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Building a furnace room in the attic

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?

Asked by BuildingNewb
Posted Dec 6, 2017 6:59 PM ET


9 Answers

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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.

Answered by Francis Thomas
Posted Dec 7, 2017 12:48 AM ET
Edited Dec 7, 2017 6:42 AM ET.


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.

Answered by Martin Holladay
Posted Dec 7, 2017 6:46 AM ET


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!

Answered by BuildingNewb
Posted Dec 7, 2017 7:02 AM ET
Edited Dec 7, 2017 7:25 AM ET.


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.

Answered by Martin Holladay
Posted Dec 7, 2017 7:32 AM ET


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.

Answered by BuildingNewb
Posted Dec 7, 2017 7:38 AM ET


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.

Answered by Jon Harrod
Posted Dec 9, 2017 3:00 PM ET


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.


Answered by BuildingNewb
Posted Dec 9, 2017 3:18 PM ET


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.

Answered by Martin Holladay
Posted Dec 10, 2017 5:39 AM ET


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

Answered by BuildingNewb
Posted Dec 10, 2017 7:37 AM ET

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