Adding attic insulation over rock wool

Discussion in 'Remodel Forum & Blog' started by bcarlson78248, Feb 24, 2013.

  1. bcarlson78248

    bcarlson78248 Member

    Feb 24, 2013
    Alexandria, VA
    My 1940 house only has about 6 inches of rockwool insulation in the attic and I want to improve the situation. The attic is very square and open, with only a few obstructions from vent stacks, and the center section (about 8 feet wide and 20 feet long) is floored. The attic also has only two fairly small gable vents and a few soffit vents. I am located in Alexandria, VA.

    The rockwool insulation is incredibly dusty, so I would prefer to not disturb it. Would it make sense to add air channels underneath the roof to keep a path open to the soffit vents, and then just lay fiberglass batts right on top of the rockwool and the joists? Is there a good way to raise the floored area so that I can also add fiberglass under there also (the floor is about 50% of the attic area)?

    I need to keep a section of the attic floored because I plan to add an attic mounted AC system this spring. It would also be nice to have some extra storage space, but right now the dust from the rockwool makes it almost unusable.


  2. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    If you place fiberglass over the rockwool, make sure it is "unfaced" without a vapor barrier. Trying to raise the floored area would be an exercise in futility
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  4. cacher_chick

    cacher_chick Test, Don't Guess!

    Jan 5, 2008
    Test, Don't Guess!
    Land of Cheese
    Assuming that the rock wool is up to the bottom of the existing floor, it would be ok to leave it. To do it right you will want to pull back the rock wool from any existing ceiling penetrations to air seal them first. A lot of the heat loss takes place out near where the exterior walls and roof come together. The channel vents under the roof deck into the soffits allow you to maintain maximum insulation depth out at that point. Blowing in cellulose insulation would be easier and provide better insulation than adding fiberglass batts over the rock wool.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    I'll second using blown-in cellulose - the whole job would probably take an hour or so after prep. If you buy enough, most stores will loan you the machine for free. You will need someone to keep filling the hopper, though, along with a good dust mask and goggles. The one I had, came with a 100' hose, so it wasn't too hard to snake it through the house into the attic. We kept the machine on the back of the pickup, which made it easier to return as well. We used a cordless phone on intercom to communicate and to pause things, if required, during the task. I did this last spring in my home. It immediately made the house feel more comfortable. After some of the recent snowfalls, my unit (a condo) is the only one with snow still on the roof as much as a week after the others have all melted off. Building a frame to raise the floor can be done - getting the wood and ply up there may be more of a pain. You could tear up the existing planks and build a frame, then reuse the less thing to try to get up in the attic...some strong-tie brackets to hold things in place and good framing should make that sturdy enough - at least as strong as the existing ceiling joists (which may not actually be very strong!).
  6. Dana

    Dana In the trades

    Jan 14, 2009
    x3 on the blown cellulose vs fiberglass batt issue. Even at 1.2-1.5lb open-blow densities the air retardency of cellulose is 10x better than standard density R38s, which keeps it from losing performance to convection between attic air and the entrained air in the fiber layer at the cool temperature extremes. Also, unlike fiberglass cellulose is completely opaque to infra-red radiation, and does a much better job of rejecting the radiated heat under a sun-baked July roof. Fiberglass absorbs the radiated heat at an inch or two of depth, where it's insulated from the convection cooling of the attic air, whereas cellulose absorbs and re-radiates most of the heat at the surface, where it is also cooled to near attic air temps.

    No matter what you use for insulation, take the time to air seal every plumbing, flue, and electrical penetrating of the attic floor before adding to the depth. It can be a real PITA, but it's critical to maintaining thermal performance and avoiding moisture issues. Once you're at 2x the R value the attic will be colder in winter, which makes it more likely to retain air-transported moisture leaking in from the space below. In fact, you may be able to spot the more eggregious air-leak areas by moisture staining of the joists or damp rock wool.

    Even if you have to special order it, use "borate only, sulfate free" variants of cellulose. Those that use sulfated fire retardents stink and are corrosive of metals should they ever get wet, and every roof will leak somewhere eventually. When that happens it's easy to scoop out the wet insulation and replace it with fresh stuff, but if it's the sulfated goods it can even corrode nails it if stays wet for many weeks or even months. (Box store goods are usually sulfated, but the same manufacturers make "stabilized formula" suitable for damp spraying, which are almost universally sulfate-free.)

    Attic mounted air handlers and ducts are universally a bad idea, since it requires making big penetrations in the attic floor/conditioned space boundary, and unless the system is perfectly balanced for supply/returns it drives air-infiltration to something many times the natural infiltration the house would have without an air handler pressurizing/depressurizing rooms relative to one another, making the "great outdoors" the pressure equalizing path for the air to take, which increases the cooling load. If the ducts and air handler aren't well sealed at every seam and joist it makes it even worse! Then there's the issue of conducted gain of the ducts in a 130F attic on a 90F day...

    If you have a basement or crawl space where the AC might be installed, that's a much better choice, since it takes a ton or so off the sizing and it'll run more efficiently. If that's not possible, ductless mini-split technology has none of the infiltration and parasitic gain issues, but it could be substantially more expensive if there are many doored-off areas that need separate zone control.

    If you really MUST put the ducts and air handlers up there, it might be better (but also more expensive) to insulate at the roof deck with spray foam. To in an Alexandria climate it would take 1-2" of closed cell polyurethane directly against the roof deck, with the rest of the R-value in fiber. Damp-sprayed cellulose (not a DIY project, unlike open-blown dry cellulose) would usually stick well enough to the polyurethane to apply it directly at full rafter depth or beyond. But one could also use a blown-in-mesh solution for the fiber layer (either cellulose, or high-density new-school fiberglass blowing wools such as L77, Spider, or Optima recommended.) See:

    Alternatively, 1" of closed cell foam followed by a full fill of open cell foam works, and is sometimes cheaper than blown-in-mesh solutions (local-market driven.) The closed cell foam is essential for controlling the moisture loading & drying rate of the roof deck, but more than 2" is both too vapor-tight, and very UN-green, since it's blown with HFC245fa, and would have a much higher lifecycle greenhouse gas cost than the carbon emissions of the energy use it offsets. Open cell foam has none of those issues, with most products using water, not HFCs as the blowing agent. But open cell foam is fairly vapor-permeable (though air-tight), and there can be long term issues with moisture in the roof deck in your climate with an all open-cell solution.

    With an unvented insulated roof deck you could then build out the floor for more storage area, which would now be inside of conditioned space. That may or may not be enough rationale for the substantial uptick in cost that an unvented attic solution would carry, but it is WAY more energy efficient to have all of the AC located inside of conditioned space. Insulating and conditioning a crawl space is a lot cheaper than an unvented attic solution, since the R-values required are but a fraction of that in an attic. (The 55-60F deep subsoil temp in your area is a lot easier to insulate against than a 130F or 20F attic.)
  7. jefferson17

    jefferson17 New Member

    Nov 12, 2010
    Bristol, PA
    Not sure what you went with but in case you didn't get to it - please make sure to air seal (fire-rated foam only) all penetrations between the attic and heated area below - and get a good air seal w/ foam strips / whatever - the attic access. Even a few small penetrations can cost you 10-20%, if left unsealed. Start with the obvious stuff - like ceiling lights/fans and any places where wires come through. Some of them may only be visible from inside the attic itself.
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