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What’s my best option for retrofiting rafter bay venting up to a brick chimney?

David Bailey | Posted in Energy Efficiency and Durability on

Hi GBA community!
I’m in the midst of rescuing a friend’s roof from poorly installed fiberglass insulation. Here’s what I’m up against:
There are 2×8 rafters 2′ o.c. on the south side of the house (a saltbox, built in the 1970’s, in climate zone 6A). The thermal control layer currently follows the 4′ kneewall, presumably but not definitely continuing under the plywood subfloor over to the exterior walls. The rafter bays are completely filled with fiberglass and there are signs of this causing mold to get a foothold. I haven’t seen rotted wood yet, but there is staining.
I’m having decent luck raking out the fiberglass and adding in 2″ rigid foam venting baffles – polyiso foam with foam ‘runners’ taped to it to create venting space – up the crawlspace behind the kneewall and into the attic. I am also running these down to the eave end to restructure the thermal control layer completely into the rafter bays. I’m going to install dense pack cellulose into the rafter bays. I’m also going to pack down the rafters in the crawlspace to beef up the insulation values where I can and will just hope that the slopes in the living space can be retrofit at some point in the future. It’s going well, so far!
My question has to do with the several rafter bays that butt into a sizable brick chimney which blocks the bays from accessing the gable end vent. Those rafter bays (2, maybe 3) are currently full of fiberglass. What’s my best strategy here? Access in the attic is very limited (less than 2′ to the peak from the ceiling joists). I don’t think I can vent those bays, practically speaking, so should I close them off at the eave and fill the entire bay with cellulose? Or is having vent baffles in those bays somehow better? If so, can anyone please explain? The roof, as I am reworking it, will be decently vented and somewhat decently insulated (I wish I could pack the slopes down in the living space, but it’s not in the cards). I don’t feel great about having a 4-6′ wide run of the roof sheathing unvented, but maybe I’m over thinking it?
Thanks for any and all feedback!

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you are helping a friend. I'm guessing that your friend's budget isn't unlimited -- although (who knows?) I might be wrong about that.

    Compromises are sometimes necessary, I know. Sometimes, if you can't afford to do the right thing, you do the best you can, and cross your fingers.

    If rafter bays are blocked by a large chimney, you can't vent those bays. That means that you need to detail those rafter bays (or the entire roof) as an unvented assembly.

    There are two ways to make an unvented insulated roof assembly in your climate zone. Option 1 is to insulate the bays with closed-cell spray foam on the underside of the roof sheathing (with or without fluffy insulation below the spray foam).

    Option 2 is to install an adequate thickness of rigid foam above the roof sheathing (with or without fluffy insulation below the roof sheathing). This option requires new roofing.

    That's the correct answer from a building science perspective. If you break the rules, you must accept risk.

    For more information on your options, see this article: How to Build an Insulated Cathedral Ceiling.

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