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Insulating a vaulted ceiling in a converted garage space

oxtart | Posted in General Questions on

Ok, first off I gotta say I love the internet for info, but this question has tons of conflicting info and it’s got my head spinning.

I have knocked down the wall between my living room and garage and I’m converting the garage into living space. The garage was unfinished and has a cathedral ceiling made up of 2×6 at 16″ and the room is approx. 12 ft wide x 15 ft long. I crippled the roof rafters to create a flat at the top of the ceiling which created about 1 ft. of empty space above the ceiling at the peak. There is no ridge vent and I don’t want to start messing with the roof.

So with this info, what would be the best (and most inexpensive) way that I can insulate this space?

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  1. GBA Editor
    Martin Holladay | | #1

    With no ridge vent, you need to build an unvented insulated sloped roof assembly (otherwise known as an unvented cathedral ceiling).

    There are two main ways to proceed. You can either insulate the roof from above (by removing the roofing, installing a thick layer of rigid foam, and then installing a layer of plywood or OSB and new roofing), or you can insulate from below with spray polyurethane foam.

    You can't use fiberglass to insulate an unvented roof.

    More information here: How to Build an Insulated Cathedral Ceiling.

  2. oxtart | | #2

    Being that the area isnt that big if i cut a few vents on the side of the peak above the flat area I created out the side of the garage and ran vents under fiberglass would that allow enough ventilation the suffice?

  3. GBA Editor
    Martin Holladay | | #3

    Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area. Manufacturers of soffit vents, ridge vents, and gable vents usually specify the net free vent area of their products on product packaging or in specifications available online.

    It's up to your local building inspector to determine whether to permit you to skip the ridge vents. If your inspector approves the use of gable vents, and if the vents are big enough, I have no problem with the plan.

    Remember, fiberglass batts are the worst performing insulation material available. If you install fiberglass batts, spend some time on installation. Do a good job, and make sure their are no voids or compressed areas. Make sure that your ventilation baffles are installed in an airtight manner.

    More information here: Installing Fiberglass Right.

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