Welcome to permies! Yes, an understandable quandary. One solution I know that people have used, is to put rigid Styrofoam insulation on top of the roof cladding, put pond-liner on top of the insulation, and a green roof on top of that (to cover and protect the pond liner).
I'm sure that elsewhere, I saw a fellow make a box-like structure on top of the roof that was large enough to hold hay-bales. Bulky but he made it work. Something similar could be done with insulation with a higher R-value (thinner but more effective insulator).
The key with whatever you do, is to consider whether warm moist air can infiltrate the space, cool and condense and then lower the R-value. This won't happen with Styrofoam, but it's toxic to make, dangerously nasty if it burns, and hard to recycle. Yurts were traditionally insulated with wool felt, but yurts could easily have the felt "refreshed" with more wool as it aged as it wasn't trapped inside a roof cavity. Wool doesn't burn easily or well, but yurts originated in "dry" geographical areas - the concept in its original form doesn't work well in wetter climates.
Whatever you decide to do, make sure your building is designed for the weight! If anything, *overdesign* for more weight than what you expect the roof to be, and for more snowload than you expect to get. That way if you find your building isn't staying warm enough, wall strength won't limit you from improving it! I'm a firm believer that if a little insulation is good, a lot of insulation is better. Yes, you get the law of diminishing returns happening, but people use a huge amount of energy to heat their homes, and a home that is well-insulated, uses less energy from the start and the savings don't go away. Also, many people don't realize how important the reverse is - more insulation can help your home stay cooler when that's an issue.
Thanks for the direction Jay! We are planning on doing a metal roof for our first building which leads me to think the box would work quite well. We wanted to use therma fleece or maybe a hemp product for insulation. Building the box really makes sense! My brother and the crew he works with do lots of timber framing and with their advice we plan on building a very strong roof. Luckily not much snow here in the upstate of SC but better safe than sorry! They primarily do squared timber framing, is round wood pretty similar or would you say it is more difficult?
John Visser wrote:primarily do squared timber framing, is round wood pretty similar or would you say it is more difficult?
Hey there. As someone doing roundwood framing. I would say it is more difficult. Just having to work with something that is not perfectly square and having to cut into round ends in order to create all of your joints adds another level of complexity and effort. I think James Mitchells The Art of Modular Post and Beam has been one of the better references I have found. Hope this helps!
Hi John;
I agree with Daniel that working with round wood is a skill all by itself.
Squared wood is much easier and faster.
In your build, I would use the round wood rafters.
Then cover your roof with beautiful tongue and groove boards that you can admire for years to come.
After that, I would use 2" thick (or more) 4'x8' sheets of foil-covered foam. Frame in the panels to allow for securely screwing down your metal.
Waterproof with suitable material and put on the metal roofing.
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