19 Aug 2015

Passivhaus: the shape of things to come

Passivhaus aficionados like to make the point that Passivhaus construction doesn’t have to cost any more than “normal” construction. This seems counter-intuitive because the Passivhaus standard demands certain rather expensive features, such as triple-glazed windows, mechanical ventilation with heat recovery and more insulation than you can imagine fitted into places you’d never dreamed of. How can that not cost more to build?

The argument seems to go like this. You learn to work the Passivhaus way and each project you do gets relatively easier and quicker and less expensive. You also learn to design in ways that make it easier to build a Passivhaus and, bit by bit, you close the gap between “normal” build costs and Passivhaus.  Plus there is the added bonus of being able to do without a space heating system or at least a large space heating system, which saves a bit. The argument goes that if all houses were built to Passivhaus standard, within a couple of years the cost premium would vanish. 

But there is a problem with this argument. The physics of heat loss dictate that, for efficiency’s sake, you need to maximise the heated volume and minimise the area of the envelope enclosing it. The heated volume determines what heat you have to deliver and the envelope surrounding it determines how much of that heat you lose. Getting the form right is one of the key determinants of effective Passivhaus design, yet is something that isn’t made explicit.

The most energy efficient shape for a building is a cylinder (it’s why Thermos flasks are shaped that way) but this is impractical to both build and live in. The most sensible conventional shape is a square box, three storeys high. Take a look at the original Darmstadt Passivhaus (pictured) built by Wolfgang Feist. Guess what? It’s essentially a three-storey square box, albeit in a terrace of four — terraces also work well because of the shared party walls. Very efficient form.

Changing a square box into a rectangular box has a small detrimental effect which gets more extreme as you add to the length and shorten the width. Reducing the structure to two-storeys also decreases efficiency a little, but neither of these two measures make a substantial difference.
But certain shapes get severely penalised by this remorseless geometric logic and none more so than single-storey house or extension. Single-storey-anything blows the ratio out of the water as the envelope needed to encase a single-storey structure is usually 30% larger than it is on a two- or three-storey dwelling of the same floor area and therefore the heat loss will be 30% greater. Just because of the shape. You could still build a Passivhaus bungalow but it would require far more insulation, and it would be much more expensive to construct. Consequently, you don’t see many (any?) single-storey Passivhauses.

So when you hear that a Passivhaus costs no more to build than a conventional house, bear this in mind. It may be true, provided you keep the design within certain tight parameters. To put it another way, the Passivhaus standard is either restrictive in what you sensibly can do, or costly to build if you want something that isn’t a plain box-shape.

To be fair, this same rule applies to the building costs of shells generally, not just Passivhaus. But because Passivhaus places so much emphasis on constructing a highly-insulated and airtight shell, it exaggerates the difference.


  1. Very good points Mark. Passivhaus does actually make explicit the relationship between volume & surface area, known as the a/v ratio. It also makes explicit the relationship between surface area and usable (or heated) floor area, known as the heat loss form factor (more on that here: http://elrondburrell.com/blog/passivhaus-heatloss-formfactor/). However I agree that it isn't talked about enough by aficionados, particularly as the form factor is an opportunity for architects & designers to have a big impact with a key element of design: the form. There are quite a few passivhaus bungalows around - just not so many in the UK or Europe!

  2. Oh I also wanted to add - I'm not aware of m(any) people making the claim that a passivhaus single family dwelling can be delivered for the same cost as an equivalent sized building regulations minimum dwelling? At larger scale, or multiunit housing, schools, commercial building etc, I would argue, as I know others do also, that cost parity for passivhaus or building regs minimum for the same size is achievable. Larger buildings benefit from a reduction in mechanical services requirements (heating & ventilation) and other cost savings can be made through good design & simplification. The key is to design with the end goal in mind from the start - IE achieving passivhaus certification in this case. This focuses the mind and the money on the critical priorities so the budget is spent where needed. Even so, you could make the argument that having developed a passivhaus compliant design, the windows, for example, could be substituted out for cheaper lower performance windows to demonstrate it would be cheaper not to achieve passivhaus.
    All that said, in other places it is different. Building regulations compliance in Ireland for example is rather costly now following 'conventional' approaches and it has been demonstrated that it is cheaper to achieve passivhaus (& therefore building regulations) instead. The previous Passive House Plus magazine had several articles on this topic including a case study with a good level of detail.

  3. Christian NialkiAugust 29, 2015

    Dear Mark,

    You are right say that most Passive Houses are not bungalows / single-storey, but quite a few are and can be seen in this PH data bank:
    http://www.passivhausprojekte.de/index.php?lang=en, including this large certified PH build by Chris Brookman http://www.backtoearth.co.uk/case-study/silverton-passive-house-devon.
    And you re right to say that a 100m2 bungalow has a ca. 25-30% larger envelope surface compared to 2-stories dwelling with the same floorspace. But, keeping all other parameters (insulation, fenestration, air tightness, etc.) equal, the heat loos should only increase by 10-15%
    as less than half of the heat is lost through the opaque elements

  4. Christian,

    That assumes you have a similar area of fenestration, which may be a fair assumption. But do windows account for half of all the heat loss in a PH?

  5. It wouldn't surprise me - windows are always a factor of several worse than the rest of the fabric (0.7 vs 0.1 is typical). So they don't have to be that big to account for half the heat loss.