Passivhaus & house shape – how much does it matter?

April, 2019|
Dart House is our most recent completed PH15 Passivhaus Project and one I frequently reference when talking about Passivhaus. Why?
Well, the house has our highest ever Form Factor (FF), that is the ratio of thermal envelope area (walls roofs, floor) to usable floor area. Simply put, the shape is not very efficient at all, and there are lots of external wall and roof areas, that is when you compare them to the internal floor area. For Dart House this ratio is around 4.0 (our most efficient PH15 project has a ratio of 1.8). The house shape includes a lower ground floor, a double height living/family/kitchen space, three separate single storey wings, and even a spiral staircase that protrudes outwards as an external wall.
This house nicely illustrates that a Passivhaus home need not be a simple cube or rectangle, and if you want a more complex shape then it can be done. I would have run a mile away from this project when I began my step-change to Passivhaus. Now, with much increased confidence in tested & robust airtightness details & good airtight products, we can deliver a design like this, and not suffer sleepless nights. It also helped that the builder, Waterhead Projects, had previous PH15 experience and were also confident from the start. Yes, there is some added complexity involved, but we still ended up with our usual two airtight test days (one when the airtight layer is all exposed, and a final test at completion). The result was a very honorable 0.35 ach@50Pa. And it feels good to see designs emerging that break the mould and usual Passivhaus expectations.
The downside of a complex form is cost related – this PH15 System cost around £300/m2 more than the PH15 project with FF of 1.8. That is a significant cost increase given there are no differences in the specification itself – we use the same products on all our PH15 projects. If building a house of say 160 m2 (gross internal floor area), then that is an extra £48,000.00 to pay for that shape.
Of course the maths will work for any house, not just a Passivhaus. But if your budget is tight you might want to consider that the shape of your house could cost you more than the overall uplift in cost to go from Building Regulations level specification up to a Passivhaus standard specification. Our PH15 System typically costs an extra 6-10% to uplift from building regulations to Passivhaus. This uplift is much less significant than the extra over for the complex form.
This article has not been written to shame complex forms, we should build Passivhauses in all shapes & sizes, and this house is right for these clients, who are now really enjoying living in it. However, we do need much more of an understanding from architects & designers on the cost implications of Form Factor especially.
Knowledge is power, and if I was self-building my home I would want to live in a Passivhaus with all the attendant benefits of comfort, low bills and healthy indoor air quality. I would definitely cut my cloth to fit my budget, and would compromise on both floor area, house shape and internal finishes, to be sure I got to the Passivhaus destination. If I was lucky to have a larger budget, who knows, I might just knock myself out with a Form Factor over 4.0!
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