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Community and Q&A

Insulating attic to be finished

Striker9 | Posted in General Questions on

I have an unfinished attic that I’m working to finish and condition.

-True 6in roof rafters
-Soffit vents and ridge vent
-Roof will need to be replaced within 5 years
-Zone 5A (Ohio)
-Head-room is limiting the rafter depth I have to work with

I plan on installing 2in Sika Rmax Pro (R13 and foil faced) between the rafters and leaving a 1in air gap between the bottom of the roof decking and the foam board. I plan on having the reflective side face outward towards the roof decking.

I then plan on having 3in of closed-cell spray foam installed to bring it flush with the edge of the rafters. 

-Should the reflective side face the roof decking?

-Is it okay to have the reflective side acting as a vapor barrier and then have closed-cell spray foam underneath it which also acts as a vapor barrier?

-All closed-cell contractors I’ve spoken to (5 well known within our area) will not spray more than 3in even though that only gives ~R21. They state that this is fine since it’s much more efficient. Adding the 2″in R13 foam-board will get me up to ~R33. Do people commonly run in to this problem with spray foam companies? How do these companies get around not meeting code?

-It seems mortgage companies in the UK are raising concerns and even refusing houses that have had spray foam installed due to rotting rafters and other issues. Is this not a concern in the US or is it starting to become one?

-I have also seen concerns about romex wiring becoming degraded by spray foam. My wiring is already in place and was assuming that the spray foam would just go over it. Do I need to avoid having the wiring encased by the foam?

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  1. walta100 | | #1

    Consider the unthinkable, blow out the current roof and adding a full second floor.

    The way I see it the half story homes tend to be horrible energy monsters.

    There is never enough room for code mininmum insulation and proper venting.

    The space always feels cramped and dark with low ceiling limiting the usefulness of most of the floor area and a few windows in dormers.

    Yes it will cost more but you will get much more useable well insulated square feet.


  2. Striker9 | | #2

    This is a 3rd story attic with about 300 sq. ft. of floor space, not counting space behind the knee walls. Raising the roof isn't an option but laying down insulation on top of the roof deck when the roof is replaced may be an option, if I can find any roofers around here who will do that. I may be able to add a thin layer of insulation between the rafters and drywall to reduce thermal bridging but don't know enough about how to do that yet.

  3. Expert Member
    BILL WICHERS | | #3

    Yes, the reflective side should face the air gap. This gives you a radiant barrier, usually considered to add about R1 to the assembly. If you're using foil faced polyiso anyway, you get that extra R1 for free, so you might as well take advantage of it :-)

    The foil facer can be a vapor barrier, that's fine. Closed cell spray foam can also be a vapor barrer. Putting closed cell spray foam against the polyiso so that there are two vapor barriers is also fine, since spray foam is fully adhered, meaning, in this case, there is no place for vapor to get "trapped" between those two materials. The problem with the vapor barrier is that you're putting it in the wrong place though. In a vented roof assembly in a heating dominated climate, you want the vapor barrier on the INTERIOR, the WARM side of the assembly, NOT the outer cold side. I would use 1/2" polyiso or 1/4" waferboard to build your vent channels here, use batts in the rafter bays, then put the 2" polyiso on the interior UNDER the rafters. This would get you a warm-side vapor barrier (the foil facing), R13 of continous insulation to help with thermal bridging (the 2" polyiso), batts for extra R value (about R23 for mineral wool batts), and possibly another R2.7 or so if you use 1/2" polyiso instead of 1/4" waferboard. This means a full assembly R value of around R36-R39 or so. With the continous insulation, if you go for U value, you might actually be OK, although I'd still consider it a bit light on overall R value.

    The "spray foam is more efficient so it needs less R value" is spray foam industry BS. R value is R value, period. While it IS true that spray foam gets you a good air seal, and less air leakage is better for energy efficiency, R21 is still only R21. R21 with good air sealing will outperform R21 worth of something leaky like fiberglass batts, but you can airseal and get those R21 batts to perform the same. Building codes don't accept that "spray foam is better" argument, either. If you go with spray foam, make them install whatever R value you need, don't buy into the "I need less R value because it's spray foam" stuff.

    The mortgage issue in the UK is apparently a thing, at least according to a recent poster here on GBA. I'm not aware of that being an issue in the US, and many builders are using spray foam as an added cost "upgrade", so a lot of "regular" builds use it. I think the issue in the UK is that they have have had issues with the wrong kind of spray foam (open cell) used in roof spaces, which has been known to result in moisture retention issues. Why they chose to deal with this as a mortgage issue and not a building code issue is strange to me, but it is what it is, apparently. In the US, there is no such issue -- spray foam does not detract from the value of your project, many people even think it's an upgrade. Note that I am not one of those people though. If you want to do a vented roof assembly, you don't need spray foam. If you want to maximize ceiling clearance, then you might need to do an UNvented assembly, in which case closed cell spray foam is your best (and really, only) option. I would try to build a vented assembly unless you have no other choice.


    1. Striker9 | | #5

      Would the .5in polyiso used for the vent channels need to be unfaced? I haven't been able to find unfaced polyiso, only XPS/NGX.

      I'd also need to compress 5.5in mineral wool to fit in the remaining 4.5in rafter bay space, any problems there? This probably also means adding additional blocking in the middle of the vents to avoid compressing the .5foam board against the roof deck.

      1. paulmagnuscalabro | | #6

        I believe all polyiso is faced, unless it's cut from larger pieces (ie, cutting out polyiso from between facing layers). Not sure on the details, but I think the facing acts as a substrate onto which the polyiso is applied.

        1. Striker9 | | #7

          From what I've found so far I would agree. Seems like XPS is the way to go to avoid sandwiching the mineral wool between two vapor barriers and allow for drying outward toward the roof.

      2. Expert Member
        Akos | | #8

        If there is no code or anything like that you need to meet, the simplest is to install the batts carefully to form the vent space. High density batts, MW or FG, don't suffer much from wind washing so the baffles aren't doing all that much.

        You can add a bit of extra depth by stapling 1x2 to the rafters or 3/4" polyiso strips and use long drywall screws. This would you a reasonable vent space and you would not have to squish batts. If you are using foam strips, you have to be careful at the butt joints as the foam can be squished. If you have only a butt joint or two, I would skip the foam there and use 3/4" ply.

        If your rafter spacing is random like it seems to be with older houses, a simpler way to insulate these is to get 24" batts for steel stud walls. These are actual 24" wide, you can turn them sideways and trim to the width of the rafter bay. Much easier than trying to cut off half an inch or an inch off the 4' side.

        With this type of build, much more important than venting is getting an air tight ceiling. This should be the main focus of your efforts. An air tight ceiling needs very little venting and no amount of venting will get you out of trouble if there are big air leaks.

  4. Deleted | | #4


  5. Striker9 | | #9

    Zone 5A
    Rafters are true 6in and are variably spaced between 16in and 18in apart
    -Vapor barrier should be closest to living area and I don't want to sandwich insulation between two vapor barriers
    -Using polyiso for the vent baffles doesn't seem like an option due to its low water permeance. I'm also having a hard time finding unfaced .5in EPS
    -Rafter spacing is random so with 15in batts I'll need to cut strips to fill the space or buy 24in and cut squares.
    -I don't want to lose more than 2 or 3 inches of head room to make this viable

    Here are the options I have so far.
    Option 1:
    1in vent gap
    2in foil faced polyiso (R13)
    3in closed cell spray foam (R21)
    Total R: R34

    Option 2:
    1in vent gap
    .5in XPS (R3)
    5.25in mineral wool compressed to 4.5in (~R23)
    2in polyiso attached under rafters with foil facing towards room (R13)
    Total R 39

    Option 3:
    1in vent gap
    5.5in mineral wool pushed in but leave a 1in gap for venting. (R23)
    2in polyiso attached under rafters with foil facing towards room (R13)
    Total R36

    Option 4:
    1in vent gap
    .5in XPS (R3)
    5.5in unfaced fiberglass (R20)
    2in polyiso attached under rafters with foil facing towards room (R13)
    Total R36

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