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Community and Q&A

How effective is dirty insulation?

7yKbWZGtp8 | Posted in Energy Efficiency and Durability on

I have an 80 year old bungalow in Raleigh NC. It has previously had a thin layer (2-3 in) of fiber glass insulation blown into the attic. I have recently had an energy audit and know that I have LOTS of air leaks to take care of. The existing insulation is quite filthy (covered in black dust). I know I need more insulation, but my real question is given that it is covered and saturated with this black dust, is it doing any good? Would I be better off to brush it aside to get to all of the air leaks or go ahead and remove it all? Removal would be some cost, and it would up the cost of the planned insulation upgrade, but it would make it a LOT easier to find all of the potential air leaks.

Despite a few internet searches I’ve not found any discussions related to this topic. Any thoughts will be appreciated.

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  1. Riversong | | #1


    As you already noted, it would make it much easier to seal ceiling air leakage paths if the insulation is removed. Old fiberglass not only has less R-value per inch than modern materials, but is typically contaminated with rodent droppings and sometime dead carcases. Adding many inches of additional insulation on top of the existing insulation will likely compress it to worthlessness.

    It would be wise to get rid of what's there, seal the deck and blow loose-fill cellulose to whatever depth the framing and your budget will allow. Your climate zone (4A) requires a minimum of R-38 in the ceiling. I would recommend a minimum of 12" of cellulose, which is a far superior insulation in every way to fiberglass.


    You can push it around to air seal and not be too concerned about removing it but if you have an 80 year old bungalow in Raleigh you probably have a beaded pine ceiling so you need to consider some type of sprayed on sealing package such as spray foam or energy complete etc. with the extreme humidity we have here (I'm over in Saxapahaw NC) I would recommend JM Spider over cellulose but either is fine. R-38 is what you should be shooting as Robert says, do you have any ducts up you need to get inside the insulation? have you a plan for sealing the crawl? Lotta questions here, be sure to follow a comprehensive plan and not just shoot from the hip.

  3. GBA Editor
    Martin Holladay | | #3

    Another vote in favor of filling the Dumpster with the old insulation and doing a good job of vacuuming up the dust before beginning air sealing work.

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