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Community and Q&A

Should I fill the unvented soffits with foam?

ideaconstruction | Posted in Green Building Techniques on

We have an outbuilding that’s about 12 x 13 that we want to turn into year round writing / guest room.
It has a basement where well equipment is located. We’ve sealed and insulated the ceiling and the floor joist bays above the foundation and covered with 5/8ths wallboard. With a tiny heater that comes on intermittently in the cold weather, the basement stays about 42deg F so the pump doesn’t freeze.
We plan to improve the ventilation for the warm months when some condensation occurs.
Inside we’re insulating the roof and walls. The roof with XPS panels glued in place and the spaces filled with Great Stuff spray foam. The walls will have 1″ XPS panels mounted 1″ away from the backside of the pine siding which is nailed to the studs with no sheath in between. I am thinking of filling the soffits with foam. Is there a significant downside to doing this? Should they be vented (I have these 3″ round copper vents.) or is OK not to!

Thank you v much,
Rob Cramer

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure how you insulated the roof. It sounds like you either (a) installed strips of rigid foam between the rafters, up against the roof sheathing, using the "cut-and-cobble" method, or (b) installed a continuous layer of rigid foam under the rafters.

    Method (b) would have been better than (a), by the way.

    As I'm sure you know, your method of insulating this building does not meet code. Your R-values are much too low. If you told us where you live, or mentioned your climate zone, we could tell you what your minimum R-values should be.

    In general, you want to have a continuous barrier of insulation surrounding your conditioned space. While there is often a discontinuity where the tops of the walls meet the roof insulation, there are various ways to limit this discontinuity. You should strive to have as few gaps as possible between your wall foam and your roof foam. All gaps should be sealed to prevent air leakage.

    In almost all cases, there is no advantage to insulating the soffit. However, if that's the only way to make sure that your wall insulation is continuous with your roof insulation, there is no harm in insulating your soffit -- especially if you insulated your roof using method (a).

    If you insulated your roof using method (b), you probably want to leave a ventilation channel from your soffit to your rafter bays.

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