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Community and Q&A

Cellulose density in rafters

Matt_Michaud | Posted in Green Building Techniques on

In a retrofit project we are looking at insulating 24″ deep Larsen-esque rafter cavities with cellulose. This will be vented with baffles. I have been doing some research to choose a target density without putting blowing in more than necessary. It is a very low slope (2/12) so that should work in our favor and will be blown behind Intello air barrier paper. In “How to Install Cellulose” comment 35, Bill Hulstrunk advised for similarly deep rafters, 3.8pcf if it is unvented or loose fill if it is vented. Why the difference? If National Fiber were still around, I would ask Bill directly, but maybe this is obvious to someone else! It seems that at ~2.3pcf loose fill stops settling, so that would be great savings compared to 3.8pcf…I just don’t understand why its different. Anyone have experience or examples of loose filled rafters?

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  1. GBA Editor
    Martin Holladay | | #1

    Bill Hulstrunk tells builders that it is OK to dense-pack rafter cavities with cellulose, even when the rafter bays are unvented. This is controversial advice, and the topic has been debated on several GBA pages. Suffice it to say that building codes don't allow Hulstrunk's method. If you want to install cellulose between rafters, the code says that you need either (a) a ventilation channel between the top of the cellulose layer and the underside of the roof sheathing, or (b) an adequately thick layer of rigid foam above the roof sheathing.

    Hulstrunk's advice on insulating unvented rafter bays with cellulose would be even more risky if the cellulose weren't dense-packed. It is the dense packing which saves these roofs from quick failure. (The dense packing limits air movement, which helps protect the roof.)

    If you have a ventilation channel, then (a) the assembly meets code requirements, and (b) the assembly is at less risk for moisture accumulation than it would be without the vent channel, and (c) dense packing isn't required (and may even be difficult).

  2. Matt_Michaud | | #2

    Martin, thanks for the response. That makes sense. So, with ventilation, are there any guidelines for density requirements? I would think the steeper and deeper, the higher density recommended as it becomes more similar to a double stud wall. The lower the slope, the closer it becomes to an attic, but its not as if I can go in the rafters later and add a little more if it settles.

  3. GBA Editor
    Martin Holladay | | #3

    The final density of your cellulose installation will depend on the strength of your ventilation baffles, the attachment details on the Intello membrane, and the skill of your installer.

    Most dense-pack installations use InsulWeb membrane, not Intello membrane. I've never seen a dense-pack job with Intello, but I assume you would need close fastener spacing or furring strips to keep the Intello membrane in place.

    The denser the insulation, the better. You can't use polystyrene ventilation baffles -- you'll need something sturdier, as Hulstrunk noted when I interviewed him.

  4. GBA Editor
    Patrick Mccombe | | #4

    I have seen dense pack jobs with Intello and the material is so air impermeable you need to provide relief holes so the air can escape as the cavities fill. Insulweb is far more air permeable. Chris Corson uses Intello to contain the insulation layer in his panelized Ecocor Passive houses.

  5. Expert Member
    Michael Maines | | #5

    Although I have been involved with projects that have unvented dense-packed cavities, and they are probably safe if the dense-pack is done well and there is a well-detailed variable permeance air barrier on the interior such as Intello, like Martin I don't recommend it because there are too many things that can go wrong. If you choose this route, another reason why it's important to properly dense-pack the cavity is that cellulose acts like a sponge, pulling moisture away from the roof sheathing, but because it is treated (ideally with a 100% borate additive) it won't develop mold or fungus when damp.

    I have seen dense-pack behind Intello; Jon Riley, an insulator in Maine who recently wrote this article (and consulted on my basics of cellulose article,, and like me has consulted extensively with Bill Hulstruck) recommends leaving out a strip and replacing it with Insulweb, to get the appropriate air evacuation, and double-blowing the cavity--first filling the cavity with a big hose, then going back with a small, rigid hose to make sure everything is tight. In a large cavity you need to blow more densely than in a small cavity, up to 4.25 lbs/ft³. Insulweb is cheap, though, so in most cases it's probably easier to blow behind Insulweb and then come back with Intello if your assembly calls for it.

    The only rafter vent I know of that can hold up to that pressure is Brentwood Industries' Accuvent. I spec their 1 1/2" vent, rather than their standard 1" vent, for better air flow. I've heard from some installers that even Accuvent can crush under heavy pressure, but others swear by it. I have not taken apart a completed assembly to see for myself what happens. It's pretty stiff so I imagine there is some crushing (another reason to start with 1 1/2") but not complete failure. The other option is to build your own vent chute, or to over-vent the roof, on the outside of the roof sheathing. Over-venting is my go-to with this type of assembly when it will work--it's easiest with metal roofing.

    Both Hulstrunk and Riley say that with a deep cavity it's important to break the cavity into "cells," by running Insulweb to create two 12-inch cavities, or even more, smaller cavities, in order to get a proper dense-pack.

    I imagine what Hulstrunk meant by loose-blown ventilation is that when possible, the least expensive and most forgiving assembly is a ventilated attic with loose-blown cellulose on the attic floor.

  6. Matt_Michaud | | #6

    Michael, thanks for the response. I was never considering unvented, only comparing the advice Bill gave in the article I referenced (comment 35) regarding density. Your concern over a baffle handling as much as 4.25pcf demonstrates my point - I haven't come across examples of installations less than 3.5pcf in rafter bays despite Bill's recommendation. He is clearly talking about a procedure for loose filling baffled rafters, not a vented attic. Sure, "denser is better", but if its not changing R/in significantly and there is already a taped AB, it just seems like a waste of money. In a vented assembly, shouldn't it just be "a little more than the lowest density that won't settle, is better"?

    Baffles will be made on site with PIC and the vertical insulweb separating truss cavities should provide the path for air to escape (up until the last cavity). It shouldn't be any more air resistance than blowing behind drywall...

  7. Expert Member
    Michael Maines | | #7

    Matthew, I see where you read "loose fill", at the end of Bill's comment. If you go back up in Martin's article and read the section under, "Q. Does the same technique work with 12-inch-thick double-stud walls?", I believe that Bill means to loose-fill the bays first and then come back with a smaller wand to dense-pack. I have an old fact sheet that Bill wrote for National Fiber, about installing cellulose in blind cavities in new construction, which is all about the fine points of achieving a consistent 3.5 lbs/ft³. But maybe in a vented roof situation the assumption is that if the cellulose won't settle as easily and it's less important for the insulation to remain in contact with the vent baffle, so a looser fill is ok. Personally, having seen a lot of new and old cellulose installations, I would never do anything but a dense-pack in a compact roof assembly.

  8. Matt_Michaud | | #8

    Michael, I think you are probably right that the loose fill was justified by the lower settling risk and the unnecessary vent contact. Vent contact may be more important for me since I don't want the PIC's R-value to be degraded by I'll just have to make my own risk/value assessment. Thanks.

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