At the point where the roof rafters of a yurt meet the upright walls (particularly in a solid wall yurt design), there will be thermal bridging. This leads to condensation and cold spots in cooler months. Yurt design generally fails to compensate for this loss of heat.
In the solid wall yurt that I constructed, even though I used foil backed insulation and bubble foil insulation to minimize the extent of heat transfer, the results last year were largely inadequate. However, this year, I believe I have found the solution, and it is, in part, due to the supplementary restraint system that I incorporated in the design.
In prior articles, I discussed how I had installed a dome tarpaulin that overhung the walls by about eight inches. This reduced air infiltration during wind, allowed for a greater ability to shed “horizontal” rain, protected against pest intrusion, and allowed us to create a small overhang above the windows.
I also installed another feature: ratchet strap tie downs around the upper perimeter of the walls. While the ropes that tether the roof tarpaulin to the unit generally are adequate, and the sole restraint system in many conventional designs, the ratchet straps can be adjusted around the circumference to further resist the parachute tug of high winds. At a cost of less than $40 for ninety feet of strapping, it is an inexpensive solution. That strapping also allows me to install a flexible rain gutter (see prior articles). However, its greatest benefit is in the ability that it provides to me to resolve the thermal bridging problem in the yurt.
Thermal bridging occurs, quite simply, where a harder surface that transmits hot or cold easily is exposed to the elements and to a conflicting heating or cooling source. Think of that metal counter top, and how cold it seems to the touch in winter, how hot in summer. Wood, although offering less transfer capacity, still acts as a bridge. In houses, R-factor of insulating walls is lowered, if the studs meet the outside and inside walls with no insulative materials between them. The same happens in the yurt.
To resolve the thermal dilemma, I cut pieces of two-foot wide by one inch thick rigid polystyrene insulation into five-inch lengths. Sliding these under the ratchet strapping and to the apex of the walls, all around the circumference of the yurt, I provided an R-5 insulation barrier between the top plate of the walls and rafter joints and the outside air.
Although, to date, temperatures have remained moderate by late autumn standards in the region, I have experienced no condensation in these areas, where I did so last year when the temperature neared freezing. Although an infrared thermometer shows a five-degree difference in temperatures at the bridge point versus the rest of the wall area, this differential is insignificant. Accordingly, I completed installation of similar strips of rigid insulation along the bottom perimeter of the yurt, where wall meets floor.