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Community and Q&A

Cellulose retrofit

robert79 | Posted in Green Products and Materials on

I recently acquired a late 40’s vintage brick veneer house located in southwestern Pennsylvania. The house is a two story 30′ x 22′ rectangle topped with a hip roof. Like virtually every other house in this area, it has no overhangs other than oversized half-round gutters. Having said that, the bricks all seem to be in very good condition, and there are no signs of water damage on the interior walls or ceilings. Additionally, there is no insulation in the exterior walls. The attic is insulated with a combination of old rock wool batts and blown fiberglass; however, I’m in the process of removing this material and replacing it with sixteen inches of cellulose (after careful air sealing of all penetrations).

Now for my question. I would like to insulate the exterior walls and have talked with a number of insulation contractors about the best way to accomplish this. Basically, I’ve been given two options (both involving dense packed cellulose). The first being to drill 1 1/2″ holes through the rock lathe and plaster through which the material will be blown. This interior approach would also involve drilling a number of holes in the first floor ceiling in order to insulate the second story rim joist. This approach seems straight forward but messy. The second approach involves drilling a number of 3/4″ holes through the mortar joints and the underlying sheathing boards (two holes per stud bay on both floors, and additional holes for the second story rim joist). I’ve researched this second option, and apparently it is quite common in Canada. My concern with it is that it will leave a number of holes in the felt paper that covers the sheathing boards and perhaps make them more prone to rotting. Does anyone have any experience with this method, and do you think my concerns about the penetrations in the felt paper are warranted?

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  1. Expert Member
    Dana Dorsett | | #1

    What (if any) sheathing is there on the exterior side of the studs, and is there an air-gap between the sheathing and the brick?

    In general there needs to be an air gap /cavity from the brick to the next layer as a capillary break, and blowing from the exterior creates a likliehood of some cellulose getting into the cavity, which would be a capillary bridge drawing water from the brick to the rest of the assembly.

    If the sheathing is in good shape and the cavity is free of crud, you can go ahead and blow cellulose into the cavities from the interior. It's not as messy as you might think. Competent installers will usually drill the interior walls with a hole saw to make it a clean & easy repair. A plug of blue-board glued into the hole with at least 1/4" between the surface of the blue-board and the finish plaster is enough, and you can use a fast-setting patch such as plaster of paris for the repair rather than a lime plaster. In some cases it's possible to carefully pull a kick board or other trim and drill where it'll be covered.

  2. GBA Editor
    Martin Holladay | | #2

    I have never heard of the technique you describe (blowing cellulose through 3/4-inch-diameter holes). Anyone else?

    I'm guessing that a different type of insulation -- something other than cellulose -- is blown through those 3/4-inch holes.

  3. Dana1 | | #3

    There are cone shaped nozzles for cellulose blowers that are about 3/4" I.D. which works with a 1" -1.25" hole. But you can't dense-pack with a 0.75-1" O.D. tube since it would be clogging constantly. Using multiple holes per stud bay and a high air ratio on the blower you can fill the stud bays blowing directly with the nozzle but it won't be dense packed, probably ~2.7-2.8 lbs density best-case. But it would also fill the masonry cavity, which is not a good idea. Maybe with very deep roof overhangs to limit direct wetting of the brick you could get away with having the cellulose in contact with the brick, but I'm a bit skeptical.

  4. robert79 | | #4

    Dana Dorsett,

    The exterior side of the stud walls are sheathed with "1 x6" boards nailed on the diagonal and covered with a layer of felt paper. There is an approximately 1 1/4" air gap between the bricks and the sheathing. The lack of overhangs make it more or less impossible to visually inspect the cavity for crud. Additionally, there are no weep holes or vents in the brick work (I do plan to add some if I go forward with the insulation).

    One of the insulation contractors I've spoken with says he uses a "dustless" rig for drilling the holes in the interior. It's basically a right angle drill fitted with a dust collection shroud that is attached to a large commercial vacuum.


    I had never heard of blowing cellulose through the mortar joints either, but it is apparently becoming quite common.

    Basically the contractor would use a rotary hammer drill with a coring bit to drill holes at the T-intersections in the brick work. Another drill and bit is then used to pierce the sheathing, Next, a flexible 3/4" hose is then inserted into the cavity an pushed up against the top or bottom plate. The material is then packed to the half way point before the hose is pushed against the opposing plate and rest of the cavity is filled. The holes in the mortar are then filled with a premixed mortar mix and the joints repointed.

    The contractor I spoke with said that the process takes approximately three to four times longer than is required to do the work from the interior via of a larger openings. However, he said the homeowner doesn't need to remove or cover all of their furnishings. It's also a viable option if the interior wall surfaces are fragile or covered with something like lead paint.

    Apparently it's not just cellulose that is being installed this way. Johns Manville has a pdf file detailing a similar process for their Spider fiberglass product.

    D. Dorsett,

    The contractor I spoke with guarantees he can hit an average density of at least 3.25 pounds per cubic foot (measured by the amount of material being pushed into the cavity). I'm skeptical of this claim, but have no real way to know if my skepticism is warranted.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for sharing details of this technique. I can't comment on its effectiveness.

  6. Expert Member
    Dana Dorsett | | #6

    This is definitely one to do from the interior if you can, which is pretty straigthtforward with plaster & lath. It's hard enough to get a 1" I.D. dense packing hose to not turn back on itself in a cavity, let alone something that has to be less than 1/2" I.D. Taking only 3-4x as long to for installation is probably optimistic , and even if an average density over 3lbs might be achieved, it's probably well under 3lbs in the corners. (The only way to tell for sure would be to take cores and weigh them.)

    It'll be quicker, better, and probably even cheaper to do it from the interior, including the patch & paint time/expense. Most installers are cognizant of the cleanliness factor when installing from the interior, but get references from recent clients if you can.

  7. robert79 | | #7

    Thanks to everyone who has taken the time to comment and offer suggestions.

  8. AlanB4 | | #8

    What is blue board?

  9. Expert Member
    Dana Dorsett | | #9

    Blue board is gypsum board with rougher paper facers designed for sticking to plaster rather than paint.

    Guessing the color of that paper facer wins you a kewpie doll. :-)

  10. AlanB4 | | #10

    I wonder where in Ontario Canada sells it, i could use some to repair my plaster from the cellulose i got done last year

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