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Dense-packed cellulose in attic rafters with Ice and Water Shield on exterior

BuildingNewb | Posted in Energy Efficiency and Durability on

After having an energy audit perform, it was recommended that I have the attic rafters dense packed with cellulose opposed to insulating/air sealing floor in order to bring the HVAC equipment up there into the conditioned space. Also, 3 cathedral ceilings going into the attic made doing the floor too challenging.

The home is located in upstate NY and the attic is currently vented with kraft faced fiberglass batts on floor. The problem is that the entire roof is covered in ice and water shield which is obviously vapor impermeable (asphalt shingles on top of that). The contractor recommended converting the vented attic into an unvented attic and maintaining the openings at the ridge and soffit (no baffles on the underside of roof deck).

The contractor told me that when cellulose is dense packed to the proper density, air and water vapor shouldn’t be able to easily get through it from the interior. In the event any moisture does get into the cellulose, it will disperse throughout the material and find its way to the openings at the soffit or ridge rather than trying to diffuse through the wood which obviously has a vapor impermeable membrane on the other side. He said that the 2×10 rafters would be insulated to roughly R35 (effective r value less due to thermal bridging) and that the roof deck should be kept warm enough to prevent water vapor from condensing on the sheathing.

I just wanted to get some of your opinions on this route. Am I asking for trouble by eliminating venting and only allowing drying to the interior? I’d really like to make the attic conditioned but obviously don’t want to cause any rotting potential.

Am I better off just accepting the air leaks on attic floor and furnace in attic? If I chose not to insulate rafters, I would upgrade flex ducts from R4.2 to R8 and install a 2 stage furnace which would increase efficiency but it obviously can’t be a condensing furnace in an unconditioned space.

I’m sure some of you will recommend using spray foam instead but we are not interested due to potential health concerns. Thanks for your thoughts in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    Building Newb,
    I think that the approach suggested by your contractor is risky.

    Even though you have Ice & Water Shield installed on your entire roof, you can still include ventilation baffles under the roof sheathing to provide ventilation. (Of course, you can only create this type of vented roof assembly if your roof has no valleys or hips. You want a straight shot from the soffits to the ridge.)

    If you can't (or don't want to) install these ventilation baffles, don't use cellulose between your rafters.

    If you have your heart set on an unvented conditioned attic, you have to follow the rules. If you don't want to install ventilation baffles as I described, you have just two other options: rigid foam above the roof sheathing (an approach that requires new roofing) or closed-cell spray foam on the underside of the roof sheathing.

    All of these options are described in this article: Creating a Conditioned Attic.

  2. BuildingNewb | | #3

    I agree that a vented roof assembly would make me sleep better at night. The problem is that with 3 values ceilings, a considerable amount of drywall would have to be removed in order to replace the current Styrofoam baffles with rigid baffles that can withstand the pressure of dense packing.

    The contractor (who has been dense packing roofs for many years in my area) swears that the baffles are unnecessary since the hygroscopic properties of cellulose will bring moisture to the ridge/soffit.

    If I did go with an unvented assembly, would putting a humidifier in the attic solve any potential issues? Currently, with the forced air system we have, relative humidity seldom exceeds 22% unless I put a humidifier on. I'm not sure how much this number will increase once I'm done sealing rim joists, cantilever, and attic.


  3. BuildingNewb | | #4

    It should also be noted that the contractor has been in business in my area for over 30 years and provides a lifetime warranty on the work any any potential damage. They do roofing, siding, and insulation.

  4. GBA Editor
    Martin Holladay | | #5

    Building Newb,
    Every homeowner has a different appetite for risk. Your appetite seems more robust than mine. It's your decision.

    Adding a humidifier raises the risk rather than reducing the risk.

  5. BuildingNewb | | #6

    I would add a dehumidifier rather than a humidifier.

  6. Expert Member
    Dana Dorsett | | #7

    "The contractor (who has been dense packing roofs for many years in my area) swears that the baffles are unnecessary since the hygroscopic properties of cellulose will bring moisture to the ridge/soffit."

    There is some merit to designing in a DIFFUSION vent at the ridge with dense packed cellulose, but that requires replacing a significant amount of area of the sheathing at the ridge with something more vapor permeable than plywood/OSB/plank, not just the typical narrow slot or a series of drilled holes and some roll ridge venting mesh.

    Some discussion of the approach can be found here:

    And a bit more here:

    If going that route replacing the sheathing at the ridge with something fairly vapor permeable at the ridge such as MDF or asphalted fiberboard (top side only) and removing the Ice & Water Shield from that portion would mitigate some of the risk.

    Do NOT add a humidifer, which would only increase the risk.

  7. BuildingNewb | | #8

    I was saying that I could add a dehumidifier in the attic as a safety measure to protect against high humidity. Overall, do you guys think I'm just better off leaving the R30 first erg lass batts I currently have on the floor and paying more attention to air sealing the leaking hvac ducts and upgrading to a 2 stage furnace?

    Dense packing the rafters without ventilation baffles seems to be kinda risky. All of the hvac and ductwork up there makes air sealing the floor near impossible.

  8. Expert Member
    Dana Dorsett | | #9

    "Upstate New York" covers a wide range of climate, from zone 4A to zone 6A, even 7A at altitude in the 'dacks, and location matters if going with an unvented roof.

    You can get there with closed cell spray polyurethane against the roof deck, with batts or cellulose below that, but the fraction of the total R that must be foam to have adequate dew point control at the foam/fiber boundary varies:

    Zone 4A---30%+

    Zone 5A---40%+

    Zone 6A---50%+

    With 2x10 rafters you have 9.25" to work with sooooo...

    In zone 4 with 2" of R6/inch foam on the underside of the roof deck (R12) and 7.25" of cellulose or mid-density fiberglass (R27), for R39 total, and you would have about 31% of the total R as air-impermeably low vapor permeance foam, and it would be fine.

    In zone 5 with 3" of foam (R18) and 6.25" of fiber (R23) you'd have R41 total, with 44% of the R being the foam.

    In zone 6 with 4" of foam (R24) and 5.25" of fiber (R19) you'd have R43 total, 56% of which is foam.

    Closed cell foam runs about a buck a board foot (installed, all-in price) in my neighborhood, so 4" would be $4 per square foot, which gets to be pretty pricey, and it's not all that green, particularly if it's blown with the industry standard HFC245fa, a powerful greenhouse gas. There a few vendors with ~ R7/inch closed cell foam blown with HFO1234ze, which is far more benign (despite being a similar combination of letters & digits :-) ), but it's still quite a bit of polymer per R, so using the least amount necessary for dew point control is generally greener.

    At 4" closed cell foam is pretty vapor tight, but still not a true vapor barrier, and still provides a drying path to the interior for the roof deck. But it's good to check the moisture content of the roof deck in several places before applying more than an inch of closed cell foam.

  9. BuildingNewb | | #10

    I'm in climate zone 5 but do not want to use spray foam due to potential health concerns (I've seen some bad applications but don't want to get off topic).

  10. Expert Member
    Dana Dorsett | | #11

    Without the spray foam it has to be reasonably vented to be moisture safe.

    With a 9.25" cavity depth and standard spacing there is just enough room to accommodate R30HD batts with a 1" code-min vent gap, or R25 mid density batts. Ideally there would be an reasonably air tight baffle on the exterior side of the batt to guarantee that the vent space remains open for the longer term.

    The, with 1.5" or 2" continuous rigid foil faced polyisocyanurate board cap-nailed to the under side of the rafters, seams taped, followed by half-inch sheet rock through-screwed to the rafters with 3" or 3.5" screws (depending on foam thickness) or nailed with 12d ring shank nails it would be R34+ at center cavity (as much or more R than 9.25" of dense-packed cellulose), with at least R9 of thermal break over the rafters further enhancing performance. It's probably cheaper than dense packing too, unless the roof lines are complex or the space it too cramped to work in.

  11. BuildingNewb | | #12

    The space is very difficult to work in and awkward. That's why dense packing from attic space and soffit would be ideal. I just don't know if it's safe to be unvented with ice and water shield on the exterior of the sheathing.

    Maybe it's better to just replace the R4. 2 flex ducts up there with R8 and roll out some unfaced fiberglass perpendicular to the current batts in the floor joists?

    Perhaps the lesser of the evils is just succumbing to the fact that the furnace is in the attic and just insulating the floor where I can.

    The current batts have kraft faced paper on them and are stapled to the joists. Is it safe to tear these to remove the batts in order to air seal what I can or is this more trouble than it's worth? Roll out the new fiberglass over the existing and call it a day?

    I'd love to make some sort of furnace room up there to accommodate a condensing furnace but the layout is just too awkward and would require substantial modification of gas line and location of furnace.

  12. GBA Editor
    Martin Holladay | | #13

    Building Newb,
    If you decide (for now) to leave your attic as a vented attic -- and to simply improve the air sealing and install new ductwork with better duct insulation -- you should resolve to convert your vented attic to an unvented conditioned attic the next time you install new roofing. At that time, you can install exterior rigid foam above the roof sheathing.

    Here is a link to an article with more information on that option: How to Install Rigid Foam On Top of Roof Sheathing.

  13. BuildingNewb | | #14

    Thanks for the input, the unfortunate part is that the roof was recently done. The contractor recommended ice and water shield for the entire roof which I now realize is a bad idea. Nevertheless, foam on the exterior is not an option.

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