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Venting cross gabled Cape Cods?

hertzr | Posted in General Questions on

I have recently re-worked my zone 4a (central MD) Cape Cod’s attic insulation. Am now shifting focus to attic ventilation, which is 1) currently lacking and 2) complicated (see attached figure).

The insulation project involved:
– plugged floor joists under kneewall using cut-and-cobbled 2″ XPS blocks
– air sealed everywhere
– installed site-built XPS baffles at top plate/soffit areas
– placed R30 rock wool in floor joists with another perpendicular layer above (R60 total)
– put R15 in knee wall cavities, then secured with 4″ XPS foam (R35 total)

This project has hopefully improved the 4,640 cfm50 measured prior to this work. However, am now concerned the lack of ventilation may come back to haunt me.

My question – what’s the best way to vent my attic area, which includes intersecting gable roof lines (the attached figure’s blue and yellow shaded areas) and a porch overhang (orange)? We currently have 2 small soffit vents and a gable vent in the attic, with 2 additional gable vents above the living space.

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  1. hertzr | | #1

    Here is the attached roof line graphic. Thanks all!

  2. GBA Editor
    Martin Holladay | | #2

    On the illustration, you wrote "Attic space from dashed line forward." So how would you describe the yellow area on the other side of the dashed line? If it isn't attic space, what is it? A finished cathedral ceiling, perhaps?

    The reason I'm asking is that your illustration shows two gable vents where the green area meets the yellow area. If these gable vents aren't in an attic, where are they?

    I'm guessing that the gable vents are in a third-floor attic, and that the "Attic space from the dashed line forward" is a small triangular attic behind kneewalls. Is that correct?

    If you want to improve the ventilation, you have opportunities to do so. Obviously, you could install soffit vents at the eaves of the blue area. You could also install a continuous soffit vent at the eave of the yellow area (where you have two small vents now).

    Finally, you could install two gable vents in the yellow area that you have described as "Attic space from dashed line forward."

    All of that said, you may not have a problem. Attics won't have moisture problems unless there is an air leak that connects the attic to the conditioned space below. Usually, it makes more sense to track down these air leaks and seal them than to spend a lot of time improving attic ventilation. For more information on this issue, see All About Attic Venting.

    One last point: If my guess is correct that the "Attic space from dashed line forward" is a triangular attic behind a kneewall, then one solution -- the best solution -- is to bring this area into the home's thermal envelope by insulating the sloped rafters. For more information on this approach, see “Two Ways to Insulate Attic Kneewalls.”

  3. hertzr | | #3

    You're assumption is correct - the area in front of the dashed line is a knee wall attic that is triangular shaped on the right hand portion (yellow), and opens up into a triangular/gabled space to the left (where the blue, orange, and yellow sections meet). We have a tiny 3rd floor attic above the living space, but this area is inaccessible and likely only ~8" high in some areas.

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