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Duct soffits and fire blocking

Matt F | Posted in General Questions on

I need to install two ducts in soffits that run along part of my unvented cathedral ceiling. I am trying to keep the profile of the ducts soffits as small as possible

First one is at the gable end, runs in the corner of the cathedral ceiling and gable wall up to the underside of the flat ceiling and down the other side to the knee wall. It will be 6″ round duct. With mineral wool filling the bays, do I need any fire blocking besides fireblock spray foam to seal around the duct at the ends of the soffit? The surfaces with exterior exposure be lined with 1″ polyiso and open spaces on exterior sides will be stuffed with mineral wool.

The second one is 3.25″x10″ duct running up the cathedral ceiling, above the flat ceiling, and down the other side. The sides will be frame framed with 1x reasonable tight to the duct and the outside surface lined with 1″ polyiso. There will be essentially no airspace in the soffit besides the duct and its insulation wrap. Do I need to worry about any fire blocking anywhere besides the end openings?

I was thinking I needed drywall or 3/4″ plywood on the outside surfaces, but given that I have mineral wool in the walls already, it seems that I may not need that.

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  1. Expert Member
    Malcolm Taylor | | #1


    This is very location dependant code advice, so take mine with a gain of salt. Our code would only require fire-stops between the soffit and ceiling if there was no insulation in the stud cavities - so typically only on interior walls.

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