How to work out the Passive House Form Factor

The Passive House Form Factor quantifies the relationship between the living area of the building and the total amount of surface area that heat can escape from.

The calculation is simple:

Total heat loss area ÷ floor area = Form Factor

The Form Factor of a building is key in low energy design because it tells you how thick your insulation has to be. If you can halve the form factor (ie, simplify the building’s shape) you can halve the wall insulation you need to get the same thermal performance. The lower number the better.

The average semi-detached house has a form factor of 3. Here’s an example of an apartment block with a form factor of 1.75.

A block of flats in London to demonstrate the concept of Form Factors

Why is a block of flats so good?

Imagine this block of flats in London is 20 metres wide, 40 metres high and 10 metres long. To get the total Heat Loss Area we work out the areas of each of this buildings six sides:

[(20 x 40) + (40 x 10) + (10 x 20)] x 2 = 2800 square metres

Then we need to work out the total floor area for the building. This is an 8-storey building, so each floor area has to be counted:

10m x 20m = 200 square metres x 8 floors = 1600 square metres

The Form Factor is the heat loss area divided by the total floor area, so:

2800 ÷ 1600 = 1.75

1.75 is a great form factor. Now look at this building:

Stunning cubist architecture on the St Lawrence river in Montreal

Stunning cubist architecture on the St Lawrence river in Montreal

Because of its incredibly complicated architectural design, it has a lot of surface area for heat to escape from. This probably gives it a Form Factor of about 4.5, so it will require over twice as much insulation thickness as the block of flats.

That really sucks considering that this beautiful Cubist building was built on the edge of the St Lawrence river in Montreal, which regularly has winters of -30DegC. At least it looks really cool.

If you’re investigating Passivhaus design for your renovation or new build project, please feel free to contact me with any questions and I’ll be happy to help.


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