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Community and Q&A

Metal roof – foam and a ridge vent

ATXMatt | Posted in General Questions on

We are having a metal building (46×36) built to serve as a shop/garage and short term a small apartment (26×26 in the corner) while we plan and build our very green main house. Central Texas – on the 3a/3b line – Hot but not crazy humid. But can be super humid in the cooler months (house on a ridge it’s like living in a cloud sometimes)

This will be a pole barn style construction, with a truss roof, without any sheeting, just metal on lathe. We plan to foam the entire interior of the shell as both a vapor barrier, waterproofer, and starter insulation. When we skin the interior walls we may throw in some fiberglass bats just to add a cheap bit of extra insulation. The apartment, built into the corner, will be freestanding and will get full insulation (ceiling, interior to the shell and exterior shell wall). The apartment will have a mini split and will be set to maintain that space even when we are not around. For the shop my goal with the insulation is just to keep it from being a hot box.

For the shop area (20×36) there will be a standard garage door which we can insulate and a roll up door (harder to insulate).   The builder wants to install a ridge vent, but there will be no soffits or any other way for air to get in other than leaking around the garage doors. If we are going to foam the whole shell, shouldn’t we leave out the ridge vent? I am not sure today if we are going to sheetrock the ceiling, or just leave the truss work exposed. 

So should we have a ridge vent or not? Should I drywall the ceiling and blow in insulation too? Should I plan to condition the shop, even just a little to manage humidity?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi ATXMatt.

    It doesn't sound like the ridge vent makes sense. However, it looks like you could install a drywall ceiling on the bottom chord of the roof trusses, which could be air sealed and insulated with cellulose. That's a much more cost-effective and environmentally friendly approach that spray foam on the ceiling. Then, you could vent the attic space.

    Another thing to consider is that an apartment inside a garage should be surrounded by an air barrier so that potentially hazardous fumes from the garage do not infiltrate the living space. But once that apartment is sealed up tight, you'll probably want to find a way to bring fresh air in.

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