Insulating and air sealing lath and plaster celing with ventilated attic
Hi, I want to seal and insulate my ceiling this summer. Old house with lathe and plaster, 2×4 joists, ventilated attic, and 3-4 inches of what looks like mineral wool insulation. Plan was to remove old insulation, seal any holes and cracks, and seal over interior wall partitions. Then add as much blown cellulose as possible though not sure yet if 2×4 joists will limit how much cellulose I can add weight wise.
In below JLC forum thread it was stated that you can’t use lathe and plaster as an air barrier.
“Air seal any obvious holes/penetrations, but lathe and plaster, unlike sheetrock, can’t be part of a good air barrier.”
Is this due to crack issues in the plaster? If cracks are painted over will this block air flow sufficiently for the paint to act as an air barrier (plaster has multiple layers of paint without obvious cracks, but I am sure that some of the plaster is cracked underneath) If cracks through paint are apparent, would sealing with dry wall compound and repainting be sufficient to seal the cracks.
Or is it advisable to try and seal the attic side of the ceiling (not the roof) using drywall or sheathing of some sort (homosote/plywood/…?)
At the eaves there is little room for adding cellulose, is it recommended to use a material with greater R value ie spray foam or fitted solid foam? There full length vent at peak of roof, but there are no eave vents. Some air flows through the attic siding, as there is no sheathing under the siding.
House is in Seattle.
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