What kind of insulation materials should I use to insulate the hot-air pipes
and ceiling in a furnace room?
The house is about 5 years old with 6 inches of blown-in cellulose
insulation in the walls and ceilings. The furnace room is on the second
floor and has a finished drywall ceiling and walls. But the room is very
warm during the winter and snow on the roof above the furnace room melts
before the snow on the rest of the roof.
I've sealed the furnace pipe joints with mastic to stop hot-air leaks; but
now I'd like to wrap the pipes with insulation and add a layer of ceiling
insulation inside the room and maybe the outside wall too.
Good points. There is no system air return in the furnace room, so that's
an excellent idea.
The furnace uses outside air for combusion. The walls are 2x6 construction
as is the ceiling. 6" is the mimimum ceiling insulation thickness as the
roof meets the eave right above the furnace room. In other areas of the
ceiling, the insulation thickness is several inches more (as much as the
contractor could blow in). The drywall is 5/8". The furnace room door
(bathroom adjoining) is louvered. I leave that door open to increase air
What I think I'll do is put a small flexible vent pipe into the air return
plenum with the end of that pipe at the ceiling level. That will draw off
the hot air rising from the furnace and pipes. But, I'd still like to know
which insulation materials to use for those pipes and for another layer on
the ceiling. The ceiling material, of course, should meet fire code
There are several possibilities. See what your local DIY store has.
Fiberglass bats (1") or so should do it. You can use layers if you want.
There are other products available if you look around. Frankly I doubt if
you are going to gain anything by insulating the ducts. Insulating ducts in
areas where the air is conditioned is usually a waste of time.
How about venting the room to the rest of the home? Is the air pressure
in that room higher than or lower than the rest of the home? Assuming it is
gas or oil fueled, don't allow the room to go to a negative air pressure,
but you should be able to allow greater air exchange with the rest of the
Think about it. The heat in the furnace room must go somewhere. If the
room is too well insulated it will get hotter and hotter until the heat loss
equals the heat from the furnace walls. This will cause the furnace to be
hotter, maybe shortening the life of the motor. You might put a small
opening in the inlet duct to draw house air into the furnace, making sure
there is space under the door for the air to flow in.
First check your local codes about the benefits of installing a return vent
in a furnace room. It may not be legal because of the potential of creating
negative air pressure in the room resulting in the flames not staying where
their supposed to be in the furnace, and the possibility of drawing carbon
monoxide and/or even natural gas or propane into the household air
distribution ducts if there should be a leak.
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