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Can I add insulation to a “cathedral” ceiling that is poorly insulated and has no eaves?

user-5175006 | Posted in Energy Efficiency and Durability on

We recently bought an old 3 story 4 flat. One of the units is empty and we have 18 days to improve the insulation in the upstairs that was converted from an attic. The joists are irregularly spaced some 16 some 20 or 24 ” apart. I think they are 2 X 6’s but may be 2 X 4’s. The current fiberglass insulation batts (probably R11)are thin and sometimes smaller than needed. For example a 15″ wide batt in a 20″ space. The rooms are hot in the summer. Can I blow in some fiberglass or possibly slide some properly sized width batts in the cavities for now without causing damage from moisture? There is a ridge vent but no eaves. (I would cut a 12″ strip near the high point of the cathedral ceiling and slide properly sized batts toward the knee wall attic space.) I would upgrade the insulation in the knee wall areas too. Later in maybe a year or 3, our plan is to go back in and upgrade and improve walls electric and insulation.

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  1. Expert Member
    Dana Dorsett | | #1

    The ridge vent with no eave vents just increases stack effect infiltration, and put the decking near the vent at risk of accumulating huge wintertime moisture loads.

    Without proper eave to ridge venting and a minimum 1" of space between the fiber & roof deck you can't just stuff batts in there or blow it full of fiberglass. If you gut the ceilings and use 1" of closed cell spray polyurathane foam (ccSPF) against the roof deck you can then fill the remainder with fiber insulation safely. In cold/very cold climates it may take 2" of ccSPF to get there. The result would be well below code minimums anywhere in the US, but better than what you have right now.

    With random and non-standard rafter spacings batts aren't the best way to go, but with obsessive diligence it's still possible to do a good job.

    Air sealing kneewalls is something of a fools errand, and difficult to do well. (Trust me, I've played the starring role of "fool" multiple times in that movie! :-) ) . It's usually easier/better to go unvented behind the kneewall attics too, insulating at the roof deck, not the kneewell and mini-attic floor.

    If you can share your location we can be more specific on the most reasonable ways to upgrade.

  2. user-5175006 | | #2

    Thank you. The Location is Northern Illinois.

  3. Expert Member
    Dana Dorsett | | #3

    Northern IL is US climate zone 5, where IRC 2012 code for attic is R49 minimum (or U0.026 maximum, which is R38 whole-assembly, with all thermal bridging factored in.)

    You can't really get there with 2x6 framing unless you put foam on the exterior of the roof. In zone 5 with a ccSPF/fiber combination at least 40% of the total R has to be the foam next to the roof deck. In a 5.5" milled 2x6 depth with 2" of ccSPF you'd be at R12-E13 for the foam, and have 3.5" of space left for a high density fiberglass or rock wool batt (R15) or dense-packed fiberglass (also about R15). The center-cavity R would be R27-28-ish, so you'd be fine from a dew-point control perspective with only standard latex paint on drywall as the interior side vapor retarder. (R12 is 44% of R27, you only need 40% to keep the fiber insulation dry.)

    With more ccSPF you don't really gain much performance due to the high losses through the rafters. Using any more than is necessary for dew point control is something of a waste, since the stuff is the opposite of green. Even a full cavity fill is unlikely to deliver more than the equivalent of R1 "whole assembly" performance over what you get with 2" foam and 3.5" high density fiber, despite the much higher center-cavity R. It's better to save the foam budget for continuous rigid foam over the exterior.

    The U-factor would be about 1.5x code max but next time you re-roof you could put some rigid foam above the roof deck to bring it in line.

    If you want to bump it up higher now it means giving up some headroom on the head-banger ceilings. If you cut 2" foil faced polyiso into 1.5" wide strips and glued them to the rafter edges that would allow enough depth to use R23 rock wool or R21 fiberglass batts, but it would also require you to use a "smart" vapor retarder to keep the rafter bays dry, or add another inch of ccSPF (which would be a waste.)

    Just the 2" ccSPF + 15 fiber is going to be a huge improvement over poorly installed R11s and a leaky venting scheme, even if it doesn't meet code. The 2" ccSPF is going to cost about $2-2.50 per square foot, but it's worth doing the whole roof, even behind the kneewalls. Batts could be a DIY sort of thing, but 1.8. lb density fiberglass blown in mesh would be better (but also more expensive.) Sometimes open cell foam is cheaper than dense-packed fiber, and 3.5" of open cell sprayed onto the closed cell is not a bad option. You could also consider 5.5" of open cell foam + interior side "smart" vapor retarder, but you can't put anything too vapor tight or too vapor open on the interior side of an unvented insulated roof. Open cell foam on it's own is a bit too vapor open at 5.5", but a smart vapor retarder is vapor tight during the dry-air winter months, but becomes vapor open allowing any trapped moisture out in the warmer more humid spring & summer.

  4. GBA Editor
    Martin Holladay | | #4

    Solving your problem won't be cheap. There are three possible approaches -- thickening the rafters and installing R-49 insulation with a vent channel; thickening the rafters and installing spray foam or spray foam plus fluffy insulation to R-49 (without a vent channel); and adding rigid foam above the roof sheathing plus new roofing (also without a vent channel, unless you want an optional vent channel above the rigid foam).

    For more details on this work, read this article: How to Build an Insulated Cathedral Ceiling.

  5. user-5175006 | | #5

    I appreciate this education and will likely add foam above the roof when we update the roof. But now I can't do that or the other solutions suggested. So, it comes down to this - for now I have the option of leaving it as it is or substituting fiberglass batts that fit in place of those that don't fit . So could you tell me which is better? Leave it alone or better fiberglass batts?

  6. GBA Editor
    Martin Holladay | | #6

    In your climate zone, installing fiberglass insulation without a vent channel between the top of the insulation and the underside of the roof sheathing is risky. Every year that you leave such a system in place, the more that your roof sheathing is at risk for moisture accumulation and rot.

    The more months with damp conditions, the likelier the rot.

  7. Expert Member
    Dana Dorsett | | #7

    Unless you're going to squirt at least an inch of closed cell foam onto the underside of the roof deck you can't fill that cavity up completely with any safety margin. If the southern facing pitches are not shaded you'll probably get away with it there, but not on the north side or mostly-shaded pitches.

    Adding an inch or two of closed cell foam now does not preclude you from adding insulation above the roof deck later, since at 1" most 2lb foam is not quite a Class-II vapor retarder, and at 2" is in the mid to high-permeance range of Class-II. That means the assembly can still dry toward the interior, but the roof deck will not accumulate much moisture over the winter unless you keep the relative humidity in the conditioned space north of 40% @ 70F. Even one inch of ccSPF is highly protective of the roof deck in a zone 5A climate (even though it doesn't meet the letter of the IRC.) See:

    Installing carefully fitted high density R15s would leave a 2" vent space, and as long as you make the interior side reasonably air tight (not necessarily vapor tight to water vapor) it'll be fine- at least no worse than it is now. But that gap would need to be filled & sealed later prior to installing anything above the roof deck.

    Slightly riskier, but not super-high risk (and NOT compliant to the letter of code) would be to install snugly fitted R23 rock wool or R21 fiberglass and an interior side smart vapor retarder (MemBrain, Intello, et al) behind the gypsum, detailed as an air barrier. When the air next to the smart vapor retarder's relative humidity is low (which it will be in winter) it's a class-II vapor retarder, but in the spring when the warmer temps start cooking the moisture out of the roof deck it raises humidity of the entrained air in the insulation, which makes the vapor retarder vapor-open, allowing the roof deck to dry toward the interior. This will moisture-cycle the roof deck somewhat more than if the roof deck was properly vented (which it isn't), and may be equal to or better than what it's doing right now from a moisture point of view. Key to making this work WELL is obsessively tight air sealing. This stackup also does not preclude exterior foam later.

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