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Community and Q&A

Building a furnace room in the attic

BuildingNewb | Posted in General Questions on

What is the best way to build a furnace room in the attic?

How big should the framing be? 2x6s? Also, should I attach plywood to the framing or just nail polyisoprene directly to framing? How many layers of R13 Polyiso? Is it better to do 1 layer of foil faced polyisoprene on the inside of the framing and then just put fiberglass batts on the outside?

Thanks a lot?

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  1. Francis Thomas | | #1

    The air duct into the room is for the water heater combustion air. It become initially mounted leading down from the attic. This changed into extremely good for introducing air however it allow all the hot air from the furnace room into the attic. I removed it, sealed the hole and mounted a new make up air from the out of doors.The furnace go back duct is hooked up to a sparkling air makeup that is separate. The combustion air for the furnace is a separate % vented directly into the furnace. The furnace exhaust vent has a drain for condensation that drains at once right into a condensate pump in which it's far pumped to waste. I don't assume the moisture is from the exhaust but I'll get on the roof the next day anyway and test.

  2. GBA Editor
    Martin Holladay | | #2

    Building Newb,
    While I don't understand most of Francis's response, he brings up a good point concerning combustion air. You can't build this type of room in your attic unless your furnace has a sealed-combustion burner with ducted combustion air. Otherwise, you will be starving your furnace for combustion air. One possible solution (if you have an atmospherically vented furnace) is to install a Fan-In-A-Can wired to the burner. That's a possible solution, but not ideal.

    The furnace room needs to be big enough to enclose the furnace, with easy access for maintenance personnel and filter changes. You'll need an airtight door with good weatherstripping.

    The details will vary depending on your climate and budget, but basically you will be building insulated exterior walls and an insulated ceiling. Of course, you'll need drywall on the interior for fire safety and to meet code.

  3. BuildingNewb | | #3

    The new high efficiency furnace I'm looking to install will draw its combustion air from outdoors.

    I was hoping to put rigid, foil faced Polyisocyanurate boards on the interior and tape the seams. Is this not allowable by code? I'm referring to the Super TUFF-R product that says is acceptable as wall sheathing.

    Furthermore, does this room need a connection to the conditioned space? Or am I just relying on the furnace itself to keep the room warm?

    Will it be hotter in this room than the rest of the attic in the summer? Will this make the evaporator coil in the room work harder? Will there be any moisture issues if the evaporator coil is located in an airtight room?

    Thanks so much!

  4. GBA Editor
    Martin Holladay | | #4

    Building Newb,
    I'm assuming that a mechanical room with a combustion appliance (like a furnace) will be required by code (and common sense) to be finished with gypsum wallboard. If you have any doubts on the issue, consult your local code official.

    This type of mechanical room won't need supplemental heat in the winter. The operation of the furnace will keep the room quite warm.

    The mechanical room does not need to have a connection with other interior space, but you need to make the room large enough for maintenance personnel and human access.

    If your ductwork / air handler includes an evaporator coil, and the air conditioner is operating, the mechanical room will be cooler rather than warmer than your attic.

    The extent to which the walls and ceiling of your mechanical room save energy depends on the R-value of the insulation you install and the airtightness of the new enclosure.

  5. BuildingNewb | | #5

    Understood. I'm in Climate zone 5 so I was going to insulate it to approximately r60 on walls and r50 on ceiling (Four 2" R13 boards with a 2" ventilation gap in the 2x10 rafters).

    Are there any potential moisture issues by having the evaporator coil located in an airtight enclosure? It's obviously connected to a drain that runs out to the gutter but I'm not sure if any moisture on the coil itself will cause any issues.

  6. Jon Harrod | | #6

    The evaporator coil itself (meaning the copper tubing through which the refrigerant moves) is designed to attract condensation; that's the mechanism by which AC removes humidity, and the liquid should drain away through the condensate piping. There needs to be a secondary drain pan in case the primary drain fails, and this should have either its own drain piping or a water-detection device that cuts off power in the case of overflow.

    I would expect the potential for condensation on the cabinet that houses the evaporator coil to be much less in a tempered, enclosed space than in an attic open to hot, humid air. The possibility of condensation can be further reduced by insulating the cabinet with appropriate insulation materials.

  7. BuildingNewb | | #7

    Do you think it's better to make an insulated cabinet directly around the furnace /evaporator with Removable panels for servicing or make a whole mechanical room? I just question how airtight a cabinet can be if it has Removable panels.

    Furthermore, do 96% modulating furnaces make a lot of heat in the cabinet itself? I wonder if the furnace can product enough secondary heat to keep the room adequately warm to prevent condensate freezing.


  8. GBA Editor
    Martin Holladay | | #8

    Building Newb,
    Q. "Do you think it's better to make an insulated cabinet directly around the furnace /evaporator with removable panels for servicing or make a whole mechanical room?"

    A. Trying to build an insulated airtight cabinet around a furnace is a terrible idea. A furnace is designed to be installed in a mechanical room. You can't alter the furnace in the way you suggest without voiding the furnace warranty and risking all kinds of negative unforeseen consequences. The furnace needs adequate clearance all the way around it.

  9. BuildingNewb | | #9

    The only reason I said that is because my hvac contractor said the model I was looking at needs virtually zero clearance.

  10. TahoeJim | | #10

    Hi, I realize I'm resurrecting an old thread, but I have this same problem.
    My house is in Tahoe @6800 feet.... in big snow years we'll have 4-6ft of snow sitting on the ground. Last year I had 12" thick ice dams w/ 10' icicles off the eves.
    Mine is two story house built in '91 with two furnaces... one under the house for downstairs and one in the attic for upstairs. I've assumed the cause of the ice dams is the atmospheric gas furnace in the vented attic, along with 20-odd, unsealed can lights. :) Floor of attic has fiberglass baffle insulation. The furnace has a metal exhaust flue straight up and out the roof, and as noted there are large gable vents on each end of the attic.
    Because building a mechanical room and sealing the cans will cost a few thousand dollars, I ran a test.... I put a temperature sensor in the attic on a rafter near the roof peak a few rafters away from the furnace, and one on a rafter under the eve outside. So I have the attached heat graph of upstairs (Nest) temps, attic temps, and outside eve temps.
    I *think* there are two problems:
    1) Today it started snowing and you can see it is cold outside (34 in graph, likely a couple degrees off), 46 in attic and 68 in house envelope. This is what I've been waiting to confirm... attic being warmed likely by furnace first and ductwork second, and then likely cans third? Does that causation sound right?
    2) However, if you look at prior days when we were away (inside furnace heat off) and it was sunny and nice (no snow for last couple weeks, high outdoor temps in the 40s/low 50s) you can see that the attic got up to 70 degrees (again with NO furnace running!) So the sun on our asphalt roof is heating the attic considerably, and the gable vents seem to be ineffective at exhausting the heat.

    I should also add that there are no eve vents (picture attached).
    Given the data and my situation, what is the most effective course of action? And in what priority order?
    My goals are: 1) prevent or mitigate icedams, 2) make house more temp comfortable, for the least amount of money. Would appreciate expert help prioritizing an attack plan that gives biggest benefits first, for least $$ spent. :)

  11. TahoeJim | | #11

    Coming back to this. So what I'm seeing is roughly a 20 degree difference from outside to attic to inside (ie, 30 F outside, 50 in attic, 70 inside). On warmer winter days I'm seeing more of a 10 and 20 degree difference (ie 40 outside /50 attic /70 inside). Attic temps seem to mostly track inside temps. When the heat goes off late at night and inside temps decline, the attic temps also decline.

    Also, I was partly heating with a wood stove, so the furnace in the attic was running less generally than it would run if I was heating only with gas. Attic furnace was running 2.5 to 3.5 hours/day instead of 4-5 hrs/day.

    Are my numbers typical? (I thought unconditioned attic should be close to outside temp, not 20 degrees above?).
    If these numbers are poor, do you think that the can lights are the main culprit heating the attic? or is it just poor insulation overall (ie bats not enough?).
    I was convinced that the attic furnace was the culprit, but furnace runtimes seem only loosely correlated with changes in attic temps.
    I guess I'm not sure where to start... sealing can lights, enclosing furnace, or do I need to plan for more insulation as well before I see an improvement? (or is this normal and as good as it gets???)

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