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codyish t1_jab4mq2 wrote

Some places have specific requirements to meet fire code when filling that space. For example, where I live you can't just place insulation bats between the studs because the narrow gap that would be left between the bats behind each stud is considered an airspace that could allow a fire to transit to the floor above. I do live in fire country and using bats doesn't look practical in your situation, this is just an example. I would just double-check the fire code to make sure that exactly how you are going to do it won't run afoul of that.

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Thrawn89 t1_jacrxbn wrote

The IRC allows this, but you need to provide fireblocking with an approved material (1/2 gypsum board, 3/4 ply, fire spray foam for gaps are most common). The blocking material must be every 10 feet both horizontally and vertically as well as at every horizontal to vertical cavity transition (ie. wall intersects with soffit).

This means that you'd need to add 3/4 ply or something to close the gap at the top plate to the exterior top plate, and you'd need to add this every 10 feet along the wall between the interior stud and exterior stud from the sole plate to the top plate. Though I think stuffing fiberglass in the gap is allowed if it's secured mechanically.

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