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Community and Q&A

Practical impact of varying rectangular building shapes

Craigonian | Posted in Energy Efficiency and Durability on

Hello. I am a keen reader of the GBA site and even more so as we have recently purchased land to build and are saving to build our own house in the next couple of years. From this site and others similar, I have some understandings of large concepts and how they play out in the building process, but with no experience building, it is all largely academic. I do have a question though that I have been considering for a while (as someone dreaming to build in Northern Michigan, zone-6).

I understand that there is an impact from the building’s shape on energy efficiency and that there are a number of factors that affect this. A dome is probably the most energy-efficient (all insulation factors being the same) shape for retaining heat. Hexagonal shapes next, then an even square shape, as they are the decreasing ratios for surface area vs. volume. Further, I understand that there is a balance to be struck between solar gain and heat retention, meaning that a rectangular shape, if done well, may give a net gain for heat. I’ve seen it suggested that 2:1 (length:width) may be the most appropriate rectangular shape 
for a cold-weather house that is taking advantage of solar gain. 

There are many factors that go into such a thing and I understand many of these things technically, but my question is more about what it might mean practically.

Aesthetically, I love a long, narrow house shape, like a Scottish longhouse (maybe 2.5:1 or even 3:1). I don’t want to place all of my house design decisions on the math, as there are lifestyle considerations, but I also know that there are very real environmental conditions to be dealt with in the zone-6 climate and I don’t want to pay ridiculous amounts to live in a narrow house.

The question is: would a simple rectangular longhouse plan (3:1, length:width), air-tight and insulated to say r-40 walls and r-70 roof, appropriate glazing, etc. cost a lot more to keep than a comparable 2:1 house? I know that the math would give various percentages for the area-to-volume ratio, but I’m not sure how it translates practically.

Is 3:1 rectangular shape in a well-built house a huge deal or not that big a deal for my winter heating costs. I suppose I’m looking for “Pshht,  it’s really not a big deal at all practically” or “Pshht, you’d be crazy to build a house with those dimensions in zone-6… you’d be throwing money away” sorts of responses ^^

This is relevant as we are talking about style things for building a house someday, but it is also just something that I have wondered for a while now, having read many builder discussions about house shape things.

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Replies

  1. this_page_left_blank | | #1

    My opinion is that of all the architectural shapes out there, a long rectangle is far down on the list of egregious efficiency choices. If you keep the shape simple, you'll be ahead of all those houses with outcroppings, dormers, etc. We made the choice to have pretty much the ideal energy shaped house a few years ago (two story rectangle) and I have regrets just about every time I go up the stairs that we didn't do a bungalow. The couple of percent energy savings aren't enough to offset that. If the 3:1 house is your ideal house just go for it.

  2. GBA Editor
    Martin Holladay | | #2

    Craigonian,
    Do what you prefer -- but to me, a long skinny house feels like a single-wide mobile home (trailer) -- and doesn't feel right. But everyone has opinions.

  3. Expert Member
    BILL WICHERS | | #3

    You are right to avoid the complex dome/hexagon shapes. In theory, you’re correct: the shape with the minimum surface area for the enclosed volume will be most energy efficient. In reality though, the complex framing required to build those fancy shapes will more than cancel out (due to thermal bridging, air leaks from poor sealing, etc) any theoretical advantages.

    In terms of the basic shape, I’m with Martin here: boring rectangular structures are pretty limiting and you’re likely to wish you had done it differently. Don’t focus entirely on energy efficiency, it’s not the only thing that’s important in a home. Don't go overboard with bump outs and dormers and other stuff, either. Come up with a basic floor plan you like, then work out the insulating details. You’ll end up with a house you like living in, and you’re not going to have so much more energy consumption over a basic rectangle to break the bank with operating costs.

    I’d seriously look into thick exterior foam (my favorite), or double stud walls. Triple pane windows of full 1-3/8” thickness, especially for any windows with a large glass area, are really nice for both comfort and energy efficiency. Read all the articles on GBA about all the little details that are often overlooked, then select a GOOD builder who has some familiarity with energy efficient home construction. That last step is critical: builders unfamiliar with that more complex detailing needed to build a really tight home are likely to fight you or skimp on the details that “they never do this way” because they won’t think they’re important. There are many Q and A posts here about people having problems like that with their builders.

    Bill

  4. Expert Member
    NICK KEENAN | | #4

    See "Reassessing Passive Solar Design Principles": https://www.greenbuildingadvisor.com/article/gba-prime-sneak-peek-reassessing-passive-solar-design-principles

    The short answer is that you gain more from insulating than you do from passive solar. So a design that maximizes insulation efficacy is more efficient than a design that maximizes passive solar. To boot, a house that is easy to insulate is usually cheaper to build anyway because it has a smaller, simpler surface.

    But life is more is more than efficiency. Start with a cube with no windows. Figure out a way to estimate construction cost and energy usage. Add windows and stretch that cube until you no longer feel comfortable with the impact on cost. Generally it's not that expensive to make a house super insulated and super tight. From there it's not that expensive, either in construction cost or energy usage, to make excursions from the ideal shape.

  5. malady | | #5

    I too see this as mostly a question of style and preference and, for what it’s worth, I like the look. A few observations:
    1. Your roof and foundation will cost more than a similarly plain more compact design but that difference would erode with all the doodads people stick on houses these days.
    2. To my eye, the higher the roof pitch the better a long thin house looks. Some like the shallow 60s look. It is cheaper but to Martin’s point, the house will look like a trailer. I think a 6 pitch strikes a nice balance between cost, appearance and function. Martin, I suspect would go higher.
    3. I wouldn’t follow the current fashion and omit broad sheltering overhangs and rakes. This forum is full of the reasons why.
    4. With the right layout, a long thin house can offer greater privacy in the rooms at either end than other modestly sized houses.
    5. Give a thought to the length of your plumbing runs and hot water heater(s) placement, not just because of cost but as to function.
    6. Paint it white and go classic or black and go current and enjoy your iconic new home.

  6. Expert Member
    MALCOLM TAYLOR | | #6

    Craigonian,

    When you design your own house you should try to use a similar process to the one you would if you had employed an architect. At the first design meeting you would discuss the site and how you wanted the house to relate to it, the appropriate architectural response to the climate, your needs and desires for the house, how the various spaces relate to one another, how light will be brought in, energy efficiency - and a score of other things. Even at that p0int I can't imagine the discussion turning to the proportions of the rectangular plan. It's simply not something important enough to influence the form of the house.

    Having efficiency drive the architecture was the big mistake early energy conscious builders made in the 1070s. They designed machines for generating and conserving energy, and asked people so somehow find a way to live in them. That dead-end set back the adoption of energy efficient housing for decades.

  7. KauaiBound | | #7

    Keep it simple! Way too many go deep in the weeds trying to build super-efficient homes and ignore the practical things. Find out what materials are available locally so you don't spec expensive-to-obtain products that involve delivery fees or inflated prices due to limited supply. Simple things like a choice of insulation can be thousands different. Find a good builder in your area and see if you can pay him for a few hours of his time to give practical advice for cost-effective products/design. Be wary of advice on forums full of 'believers' - they tend to exaggerate the pros and dismiss the cons (mostly cost/time/complexity).

    1. Expert Member
      MALCOLM TAYLOR | | #8

      Solid advice.

  8. Expert Member
    AKOS TOTH | | #9

    The building shape can be very flexible with minimal impact on energy use and build cost.

    The one area you want to watch is roof shape. Avoid origami roofs. Seems to be the bane of modern suburbia.

    You don't want to go down the road with a building design that needs a complicated roof shape. These are hard to air seal, prone to ice buildup and water leaks.

    If you want to condition the attic, design this in from the beginning. Avoid using room in attic trusses as these are impossible to air seal.

    1. Expert Member
      MALCOLM TAYLOR | | #10

      Ban origami roofs!!!!

  9. 730d | | #11

    Only about a 5% difference in outside wall length.
    Let your developing floor plan steer you as to 2:1 or 3:1, not the other way around.

  10. Craigonian | | #12

    Thank you, Everyone, for your responses. These are just the kinds of answers I was hoping to get back. It sounds to me like a simple long rectangle (with a simple roof shape as well) will have a minimal effect on the overall heating efficiency, all things being the same, over a square house... certainly not enough to deter me if I enjoy the shape.

    Thank you again for the direct responses and for the "other considerations" information supplied. It is much appreciated, and please feel free to offer any other insights you may have on the topic.

  11. user-723121 | | #13

    We had this (heated) discussion here years ago on house shape and efficiency. From a simple math standpoint a square house is the most efficient in terms of material use and ratio of floor space to exterior wall. The old four square farmhouses are an example of this shape efficiency.

    That said a reasonable rectangle gives up little in terms of extra surface, is easy to build and can be very efficient. I would prefer 30' x 40' as opposed to 20' x 60'.

    1. Expert Member
      MALCOLM TAYLOR | | #14

      Doug,

      Last summer I designed a 1500 sf house that was just over eighty feet long. It was for a sloped, south-facing site overlooking the ocean, and incorporated two covered courtyards. it was a direct response to the site conditions, and the mild but wet PNW climate.

      I don't think there is generically a best house shape, only an appropriate one for the specific circumstances.

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