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Using my nifty thermal camera, I discovered that my furnace closet's combustion air duct (visible in this question) is getting really hot when the attic heats up, and then (being sheet metal) it radiates that heat into the furnace closet. Here's the combustion air duct:

enter image description here

The furnace closet has no insulation to thermally separate it from the rest of the house, so the obvious thing happens: the heat escapes. Here's a thermal view of the wall closest to the duct when it's hot out:

enter image description here

In order to combat this heat transfer, I would like to insulate the stud walls that frame the furnace closet, which are currently empty. The furnace closet sits above a return air plenum that is un-drywalled, giving me a place where I could stuff something rigid or semi-rigid up into the cavities from the bottom. I am considering using Roxul mineral wool insulation batts, which I hear are more rigid and dimensionally stable than fiberglass batts. Would this work? are they rigid enough that I could actually stuff them up into the cavity? Or would this be a waste of time and money because they'll just snag and compress?

In addition, would a radiant barrier help? (asked as a separate question)

Before anyone asks, I don't want to fill the stud bays with loose fill cellulose because it would require drywalling and sealing the open sections in the return air plenum and blowing the material in from the attic, which is scary and I don't like to go up there. It's also flammable and it would be used to insulate a furnace closet, after all.

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  • How about insulating/shielding the vent pipe instead? Actually, if the attic is getting so hot that heat transfer is an issue with one pipe, it may make more sense to consider insulating the attic better.
    – DA01
    Commented May 21, 2014 at 22:43
  • Since the combustion air duct deliberately pulls air from the hot attic, I doubt that better retarding heat flow from the attic into the conditioned space will help--the duct itself will still be exposed to the attic's temperature, after all. As for insulating the attic better, it's on the priority list. :-)
    – iLikeDirt
    Commented May 21, 2014 at 22:50
  • For combustion air, you should be able to use PVC which wouldn't conduct heat as much as metal.
    – DA01
    Commented May 21, 2014 at 22:54
  • When the furnace is eventually replaced with a high-efficiency sealed combustion unit, that is definitely going to happen.
    – iLikeDirt
    Commented May 21, 2014 at 23:10

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Roxul batts do not compress a lot, well not nearly as much as traditional fiberglass. They are also pretty rigid. They can definitely go 6 feet or so in a wall cavity. I have used them to do what you are doing, when there aren't things in the way.

I am guessing you believe you don't have cross bracing or electric/plumbing or obviously this won't work. I would unroll a batt, cut to height size (possibly cut to width), push up slowly starting at top of batt. If it snags and is resisting I would cut it. Then push up with broom or something similar and just keep on going until you fill it. Even if the Roxul slides a little you can stuff the bottom to hold it. The stuff really compacts well and holds its place. Why I tend to use it in basement joists.

Also if you place a small screw in the top side of your batt (just push it in so it isn't hanging out) you can use a magnetic stud finder to see where you are at.

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  • Thanks! I can see into the cavities, and only one has anything in the way: an electrical receptacle.
    – iLikeDirt
    Commented May 22, 2014 at 2:51

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