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.
