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How important is form factor?


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I’m in the early stages of planning/designing a Passive house inspired 3-bed self-build (plot already lined-up and planning looks favourable).


I’m trying to understand the importance of building form factor for energy performance.


I understand that the optimum building shape would minimise external envelope area compared to internal volume or floor area. What I don’t understand are the implications of a less compact form on insulation requirements and building costs. They would go up. But prohibitively so?


I’ve been looking at this really good blog that has a useful graphic on form factor (http://elrondburrell.com/blog/passivhaus-spreadsheet-phpp-design-tool/)

In the graphic example, going from a box to an L-shaped box increases the area/volume by less than 10%. So to my mind, increasing insulation performance by 10% could offset this. Something like going from an average U-value of 0.12 to 0.105 for example. Does that make sense?


I’m keen to do a moderately complex form, like a T or and L-shape. Would that make it really expensive to achieve near Passive House performance?


Any ideas or wisdom appreciated!

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I guess you understand that this is about surface area through which heat can escape.

Thus a dome has very low surface area

While a box is more.

The t and l shapes ( think multiple boxes next to one another) is more.

The greater the surface area the greater the volume of materials. And there is your cost driver.

Complex forms can be done, but more materials and more cost. You could try the looking at curved roof to minimise surface area.

Placement/size of windows, sun protection also are factors.

The phpp spreadsheets prefer low surface areas - boxes, curves.

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You also have to think about wall junctions and any supporting beans that can cause cold bridging to the outside.

Then there is orientation.  If you have an L or a T shape, with the 'leg' pointing south, you will get a lot of shading.  And if you have it pointing North, you have a greater area exposed to no sunlight, so cannot take much advantage of solar gain.  May make a roof unsuitable for PV as well.

Then there are all the internal things to worry about, pipe, MVHR ducting and cable runs, extra doors or corridors.  Waste pipes, which are large, could be a problem.

Oh and any UFH pipework needs to be in exactly the place you want it, before you built the walls.


There is a reason that houses are basically rectangular.  It works at most levels.

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