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Should I fill up unvented soffits with cellulose or block them off?

progmac | Posted in Green Building Techniques on

Hi all, I have a c. 1915 house with two gable vents. Until about a month ago, the attic had no access. I installed an access and have been busting my butt getting rid of the knob and tube wiring so that I can insulate. My ultimate goal is 18″ of insulation, r-60 i believe. My climate zone is the zone four, but one county to the north is zone five.

Now, to my questions…

(1) my two-story house has an approximate 10″ soffit that is not vented. Can I just fill the soffit areas with cellulose as I fill up the attic, or should I create a dam from the top plate of the exterior walls to the roof deck with rigid board or some other material so that the cellulose does not spill into the soffit?

(2) the rear part of my house has a shed-type roof. the vertical area between the attic “floor” (the top of the plaster ceiling below) and the roof deck gradually decreases from about 36″ down to less than 6″ by the time you get to the exterior wall top plate. From there it opens into the soffit, which I describe above. Am I okay to jam-pack this area with cellulose, from the attic floor to the roof deck? Or should I be removing all of the dirty rock wool insulation and doing something different until I get to the area with 18″ clear heigh?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I just fill the soffit areas with cellulose as I fill up the attic, or should I create a dam from the top plate of the exterior walls to the roof deck with rigid board or some other material so that the cellulose does not spill into the soffit?"

    A. Either way will work. If you choose the latter, the carpenter who has to replace your soffit in 40 years will be grateful.

    Q. A question about the cramped attic where the access height "gradually decreases from about 36 inches down to less than 6 inches by the time you get to the exterior wall top plate."

    A. This type of attic, and various approaches to insulating this type of attic, are discussed in the following article: Insulating Low-Slope Residential Roofs.

  2. progmac | | #2

    Martin, thank you so much for the reply! The link you posted was very helpful. A quick question...understanding that I have never blown an ounce of cellulose so I am not familiar with how it installs...

    if I decide to dense pack a portion of my attic where I have less than about 18" clearance, would I need to install some type of blocking on the open side, so that the cellulose has something on this third side to dense pack against?

  3. GBA Editor
    Martin Holladay | | #3

    First of all, it's helpful to have a powerful commercial-sized insulation blower -- not the DIY type of blower you see at tool rental outlets -- to dense pack cellulose. For more information, see:

    How to Install Cellulose Insulation

    Borrowing a Cellulose Blower From a Big Box Store (Read the comments on this page as well.)

    If the have the right equipment, the insulation can be dense-packed fairly well on the low side of the attic, but of course the density will decrease as the insulation makes the transition to the side of the attic with more head-room.

  4. progmac | | #4

    Great! My preference would be to do all of the air-sealing and prep myself and then pay someone with equipment and capacity to come in and do the actual blowing. I just want to make sure I set them up for success.

  5. Expert Member
    Dana Dorsett | | #5

    I'm not convinced there is sufficient additional benefit here to dense packing the thin-wedges a the edges to warrant the additional cost in a zone 4/5 climate. The primary benefit to dense packing is reduced air retardency- the actual R-value of cellulose peaks at about 2.6-2.8 lbs, which doesn't take a 2-stage blower by any means. If you install some blowing mesh (or landscape fabric) at the boundary of where the wedge gets too thin and give it a 4" slit for inserting a 3" hose then just blowing until the 1-stage rent-a-blower stalls you'd be "good enough".

    The other benefit of dense-packing it to prevent settling over time, which is function of seasonal moisture cycling of the material. Here again, in a zone 4B climate 2.8lbs of any cellulose would be more than sufficient, but in zone 4A you can still do fine if you use "stabilized formula" cellulose, which has moisture activated adhesives. Stabilized formula would be the preferred material for the low-density open blow area as well, but would usually have to be special ordered if going through a box store. Price-wise it's not a big uptick, and has the benefit of having only borate fire retardents, with no corrosive-when-wet sulfated fire retardents.

  6. mackstann | | #6

    I'm so grateful there is at least one place on the web with level-headed advice. Most people will incessantly preach about how important soffit venting is.

    I had my attic air sealed and insulated. The utility rebate program required that attic venting be improved to modern code, but my contractor was able to get a waiver on that. I specifically wanted them to fill the little wedge area at the eaves with insulation, since every precious inch is valuable there. The attic is well air sealed so I'm not worried about moisture problems. I do expect the cellulose to settle and some space to open up between the top of the insulation and the roof sheathing. I plan to top it off someday.

  7. Expert Member
    Dana Dorsett | | #7

    The moisture cycling of the material isn't (or at least shouldn't be) from air leakage at the ceiling below- it's from the vented outdoor air. In zone 4A the summertime outdoor dewpoints are well into the 60s F, with excursions well into the 70s, whereas in zone 4B they tend to stay in the 40s or 50s, well below the temperature of the conditioned space ceiling. The comparatively close temps at the bottom of the cellulose relative to the dew point of the vented air space above in most 4A climates means the moisture content of the cellulose will be quite a bit higher than in most 4B climates.

    The dimensional changes in the fiber between the humid summertime accumulation and the drier cold season causes mechanical creepage leading to settling over time, unless the density/pressure of the cellulose is high enough to prevent it. This has been studied and modeled fairly well (with the modeling well correlated to field tested assemblies) by researchers at Aalborg University in Denmark.

  8. progmac | | #8

    Thanks again all. FWIW, I am in zone 4a. The big box store nearby carries the borate-treated cellulose.

  9. GBA Editor
    Martin Holladay | | #9

    I answered a question about whether cellulose would cause any problems in the soffit. I never answered any question about attic venting.

    Attic venting is sometimes useful, especially in regions where ice dams occur. Attic venting is a complicated topic; if you want to learn more on this topic, see All About Attic Venting.

    Concerning the question about insulating low-slope residential roofs, I referred you to my article on the topic. In that article, I referred to the method you seem to be considering as "a controversial approach."

  10. Expert Member
    Dana Dorsett | | #10

    Adam: All cellulose products in the US have some amount of borate as an anti-mold agent, which is also fire retardent.

    But most of the box store goods also contain (cheaper) sulfate fire retardents rather than larger quantities of borates to meet the fire retardency specs. If you're not sure, dissolve a pellet or two of Drano (sodium hydroxide) in cup or two of water, and stir in a couple tablespoons of the cellulose insulation. Your nose should tell you pretty quickly whether it contains ammonium sulfate, as the resulting chemical reaction releases ammonia gas. (Rinse out your wife's coffee cup thoroughly after conducting this test, independently of the result. :-) )

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