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Rookie question about Roxul Comfortbatt insulation

lfelicio | Posted in Energy Efficiency and Durability on

I am insulating the attic ceiling. My house was built in 1907 and has 2×4 rafters spaced around 22 inches apart with some variation in the distances. I purchased 23″ Roxul Comfortbatts. The question is can I compress 23″ into my 22″ space or do I need to cut an inch off the batt for optimal insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    First, I hope that you are providing vent channels in these rafter bays before you begin installing insulation. For more information on this issue, see How to Build an Insulated Cathedral Ceiling.

    When installing batts in an older house with framing bays that vary in width, the principle to remember is that you want the batt to fill the space completely, without any gaps, compression, or buckling. So you'll need to use your judgment. In most cases, careful trimming results in a higher quality job -- but the need to trim depends on the springiness and compressability of the insulation. If you can compress a batt without creating buckling or arching, everything is OK.

  2. lfelicio | | #2

    Thank you Martin for your response. Since the rafter bays are 4 to 4.5 inches deep and the insulation and 3.5 inches thick, there will be between half an inch to an inch in space behind the insulation. The is a continuous perimeter vent going around the 4 sides of the roof about 2 to 3 feet above where the rafters meet the floor joists and there is a ridge vent. Is the space behind the Roxul Comforbatt enough ventilation?

  3. Expert Member
    Dana Dorsett | | #3

    Try test fitting it. If the batt has a tendency to buckle & bow it may need a trim, but it still wants to be a compression fit, so you don't want to whack a full inch- a half-inch will do. The batts are manufactured with lower density at the sides to accommodate some side compression, and are manufactured about a half inch wider that standard framing cavity widths. With the batt still in the framing it's sometimes to shave the edge of the batt with a batt knife to get it to flatten out.

    Also full-dimension rough 2x4s have deeper cavities than milled 2x4s, a half inch deeper than the manufactured loft of rock wool batts, but they'll still work, but require a bit more "fluffing". As with any batt insulation it has to first be tucked firmly in at the corners and along the rafters to ensure that it contacts the sheathing fully, then gently tugged back until it is just proud of the edges to make it a compression fit when interior side gypsum board is applied.

    With batts between rafters you need either a full inch of vent space to the roof deck, or sufficient insulation above the roof deck to prevent the roof deck from accumulating excessive moisture over the winter. The minimum amount of above-deck insulation needed varies with local climate. See:

  4. Expert Member
    Dana Dorsett | | #4

    With a vented roof and full dimension 2x4s you may want to go with sound abatement batts, which are manufactured with a bit less loft, and will compress more easily against your batt-baffles. Installing them without an exterior side baffle can't guarantee the 1" minimum space.

  5. lfelicio | | #5

    Dana: thank you for your answer. About accumulation of deck moisture in winter: I live in Zone 3 and we never have temperatures in the freezing zone. Afternoon temperatures are usually at least in the 60s year round. My big problem is that our furnace and all conduits are in the vented attic and there is a temperature differential of 40 to 50 degrees F in the afternoon because of radiant heat, making the AC work very hard to go from 80s to 70s inside the house. I think moisture accumulation is not as big a problem for us as other regions.

  6. Anon3 | | #6

    Once you are done, please update us if you run into dust issues or not.

  7. GBA Editor
    Martin Holladay | | #7

    There are several issues here.

    1. A 1/2-inch-deep vent space is insufficient. You need at least 1 inch, although 1 1/2 inch or 2 inches is better.

    2. You need an airtight baffle between the top of your insulation and the vent channel to prevent windwashing from undermining the thermal performance of your mineral wool insulation.

    3. In your climate zone, you need at least R-38 insulation in your roof, not the R-13 insulation that you are about to install.

    I urge you to rethink your plan. There is good advice in this article: How to Build an Insulated Cathedral Ceiling.

  8. lfelicio | | #8

    Martin: Thanks for your advice. I will re-read your article on insulating cathedral ceilings. R13 is all I have room for. If I had known about this website two years ago when I replaced the roof, I would have added insulation to the outside of the roof decking, but I did not know and my contractor didn't either.

  9. GBA Editor
    Martin Holladay | | #9

    There are many ways to provide more room for insulation when your rafters aren't deep enough. For example, you can install new 2x4s, with each new 2x4 installed below (and in the same plane with) each existing rafter, and you can connect the new 2x4 to the rafter with plywood gussets.

    With the technique I'm describing, you can create an insulation space as deep as you need.

  10. Expert Member
    Dana Dorsett | | #10

    In zone 3 the IRC code-minimum is R38 (if the insulation is thermally bridged by rafters or joists), and prescribes a minimum of R5 of that to above the roof deck for unvented roofs.

    The same R5 applies if it's closed cell foam applied to the underside of the roof deck, with the rest as fiber insulation. One could close up the venting and apply 1" of closed cell spray polyurethane foam (~R6) on the underside, then add fiber or open cell foam from below, as long as it is in contact with the low permeance closed cell foam layer.

    Using Martin's gusseted built-up truss approach works great with blown fiber or open cell spray foam, but requires a lot of detailing to get batts to fit without voids. An alternative would be to use the compressed R15s in the 2x4 layers, but add a set of 24" o.c. 2x6s perpendicular to the 2x4 rafters, and install R23 rock wool, between the 2x6. Even with R15s compressed to 2.5-3" in a vented roof (no closed cell foam) the R15s perform at about R12-R13 due to the compression, so with R23 in the 2x6s you'd be at only R35ish, not R38, but since both the 2x4s and 2x6s are thermally broken by the rock wool on the adjacent layers, the thermal bridging is far less than a 2x10 that runs fully through the rock wool layer, and from an overall performance point of view it would meet or exceed code minimum.

  11. lfelicio | | #11

    Martin: I would love to add more depth to the rafters, but I am unsure about the structural soundness of the roof support and whether it could hold the added weight. There are very long pieces of 2x4 that go all the way from the soffit to the roof ridge with mid span supports that look like were added much later perhaps when the roof was sagging. I know this roof has survived 110 years but I am afraid to add load to it. The R13 Roxul would add almost 1,000 lbs already. I am sending you two photos: one looking at a corner towards the soffit and the other looking at the ridge. What do you think?

  12. GBA Editor
    Martin Holladay | | #12

    Q. "What do you think?"

    A. I think you should consult an engineer. Look at the bright side: If the engineer informs you that you need to install new sistered 2x10 rafters, you'll have more room for insulation.

  13. lfelicio | | #13

    You made me smile. I will call an engineer and will cry when I see the price of the fix. Thanks Martin.

  14. Anon3 | | #14

    Radiant barrier + turbine vent is going to be much more cost effective btw. They won't add any weight compare to the alternatives.

  15. Expert Member
    Dana Dorsett | | #15

    You have skip-sheathing (designed for tile, slate, or wood-shingled roof) that has been overlaid with OSB for some other type of roofing. Batts won't be able to snug-up to the OSB with the intervening skip sheathing planks, but blown fiber or spray foam would.

    Sistering-on 2x10s would work, but it's more thermal bridging and would require even more batt-trimming to get a decent fit with batts. An add- on truss approach (use 2x3s for the inside chords, for less thermal bridging) would add a lot of structure to the assembly, probably sufficient to support insulation & interior gypsum (ask the engineers). Ten inches of half pound open cell foam applied directly to the roof deck (installed in 2 lifts of 5") in a truss setup would get you to code min performance, even though it comes up shy on center-cavity R (only R35-R37, not R38). It's quite a bit lighter than rock wool of equivalent R.

  16. Expert Member
    Dana Dorsett | | #16

    Anon3: A turbine vent depressurizes the attic, making it critical to fully air seal the ceiling plane below, as well as the ducts, duct-boots, air handlers, etc, Without meticulous air sealing it could raise cooling season energy use, but with perfect air sealing it can potentially lower cooling energy use. Air sealed it would raise the heating energy use.

    Radiant barrier would add a miniscule amount of cooling season benefit (since it appears the ducts are insulated), but would also add (slightly) to heating season energy use.

    Parts of Zone 3 are slightly heating dominated climate in terms of total HDD/CDD, others slightly cooling dominated, but adding code-min-R at the roof assembly would lower both cooling & heating energy use.

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