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Community and Q&A

Cellulose insulation up against roof sheathing and in soffit in Mass.

Cam13 | Posted in Energy Efficiency and Durability on

Hello, I am a plain old homeowner with minimal building experience and not much money or time (5 kids) . I live in an old house (1890) and we pay a lot in energy bills and our hot air furnace is constantly running.

A contractor for the “state energy saver program” came out and began adding more insulation in the attic, the guy said he wanted to take down the styrofoam attic baffles that I had put up the night before, and dense pack the eaves with cellulose.  I told him I don’t think that cellulose is supposed to up against the roof sheething like that or fill up the soffit. He got irritated at me not trusting his experience, then offered to loose pack it but it still seems like he’d just be filling up the soffits.  I said I wanted to plug up toward the bottom of the eaves.  He pointed out that the roof was not really vented in the soffits and had plenty of venting in the gables and towards the top of the attic (which is true). I had just read form articles on here that fiberglass and cellulose is not supposed to be in contact with roof sheething when possible. Then, tired to the back and forth he said fine we’ll cut it from the contract.

So now I have an attic floor that is very well insulated (but hard to navigate because of the 16″ cellulose and 1″ by 8″  underneath).   But I have no insulation in the eaves which is roughly 1/2 of some of the 2nd floor walls.   And no insulation in the  vertical walls because they could not insulate because its an old house with double lathe walls and only a 2.5″ space in the walls (the house would not fit).

Was I right to ask him not to spray cellulose in the eaves if he wasn’t trying to keep it out of the soffit or use the baffles to keep it from the roof sheathing? I may have extended the life of the roof for 6 months but I’m afraid my energy bills will stay sky high. I had wanted to stuff a small piece of fiberglass batt down to block the cellulose from the soffit and have him spray on top of it but he was not interested in waiting.

What should be my next step for trying to insulate the eaves/bays? I was thinking about trying to stuff r13 mineral wool in each of the sloped bays because there is roughly 3.5″ of space under the attic baffles.  It will be a nightmare getting to each one with the cellulose over the attic joists but I need to do something but want to make sure it’s the right thing.

Any advice is helpful. Attached is a quick diagram that I sketched but I got tired of the software.  Thank you.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Cam.

    By "eaves" do you mean the part of your ceiling that slopes with the roofline?

    1. Cam13 | | #4

      Yes Brian, the part of the ceiling that slopes with the roofline. I'm trying to insulate those slopes. Thanks

      1. Expert Member
        Dana Dorsett | | #6

        And there is only 2.5" of space between the roof deck & plaster/lath sloped ceiling?

        1. Cam13 | | #7

          Hi Dana, The roof rafters are 2x6s. Without baffles along the roof sheathing its a little over 5.5 inches of space to the plastered sloped ceiling, and with the baffles (which are mostly in place) its roughly 3.5 inches of space because the baffles give 2 inches of space for air flow. Thanks.

          1. Expert Member
            Dana Dorsett | | #15

            Without both soffit & ridge venting the 2" vent gap isn't going to do a whole lot for protecting the roof deck, and 3.5" of insulation isn't much. It's safer/better to go with an unvented approach.

            To meet the IRC code-prescriptive ratios in Chapter 7 of the IRC for an unvented roof in zone 5 (all of MA is zone 5), at least 40% of the total R needs to be an air-impermeable (closed cell spray polyurethane works). So 2" of closed cell (R12-R14) up against the roof deck, and 3.5" of fluffy stuff ( R12-R15) would have plenty of margin.

            An alternative approach that's still protective of the roof deck, if skirting the edges of the code a bit would be 1" of closed cell foam (R6-R7) and 4.5" of cellulose (R17-ish), with a "smart" vapor retarder on the interior side, such as 2-mil nylon (eg Certainteed MemBrain.) The 1" of closed cell foam is sufficiently vapor retardent to protect the roof deck, while sufficiently vapor open for the roof deck to dry seasonally toward the interior. But the foam/cellulose boundary will still dwell below the indoor air's wintertime dew point long enough to accumulate some moisture.

            Cellulose can buffer a substantial amount of wintertime moisture without damage or loss of function, but installing a smart vapor retarder reduces the moisture uptake, since it's quite vapor retardent when the proximate air is dry (which it will be in winter), yet becomes more vapor open than standard interior latex paint when the moisture content is high enough to support mold.

            Rock wool batts or high density fiberglass will work too, at a slightly higher R, but also with a slightly higher mold risk, due to dramatically lower moisture buffering capacity.

            If going that route, PLEASE try use only HFO blown closed cell foam, which is substantially greener (and slightly higher R) than the more common HFC blown foams. It's somewhat more expensive (about $1.35 per square foot per inch of depth v.s. $1.00- $1.20 per square foot per inch). DIY kits are all HFC blown goods.

            As a sanity check on the relative safety of the roof deck with only 1" of cc foam + fiber, read this document:


            If you want to cut to the chase, see Table 3, particularly the column titled: 1" ccSPF + spray fiberglass".

            Note that in all climate zones that is enough to protect the roof deck as long as the roofing material isn't a light "cool roof" type of roofing. The interior side smart vapor retarder adds "belt & suspenders" redundancy protection to the roof deck, as does the moisture buffering capacity of cellulose. Note also in the column titled "cellulose", cellulose alone is not sufficient protection for the roof deck north of zone 3, at least not on the north facing pitches that were modeled in that study.

  2. T Carlson | | #2

    Those are difficult areas to retrofit. A lot of times I fill those areas but use a piece of fiberglass if it has a large soffit area to keep the soffit clear. Since loose cellulose settles my theory is the cellulose will lay down and create a pathway between the roof deck and cellulose although eventually some areas will windwash so it might need to be touched up every so often. But usually I do it with proper vents in place and back off after they start to deform to get a pathway plus fiber as dense as possible.

    The Wx program I used to be a contractor for called for dense packing those types of situations whether it was your style you illustrated or a knee wall attic situation, Ive also done that and haven't had any issues with homeowners calling me.

    1. Cam13 | | #5

      Thanks, T Carlson,
      So I'm hearing from you to stuff a piece of fiberglass towards the bottom, but above the soffit to keep the soffit clear, then blow cellulose as dense as possible down the slope but back off once the attick baffles (vents) start to to deform. That's what I was leaning towards especially since I put all the work at putting the baffles (roof sheething vents) in. The contractor seemed to be in a rush did not want to stuff a "stopper" in nor wait for me to do so.

      So using cellulose in those slopes is much preferable to fiberglase or mineral wool cost wise? r value wise? ease wise or all three?

      Then in your last paragraph you said before as a contractor you used to dense pack everything including and, although it doesn't seem right, hadn't had any issues with home owners complaining. Which would mean I messed up by not just letting the man dense pack everything including the soffits. Thanks for replying.

      1. T Carlson | | #9

        You wont get a true dense pack against a piece of fiberglass, you'll blow it out. You either fill the soffit which Ive done on the <6” unvented soffits common on capes or like I said fill it dense but not true dense pack dense, or you can fill a plug of cellulose in an onion bag and then dense pack. My preferred method of filling slightly under dense pack would be with no baffles since the cellulose will settle down and gap off the sheathing creating its airspace, especially when you dont have much depth. Ive confirmed this technique going back to some installs.

        You don’t have a lot of depth with the 2x6, so do you maintain an airspace with a little bit of insulation or do you just fill it? The Wx program I was involved with said fill it and I would tend to agree with them and by doing so I never had any moisture issue callbacks, your roofing will be fine assuming it has been done correctly.

        I think fiberglass is a terrible product unless it is contained in a 6 sided sealed box, cellulose is great in attic applications but I wont use it in walls because the density required is too great for an efficient install. You can easily blow drywall or plaster off studs trying to get a dense pack. Thats why I use blown in fiberglass in walls and I seal my wall cavities as best I can, the lower density is faster and easier to install. Worked very little with mineral wool so no comment.

        Kind of random, hope it helps you out. End of the day don't overthink it, 99% chance you wont rot your house out with any of your options.

      2. T Carlson | | #10

        Im still a general contractor btw, and I still operate an insulation division within my company, I just quit the Wx program for other reasons.

        1. Cam13 | | #11

          Thanks, If possible I'm going with another energy saver contractor, (for other reasons) and I'll take their advice on it and be ok if they lean towards just dense packing everything. One thing that my house has going for it is that the warm air near those slopes is being heated by a very dry furnace so while there will be vapor and condensation, not as much as if the baseboard baseboard and radiator heat. But your right I'm over thinking it if mold/rot ever becomes an issue I'll deal with it then.

          If I can't get another discount program out out I'll try do it it my self (with a friend) with cellulose but pack it as tight as I can while leaving a little air space. The onion sounds very clever... It will but tough being that I'm a rookie at least I'll have tried.

          Thanks for all of the wisdom.

          1. T Carlson | | #13

            For mold and rot you need an active moisture source, so you would need a roof leak(s) or out of control indoor humidity. Thermally, you may get some condensation in some spots under certain conditions maybe on the roofing nails but so do most houses, same as siding nails, people just don't realize it because they don't go up in their attics to look or rip off a piece of drywall in their living room and inspect the sheathing. And it dries out on its own and you'll never know the difference and all is well.
            You may get some more ice damming by having insulation in direct contact with the roof sheathing, but a durable roofing job and clearing the roof of snow will take care of that.

  3. BFW577 | | #3

    I have a similar issue. My house had cellulose blown in the attic but it was poorly done. Many of the rafters are missing cellulose especially near the soffits in the rear. I have been contemplating blowing more in completely to the soffits. My soffits have no vents just the 2 gable vents. Here is a rough sketch showing the outside and inside. The blue lines inside are were there is little to no cellulose. Using my thermal imager there is serious heat loss in this area.

    I am hesitant to blow more in and completely fill the rafters and soffit as I have read about moisture. Though I get full sun and have solar panels on the roof so I think moisture wouldnt be an issue.

    1. Cam13 | | #8

      Thanks let me know what you do. That IS a long distance of a roof line to figure out, I can see why it would be tough to insulate the slopes but not the soffits, because it would be tough to stuff something down at the end to block cellulose from falling into the soffit.

    2. T Carlson | | #12

      Your attic looks pretty tight. On houses like yours I have accessed those cavities from outside and fed a dense pack hose up the cavity. Sometimes the little strip of soffit plywood gives enough room, or we drill through the fascia and then clad it with aluminum when we are done, or a couple times Ive removed the top piece of siding and drilled at an angle through a bit of the top plate to get in, then replace the siding.

      1. BFW577 | | #14

        Here are some more pictures. I can acess the rafter cavities from the small attic to the right in the first picture. 2nd one is looking down one of the rafters. There is a gap to the roof sheathing but at the bottom near the soffit there is poor coverage. Some areas have no cellulose at all. Its seems the person tried to just blow it in without putting the hose down the rafter cavity.

        Can I just use a long pole to push the cellulose down into that area near the soffit and blow more in as needed? I would try and leave a 1 or 2 inch gap.

        1. T Carlson | | #16

          Thats not going to consistently work to push cellulose with a stick.
          I would just have someone fill it using a dense pack tube. Short of some major surgery its a better option than leaving it empty.

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