19 May 2010

Some Passive House Maths

What effect do 300mm cavities have on floor area? I’ve just been putting my new benchmark house through its paces and have done a few calculations. As it stands (or rather will — it’s still under construction), it is a four bedroomed detached jobbie on two floors with an internal floor area of 147m2.

It’s basically a brick and block construction with a cavity width of 100mm, designed to be fully filled with DriTherm, designed to give a U value of maybe just under 0.3 (designed to pass Part L 2006 version).

Now if the cavities are widened to 300mm, without any changes being made to the design, the internal floor area falls to just 131m2.

So here it is:
• The external area of the house is 173m2
• The built internal floor area is 147m2 (85%)
• The Passive House version would be 131m2 (75%)

It’s a whole lot of wall, and as such it remains a huge hidden cost of building to very low energy standards.

One conclusion could be that you shouldn’t be designing walls that are 500mm thick, and that it’s time to embrace wall systems which are more efficient.

Another is that you might be able to overcome the space constraints by good design – the Small is Beautiful argument.

Yet another is that when you reduce the process to number crunching metrics (like £x per m2), you inevitably get some strange results and that you shouldn’t take them too seriously.

But next time you read that “Passive Houses only cost slightly more to build than conventional ones”, perhaps you should ask if we are really comparing like with like?


  1. Mark,

    I remember hearing about a study in one of the more expensive London boroughs. It was looking at the cost of retro-fitting internal insulation and explicitly included the cost of lost floor area. The findings were that, at least in most homes, the loss of value far outweighed the energy savings.

    I think it's less of an issue in new build, particularly where there is the potential to grab a bit of garden, and also places where external insulation is possible.

    Very glad you posted this now though as it's reminded me of something to add into a proposal I'm working on!


    Jamie Bull

  2. Good analysis Mark and you answered some of your own questions.

    Another consideration is that the Passivhaus methodology rewards useful floor area and compact form. Also I expect you have seen this:
    http://www.scribd.com/doc/31384556/Costs-Passive-Houses-Treberspurg-Smutny-14th-IPHT looking forward to seeing the full paper.

    For a terraced home the party wall thickness is set by acoustics and the thermal requirements for the small amount of wall between doors and windows in the facades is less than for a detached home so where space and budget is tight, terrace is good.

    I'm sure a developer wanting to squeeze as many boxes onto a site as possible will see a big cost associated with extra foot print but for the projects I see, the size is set by what size is needed or can be afforded rather than being limited by precise external dimensions that mean the house can't be 300mm longer and deeper - assuming thicker construction methods such as cavity.

    None of this undermines your analysis but I'd hate to see it taken out of context by the Daily Mail.

  3. Dave HoworthMay 19, 2010

    A very good point, Mark. Coincidentally, there's another posting today about an issue that makes life difficult when building a Passivhaus - planning (no surprise there then!) The posting is at the Passivhaus Refurb blog.

  4. Mark,

    A very considerate piece. On our Passivhaus cavity wall project at Denby Dale the financial constraints were the build costs not land area as the client owned his own garden. However, what is the 'cost' of unbridled climate change, peak oil and fuel poverty?
    What would be better, 147m2 floor area with a 75kwh/m2/annum space heating need (well built Part L), or 131m2 at 15kwh/m2/annum (PassivHaus)with its future fuel bills and proven comfort?

    On a more constrained site maybe different construction methods could be considered. For example all the different timber frame designs or silid block with external Eps insulation and brick slips.


  5. A factor you don't mention is the reduced cost of the heating system (the installation costs rather than the running costs) for a passive-like house.