Wednesday, February 5, 2014

Why are passive houses so cute and boxy?

OK, now it’s snowing like crazy. Looks like there won’t be any progress on the construction for a while. In the meantime, I thought I would begin to explain some of the special traits of the Potwine Passive House in more detail. In the next post, I’ll show the floor plans, but first we should talk about why the home is shaped the way it is.

Passive houses tend to be cute and boxy

I love the look of passive houses. Pictured above are Little Compton (Zero Energy Design); Lancaster High Efficiency (Garland Mill Timberframes); Urban Green (Sala Architects); and R-House (ARO and Della Valle Bernheimer), clockwise starting from the upper left.

Less surface area saves energy

You might have noticed a common theme — these houses take the form of a box, more or less. Here’s why: it’s an attempt to achieve a low surface area to volume ratio. Imagine a square box versus a long rectangle, but make sure that both are sized such that they each enclose the same amount of volume, as shown in the drawing below:

In this example, both the cube and the rectangle have the same volume, but their surface area is different. It turns out that the cube will always have less surface area than the long rectangle, regardless of whatever volume we choose. Similarly, a box will have less surface area than a series of partially detached smaller boxes, which might remind you of most house designs.
The problem is that heat energy is lost through the surface area (the walls, roof and windows), so more surface area will allow the same volume of space to cool down faster. Passive houses often employ boxy designs because they yield better thermal performance.

I’m sure that many would find the typical boxy passive house to be extremely limiting, but that’s where the brilliance of architect Matt O’Malia’s GO Home is apparent (pictured above): he’s made a boxy and plain structure seem open, interesting and bigger than it really is.

Choosing the exterior form factor and the square footage

One of the project’s first steps was to choose among the preset house designs and floor plans available on GO Logic’s website, which offers single story 1000 sq ft plans up to double story 2500 sq ft plans. I initially came to the project thinking that I would want to live in a bonafide tiny house — a romantic dot on the landscape surrounded by expansive open space — but the thought of raising a family in a tiny house eventually scared me off. I gravitated toward a three bedroom design, partly due to the recommendation of my real estate agent, Mike Seward: three bedroom homes tend to be easier to sell. A second story appealed to me because I felt that views of the surrounding landscape and wildlife would be enhanced by the extra height, and more space around the home would be available for a vegetable garden.
After discussing my budget with Matt O’Malia, it was clear that the 1300 sq ft model was the most I could afford. Although I didn’t know it at the time, the actual usable square footage is even smaller: 1100 sq ft, about half the size of the average home in the US. At this point, most people would balk at the idea of a passive house: they could have a standard 2200 sq ft house for the same upfront cost! Don’t get me wrong: I’m not saying that a passive house is more expensive. The upfront costs are higher, but the monthly costs are lower (almost zero) — one of many ways in which greener products are unfairly financially disincentivised.
The next step was to figure out how to make the most of that 1100 sq ft.