double ceiling (similar idea to double wall)
can’t find any info on this idea.
i’d like to insulate an awkward existing post and beam structure. it has a low-angle metal roof built over the original flat roof with its torch-on membrane still intact. the attic space tapers to nothing at one end, so not enough attic space for insulation above the torch-on layer. inside flat ceiling height is much taller than i need.
wondering if i could build a “double ceiling”, similar to a double stud wall, but oriented horizontal instead of vertical?
ie. fill existing 2×10 rafters with fluffy insulation, hang 2x rafters several inches below that (joist hangers on ends at beams, plus perhaps metal straps hung along the 2x’s to prevent sagging). fill the gap with a continuous layer of fluffy insulation, plus insulate between the dropped ceiling joists. would be easy to reach R50+, and no thermal bridging.
because of torch-on, has to dry to the interior, so only other option to mitigate thermal bridging would be a layer of roxul comfortboard hung below the current rafters, but attaching drywall to that would be tricky (strapping with long screws through the comfortboard, working against gravity).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part