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Roof ventilation

user-7383946 | Posted in General Questions on
Climate zone 7. Canada.
I have a 1.5 story house with an unvented unconditioned attic.
I followed the recommendations on this site and insulated the attic side of the kneewalls with 3” EPS and mineral wool in the stud bays of the kneewalls.
The sloped portion of the roof was sprayed with closed cell spray foam.
I am concerned about ventilation.  The attic behind the knee wall on the south side of the house has one gable vent on the front wall of the house near the peak. The attic behind the knee walls on the north side of the house (attic knee walls are separated by a shed dormer) each have a mushroom vent. The attic above the flat part of the ceiling has one gable end vent on the east side and one on the west side and two mushroom vents.
Two questions:
1. What should I do with the mushroom vents – keep them or remove them? Because there’s no soffit vent or ridge vent, I understand there is no draw of air (stack effect). The house is being re-shingled so there’s the ability to leave the mushroom vents as is or to have them removed. Having soffit vents installed / added is not an option.
2.  Should I install smart membrain  on the ceiling of the room on the main floor directly below the attic kneewall?  The room is being reinsulated and dry walled.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    The moisture you need to worry about comes from air leaks -- that is, leaks of conditioned indoor air into the attic space. If you do a good job with your home's air barrier, you don't really need attic ventilation. For more information on this issue, see "All About Attic Venting."

    When it comes to the mushroom vents, there is no easy answer. It's a judgment call. If your attics don't show any signs of existing moisture problems, and if you feel you've done an excellent job of air sealing, the mushroom vents are probably unnecessary.

  2. Expert Member
    Peter Yost | | #2

    Do you need Membrane on the ceiling below the attic?

    The tough thing about insulating and air sealing along part of the roof, down the knee wall and then over the ceiling/floor below is continuity, particularly of the air barrier. So, your need for getting the air barrier across the ceiling connected to the air barrier of the knee wall is way more important than your need for a smart vapor retarder.


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