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How do I insulate a garage with a cathedral ceiling

thegiz | Posted in General Questions on

First off thanks for everyone that has helped me to begin insulating my garage.  I am well on my way to insulating walls and doors in my 2 car detached garage.  I am at the point where I need to decide what to do above.  Originally my great grandfather built a ceiling that formed a mini storage attic.  Over the years raccoons and squirrels had found a way to turn it into a condo so I had to rip the whole attic floor out.  It was disgusting, prob the worse job I had to ever handle.  Not sure if I should rebuild an attic floor and insulate it because I might end up with the same problem.  I don’t know if I need to insulate roof if walls are insulated, I’m well aware that heat rises but plan to heat garage in winter for an hour at most then leaving.  I don’t want to spray insulation because I want to diy it.  I could put fiber insulation batts and close with osb but I’m confused about the whole ventilation thing.  I saw one video where a guy attached foam board inbetween joists and then used  spray foam for gaps and just left it like that.  What is most economical solution, I’m in downstate NY climate zone 5 I believe, pictures attached.

– Joe

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Joe.

    I could only speculate about how much or little insulating and air sealing you need to do to be able to make your garage comfortable for an hour a day. And I'm sure how you heat it will be part of the equation too. In any case, an insulated roof can work as both vented or unvented assemblies, but the insulating strategies differ for each. This article will get you on your way to understanding how to insulate the roof: How to Build an Insulated Cathedral Ceiling.

  2. thegiz | | #2

    Brian thanks again for your help. Read through the article and the many choices but I think closed cell spray foam is the best and safest choice. I see there are diy spray foam kits, not sure how much I would need and if the cost would get out of hand. If I did use spray foam could I leave it exposed after or would I need to cover it? Other option is insulated fiberglass batt walls and insulated garage doors but the roof left uninsulated would it still help with some heat loss? Willing to leave ceiling exposed if I could make up for it with more intense heating.

    1. carsonb | | #3

      For large areas I’m not sure DIY spray foam is advisable. There are horror stories of bad spray foam installs that didn’t cure properly and health risks with spraying it, plus the kits cost $1.5 per 1” sqft which may be about the same as hiring a pro. You might consider cut and cobble if going diy route, which is spray foaming the edges of rigid insulation.

  3. thegiz | | #4

    Carson, if I use cutt and cobble method can I glue rigid foam directly to underside of roof sheathing, how thick would rigid foam need to be? After that would I need to add anymore insulation and would I need to finish walls with osb or drywall or can I leave insulation exposed. Do I need to use same method when insulating walls or can I just use fiberglass batts

    1. carsonb | | #5

      Search the site for how to insulate cathedral ceiling, there is an article directly on the subject by Martin. There is a minimum foam thickness you need based on climate zone. There are a lot of options, fiberglass is an option fir walls and roof. You will likely need a fire break, like drywall, under the foam for the ceiling to follow code.

  4. Expert Member
    Akos | | #6

    Cut and cobble is a waste of time and is a risky assembly as it is impossible to get it air tight.

    Looks like your have a simple gable structure, which means if you need to, you can easily add soffit to ridge vents. Most likely this won't be needed, but if you end up with condensation issues, it is something that can be easily added.

    The budget way of insulating workshops is mineral wool batts that are undersized for the rafters covered by an air control layer. The undersized batts keep a channel above open so you can vent the roof if needed.

    The air barrier could be a well detailed house wrap, layer of taped OSB or drywall. Make sure this air barrier is sealed to your walls as well.

  5. thegiz | | #7

    I realize cut and cobble is not preferred method but I’m trying to save money. What about the “stack of pancakes method” stacking 2 layers of rigid foam. Then bays would be completely filled with closed cell foam. Is this method less likely to have problems because I’m not using batts in an detached unvented garage. I would use this in walls too so mold would not be a concern. I mean would this method work in a basement as well, that was my next project but I was going to use rigid foam against wall and then use unfaced batts. Basically what is disadvantage of stacking rigid foam, r value?? Seems like perfect solution wherever moisture would be a concern

  6. the74impala | | #8

    Is it correct to say that you are trying to avoid animals by doing a cathedral ceiling. A flying squirrel family of 4 I have met would likely laugh at that assumption. just put back a flat ceiling and block the holes for the critters the best you can.

  7. thegiz | | #9

    Tom, yeah this is the assumption that it will stop animals from getting up there if I didn’t have a separate attic. I think they like the darkness of attic though. Any other green ways to stop them bedsides just checking any entryway? Squirrels are not the worse, it’s the raccoons. I’m in suburbs of NYC and these raccoons are street smart. When I moved in it was neglected up there, just don’t want to be shoveling loads of raccoon feces again. What about pancake stacking as a building method though?

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