Can I install polyisocyanurate rigid foam insulation right on the interior side of the plaster or gyproc ceiling?
My sun room is quite cold in the winter, and I am sure it is not just due to the amount of window to solid wall ratio. I don’t really have any room to work above the area, do to that it is a roof over a roof situation. The sun room originally had a flat roof that was a patio on top. At some point a pitch was added sloping away from the house, and eliminating the patio. I have taken a peak up through a repair patch to see that I had about 2 inches of batting, and then the underside of the original tongue and groove roof.
This leads me to believe that there is airflow just above the ceiling, as well as airflow in the newer pitched roofed section. This would lead me to believe that even if I was to access the pitched area to add more insulation that it would be in vain as the airflow would be under the new insulation. Now as the house is all plaster and metal lathe, I am hoping to avoid the mess and frustration of ripping and cutting it all out, and if I did I am still only left with 2 inches to work with unless I sister in some 2by6 to give me more depth to insulate.
Can I locate my joists and then install the polyisocyanurate rigid foam insulation right up against the plaster on the interior side, and tape. Then add 1/2 inch gyproc to finish the room. Is this a “do-able” situation, or is there an issue I am missing?
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Your proposed solution makes sense. However, it's possible that there are hidden air leakage channels that connect your wall framing with the air channels above your ceiling, so you should also do your best to perform air-sealing work on the interior finish of your walls, especially at electrical boxes and windows.