Insulating Plank and Beam Roofing

Insulating a plank-and-beam roof is a little different than other styles without exposed beams. Log End Cottage had a fairly steep roof, with a slope of about eight-in-twelve (8:12). In that home, the “beam” component consisted of full – sized recycled three-by-tens. We planked over this with ordinary one-inch planking from the sawmill, planed one side for regularity of thickness, as well as for smoothness and appearance on the exposed interior. To insulate, we created a “double roof.” On the topside of the planking, over each of the three-by-tens, we installed a plastic vapor barrier and then another rafter made from a full-sized two-by-six.

We insulated between the two-by-six rafters with fiberglass batt insulation, and then installed a second layer of one-by planking, upon which our roofing material was affixed. This worked pretty well, but I made a couple of mistakes. First, I didn’t vent the space properly. Installed correctly, there should be about two inches of air above the fiberglass batts, and this space should be vented from below, at the eaves, by way of soffit venting, and at the ridge by a ridge vent, a fairly standard item used on peaked shingled roofs of this kind. Allowing for the vented air space above the insulation, I would have needed two-by­eights for the topmost rafter system. Second, we should have immediately closed off the soffits (space between rafters at the eaves) with rodent-proof

venting, such as quarter-inch grid wire mesh (also called hardware cloth), or wooden soffits with soffit vents made for the purpose — another common item at building supply yards. As it was, we had squirrels living in our roof during the three years we lived at the Cottage. They are fairly irritating, although, from their point of view, I should imagine that they were cozy through the winter.

Shallow-pitched plank and beam roofs can be insulated in the same way as a steeply pitched roof, but I much prefer the use of rigid foam insulation
with shallow-pitched roofs, and I’ll show you how we did this on our new addition, the “case study” subject of the next chapter.

I’ll conclude this chapter with an impressive example of an owner-builder employing timber framing techniques “for the rest of us.”

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