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Deep blown in cellulose for attic – questions

Lance Peters | Posted in Energy Efficiency and Durability on

I have a few questions regarding deep (beyond R60) blown in cellulose for an attic. Cellulose is relatively cheap and I’d like to maximise my attic insulation without causing any other issues:

1. Several manufacturers of cellulose insulation have depth/R-value charts that stop at R60. Is there any practical reason for this, or is it just because most people don’t bother going above R60?

2. Does compression above a certain depth negatively affect the performance of the insulation in any way? I.E. does your R-value/$ go down appreciably beyond a certain depth?

3. Assuming 24″ truss/rafter spacing, what practical limit on depth is there if you want to avoid bowing of the ceiling drywall with both 1/2″ and 5/8′ panels? At what point would it make sense to sheet with OSB before installing the drywall?

4. If sheeting with OSB before drywall, would you put your vapor barrier on the top side, or between the drywall and OSB?

5. If installing a loft floor in the attic trusses for storage (not conditioned living space), how much space should be left between the top of the cellulose and the floor sheet? Is a space necessary?

Thanks for your thoughts.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Lance,
    Have you seen this article yet? How to Install Cellulose Insulation.

    Read the article. It will answer some of your questions.

    -- Martin Holladay

  2. GBA Editor
    Martin Holladay | | #2

    Lance,
    Q. "Several manufacturers of cellulose insulation have depth/R-value charts that stop at R-60. Is there any practical reason for this? Or is it just because most people don't bother going above R-60?"

    A. There are diminishing returns beyond R-60. But if you want to install R-100 of cellulose, you can. Some people do it.

    Q. "Does compression above a certain depth negatively affect the performance of the insulation in any way? I.E. does your R-value/$ go down appreciably beyond a certain depth?"

    A. The R-value per inch doesn't go down appreciably. The R-value per dollar invested doesn't go down appreciably. But the dollars saved on your annual energy bill per dollar invested in insulation goes way down.

    Q. "Assuming 24" truss/rafter spacing, what practical limit on depth is there if you want to avoid bowing of the ceiling drywall with both 1/2" and 5/8' panels? At what point would it make sense to sheet with OSB before installing the drywall?"

    A. In the article I linked to, I quote Bill Hulstrunk on this point. Bill said, "We have never seen a sagging issue due to the weight of the cellulose installed above a [drywall] ceiling. That may be because some of the weight of the cellulose is being redistributed onto the ceiling joists. We have blown very high R-values, up to R-100, and never had any issues with the ceiling sagging."

    Q. "If sheeting with OSB before drywall, would you put your vapor barrier on the top side, or between the drywall and OSB?"

    A. You don't need a vapor barrier. Building codes only require a vapor retarder in this location. Most builders use vapor-retarder paint (primer).

    Q. "If installing a loft floor in the attic trusses for storage (not conditioned living space), how much space should be left between the top of the cellulose and the floor sheet?"

    A. As much or as little as you want.

    Q. "Is a space necessary?"

    A. No.

    -- Martin Holladay

  3. Lance Peters | | #3

    Yes, I had read that article, which was very informative. I actually found that article while searching for cellulose depth information.

    During that search I also turned up several instances of people claiming that their high R-value cellulose ceilings had signs of drywall sagging, hence my questions. I believe our code requires 5/8 fire rated gypsum but I'm not 100% sure on that. It may have to do with truss spacing, but at 24" I'm likely going to need 5/8, I'd assume anyway.

    Regarding vapor barrier vs vapor retarder, my plan is to use Membrain throughout the house, including the attic ceiling. Is this a bad idea? I have little confidence in using drywall to seal the entire attic space. It would seem to me that, even with meticulous detailing, the interface between interior walls and the gypsum would end up compromised over time and result in air leakage. I'd be much more comfortable with a continuous sheet of vapor barrier.

    Question, is Membrain a vapor barrier or retarder, or does the definition change based on its humidity-relative permeance? I'm very happy to find out that it has been approved for use in Canada, and is apparently planned for the 2018 version of our building code.

  4. GBA Editor
    Martin Holladay | | #4

    Lance,
    Q. "Regarding vapor barrier vs vapor retarder, my plan is to use Membrain throughout the house, including the attic ceiling. Is this a bad idea?"

    A. No. MemBrain will work.

    Q. "I have little confidence in using drywall to seal the entire attic space. It would seem to me that, even with meticulous detailing, the interface between interior walls and the gypsum would end up compromised over time and result in air leakage. I'd be much more comfortable with a continuous sheet of vapor barrier."

    A. To create an air barrier at the ceiling level with MemBrain, you'll have to install the MemBrain before the partitions are raised. That's tricky. Raising the partitions is likely to rip the MemBrain. With drywall, all you have to do is go up in the attic after the drywalling is complete, and seal the cracks between the drywall and the partition top plates from above, using caulk or canned spray foam.

    If all you are worried about is vapor diffusion (rather than air leakage), you can be sloppy with your installation of the MemBrain (or sloppy with the application of your vapor retarder paint). Even if 5% of your ceiling lacks a vapor retarder, the vapor retarder will be 95% effective.

    Q. "Is Membrain a vapor barrier or retarder?"

    A. It's a vapor retarder with variable vapor permeance.

    Q. "Does the definition change based on its humidity-relative permeance?"

    A. The definition doesn't change. The permeance of the MemBrain changes.

    -- Martin Holladay

  5. Lance Peters | | #5

    Martin, thanks again for clearing up those points.

    Regarding raising partition walls and damaging the Membrain, I had this vision for how the ceiling might go together, top to bottom:

    1. Roof trusses, 24" OC

    2. Membrain vapor retarder, continuous application using flexible sealant along truss bottoms to seal fastener/staple perforations

    3. OSB, continuous application

    With the OSB installed I can raise my partitions without fear of damage to my vapor retarder, and it should prove more than capable of supporting any cellulose load I desire. Once the walls are up:

    4. Strapping, 16" OC perpendicular to trusses, creates a service cavity deep enough for wiring and octagon boxes for ceiling lighting

    5. 1/2" gypsum (5/8" if required for fire rating).

    Are there any perceived issues with this approach?

    I would really like to focus on airtightness for this build; a well supported, continuously sealed sheet of Membrain with the fewest attic perforations possible, none being the goal, should be a big benefit. I'm planning no bathroom fans (HRV exhausts), a through-wall exit for the 2nd floor drier vent, through wall plumbing vents if possible, as well as 2nd floor attic access through the soffit area where the garage roof meets the 2nd floor roof. I may need a few wiring penetrations, but hopefully that will be it.

    My thoughts are to keep penetrations running horizontal for two reasons: to keep "stack effect" from increasing leakage, and to keep detailing of penetrations in the walls where they're easier to access and do properly, as well as keep them accessible should they need service in the future.

    I'm also planning 9-foot ceilings on the second floor with the main reason being that I can drop the ceiling height where necessary for plumbing and ductwork; no need for 9 foot ceilings in bathrooms, closets and the laundry room, where much of the plumbing and ductwork will be needed.

    I'm hoping if I approach the build with airtightness in mind the details will come together much better and with less work as construction progresses. Of course, any insight from seasoned builders is welcomed!

  6. GBA Editor
    Martin Holladay | | #6

    Lance,
    You are making your life more complicated than it has to be. If you want to install OSB, that's OK. But OSB is a perfectly good vapor retarder, so there is no need for the MemBrain. (OSB, like MemBrain, is a "smart" vapor retarder with variable permeance.)

    If you tape the seams of the OSB, you'll have an air barrier as well -- an air barrier that is far more durable than MemBrain, which is fragile and easily damaged.

    A few Passivhaus builders have noticed that some brands of OSB leak air, but those cases are rare. Most brands of OSB are airtight.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    What he said- OSB is more vapor tight than MemBrain (at any humidity level), and is a perfectly adequate interior side vapor retarder. OSB doesn't break 5 perms until the proximate RH is averaging ~75% (which is fine) whereas MemBrain hits 5 perms @ ~55% RH (which is also fine.)

    The 5 perms is only a relevant "order of magnitude" number, since that's roughly the vapor permeance of standard interior latex paint on gypsum board. Once the variable vapor retarder is much more vapor open than 5 perms it doesn't change the drying rate by very much, if there is latex paint between the vapor retarder and the interior.

    https://naturalspacesdomes.com/dome_store/dome_insulation_systems/images/Membrain3.jpg

    https://www.greenbuildingadvisor.com/sites/default/files/bsi-038-mind-the-gap-permeance-of-plywood-and-osb.jpg

    Make sure it's detailed as an air barrier, and you're good to go.

  8. Jon Harrod | | #8

    Lance,

    We've worked with a couple passive house builders who use carefully taped Zip sheathing as their attic air barrier, keeping all the wiring and partition walls beneath the attic plane as you've described. It seems to work well. I agree that the Membrain is not necessary.

    For several of these projects we've blown attics to R-90+. It's not cost-effective, but there's no downside other than the cost. One thing we learned is that the coverage charts (bags of material vs. insulation depth) aren't totally linear. When you start piling in 24" of material, it starts compacting under its own weight, so you may need to budget for some extra material to get to your target depth.

  9. Lance Peters | | #9

    Martin/Dana, after reading a few articles such as this one regarding the "airtightness" of OSB sheathing, I'm a little hesitant to depend on it as an air barrier:

    https://www.greenbuildingadvisor.com/blogs/dept/musings/osb-airtight

    If you think about how OSB is made, it's a completely random process where controlling the permeability of the product is likely a futile effort... I would bet that's why nobody is willing to assign a permeance specification to their OSB product. If they could, they surely would as it would be a tremendous marketing advantage in today's construction market. Especially a company like Weyerhaeuser who would be interested in clearing their name!

    Per Jon's comment, a coated OSB product like ZIP might work well for an attic application. Speaking with a code official at the local city office, he said I would have to give a permeance spec for any material I wanted to use as an air barrier.

    Does anyone know off hand if ZIP sheathing (or similar) is available in Canada? I did a quick search and didn't come up with anything other than an article from last year suggesting that Advantech products may start being manufactured in Canada (Quebec) in the near future through a joint venture with a local company.

  10. Lance Peters | | #10

    Jon, thanks for your comments. I suppose I could get some more accurate depth/R-value information directly from the cellulose supplier as well.

    Thanks for the ZIP attic barrier suggestion.

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