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Insulating a void in basement.

user-1098359 | Posted in Energy Efficiency and Durability on


I was wondering if the experts here could help me with an insulation question.

I have a fireplace in the basement that we do not ever use. It is a slight bumpout from the house. There is a bathroom just above it on the first floor (the chimney angles up to the left and does not interfere with the bathroom). As a result, there is a void above and behind the fireplace and below the 1st floor bathroom (roughly 40 inches to the foundation wall).

The bathroom is always cold–many degrees below the rest of the house. From what I can tell, there is insulation behind the drywall of the mantlepiece and basement wall above it but the void above/behind the fireplace and directly below the 1st floor bathroom does not seem to have any insulation. Said differently, the void is outside the thermal envelope.

I’ve had a few insulation installers come out. A few of them have suggested blowing in mineral wool to densely pack the void and use open cell foam along the foundation wall and top-plate.

A few questions:

1) I have never heard of mineral wool before. Is it safe? Any off-gassing or airborne fibre issues?

2). What would be other alternatives I could use? Could blown in cellulose also be used to densely pack the void?

3) Aside from Icenyne open cell foam what else could be used? I am wary of the issues people have had with these products.

Many thanks,


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's hard to understand or visualize the void from your description. Is this a pre-fab metal fireplace or a brick fireplace? Is it wood-burning or gas-burning?

    If there is any risk of the insulation getting close to a hot flue, mineral wool is the best insulation for the job. Mineral wool is safe at a higher temperature than any other common type of insulation. For more information on mineral wool, see:

    GBA Encyclopedia: Batt and Blanket Insulation

    Installing Mineral Wool Insulation Over Exterior Wall Sheathing

    Some GBA readers have worried about formaldehyde off-gassing from mineral wool. However, according to a leading manufacturer of mineral wool, Roxul, "a high-temperature curing phase virtually eliminates volatile compounds. The result is no measurable free-form formaldehyde in the final product and no volatile organic compounds that can off-gas." Like all statements made by product manufacturers, we would all feel better if this statement were confirmed by a disinterested third-party lab.

    Concerning your final question about alternative insulation products: there are lots of types of insulation out there. Whether other types of insulation can be used between your basement and your bathroom depends on (a) access to the void, which is hard to determine from your description, and (b) how close the insulation needs to be installed to a hot flue.

  2. Expert Member
    Dana Dorsett | | #2

    Codes around masonry chimneys require 2" air gap from the masonry to combustibles, which rules out the use of open cell foam any closer than 2" to the masonry, but it's fine to insulate rim-joists and foundation sills with.

    Code also demand that the 2" clearance to combustibles be empty, but you may be able to get a variance if it's stone wool, which is completely fire-proof itself, and about as insulating as open cell foam when dense-packed. The fire retardents in cellulose do not give it a sufficient fire rating for this kind of application, and the strong moisture wicking characteristics of cellulose make it a poor choice for use in contact with masonry or concrete.

    Rock wool has none of the airborne fiber issues associated with fiberglass

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