you if that 60m3/h was being dragged in from outside at 0C then the heat you need to add is about 400W.

    Maintenance wise there is little to do. the fans are brushless long life and should run for many years. Filters are the main grumble but can be changed by householder. I am sure we will see innovations such as washable pre-filters or more radical solutions once mechanical ventilation becomes more mainstream.

    I need to put my money where my mouth is and retrofit MVHR to our passively vented house.

  3. According to the Stockholm environmnet Institute the carbon footprint of a typical masonry house is in the UK is 56 tonnes . The average carbon footprint for a resident of the UK is just over 12 tonnes. Therefore, the footprint of a new house is equivalent to 4.7 person years of consumption.

    Whilst, I agree that, over a 400 year or so lifespan 56 tonnes isn't all that much it's still a significant quantity of carbon. Furthermore, it's carbon that is being spent NOW meaning that it's effect in the atmosphere is more pronounced than it would be if it were being released slowly over the 400 year period. Furthermore, if the house is truly passive and minimal energy is being consumed then, ironically, embodied energy becomes a relatively large part of the building's lifetime CO2 emissions.

    Finally, with regard to his point about the availability of energy, Dieter Helm argues that the problem may be too much fossil fuel availability, not too little (www.dieterhelm.co.uk/node/993).

  4. A wood stove hardly runs itself - it needs feeding with wood regularly. However the heat can be distributed with old fashioned radiators if you have an Aga style unit with a heat exchanger.

    In my opinion the problem is that it's dirty and probably not great for air quality in a sealed house.

    The trouble with MVHR is that it doesn't seem suited to the domestic environment - or maybe we just need to get used to them...

  5. Under the Code for Sustainable Homes, which us now being used on most English developments there are significant advantages to using MHRV, and the highest levels are difficult or impossible to achieve without it.
    There are units available in the UK that can run on as little as 9W for a small flat, and achieve 98% thermal efficiency.

    It has been installed now in huge numbers in the big home developments all over the country... An example is the Athletes Village. But there are many smaller developments that use it.

    Why keep harping on about the German Passivehause standard when while different, the CfSH code level 6 is equivalent. (if sufficiently different to be incompatible).?