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Community and Q&A

Best way to insulate a Four-Square attic

TTub | Posted in Energy Efficiency and Durability on

Situation: In-laws have 4-square house with uninsulated attic, no roof ridge vents, and plenty of air gaps where original 1x roof sheathing meets attic T&G flooring. Hard to explain but rafter frieze boards exist only at two dormers; all other roof-to-attic floor junctions are a tight vee. I do not know how unvented soffitts were built but they are not accessible from the attic (except at dormers). Pull-down stairway is non-standard size and leaky but I recently built a weighted 2″ rigid foam board box to cover opening and have stopped the majority of air leakage (uneven T&G flooring makes tight air seal a challenge). There are multiple penetrations through the attic floor into the 2d floor ceiling and insulation appears to incomplete.

Solution: I see two options — 1) insulate underside of 1x roof sheathing with mix of rigid foam (w/seams sealed) & fiberglass or 2) lay down a layer of rigid foam over attic flooring, cover with 7/16″ OSB, and follow Martin’s pointers in 2013 “Air Sealing an Attic” post. Option 1 is preferred as it pulls the attic space into the interior and seals all air gaps around the attic perimeter but I do not know what would happen to the roof with insulation attached directly to the underside of the sheathing with no air channel. Option 2 is easier/faster but I lose the attic to outside temps and airflow.

All suggestions are welcome. Thank you.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, if you decide to insulate your attic floor, I wouldn't worry too much about attic venting. As long as your roof sheathing boards show no signs of mold or moisture damage, everything should be fine without any vents. For more information on this issue, see All About Attic Venting.

    If you decide to create a conditioned attic, you have to be a little more careful about the venting issue. Since your rafter bays are unvented, you certainly don't want to use fiberglass batts between your rafters unless you have an adequate thickness of rigid foam or spray foam above the fiberglass batts.

    For more information on conditioned attics, see Creating a Conditioned Attic.

    It sounds like you are tempted to use the cut-and-cobble method of insulation to create a conditioned attic. I don't recommend the cut-and-cobble method for unvented ceilings, because of several reports of failures. For more information on this issue, see Cut-and-Cobble Insulation.

  2. TTub | | #2

    Thanks Martin. I have read all your links and admit to still not being certain of the best method to use.

    You are correct about me leaning toward a cut-and-cobble install and, after reading the article on same, I am not sure this would not work in my situation (the labor is mine and the in-laws just have to give me a room and feed me). The other articles lead me to believe there is no real issue regarding shingle performance if I were to insulate under the roof sheathing. However, gaps in the current 1x roof sheathing, which expose the underside of the roofing paper, lead me to leaving the roof alone.

    Ultimately, insulating the attic floor is the cheapest, easiest and fastest way to go. Venting is not an issue as the existing wood shrinkage of the roof assembly leaves plenty of gaps for airflow. And keeping the attic storage area unconditioned is not that big an issue.

    Thanks for all the info...Terry

  3. Rec1Construction | | #3

    Why not blow cellulose into the 2nd floor ceiling joists from the attic? Save you the trouble and hoakyness of laying foam (and osb-ugh) and also create something of an air seal in the ceiling.

    The other option, which I've seen turn out pretty nice (so far), is to use spray foam in the rafter bays & finish the attic. We did that on a house at the Oregon coast - sanding & finishing the skip sheathing attic floor and it turned out fantastic. Not cheap, of course.

  4. BobHr | | #4

    before insulating know what type of wiring is in the house. It was probably built with knob and tube. Make sure that as all been disconnected prior to insulating around it.

    If I read it right the house has a walk up attic with board floor over the joists. I would dense pack cellulose under the attic floor.

    Martin could he then put some type of permeable wrap above the floor and air seal that. Then cover that with more insulation.?

  5. Expert Member
    Dana Dorsett | | #5

    The knob & tube hazard is so far purely theoretical, though barred in the National Electrical Code based on the theory. There have been no known house fires caused by insulating over K & T, though the theory is that insulating over poorly twisted or corroded splices could possibly cause a fire since it would not be able to dissipate heat as well as when it was in open air. (This is actually difficult to do even in a laboratory.)

    Several US states (California, Washington, Nebraska, and Oregon) will allow you to insulate over live K & T as long as it has been inspected- if the wiring is not corroded or loose at the splices, it is specifically allowed. There may still be issues with insurers though.

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