Best way to insulate with a furnace in the attic

techedSeptember 16, 2011

Hello, I have never posted in this forum, but I am not new to GardenWeb. I am hoping the collective wisdom in this forum can help me in a new situation.

We just purchased a home in northern Illinois. There are 2 attics and 2 furnaces. Both furnaces were replaced in 2010. The older part of the home has a larger attic with maybe R3 of old rockwool. The furnace is in the basement. The newer part of the home has a small attic with maybe R15 of rolled fiberglass. The furnace is in the attic here. Both attics have some duct work (which is encapsulated in insulation).

We know we need to insulate. I have talked to traditional (blow in fiberglass) and spray foam contractors. The roofer says the spray foam will reduce the life of the roof. I'm worried about that furnace in the attic. I sort of think we should spray foam that attic space because it seems to me that the furnace should be in an "indoor" space. Of course, spray foam is very expensive, so I am not sure I want to spring for it in the larger attic, which could be addressed with traditional insulation.

What do you all think? Should I worry about the furnace in the attic? If we insulate to R60, that attic is going to be mighty cold in the in winter and hot in the summer.

Thanks in advance.

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Though you do not state it specifically, you appear to be considering sealing up the attic and foaming under the roof deck. The roofer that says that will reduce the life of the shingles is full of it, but most all of them say that. It might increase the temp of the shingles a little, but they are already hot and will not get that much hotter.

I can not comment on the wisdom of sealing the attic in your climate. It is advised where it is hot and humid on the Gulf of Mexico coast. Sealing the attic would transform your attic from outside to inside and cut duct losses. In addition, it makes all those leaks between the living space and attic largely irrelevant, holes drilled for cables, can lights, etc. It could, however, cause moisture problems. You see, in my climate, the excess moisture is almost always outside. In the winter, where you live, it can be inside. Go to and see what you can find about sealed attics in your climate.

You are right to worry about your furnace. Unless it has its own air intake direct to the combustion chamber, you need to supply outside air if you seal the attic. Fortunately, there is an easy solution. An interlocked fresh air damper can be installed. These devices will not allow the furnace to start unless the damper is in the open position.

    Bookmark   September 16, 2011 at 3:04PM
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You can build a small enclosure around the furnace with rigid foam insulation, then pick up the insulation on the ceiling inside the enclosure.

This puts the furnace in at least semi-conditioned space.

If the furnace is a fuel burning one be sure you provide air for combustion.

    Bookmark   September 21, 2011 at 3:55PM
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