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What to do with a thermal disaster?

jackluminous | Posted in Energy Efficiency and Durability on

I am trying to determine a cost-effective strategy for improving the efficiency of my home, specifically the roof.

It is a 1969 ranch in Tucson, Arizona. The house is a thermal disaster if there ever was one.

Here is how the roof is constructed: Low-slope (half inch per foot), 2X10 rafters (built-up membrane roof above, drywall ceiling below), insulated with foil-faced batts of a whopping R-9.6. There are eave vents but no ridge vent. Ceilings are code-minimum low at the eaves, 8′ at the ridge, with ductwork in a dropped soffit under the ridge. I guess you’d call it a vented cathedral attic but with ceilings this low, “cathedral” is not exactly a word I would choose. The closest thing the house has to an “air barrier” would be the drywall, which of course is poked with a million holes and leaks air like a sieve.

I’ve thought of three options. Option 1 is to attempt to pull as much of the foil-faced batts out as possible, dense-pack the rafter bays with cellulose, close the vents, move the air barrier to the exterior, and pray. Tucson is in climate zone 2B so an unvented attic like this can work pretty well. It’s simple and the work is easy-peasey lemon squeezy. Unfortunately I won’t be able to remove all the foil; some of the rafter bays are 20 feet long. I should be able to get maybe half of it out without removing drywall. Just how concerned should I be about the foil facing that gets left?

Option 2 is to leave the assembly more or less alone, and use my insulation/air sealing budget to install extra solar panels to cover the ridiculous heating/cooling load.

Option 3 is to remove all the drywall ceilings and replace all the insulation. I don’t think this option has a payback period, ever. It’s also not nice to the landfill.

I’m leaning strongly toward Option 1 but I have this nagging fear that it could be a Really Dumb Thing, which is why I am posting this here.

Thank you for any ideas or thoughts you might have!

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  1. user-2310254 | | #1

    Ryan. Any plans to re-roof? Ideally, you would add 1.5 inches of EPS rigid foam (R-5) to your exterior sheathing, seal the attic, and then add as much air-permeable insulation as possible to the rafter bays. But it sounds like you already have a solar array, correct?

  2. Andrew_C | | #2

    A couple of thoughts -

    1. Air sealing should be high in your list, sounds like it is. A blower door test that you get to help set priorities will likely be useful.

    2. Most Tucson homes have ductwork outside the conditioned airspace. If that's true in your case, you could make a big difference by bring them inside, or lacking that, going to town with mastic to seal everything up prior to insulating them. If your ductwork and other HVAC elements are outside the condition, you might consider having your ductwork tested at the same time as your blower door test.

    IMHO, hot attics with little insulation allow the drywall ceilings to get hot. Once they do, they become big radiant heat panels that continue to radiate heat after sundown. Even if your air temperature says that it should be comfortable, it won't be. In Tucson, with high temps and lovely clear skies, this can be a big deal. So, I don't think option #2 will work, even if you get a really large solar array.

    Good luck with that nice warm weather.

  3. jackluminous | | #3

    Thanks Steve and Andrew.

    I think my primary area of concern is the foil facing on the fiberglass batts. Foil is vapor impermeable. My roof membrane is vapor impermeable. My framing and roof deck are beautiful because the attic is ventilated. Sealing the attic (which would bring the ductwork into conditioned space) would be a no-brainer if it weren't for the foil, which is a class I vapor retarder. With the foil in place, sealing my attic is a code no-no. So I want to rip out as much fiberglass as I can to get rid of the foil. I just can't access all the rafter bays to remove all the foil, some of it is going to get left. Unless I want to rip out all my drywall ceilings (which were recently retextured). That's a lot of $$$, probably more $$$ than I have.

    Can I rip out half the foil, pump the rafter bays full of cellulose, and not have done a Really Dumb Thing?

  4. user-2310254 | | #4

    Ryan. Is the foil properly or poorly installed. As a rule, fiberglass batts are usually poorly detailed. In your assembly, poor detailing may work in your favor and help to promote drying to interior.

  5. jackluminous | | #5

    It varies. The house was part of a large eastside development in the late 60s early 70s. The insulation was done to standards of the day for production building. So, yeah, not exceptionally well detailed.

    We took down enough drywall to access the ductwork and pulled out as much insulation as we could. A couple batts came out easily all the way from the eave. Others just tear and we would only be able to remove what we can reach.

    Assuming we can get about half the insulation out, some bays more and others less, and have a fairly leaky ceiling, one would hope that there wouldn't be much moisture entrapment. Dense-packing cellulose really cuts down on circulation though. The low slope may suggest I should just rent a home-store blower and not pack the insulation so densely; it probably won't settle much, and would permit more air movement. It would just be harder to exceed R-30 code minimum in 9 inches of space.

  6. STEPHEN SHEEHY | | #6

    Ryan: I think your roof deck is in good shape because of your dry climate. A low slope roof is never going to be well ventilated and without a ridge vent, you're getting little or no benefit from the vents you have at the eaves.
    I'm confused, though. You refer to an attic, but it sounds like your ceiling drywall is right against the rafters.

  7. jackluminous | | #7

    Stephen: You are correct, the ceiling is right against the rafters. So it's not an "attic" the way we commonly think of one, but if I'm not mistaken the code still considers it an attic.

    It appears the ventilation is continuous from one eave to the other, but it does not take advantage of the stack effect so it's probably not as efficient as it would be with a ridge vent. But yes, I agree, the condition of the roof deck is certainly blessed by the dry climate. Whether that blessing continues if I block the vents, well, that's the gamble I'm considering.

  8. GBA Editor
    Martin Holladay | | #8

    I wouldn't worry too much about the foil facing on the batts. The foil facing can be left where it is without causing any problems.

    My advice is to remove the existing roofing, add as much rigid foam above the roof sheathing as you can afford, and then to install a new layer of OSB or plywood above the rigid foam, followed by new roofing. If you take this advice, you'll also need to seal the soffit vents.

    For more information, see these two articles:

    Insulating Low-Slope Residential Roofs

    How to Install Rigid Foam On Top of Roof Sheathing

  9. jackluminous | | #9

    Martin: Thanks for the reassurances about the foil facing, it's a load off for sure.

  10. EstherJones | | #10

    I think it is good to take professional help.

  11. Dana1 | | #11

    When re-roofing a low-slope roof in Tucson it's worth selecting a roofing that has a high solar reflective index, which will reduce peak roof temperatures considerably. The Cool Roof Rating Council mainains a searchable product list/ database with third party tested short term and 3 year SRIs indicated.

    For your roof ideally you would want a 3 year SRI greater than 70, but even SRI 50 would be way better than typical roofing.

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