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Community and Q&A

Best approach for attic insulation?

lieam2 | Posted in General Questions on

I recently moved in to an early ’70s Colonial in zone 5. The attic is poorly insulated with a couple of inches of loose-fill that looks like fiberglass.

The attic has newer AC (not heating) equipment, including an air handler and some flexible insulated duct on the floor (not suspended). It’s also poorly ventilated, with 2″ ventilation grilles in the soffit about every 10 feet, no ridge vent, a gable vent on one end and a small vent in the roof on the end opposite the gable vent. Finally, the asphalt shingle roof is in decent shape but approximately 17 years old.

Given the location of HVAC mechanicals and already-poor ventilation, I’m considering removing existing loose insulation on attic floor and insulating the bottom of the roof deck, making the attic conditioned space.

Is this a generally a good idea? And if so, what is the best specific approach for my climate zone? (e.g., closed cell foam directly on bottom of roof deck with no ventilation followed by a fire barrier, etc). Also, is it critical to replace the roof before doing this project?

Please note that replacing or relocating the HVAC mechanicals is not in the budget.

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  1. GBA Editor
    Martin Holladay | | #1

    Moving the insulation to the roof slope, as you propose, sounds like a good idea in your case.

    While it isn't necessary to replace the roofing, it sounds like your roofing is nearing the end of its life. If you decide to put on new roofing, you have the opportunity to install one or two layers of rigid foam above the roof sheathing. That's the best way to insulate a sloping roof, because it addresses thermal bridging through the rafters.

    If you don't replace the roofing at this time, you can insulate from below with spray foam. However, this approach doesn't address thermal bridging through the rafters.

    For more information, see Creating a Conditioned Attic.

  2. Expert Member
    Dana Dorsett | | #2

    First you don't have to remove the paltry bit of attic floor insulation except where it's in the way for air sealing. at the soffits. It might only be R11-R13 (R8-R9 after thermal bridging), but it does provide some benefit even after insulating at the roof deck.

    In Zone 5 with composite shingle roofing even 1" of closed cell foam on the underside of the roof deck is sufficiently protective to allow you to go full rafter-depth (whatever thickness that is) with much cheaper fiber for the rest with only latex paint as the interior vapor retarder. You can then put half-inch gypsum on the underside of the rafters and give it a coat of standard latex primer as the interior vapor retarder. That stackup doesn't meet the letter of code, but it has a pretty good resilience. See:

    (1.8lbs density fiberglass, or any density cellulose is good. Damp-sprayed Spider or damp-sprayed cellulose is cheaper than dense-packing in netting.)

    Then when it's time to re-roof, it's worth putting the code-prescriptive R20 in rigid foam or rigid rock wool above the roof deck. See:

    With only 1" of closed cell on the underside the drying capacity of the roof deck toward the interior is pretty good, at about 1-perm. If you go with more than 2" under the roof deck you end up with a moisture trap at the roof deck due to the low vapor permeance of thicker foam, and the extremely low vapor permeance of the roofing. Keeping it at 1-2" is the right way to go. An inch is sufficient for air sealing.

  3. lieam2 | | #3

    Martin, thanks for the thoughtful response and excellent article. I probably should have found that before posting.

    Can anyone weigh in on whether rigid foam above the roof deck is likely to be cost-effective, assuming i need to re-shingle soon anyway? Attic insulation is competing with other needed energy upgrades, so cost/performance is important.

    I realize this is probably a complicated question with lots of variables. I'm in the long island sound area, where labor is pricey. thanks!

    edit: thanks for the follow-up Dana. very helpful.

  4. GBA Editor
    Martin Holladay | | #4

    The cost of rigid foam insulation (per R) is usually less than the cost of spray foam. But many roofers are reluctant to bid on residential jobs with rigid foam above the roof sheathing -- for reasons that are unclear -- so it's hard to predict your costs. You'll have to get some bids.

  5. Expert Member
    Dana Dorsett | | #5

    Only at the point of re-roofing would adding the R20 have minimal cost consequences. Not adding it at that point is a lost opportunity that won't come around again for another 20-25 years.

    The material cost of R20 roofing iso (4.25-3.5" thick, typically) is about $2 per square foot. To that you'd have to add the labor cost of installation, and the cost of adding an OSB nailer deck (half inch is fine) above the iso.

    It's somewhat better to hold the foam in place with 2x furring through-screwed to the rafters 24" o.c. with pancake-head timber screws 24" o.c. and put the nailer deck above that, since you can then vent the nailer deck making more resilient to roof leaks, and avoid the 10,001 nail punctures to the foam, and the associated thermal bridging.

    The simpler the roof lines, the cheaper & easier it is. But with complex roof lines the benefit is higher, since the additional framing of a multiple dormer/compound angle stuff leads to higher thermal bridging and more potential air-leak points.

    Add it all up, and it's not cheap, but it's a very resilient and high-efficiency stackup. If you have 2 x10 rafters or deeper the additional performance of the exterior R20 may have a difficult financial argument, but at 2x8 or smaller it's there.

    Two layers of 2" foam with seams taped both layers, and staggered between the two layers tends to perform better in the long term than a single layer of 4", since the thermal faults at the seams are broken by the other layer. It adds a bit to the labor costs though.

    There are multiple vendors dealing in reclaimed roofing foam from commercial demolition in my area, which can reduce material the cost of the R20 to about 50 cents/foot. Some seem to advertise only on craigslist, but will ship nation wide (for a price) if the quantities are high enough.

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