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Thread: Attic insulation

  1. #1
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default Attic insulation

    Right now, I have about 6" of blown-in cellulose with some R-19 fiberglass batts layed over top. I'm toying with blowing in some more cellulose. Will it be worth the effort to move the fiberglass, blow more in, then put the fiberglass back, or just leave the fiberglass there and blow over it. Goal is to get to about R-55/60 or so, so it looks like about another 7-8". Do I have that figured right?

    I have four dormers that regularly get ice accumulations in the winter. Can't get to that roof, but am thinking about removing the ceiling from inside in those areas, spray foaming to seal the joints and stuff foam panels in. These dormers contain the only soffit vents in the roof system, and I'm pretty sure they're all plugged up with blow-in insulation. One of the goals is to clear that path out, ensure the baffles are intact and clear, then insulate as well as I can. Can't do all that much with the 2x6's there, though, so foam (minus the baffle path). I might be able to finagle a layer of foam on the bottom of the rafters, then put drywall back up. What would be the minimum thickness I should use there? I might need to make a step rather than having it go from horizontal to sloped, but I think the additional insulation and stopping the bridging of the wood would help. the goal is to not melt the snow, and eliminate the ice.

    Right now, they installed the insulation with a layer of plastic against the wall/ceiling, then either used batts in the walls or blow-in in the ceiling. To avoid moisture problems, what would be the best stackup of materials and type of foam boards to use in this area? When the roof was redone awhile ago, they put in a full ridge vent and ice and water shield, wrapping the entire dormer and 6' up the roof, so at least now water's not getting dammed up into the walls or attic. But, the size of the icicles can get daunting!

    I'd like to blow stuff into the walls, but I've got faux finish on the inside that would be impossible to match and the outside is vertical shiplap that could be 20' long and hard to remove without breaking. So, I'll probably just play with foaming the rim joist when I get around to it. Iknow I'll get more bang for the buck if I move the fiberglass to the top, but not sure it's worth it.
    Last edited by jadnashua; 02-01-2012 at 10:55 PM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #2
    In the trades Dana's Avatar
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    R19 fiberglass is low-density, and would perform BETTER with the cellulose blown over it, since cellulose is far more air retardent and doesn't lose R-value to convection currents with a cold-side up configuration the way low density fiberglass does, and it's opaque to infra-red, unlike fiberglass. Even if it compresses the 5.575" batts to 5.5", the batt would then perform at a more stable R18 across a wide range of temperatures.

    To meet all of your ice-reducting goals the rigid foam under the rafters, and empty rafter bays, soffit vents cleared you'd need ~R20 on the inteior, but R24 would be better. A layered up 3.5-4" of reclaimed iso from the usual sources works. If you leave at least 1.5" between the roof deck and insulation, and the soffit vents are truly clear, with 2x6 rafters you could put high density R15 "cathedral ceiling" batts between the rafters or cut'n'cobbled 3" rigid foam (any type) and only 2" iso (R12) or XPS (R10) on the interior side of the rafters for similar results. Whether it can tolerate that much loss of headroom is your call, but a 2x6 with interior gypsum and half-inch roof deck is an R6 thermal short, and in this climate you'd need a whole-roof R of at least R20 to have much of an effect.

    Got any pictures of the walls with the plastic + blown/batt?

    [edited to add] to hit R55-R60 with cellulose/fiberglass/cellulose sandwich, the entire depth of the sandwich needs to be initially in the ~18" range, and assume it will settle to ~16-17" over time.
    Last edited by Dana; 02-03-2012 at 03:58 PM.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    I may cut the ceiling off one of the dormers this weekend and see what I have to work with. No snow in the near future, so there's no big rush. I've got window seats in those dormer areas, so a few inches of headroom loss are not a problem - you'd only have a problem if you stood on the seats. I think part of the problem is the windows are fairly large, and there's probably mostly header above them, so there isn't much of any insulation the last 6" before the roof, either. I'll try to remember to take some pictures and let people see what I had and what I did.

    As to the cellulose, I've seen some comments about the two most common fire retardent materials used: ammonium sulfate or borates (think I got that right). Both the big box stores carry the same brand, and the version they have is based on the ammonium sulfate (although they do make it with the borate as well, supposedly costs more, thus not a big seller (not stocked) at the big box stores). Is it worth it searching out one that uses borates?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Kamco is a New England distributor for National Fiber, a cellulose vendor that only uses borates, but only have one NH location. ANY vendors' wet-spray "stabilized formula" will be sulfate-free/borate only, and works just fine when dry-blown. You'll probably have to buy it through distributors geared toward contractors, not DIY home centers, but cash is cash- they will sell to DIYers, but they aren't in the business of advising & directing the way the retail box stores are, and don't usually have rental blowers, etc. The difference in material cost is truly "in the noise" at the wholesale or retail price to the contractor/DIYer, but it's measurable in the manufacturer's margin. (In my experience Nat'l Fiber's Cel-Pak dry blow through distributors is usually price-competitive with box-store brand-X.)

    If there are generous subsidies available for insulation (as there is in most of MA) , for an open-blow it's often cheaper to have a pro do it, with the final cost being less than the retail cost of the material to a DIYer.

    The primary issue with sulfates is corrosiveness to metals (particularly copper, but zinc galvanizing & steel/iron too) if it ever gets wet. In some sense the stink of the sulfated goods when wet can be a leak detector/warning, but without the presence an alkaline substance to react with it's not so immediate or pungent- just vaguely pissy. An easy test for sulfates is few pellets of Drano dissolved in a cup, mixing in a heaping tablespoon of the cellulose- the strong smell of the outgassing ammonia makes it obvious. Sulfate fire retardents are banned for this application in some countries (notably Australia)

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