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Closed-cell spray foam for gable vent cathedral ceiling

Jason McLooney | Posted in General Questions on

Hello everyone. I’m removed ceiling joists and replacing with rafters to create a cathedral ceiling with proper supports.

The porblem I’m having is deciding if I should use closed-cell foam on entire roof decking underneath or batts of fiberglass insulation (hell of a lot cheaper).

Heres the deal though. Based on article on here it says use closed-cell for unvented but the attic space (if you call it that), used to have knee walls and each side of house has gable vents.

The roof also has vents when roofer installed.

Woukd i just have spray foam guy spray over all the vents?

OR

Should I use batts of fiberglass insulation and keep the cable vents open? Keep in mind, there are no soffit vents in this house.

It was built around 1945.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    After you have removed the existing ceiling joists and beefed up your rafters to create a cathedral ceiling, do you intend to have a small attic near the peak of the roof (defined by a small horizontal ceiling near the peak)? Or do you intend to have a cathedral ceiling that extends all the way to the ridge?

    If you intend to keep a small attic under the ridge, that attic can be either vented or unvented -- your choice.

    It's the same with the cathedral ceiling ratter bays. They can either be vented or unvented.

    Remember, though: If your cathedral rafter bays are vented, you need to be able to vent every single rafter bay. That means no interruptions for valleys, hips, dormers, etc.

    All the information you need is in this article: "How to Build an Insulated Cathedral Ceiling."

  2. Jason McLooney | | #2

    Martin, thanks for the response!

    The house has 3 gable vents. The house used to have one large attic space which knee walls. I removed the knee-walls and opened the ceilings with rafters, but they don't go all the way to the ridge vent.

    The house has a ridge vent across entire roof line.

    The first gable vent is within a "attic" space above a bedroom which carries over above the bathroom, which it also has a gable vent.

    Part of the house breaks this attic space, and its just completely open--rafter and no attic space.

    The other large portion of house, has a gable vent in a relatively larger "attic" space but the rafters in kitchen and opposing side of room are just rafters (no air space) thats leads into the "attic airspace"

    Its sort of a hybrid roof.

    I got a quote to sprayfoam and the cost was $8400 which is significantly more than batt insulation. I'm wanting to choose the most efficient/safe and yet cost effective method.

    Below are pictures of the project. See the zip file for them all. I know this framing is probably not great so disregard. It's my house, so it's not work I did on someone else house FYI!

    1. GBA Editor
      Martin Holladay | | #3

      Jason,
      As I wrote in my first response, it's up to you to decide whether you want vented rafter bays or unvented rafter bays.

      Just remember: any rafter bay that is vented needs access to a soffit vent at the bottom, access to a ridge vent at top, and must have no interruptions between the soffit and the ridge.

      It's also important to have a plan to install enough insulation to meet minimum R-value requirements. That can be tough if your rafters aren't deep.

      If you are uncertain how to proceed, I once again urge you to read this article: "How to Build an Insulated Cathedral Ceiling."

  3. Jason McLooney | | #4

    But the question is, without soffits but having gable vents for part of the roof and a ridge vent, does that suffice? Can a gable vent satisfy rafter venting if you don't have soffit vents?

    The article doesn't explain this unique situation. It just assumes you don't have soffits or that you do haven them. My situation isn't the same and wasn't sure if you have any experience with this situation and recommendations for it.

    1. GBA Editor
      Martin Holladay | | #5

      Jason,
      An attic doesn't necessarily need soffit vents -- as long as there is an airtight ceiling under the attic. The best resource on this topic is "All About Attic Venting."

      When it comes to a cathedral ceiling (as opposed to an attic), you can't use a vented approach unless there is access to a soffit vent at the bottom of each rafter bay.

  4. Jon R | | #6

    Lower vents without soffit vents:

    https://dciproducts.com/smartvent-attic-ventilation/

  5. Expert Member
    Peter Yost | | #7

    Gable end attic venting: just remember that the "rules" for roof/attic venting (the 1:150 and 1:300) are not really science-based (https://www.greenbuildingadvisor.com/article/all-about-attic-venting).

    Attic and roof venting depends on a driving force for air flow: either wind or stack effect. Neither driving force is present all the time nor equal for each orientation. But gable to gable venting seems to have worked for many years in many climates and on many sites, handling the moisture load that the attic "sees."

    Peter

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