I am trying to insulate and seal the eaves of a vented roof over a log
house in order to protect the thermal insulation (flax batts in my case)
from the outside air and liquid moisture (not vapor). Here is a view of
my eave cavity from the inside:
The cavity is bounded by rafters on left and right, by air
insulation (Tyvek diffusion membrane) on the top, and by the soffit
boards on the bottom. It forms a deep shaft that is not going to be
thermo-insulated all through because it belongs to an overhang over
the porch. Instead, the thermal insulation should be installed more or
less flush with the wall, so I need not fill the cavity entire.
The major problem is that, it being a log house, the roof assembly may
over time slide a little on the rafters down the top of the wall, as the
wood loses moisture and settles. How can I seal these eave shafts (if I
may so call them) about 15 cm down the way (flush with the outer side of
the log) in such a manner as to prevent the tearing or breaking of the
sealing by small motions of the roof assembly along the top of wall
attendant upon the drying-up of wood?
I have thought of fixing a piece of plywood, cut to size, to the sides
of the shaft, sealing it along the sides and top with the Tyvek tape,
which adheres well to wood and membranes, and at the bottom of devising
some flexible yet sealed connection with the outer-top surface of the
log. But I am at a loss as to a simple way to achieve the latter. Can
you help?
Best Answer
The rafters form a plenum that runs over living space AND extends over the walls to the outside, outside the thermal envelope? If so, I imagine that you are trying to extend the thermal break that the wall creates through this plenum? If so, you can roll up fiberglass insulation, place in a plastic bag, then push into place right above the wall, where the rafters hang over to the outside. Follow up with expanding foam to stop air flow. I did this in my cape code with great results.
Something to be aware of though: Do you have soffit vents? If so, air is supposed to be able to be pulled from these vents as air escapes from the attic ridge or gable vents. In that case, you would need to preserve this air flow, which can be tough. Some place small PVC pipes through this plenum, from the soffit area to the main attic above. The trick it to move that air, while still maintaining good R-value on the interior ceiling. PVC pipes usually require pulling off the fascia to push through, so I would insulate all the rafter bays but for one or two to allow air flow to the main attic.
Something I have learned dealing with an older house is that you can drastically improve things, without having to make them perfect. I have lowered my electric bill from $230 to $160 per month in the last 7 years.