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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:

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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?

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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.

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    What do you mean by plenum ― a region of increased air pressure or a cavity to be filled? English is not my first language and I am not conversant in roofing terminology. Commented Jun 11, 2020 at 14:16
  • A plenum is basically a duct in this context--a passage for airflow.
    – isherwood
    Commented Jun 11, 2020 at 14:27
  • The rafters run beyond the themal envelope, but I should not call the space between them a plenum because it is not intended as an air duct. Air flow though my thermal insulaiton, which is air- and vapor-permeable, would be ruinous. Outside air enters the vent not by the soffit, but by a vent slit that runs along the top edge of the eves and leads right into the air gap between the sheathing and the diffusion membrane. That slit is protected from rain. Vapor-impermeable materials in the outside layers are in my case prohibited. See Vented Roof Design here. Commented Jun 11, 2020 at 15:54
  • The comments are too short, so another edit is probably in order. The link in my previous comment demonstates the general idea of a roof with a single vent that should let vapor out of the insulation. A vapor barrier (class II in my case) is recommented on the inside, to protect the insulation from vapor in the internal air, which (generally, when its colder outside) has a higher dew point and a higher partial pressure. The diagram also shows venting through the soffit, but that is not about my roof. Commented Jun 11, 2020 at 16:00
  • @isherwood To summarise—I fear that your proposal will not work for the following reasons: 1) it puts a vapor barrier where it should not be, because vapor travels from the inside to the outside. I already have a vapor retarder on the inside. 2) I do not see how your proposal is going to handle the sliding of the roof assembly down along the top of the wall. This sliding may create new gap (or widen the existing one) for the leaking of cold outer air into the thermal insulation and into the house. Commented Jun 11, 2020 at 19:57
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Out of the diffusion membrane I have cut out a box-like detail, with three short flaps on the left, right, and top edges, and a long one at the bottom. The left and right flaps I glued to the rafters, the top one to the diffusion membrane that separates thermal insulation from the ventilation gap, and the bottom flap to the top of the wall in such a way as to decouple it from the roof assembly by means of a technological fold that will straigten out as the roof assembly slides down and thereby to save the detail from tearing. My insulation being relatively heavy (30 kg per cubic meter), I hope it will seal the lower edge of the rafter bay by pressing the bottom flap and fold to the log:

Air-sealing of the eves

I beg your pardon for smartphone adverticement in the photos, but I don't know how to remove it.

Update:
I am accepting this answer of my own not because I am entirely satisfied with it, but in default of better suggestions.

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