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Solar Concept: Area, Volume & Shape


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House 1-13

 

I started with this concept because I was able to consider it before we had a site picked out. I knew the family I was designing for and what we wanted to do in the house. But I didn’t want to develop a floor plan until I had a good understanding of how heat is lost and what is important in minimizing that loss. Considering this concept had a significant impact on our early design - saving us money to build and energy every year!

Concept

Minimize surface area for the volume you need. Elongate the East-West axis to allow for more South facing windows. We inhabit volume; we lose heat through surface area – read on, I’ll explain.

Heat travels toward cold – a version of the second Law of Thermodynamics. The amount of heat (BTUs) traveling (lost) is directly proportional to the area of the surface containing the heat (square feet) and the speed at which the material conducts heat (U-value, which is the reciprocal of the R-value). OK, enough physics. The bottom line is that the smaller the thermal shell or envelop per unit volume of the house and the better insulated it is, the less heat is required.

The greatest volume for surface area is provided by a sphere, but while some have built such houses, utilizing their shape is problematic and construction is not standard or typical. See comparison below, where four shapes have the same volume but different surface areas.

Volume shape area

An elongated rectangle, the longer it is elongated, the less volume per surface area is available. The greater the surface area, the more energy it takes to heat the volume. So elongation to an extreme is not good.

A cube offers a good volume to surface ratio, but it conflicts with the concept that it is good to elongate East-West axis to allow more South facing windows and to allow greater penetration of sun light on winter days. If you had minimal or no access to the sun, a cube would be easiest to build and lose the least heat of any standard shapes. However, the tradeoff becomes more complicated when you are comparing the extra surface area and the heat that passes through it, to the extra south facing windows you could have and the additional solar gains. 

Finally, bump-outs or “additions” are high in additional surface area (and cost) relative to volume. This is mostly a variation of elongation, but with extra corners that are thermal weak points.

Keep both concepts (high volume to surface area is good & the need to elongate for southern exposure) in mind when working back and forth between the overall shape and planning spaces (next concept).

Application

Originally, we wanted a cape with a 24’x36’ main section, a single floor 16’x24’ master bedroom suite attached to the west end, and children bedrooms in the cape second floor, with a shed dormer. This gave me the spaces I wanted and a total area of less than 2000 square feet (1853 SF), which was a goal. But…. it is a very elongated rectangle, with extra corners and a lot of “extra” surface area to insulate and lose energy through. In the end, we made compromises but stuck with the Design Principles, as much as possible.

Our solar house is a lot smaller than our old house, but it is not nearly as small as I would have liked it to be. One of the main reasons is kids – they need space and so do we. As a matter of fact, if it wasn’t for kids we wouldn’t have a second floor or a basement! A single floor house would be a much easier and cheaper to build and to make 100% solar. However, the advantage of building this house, for me, was that I was able to build a more generally applicable house, utilize a fuller array of concepts and techniques, and build a house that would be very suitable for a family of as many as six – two boys, two girls would fit very nicely upstairs.

House shape end front

What we settled on was a house 42’ long (East-West) x31’ wide (North-South) with fully insulated basement and roof, with a nearly full shed dormer to the South – you know, a cape, without the thermal, building and cost complexity of two or three cute little dormers (like our old house!). See the last example in the chart above, it is close to what we built. It has a very efficient surface to volume ratio – almost identical to a cube due to the added volume of using the roof for the 2nd floor north wall and reduced headroom in the back.

The shed dormer was a compromise; it gave access to the sun on the 2nd floor, but really added floor space we didn’t need with just two kids. They are happy with the biggest rooms in the house, but sometimes I regret this design decision. In designing the shed dormer roof line, I allowed form to follow function with the roof overhang shading the first floor windows. But, there was no need, except for esthetics, to shorten the dormer to allow the full rake edge of the roof along each end. So, as I have said, you win some and you lose some.  

Regarding the interior volume. We wanted to make the best use of it as possible. In our old house half of the back area was a vaulted ceiling with skylights, plus there was wasted space with an open stairway. Certainly vaulted ceilings can be dramatic and appealing, but unless there is a real need we didn't want to waste the volume or heat the unused space, so we have no vaulted ceilings in the new house. However, we did elect 9' ceilings on both the first and second floor, primarily because we will have ceiling fans in most of the rooms as part of our strategy to minimize cooling energy in the summer.

A couple of other comments on the shape. The sloped ceilings on the second floor need to be fully insulated. To get the 60+ R-value, I insulated both above and below the rafters – an insulated roof assembly. There is very little wasted volume in this design; in other words, the house is bigger than it looks, especially from the north. 

Next time, I will discuss locating functions within the Volume.

Dan Gibson is the Reporter and Chief Coordinator of Our Energy Independence Community (http://www.OEIC.us). Previously he performed home energy audits for five years in NYSERDA’s Home Performance program and new home ratings in the New York ENERGY STAR Home program. He is currently building a 100% Solar Home - no, it is not finished yet. He can be reached at DanG@OEIC.us.


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