Building Shape/Orientation

Most often a building's shape is determined by the shape of lot and zoning considerations, so this discussion is more for those who have flexibility -- alternatively this is on why you want to think about this before you buy a lot.

There are a number of competing factor that determine shape: size of exterior surface area, daylighting and solar heat gain.  A cube is the most efficient way to contain a given amount of space, because it has the smallest surface area, and hence it also has the least heat transfer surface. Because ceiling heights are around 8 feet, a two story house is closer to a cube than a one story house, hence two story houses also have surface area.  However for small buildings like houses, the difference is often quite minimal.


The above are approximate floor plans of traditional houses: the square, the rectangle, the skinny house, and two kinds of "L"s.  Each plan's dimensions were normalized a bit to make them all approximate the same square footage.  The linear length of their exteriors is (A) 128' (B) 132' (C) 160' (D) 160' (E) 136'.  If the house is only 1 story, then houses (c) and (d) use about 20% framing material for their walls (but not 20% overall since floor and roof are a constant here).  Moving to a 2 story plan (double the size, or approx 2000SF), the difference becomes more apparent.  A 2 story version of plan (A) has 128*18+1024=3328SF of above ground surface, while a 2 story (B) is slightly more at 3401SF.  At the other end of the spectrum, the equivalent skinny house (plans C,D) would have dimensions more like 20x102, for a perimeter distance of 244' and a surface area of 244*9+2040=4236SF.  While houses of these dimensions are not common, houses of this size often have a highly irregular shape, and hence their surface area approaches that of the skinny house.

Looking at cross sections of these plans, notice that there are two varieties: one room wide and two rooms wide, and the smaller dimension is never bigger than around 32'. Other than row houses, most buildings are never 3 rooms wide, and the reason is daylight (hint: row houses have problems there).

Daylighting:  the two guiding rules of thumb here at the light depth rule, and the light on two sides rule (see the daylighting section for details).   The light depth rule says that no room should be deeper (distance from exterior) than 13-16'.  This limits the thickness of the building to a maximum of 32'--it can be longer in the other dimension, but 32' is the limit for the smaller one, and since this dimension is two generous rooms, you can see why buildings are often laid out as 2 room X 2 rooms or 2 rooms X 3 rooms (plans A & B).  All the plans above were drawn with this rule in mind, yet notice that the hallway area in plans A,B&E is likely to be dark because its far from windows and largely enclosed by walls: fortunately hallways generally need much less light than other rooms.

 Next, applying the light on two sides rule,  we find that the center rooms in plan B present a problem, and it becomes obvious that moving to a larger footprint, say 2 rooms X 4 rooms will just add more problems.  Of the general solutions presented in the daylighting section, the one of concern here is the bump-out.  On a strictly energy basis, because any bump-out increases the surface area of the building, the energy saved by good daylighting is likely swamped by heat transfer thru the additional surface--at least in all but the most benign climates.  Of course light quality isn't an energy issue, so bumping out can make sense from a usability perspective even if it doesn't make energy sense. 

On the above example, the one room wide plans C&D have the best daylight, but suffer the largest heat transfer--however, as has been shown, the heat transfer difference may not be that large, and hence the more elongated building might be best.  But we must also consider solar gain--both wanted and unwanted.

solar chart
Seasonal solar gain1 for various latitudes in the US. The solid line is for the south face, the dashed lines are for the east or west face.  Since solar energy lands on both east and west, the total is double the value on the chart.

Solar gain:  because at temperate latitudes the sun's position in the sky is always toward the equator, the side of the building that faces that way will get more sun in the winter than in the summer, and an overhang over the window will make it easy to control the summer sun.  The east and west faces, however get much more sunshine on them in summer than in winter, so a building which has a longer north-south face will have a more difficult time limiting excess summer solar gain, especially once the building is more than one story tall.   This situation is can be seen from the chart at right, where April thru August the solar energy on the east and west is larger than that on the south.  While the difference is less at 48° north, it is still significant: 1100Btu/day/SF on the south and a combined 2200Btu/day/SF for east and west.

For acquiring solar gain (heating climates), a rectangular building with the long axis running east/west will allow gathering the most solar energy while providing an easy way (via overhangs) to reject the summer sun.  Exactly how much longer the east/west direction should be is dependent on insulation values, and climate.  Ideally that east/west face should point directly south, but as long as its with about 15° of it you will still get most of the benefits.

For rejecting solar gain, the same rectangular building form applies because it is easier to shade the south side than it is the east and west.  In hot climates, a north-south orientation can be better if the west sun is blocked by a hillside or other permanent barrier (for example, the west side partially dug into a hill), although doing so will still require very large overhangs on the east face (ie at least eight feet), or a few small amount on glass on that side.

Heat transfer: while a long skinny building has quite a bit more surface area than an square, when large amounts of insulation are used, the heat transfer thru the walls and roof is often no more than 1/3 of the total heat transfer, the rest coming from windows and infiltration.  If this is the case, even a 20% increase in surface area results in a less than 7% total heat transfer (ie 1/3 of 20%), hence a 5% or even 10% increase in surface area is typically not that large a penalty.

 Obviously if the walls/roof are more like 70% of the total heat transfer, then the additional surface area does become more of a problem.  This tends to happen in much large buildings--one that are at least 10X the size of very large house.

Other things: As the building shape emerges, keep in mind the connection to the street, maintaining the intimacy gradient, and how the landscape spaces will integrate with the indoor spaces near them.


1: Data from Appendix I, Passive Solar Energy Book, Edward Mazria, 1979.