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Community and Q&A

Retrofit vented attic to… unvented?

John O'Brien | Posted in Energy Efficiency and Durability on

Quick question for the board members… Currently in the prep phase for a second story addition to one part of our house, that will entail splicing into the existing roof for roof/wall flashing as well as some structural improvements to a few rafters..

The current roof structure is 2×4 rafters, sitting on the wall top plate with birds mouth cut… so… Effectively, 1-2″ of insulation if i’m lucky.

Was curious what you all would recommend…. My gut feeling is to ‘chainsaw retro’, but reading into codes (Ontario..), it’s unclear… Would it be possible to simply leave existing attic floor insulation as is, but then to slap on some exterior foam/roxul, with a vented layer above that, or does it call for removal and then reinstallation in a net’d form under the rafters….

It appears that if I were to go that route, I would need to add some ventilation supply/returns to that attic space, plus added heating, in line with a crawlspace like deployment…. All of which makes it feel like the cost effectiveness of the whole idea is laughable..

Alternatively, if someone were to just tell me to turn up the heat, go have a beer…. I’d be cool with that.. 😉

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Replies

  1. John O'Brien | | #1

    Zone 6a, ottawa, ontario.

  2. GBA Editor
    Martin Holladay | | #2

    John,
    If you want your insulation to follow the roof line (because you are creating a conditioned attic) then you'll probably need to install one or more layers of rigid foam insulation above the roof sheathing. If there is some existing insulation on your attic floor, you can either remove that insulation or leave it in place.

    In most cases, you won't have to provide a method of space heating or space cooling for the conditioned attic, although in some situations you might want to do that.

    The following three articles will explain what you need to know:

    How to Build an Insulated Cathedral Ceiling

    How to Install Rigid Foam On Top of Roof Sheathing

    How to Build an Insulated Cathedral Ceiling

  3. John O'Brien | | #3

    Thanks Martin,

    It was perhaps a poorly worded question... Essentially, by creating an unvented attic, does the attic then need to be 'conditioned', in accordance with Joe L's thoughts at http://buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics

    His comparison to conditioned crawlspaces & required ventilation is that part that gives me pause... I'll have to poke around a bit more locally.. It feels like one of those things where either way I go, the simplest solution is to just rip off the roof and drop on proper raised heel trusses, or to simply turn up the heat and carry on.

  4. GBA Editor
    Martin Holladay | | #4

    John,
    You need to condition the attic if you insulate your roof assembly with open-cell spray foam (in order to prevent moisture accumulation in the roof sheathing).

    If you create a vented roof assembly (for example, with cellulose or fiberglass insulation), or if you install an adequate layer of rigid foam above the roof sheathing, you don't need to directly condition the attic. (It will be indirectly conditioned by the floors below.)

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