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Is it a good idea to use dense pack celllulose insualtion in SE Pennsylvania?

kaczmap | Posted in Energy Efficiency and Durability on

Should I dense pack my attic ceiling?


I need to re-roof my entire house including removing all of the sheathing.

I see this as an opportunity to re-insulate an attic bedroom.

One contractor recommends (he has great references) to re-roof and then use dense-pack cellulose to fill the void between the sheathing and attic ceiling with cellulose. I like the idea – it seems to maximize the r-value.

Should I be concerned about moisture?

I am wondering about doing this without an airway to vent the space between the sheathing and the attic ceiling.

I’d appreciate any thoughts or other cost-effective options I should consider.

(I live just west of Philadelphia, PA – close to Valley Forge Park)


(I posted earlier and it was suggested that I give my geographical info for the best ideas)

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  1. Michael Chandler | | #1


    This query has been up for a few days with no response so I'll take a swing at it.

    I've used cellulose in installations similar to this (we installed a mesh on the ceiling and blew the area above full of dense pack) with no problems.

    BUT we've also had cellulose get wet due to slow recurring plumbing leaks on two occasions and in both instances the dampness was very slow to make itself known to the occupant and once the leak was discovered and the sheet rock removed the studs, sheet rock, and sheathing had been saturated for well over a week and mildew had set in and all the cellulose and sheet rock had to be removed and replaced and we had to clean and bleach the cavity and let it dry for several days before we could complete the repair with canned foam.

    Had these leaks occurred with fiberglass or open cell foam (or even, I suspect, with closed cell foam) the water would have leaked out into the living space in minutes rather than weeks and we would have had a chance to fix the leak before a major remediation was required.

    For your situation I would recommend open cell foam or JM Spider fiberglass blown in through mesh as a Blown-In-Batt. I think the risk of a roof or flashing leak is too great to risk using cellulose in this application.

  2. Expert Member
    CARL SEVILLE | | #2

    I think that insulating existing walls is one of the thorniest issues in homes today. In cold climates particularly, you can lose a lot of energy through uninsulated walls, so in theory, installing insulation is a good idea. The problem with most of these walls is that there is often no drainage plane or window flashing in place to keep bulk moisture (rain) out of the wall cavity. With no insulation, any moisture that gets in will eventually dry out, but insulation of any type will keep that moisture in the wall, allowing mold to form, reducing the effectiveness of the insulation, and frequently causing significant structural damage. Where you have masonry veneer with an air space between it and the sheathing you have a fighting chance of keeping the moisture problems at bay, but with any type of siding attached directly to the sheathing, I think that the risk is too great in wet climates to risk installing wall insulation. Work on air sealing, attic/roof insulation, crawlspace/basement sealing, HVAC improvements first. When you get the walls, make sure that you have a good system to drain the bulk moisture before you fill walls with insulation. By the way, this goes for new homes as well, since lots of contractors haven't yet figured out that they have to flash windows, doors, and every mechanical penetration properly. Apparently the concept that water flows downhill hasn't caught up with everyone yet.

  3. Bruce Wilson | | #3

    Since you need to re sheath anyway, I would do a combination system here. I would install insulated sheething, (they make both ventilated and unventilated systems with urethane insulation bonded to OSB, available at most good roofing supplies). You can also lay up your own site built system putting down the insulation then the sheathing. When installing the insulated sheating try to foam the edges together to creat an air tight seal. Then after the roof sheating is done you can do a dense pack of cellulose between the rafters. I have used the fiberglass mesh stapled to the rafters with good success. I like cellulose and use it in most of my jobs.
    As Peter pointed out, if there is a leak the cellulose will wick and absorb it so that there is mildew or mold growth.
    I tend to use ice and water sheild on the whole roof to avoid leaks during construction, it is inexpensive insurance.

  4. Bruce Wilson | | #4

    By the way I work in SE Pa and aim for an R 50-60 where possible.

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