Energy efficiency of a 36-foot-diameter structure
Hello GBA community
Friends of mine are considering a project in Cranbrook,BC area (southwestern BC, Canada). They are leaning towards a company which incorporates a beautiful 30-36′ circular section marrying into square or rectangular sections. I believe some at least would be two story built into a sloping hillside.
Here is the link to the design folder specifically ‘The ELMWOOD Series’
http://www.mandalahomes.com/Mandala_Custom_Homes_Plan_Design_Book_2017_V2.pdf
I am trying to discover if they favour a design with more or less of the wall faces exposed to the weather.
Now..my questions are mostly on the Energy Efficiency (although I am also considering the affordability comparison) of the rounded walls vs regular straight walls and the segmented circular roof vs the EE of a 2 sloped roof.
Is there great loss with circled, segments of wall?
Personally I have learnt to love Martin H’s ‘Keep it Simple’/’Boxy but Beautiful’ approach. These plans ring all the alarm bells in my head that this is not an EE approach to a structure so I am scanning back over GBA and other sources and asking your professional and considered views/opinions to see if it might be helpful to present them, my friends, with some info and urge them into a more EE direction.
All the best to all of you,
From Richard (a long time follower and 2 yr subscriber who is thankful for GBA’s work and what it has taught me)
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Replies
Richard,
Not much information in the brochure except that these homes are "Energy Star certified." A little better than code, for sure, but not extraordinary. We'd need to know more about the specifications and airtightness to judge these homes.
Martin
Where could I find a breakdown /analysis of the different system standards (or comparable)?
Minimum code/energy star/R2000/Highly EE/Passive House levels.
Has GBA tackled that?
Richard
Richard,
GBA has multiple articles on all of the "standards" you listed, so if you use the GBA search box, you'll have reading for a week.
If you think that the five descriptive phrases you listed can all be ranked or compared with some type of numerical scoring system, you're mistaken. It's not that simple.
Richard,
I can't comment on their energy efficiency, but your friends are on the right track choosing a hybrid model with some rectangular elements. Circular homes are appealing, but inevitably end up with lots of pie-shaped, unfurnishable rooms. The hybrid designs help mitigate that.
All else being equal, a circular structure has the least amount of exterior surfaces needing insulation compared to the enclosed floor area. For example, a 1,000 square foot circular structure will have just over 112 feet of perimeter wall. The same 1,000 square foot area in a square structure will have a little over 126 feet of perimeter wall. Any rectangular area will have more perimeter wall than the square. Ceiling/roof area would the same or very close depending on the type of roof construction.
The circular room would be more energy efficient than the square or rectangular rooms in the above example due to having the least amount of exterior surface area to “leak” energy. In reality, there is a lot more to consider than just the exterior wall/roof surface areas so it would be easy to have some other area of energy loss dwarf the difference between the two shapes of rooms. I suspect the circular room would be more challenging to air seal, for example, especially the roof details.
In short, I wouldn’t make energy efficiency due to the shape of the structure the deciding factor in making your choice. There are many other things to consider regarding types of construction, even if energy efficiency is your primary concern.
Bill
Thanks guys for your replies.
Martin, I will look into more specs and ask of their 'airtightness techniques and how they achieve them. (blower door tests at framing, after mechanical and after finish would perhaps be the best indicators ...wouldn't they?)
I just reviewed the podcast 'Making Green Affordable' Pt 1&2 and it brings to clarity for me some of my initial misgivings. I will blurt them out and see where they land for you all. The ppl building are a Meditation retreat centre who are pursuing many green initiatives on their property (food forest, grasslands restoration and permaculture gardening. Also I don't think they have immensely deep pockets so economy/affordability should be a concern for them .
So I feel it is incumbent upon me to play devils advocate here and insure they at least hear of a more thorough approach to achieving EE , green practices and a limiting of resources when it makes sense.
SO! My notes from the podcast and other GBA info that might effect cost,EE and a green approach.
-Simplify # of corners
-Simplify # of roof planes
-Limit amount of tricky engineering
-Limit amount of tricky framing
-Complication in this design for air sealing
-Complication in this design for insulating
-Complications leading to future maintenance work (talking 10 to 20 yr)
-Do we see 'Reduced Mechanical' layout, length of runs etc...
-What would be different if we took those similar lengths of circular walls that meet the exterior and straightened them out. Isn't that easier to build ,insulate, achieve airtightness, maintain etc.
- Please help me look for other basic approaches that may not land in the 'Simple EE, green and Affordable Approach
Thanks esp Martin,Bill and Malcolm for your comments.
And to all, I look forward to your thoughts on this (Pt 1 and 2 now)
Richard
This type of house looks pretty much identical to a Deltec home. I did a factory tour there a few years ago. If you are building the wall sections as panels in a factory, you should be able to get good air sealing. But because you should be able to, doesn't mean you do. It all comes down to how much importance they place on it. Assuming this is a factory built home, it's definitely a must to visit the factory armed with some knowledge about what it takes to make an air tight assembly, and ask some questions. As already mentioned, the circular layout presents some serious challenges for efficient use of floor space. I think that probably cancels out any efficiency you gain from reduced surface area of the walls - you need a bigger foot print to get the same useable space.
You’ll find that some of the things in your list will conflict with each other, fancy details with corners will make for tricky engineering, for example.
In other areas, there are different products that could greatly simplify things. In a round building with lots of joints needing air sealing, it might be cheaper, and would surely be easier, to just spray foam those walls instead of putting in all the labor to airseal everything. Spray foam provides perfect air sealing as something of an added bonus, and it’s really great when you have odd size/shape/angle places that need to be sealed. The spray foams added cost as an insulating material might be offset by the lesser need for air sealing labor in this case.
The last time I saw a circular building design, they kept their mechanical room near the center of the circle on the lowest level (probably a basement). They ran all the lines radially which resulted in short average run lengths, and also fit in well with the framing.
You’re probably going to find yourself making a lot of interesting design trade offs based on cost/benefit with this type of structure, and I expect many of those trade offs would be different from what you’d do in a “regular” body structure. This isn’t a bad thing, just different — what adds cost in a boxy structure might save money in a more round structure (like in my spray foam example). You’ll have to think “outside the box”, so to speak :-)
Bill
I forgot to mention another quite obvious EE detail. The 5' dia roof skylight. Even if it is triple pane, you have R7-8 where you want R60...no?
Yeah, any skylight is going to be way less insulating than a normal roof section. Chances are it won’t be a triple pane skylight, either, at least not without a big up charge. Most likely will be a double pane skylight with a mid level low-E coating. Skylights are nice if you otherwise have a dark interior space with no natural light which is probably common in a large circular structure.
Bill