Choosing Between Two Options for Insulating Attic Joists With Closed-Cell Spray Foam
We are refinishing the top floor of a c. 1910 semi-detached house. The sloped roof rafters rest directly on top of cantilevered floor joists (which extend about 18” beyond the wall below).
We are in Climate Zone 6 (eastern Ontario), and will be using ccSPF insulation to re-insulate the sloped and flat roofs. The existing condition includes joist blocking in the plane of the wall below. We will install rigid insulation on the cold side of the blocking.
See attached drawing illustrating the two approaches.
Option 1: This option involves temporarily removing the plywood sub-floor. Then, we install tar impregnated felt paper across the top of floor joists, stapled and sealed with acoustical sealant. Weight the tar paper with strips of wood/plywood to prevent upward expansion of the ccSPF beyond the top of joist level. Step 2: Once foam is cured, remove temporary wood, leave tar paper, apply acoustical sealant on top of tar paper for air sealing. Reinstall plywood subfloor in original configurations. Finalize with application of ccSPF on top of subfloor and up the rafters.
Option 2: This approach calls for cutting a seam in the existing plywood subfloor in order to apply the insulation on the top and bottom side of the remaining strip of plywood. The removed plywood is then reinstalled, adding blocking to support the new seam.
My question: while the thermal performance in Option 1 is a slight compromise over Option 2, is it nonetheless an acceptable solution (with careful air sealing)? Are there any improvements that could be made to this approach? The reason for wanting to pursue this approach is to retain the integrity of the plywood subfloor/floor diaphragm.
Looking forward to your insights and help in rounding out my thinking! With thanks! Heather
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