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Venting material for 22 1/2 inch dense pack cellulose in roof

Corkydork | Posted in General Questions on

Hey there, I’m looking for some sort of material to staple to the underside of parallel chord trusses for a single pitch 12/3 roof. The 22.5″ of dense pack cellulose will be blown in from below (after installing tongue and groove ceiling) but needs to be stopped by this breathable fabric/material. Pressure it needs to withstand is 3.5 psi. The material will also need to prevent wind from blowing cellulose up in to the 1.5″ of venting space. Any suggestions?


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  1. creativedestruction | | #1

    What's your climate zone? Are you looking for suggestions on the baffle material to maintain the 1.5" top gap, or the airtight membrane material for the bottom directly below insulation and above your T&G? Assuming you mean 3.5 pcf (pounds per cubic foot), common target density for self-supporting cellulose, you need some durable stuff for both.

    1. Corkydork | | #7

      Hey there, thanks for the reply. Zone 4. actually I meant psi for the installation per contractors specs. It will be 25 psf from below which will be held up by tongue and groove. Would be great to have suggestions for the material to maintain the gap, but also the air barrier as well.

  2. Expert Member
    Dana Dorsett | | #2

    Half inch MDF or asphalted fiberboard would work, if supported by a spacer between the roof deck & baffle mid-way between the top chords (assuming 24" o.c. spacing of the trusses.)

    All flexible fabric solutions that would have too much pillowing into the vent channel under dense-packing pressures.

    1. maine_tyler | | #3

      Dana, what might be some reasons to choose mdf or asphalted fiber board over something like osb or plywood?

      1. Corkydork | | #10

        Hey Tyler, the point would be to create a cavity to which air can pass. Roof will be sheathed with osb above air space

      2. Expert Member
        Dana Dorsett | | #21

        >"Dana, what might be some reasons to choose mdf or asphalted fiber board over something like osb or plywood?"

        Both MDF asphalted fiberboard more than an order or magnitude more vapor permeable than OSB or plywood, and more moisture tolerant too. Asphalted fiberboard can even be left exposed to the rain on construction sites without suffering damage.

        For an insulation chute, asphalted one side, with the asphalt facing the vent cavity is fine. It runs about 20 perms, give or take, compared to 1 perm for half inch OSB (when dry) , 1.5- 2 perms for CDX (when dry.)

    2. creativedestruction | | #6

      If you're to use mid-bay spacers to support baffles as Dana suggests, I would think Solitex Mento Plus could work. Not cheap, but definitely vapor open and rated for that density at spans under 20" if some pillowing is acceptable.

    3. Corkydork | | #11

      Hey Dana, you’re saying these rigid materials would definitely pillow? How much? 1/2” would be ok. Hm, with the time and effort of the spacers maybe it’s better to build it backward, tongue and groove first then loose fill from above. As I mentioned in another comment I wanted to take more time to do the ceiling myself and have the roofers/framers put up the roof first. I’ll have to consider this. Also, is the asphalted fiberboard and mdf allow air to pass? This would be ideal. Thank you!

      1. Expert Member
        Dana Dorsett | | #22

        >"’re saying these rigid materials would definitely pillow? How much?"

        Rigidity is relative- fiberboard & MDF are nowhere near as rigid as OSB or CDX. The mount of pillowing depends on the spacing, the thickness of the material (3/4" MDF or fiberboard pillows less than half-inch), and the installation pressure. Those who have tried dense packing thick double studwalls sheathed with 3/4" fiberboard found out the hard way that it's impossible to keep a flat exterior wall, and may even lose some of it's structural capacity as some of the fasteners pop under dense packing pressure.. (The fastening schedules for fiberboard sheathing are much tighter than for OSB/CDX, even at a lower specified shear capacity.)

        At 12" o.c. support spacing the pillowing would be inconsequential. At 24" o.c. it's risky, almost doomed to closing the spacing down to under the 1" code requirement.

  3. Expert Member

    Rigid baffles are going to entail a lot of labour cutting and installing, all done from a ladder or scaffold - and the materials equate to the same area as the sheathing above. I wonder if it wouldn't make more sense to move the material up above the trusses, then strap over the top chords with 2"x3"s, then sheath? That would mean choosing a material you could walk on temporarily, like 3/8" plywood, or staging the roof differently th0ugh.

    1. Corkydork | | #9

      Hey Malcom, then the space in between the trusses, which is also structural, isn’t being filled with anything. Maybe I misunderstood

    2. carsonb | | #14

      Malcolm, this is the same question I struggle with, as my builder said this would be less labor. I believe the comments in my previous QA question mentioned that this would be too impermeable, especially if it was 3/8 OSB. Is that not the case? I feel like this is a common question that 5 different experts on here would offer 5 different answers.

      1. Expert Member
        MALCOLM TAYLOR | | #16


        I agree. Right now there is no settled best approach. My suggestion of a second layer of sheathing only makes some sense when compared to using similar materials as baffles. Let's me lay out the lay out the options I can think of:

        - Baffles made from low-perm sheet goods. These take a lot of time and materials to install, and rely on the trusses or rafters to allow moisture to reach the vented cavity by diffusion.
        - Permeable baffles. Same as above, but easier diffusion of moisture.
        - A second layer of sheathing above the trusses. Quicker, fits with the sequencing better, but means all moisture has to diffuse through the lower sheathing before reaching the vent channel.
        - A layer of house-wrap or other permeable membrane over the trusses, followed by strapping the top chords and then sheathing. Great from a moisture perspective, but hard to build, and might pillow if dense-packed on steep roofs.
        - Forgo the cellulose and use batts. No baffles necessary, but a greater depth needed to compensate for voids.

        1. carsonb | | #17

          Since the advice, aside from Martin who advocated for a 6 sided air barrier, appears to be to not air seal the baffles, would it then make sense if using the sheathing over the trusses method to just cut holes in the resulting sheathing? Maybe patch them with housewrap if needed?

          1. Expert Member
            MALCOLM TAYLOR | | #18


            It that first layer of sheathing is retaining the cellulose, any holes will definitely need to be patched.
            Since the amount of diffusion through the sheathing will be a function of the area the sheathing vs the area of the holes (covered in house-wrap), you would need some pretty big holes to appreciably change the permeance of that layer.

            I'm afraid I might have helped you wander down some real rabbit-holes discussing this in the abstract. Probably Akos' suggestion to use cardboard or house-wrap baffles makes more sense.

          2. carsonb | | #19

            cardboard of house-wrap baffles would not work for dense packed cellulose. So I guess then it would need to be plywood/osb baffles per Akos' suggestion, but this was discouraged by the builder for labor costs vs just sheathing over the whole thing. I'm still not sure if a sufficiently thin sheathing layer above the rafters instead of baffles inset inside the rafters would be acceptable. If you were then putting 2x4s and another sheathing layer for the roofing, could the bottom layer be taped MDF as a more permeable layer to hold in the air and cellulose?

          3. Expert Member
            MALCOLM TAYLOR | | #24


            I wouldn't want to build that roof unless whatever you use for the lower layer could be walked on, which limits the choices somewhat.

            Akos suggested the cardboard baffles because he didn't see the advantage of dense-packing your roof. Have you thought about loose-fill? it would eliminate the problem.

          4. carsonb | | #25

            Malcolm, it’s a 12/12 cathedral roof. So I don’t think loose fill is an option. I had a thread on this earlier, but don’t have a consensus on what works yet. Would putting cdx/osb over the trusses be OK or too impermeable as Dana suggested?

          5. Expert Member
            MALCOLM TAYLOR | | #26

            Sorry - I got your's and Corkydork's roofs mixed up. I thought out was 3/12.

            No, I don't think a layer of plywood is too impermeable. Venting above the sheathing is not as effective as venting below, but in a well air-sealed roof, I don't think the difference is huge.

            If your contractor is happier skinning the roof twice, that's probably the way to go. Cutting and installing plywood air-baffles over an entire roof is something that works for owner/builders with plenty of time, or some custom contractors. If I was building your place and was given the choice, I'd much rather fly though another layer of sheathing than mess round for days on the baffles.

          6. carsonb | | #27

            Thanks Malcolm, yes that’s what my contractor preferred and honestly I agree with him for the reasons you outlined. Labor is the main cost driver and laying sheathing is fast. I’m surprised all the GBA/FHB articles don’t mention this as a good option. Further, it seems many people on here caution against it.

          7. Expert Member
            Akos | | #28


            The reason you don't hear more about this is that it isn't a robust roof assembly.

            If you look at the layers, it is pretty much the equivalent of a double stud wall with a rain screen used as a roof. The vent gap is a bit bigger, but that is about it.

            Folks have done testing on these walls, they generally work BUT there is accumulation of moisture in the first layer of sheathing (which does dry up but there none the less).

            When you take the same assembly and use at as a roof, you'll get even more air leaks becasue of stack effect, but you do get more sun, so there is more drying potential.

            So, it can work, BUT it needs to be well detailed and built. On the other hand a standard vented roof will work even with sloppy details, so it is significantly a more robust assembly.

          8. Expert Member
            MALCOLM TAYLOR | | #29


            Anything cathedral roof over 4/12 here gets batts. The rafters or trusses get strapped with 2"x4"s at 24"oc and then sheathed. The only baffles are at the exterior walls.

          9. Jon_R | | #31

            An above-sheathing vent can outperform under-sheathing. It's about more drying than wetting. So in a cold climate, make the interior side dry perms less than 1/5 the wet perms into the vent. And air seal really well (preferably interior and exterior sides).

            Do consider fiberboard roof substrate (with batts) for low cost and high exterior perms.

            Maybe not practical, but wet sprayed cellulose with glue won't create pressure and won't settle. Can even be sprayed from below.

        2. Expert Member
          Akos | | #34


          My response was specifically for the question of dense packed top vented roof. An above deck vent can work but need attention to details, so not a robust assembly.

          When it comes to cathedral celings, I'm with you, always batts. Much easier.

          1. carsonb | | #35

            Batts are easier to install, or less prone to issues than dense pack cellulose? Also, what if you put rigid foam above the first layer of sheathing? At that point I believe the venting is only there to help prevent ice damns.

          2. Expert Member
            Akos | | #36

            To dense pack, you need to create a solid enough structure to handle the pressure and contain the fibers. This is the detail that makes dense packing roofs, especially for new build, too expensive. It is also more expensive to install.

            With high density batts, you can skip the vent channel completely provided the installer takes a bit of care. It also stays put, so you don't need your ceiling up (or need a redundant OSB ceiling) before the insulation goes in. Make sequencing the build straight forward.

          3. Expert Member
            MALCOLM TAYLOR | | #37


            This thread is all over the place. We are all talking about different assemblies on different pitched roofs, interspersed with speculating about venting in general. Hopefully someone gets something useful from it.

          4. Expert Member
            MALCOLM TAYLOR | | #38


            If you put rigid foam over the sheathing there isn't need for a second layer. You have an unvented roof.

          5. carsonb | | #39

            Malcolm, that article states putting vent channels above exterior foam in two places. Though it says it is not necessary. My understanding is that this would help prevent ice dams, as snow itself has an r value and without the vent channels can actually insulate enough to melt snow. Is this correct? Or a complete waste of money?

          6. Expert Member
            MALCOLM TAYLOR | | #43


            An adequately thick layer of exterior foam will need furring above it to secure the sheathing. You end up with a ventilated channel, but don't need the first layer of sheathing.

  4. Expert Member
    Akos | | #5

    12/3 roof does not need to be dense packed. You are spending a lot of money on extra insulation and install cost that is not buying much at all.

    I would install cardboard or housewrap baffles and loose fill the cavity. The amount of settling you'll get with this and the resulting insulation value loss is noise when it comes to 22" of cellulose. The extra cost of dense packing will NEVER pay in energy savings.

    You'll also need a proper air barrier above your T&G ceiling. This could be one of the many membranes but a thin layer of OSB with the seams taped is probably the simplest/cheapest.

    1. Corkydork | | #8

      Hi Akos, I agree. The issue is that I am paying a roofer/framer and will do the tongue and groove myself. So I’d have to put a hold on the project as we loose fill from above while I finish the tongue and groove, all without a roof over it. Can be done but I’d have to change the staging. Also I haven’t ordered trusses so yeah I could make it 17-20” instead.

      1. Expert Member
        Akos | | #12

        There are many many ways of skinning that cat. Probably the simplest is to frame up a mini attic near the ridge (2' to 3') tall or have the trusses built with a mini attic.

        After trusses and roof deck is installed, install vent baffles followed by the OSB under the trusses. You can than loose fill the whole thing through from the mini attic. You can access the mini attic either from the gable ends or a regular attic hatch.

        As long as you over fill the mini attic, the extra insulation will also help to keep the cathedraled section from settling.

        You can't just use T&G as the ceiling, there needs to be an air barrier.

        1. Corkydork | | #13

          Hey Akos, thank you for your well thought out reply. This is a single pitch 12/3 roof, no attic space, not a cathedral ceiling.

          1. Expert Member
            Akos | | #15

            With mono slope, it is even easier. You can fill the roof from the eaves. Just leave the blocking out till the insulation is blown in. Depending on the length of the roof, you might need to blow in from both ends to properly fill each cavity.

  5. maine_tyler | | #20

    I didn't realize that plywood was considered to have too low a vapor permanence by many. (The answer to why mdf / fiberboard vs ply I suppose).

    So the idea is we're relying on the truss/rafter dimensional wood for moisture transport?

    Is there significant evidence that ply is insufficient at transport in this scenario, or is this one of those things people have merely developed a feeling about based on the numbers?

    1. carsonb | | #23

      Tyler, here is Dana’s previous reply to my question.
      I’m still confused on many points. He states for instance that the ijoist webbing would be able to wick moisture into the vent channel, but that OSB sheathing over the trusses is too impermeable. But ijoist webbing is OSB. I don’t know what a good assembly with above truss venting would look like.

      “ >"My confusion here is how the vent channel above the top sheathing dries out the dew accumulating on the underside of the sheathing."

      It doesn't, at least not rapidly enough for your climate zone. A layer of dry OSB (such as ZIP) is a Class-II vapor retarder- less than 1 perm. It would be classified as a "vapour barrier" under Canadian code. Even when the moisture content is high enough to grow mold in the OSB it's still well under 10 perms.

      Venting UNDER the roof deck (per code) puts a layer of dry outdoor air between the insulation and roof deck, and cellulose is VERY vapor permeable. A thin layer of exterior side air barrier between the cellulose and vent channel is fine as long as the conditioned space side of the assembly is low permeance (< 1 perm). The web of an I-joist would wick and distribute enough moisture from the insulation side into the vent side, even if the air barrier/chute is vapor impermeable.

      1. maine_tyler | | #30

        Thanks Carson.
        I think part of that has to do with the orientation of the OSB, meaning it can transport moisture (and I believe heat as well) more readily along the axis of the web than through the thickness of the sheet. Don't quote me on that, but that is my take on it.

        It definitely sounds like Dana is suggesting the drying potential through the rafters/trusses/i-joists is significant and necessary if the 'baffles' are low perm: including OSB and Ply. I did not realize ply was considered too low perm for this.

        I was mostly curious if there are many documented cases of issues with over-roof venting using something like ply as the first sheathing layer. Trouble is, there are always many variables. Having a super air-tight and low-perm barrier on the conditioned side definitely seems like a key if going with this approach.

        As Akos says, it's essentially a fat double stud wall—in concept—placed on slope for use as a roof. At the least, it would seem sensible to use ply vs OSB in this scenario. Or go full hog and use a much more vapor open product, that likely won't be walk-able (the concern Malcolm raises).

        1. carsonb | | #40

          I don’t know. I could believe osb could absorb more moisture vertically similar to wood fibers, but even if it was 10x, the surface area is decreased by over 1000x. Without some sort of study this is purely speculation though.

          1. maine_tyler | | #41

            I see your point regarding surface area, though not sure how you came up with 1000x. Seems it'd be more like 50-60x. (24/(7/16))

          2. carsonb | | #44

            I didn’t, I should have just said “more”:). Yes your 50-60x seems about right depending on oc spacing and web thickness.

  6. burninate | | #32

    Is this bulletproof?

    If you by T&G you mean decorative beadboard or something that has to run perpendicular to the slope of the roof, like I'm imagining in my head...


    1) Sheath with T&G 3/4" plywood (which is reasonably vapor-permeable, especially wet). Tape. Needs to be thick-ish to allow rooftop walkability over 24" trusses.

    2) Screw on 2x3's through to top chord. This is your eave to ridge vent. Add pest screen to top and bottom of run. Openings must be clear and *well away* from rain, so cantilever the 2x3's past the edge of the ply a good bit. Upgrade from 2x3's to 4x4's for structural strength if snow or wind loading require it, or if you want to make this cantilever especially large, or if you're having issues getting gutters attached.

    3) Sheath with roof style ZIP sheathing over 2x3's. Tape.

    4) Apply desired cladding over that.

    Interior, afterwards:

    1) Staple on insulweb netting to underside of bottom chord.

    2) Screw on 2x3's into underside of bottom cord, perpendicular (gable to gable), through netting, 12" OC. Add 2x3 blocking to same layer, in contact with remaining bottom chord in parallel direction ("fire block" / "Mouse block"). Now you have a grid of 12"x24" rectangles.

    3) Have all-borate cellulose blown into the truss cavity from below, using the standard insulweb holes & seals.

    3) 1/2" OSB screwed into 2x3's and taped, or airtight drywall. Your highly durable air barrier & vapor retarder. Up to you whether you use full-size sheets or chop them up for easier maneuvering as long as you tape them meticulously afterward, their role as a structural membrane is minor because of all the strapping and they have plenty of edge to grab onto since the strapping is 3 wide instead of 2 wide.

    5) More 2x3's below *that*, perpendicular to the first set (eave to ridge), using extra long screws to dig all the way into the bottom chord. Now you have a ceiling service cavity, inside your air barrier.

    6) Run wiring & electrical/junction boxes needed for low profile ceiling lights & fans, in between these boards. Add pest screen at top and bottom to prevent this becoming mouse run.

    7) T&G interior finish cladding element at your leisure. Before completion, attach wall and ceiling air barriers as sequencing requires.

    8) Revisit this a year down the line, boroscope into the upper rimboard to see if there's been settling, and if so top it up through holes in rimboard. With 3:12 and a full >20" of cellulose packed as densely as practical this shouldn't be a huge deal, but packing big open spaces is hard, doubly so if there are obstacles like truss webbing.

  7. Expert Member
    MALCOLM TAYLOR | | #33


    There are too many labour intensive layers that don't add much.

    1. burninate | | #42

      #33: It feels like running between-truss baffles, or custom-cutting things around the truss webbing, is potentially worse than sticking to sheet goods, even it it requires extra layers of sheet goods. And it fulfills the sequencing requirement of fast dry-in and indefinite time to complete the T&G. And it gives a great air barrier, which is important in a cathedral ceiling. And it gives roof venting to prevent ice damming, which is important in a cathedral ceiling.

      You could leave out the under-roof decking in exterior step 1, but you'd need to put in insulweb and purlins down to hold the cellulose before the over-roof deck, and have the roofers step very carefully while lugging around those sheets, and you'd require meticulous rodent control, since they can dig right into the insulweb. You're also looking at significant issues if you ever have a bulk water leak, because the cellulose will soak it all up and wick it around until it weighs tons, instead of allowing it to roll downhill.

      You might leave out the spacers / straps between the insulweb and the separate interior air barrier in interior step 2, but it sounds more difficult to dense-pack a huge, cavernous space that's ~airtight on all sides vs one that's not airtight, and then add the labor of cutting and filling holes in the air barrier (or rim board) effectively. You can't put the air barrier on after blowing without causing some wavyness from the big 24" wide insulweb hammock holding the cellulose in; Maybe that's fine if it's not gonna be exposed or & you add spacers below, but it will leave you with some small gaps between sheets.

      Maybe intrusions are less of an issue in a roof than in a wall, due to winter vapor drive being balanced out by extreme summertime roof temperatures? This would vary with the cladding, though - if you do away with the venting and use impermeable roof cladding, how does the moisture get out?

      You could skip the lower run of spacers in interior step 5, but are you going to have space for lighting in there?

      In a warm climate, I would probably leave out the whole vented over-roof and apply ZIP directly to the top chord. Is this warm enough? Since we have one massive body of absorbent cellulose, the consequences of undetected bulk water intrusion that we can't even witness directly (because cathedral ceiling) without tearing the roof apart seem dire.

      1. Expert Member
        MALCOLM TAYLOR | | #45

        1. You don't need 3/4" plywood . 7/16" is walkable, and weighs about half as much. 1/2" meets code without needing H-clips.
        2, 3,4. You are back to a vent channel over the sheathing, with all the disadvantages we have discussed above - but I'll give you that one.

        2,3. If you are using insulnet, why the strapping? And why the incredibly time consuming blocking? If you want to strap, why not use 1"x3"s at 16" oc? That's what is typically done by all East Coast builders.
        4. If you are sheathing the underside of the trusses, what advantage do you get from also having 1,2 and 3? Tape it and be done.
        5. These shallow service cavities that only eliminate a few penetrations for ceiling boxes make little sense. It's also unclear whether they meet code requirements for cover for the wires.

        From your second post: "if you do away with the venting and use impermeable roof cladding, how does the moisture get out?"
        If you do away with the venting, you build an unvented assembly, with all the attendant detailing necessary. That's a completely different animal that anything we have been discussing.

        "In a warm climate, I would probably leave out the whole vented over-roof and apply ZIP directly to the top chord."
        You can only do that do that with a roof insulated with cellulose by providing ventilation under the sheathing. I don't see where the vent channel is located makes much difference to how a roof will deal with bulk water intrusions.

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