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Community and Q&A

Insulating an attic furnace room

MiHomes | Posted in General Questions on

I have a furnace in my attic around which the builder constructed an enclosure from 2X4 and foil-backed fiberglass insulation. I’ve been told the R-value of the insulation should be sufficient, but there is clearly air getting through at the seams and creating hot spots in the attic which then leads to ice dams etc. What is proper protocol for insulating these attic “furnace rooms”?

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  1. Expert Member
    Dana Dorsett | | #1

    The "proper protocol" is to not have the thing in your attic, unless it's an insulated conditioned attic.

    Are the ducts in the attic too?

    Where is the attic insulation, and how thick is it?

    Where are you? (References to ice dams implies it's a cool/cold climate but a zip code or nearest city name would narrow it down.)

    Air sealing the all the duct joints & seams with duct-masting and the duct penetrations is job1.0, taping the seams of the air-handler cabintry is job 1.1, and air sealing all of the duct boots to the conditioned space gypsum (or the penetration into some duct-chase) is job 1.2. Insulating both supply & return ducts to at least R8 is job 2.0.

    After that we can start figuring out what to do with your furnace-shack in the attic.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, it's always a bad idea to put a furnace in an attic. But you probably already know that.

    One major problem is providing enough combustion air for the furnace burner. I hope that this is a sealed combustion furnace with a combustion air duct that pulls combustion air from the outdoors. If not, you've got a big problem, and you should not tighten up your furnace room until you decide what to do about the furnace. Basically, you're in a pickle if you have an atmospherically vented furnace in your attic.

    If you want to improve the insulation around your furnace, you'll need to pay attention to R-value and airtightness, especially concerning the ceiling (or roof) insulation. Many builders install insulation that is below required R-values in rooms like the one you describe, and very few builders have the foggiest idea about building walls and roofs that are airtight. I'll repeat my earlier warning: while airtightness is a necessary step to reducing heat loss, you can't make the walls of this room more airtight if the furnace doesn't have a source of combustion air.

    If the roof above this room is insulated, you'll need to follow the recommendations in this article: How to Build an Insulated Cathedral Ceiling.

    Once you have improved the insulation values and airtightness of your furnace room -- again, assuming you can -- you'll need to face another problem: making sure that all of the ducts in your attic are properly sealed and insulated.

  3. MiHomes | | #3

    Appreciate the quick input!

    Few clarifications on current situation:


    We are located in Southeast Michigan, so it certainly gets cold

    Furnace situation:

    It is a sealed unit that brings in combustion air via PVC pipe through the roof (and exausts through another PVC pipe that goes through the roof)

    Attic insulation:

    Floor of the attic (outside furnace room) is a blown-in insulation (looks like cotton?) that piles above the floor joists by ~10"

    No insulation along the roof itself, there are soffit and ridge vents (believe this is called "cold" roof?)

    Duct work:

    Duct work is all "class 1 flexible insulated ducts). Outside of the "furnace room" most of this is also covered by the aforementioned blown-in insulation

    "Furnace room" insulation

    This is a Ottowa Fibre product called "FSK 25" Appears to be a flame-retardent foil coating with ~8-10" of fiberglass behind it. This is stapled to the 2X4 frame of the furnace "room" every couple inches. There is no sealing tape of any kind.

    House was built in 2007, so material is relatively new (I'm the 2nd owner of the property)

    Thanks for the help!

  4. GBA Editor
    Martin Holladay | | #4

    The walls of your furnace room need one or two air barriers -- I suggest gypsum wallboard on the interior (taped) and rigid foam or housewrap on the exterior (also taped). And you need to install roof insulation above the furnace room.

  5. Expert Member
    Dana Dorsett | | #5

    So if I understand this correctly the furnace room has 8-10" batts dangling on 2x4 framing, with no air-barrier on one side of the fiberglass (just naked fiberglass) and an FSK facer on the other?

    Or it is rigid 1" fiberglass board with a foil facer, and 8-10" batts hanging off 2x4 framing? Ottowa Fibre makes fire-rated foil clad rigid fibeglass board, don't know if they make fire-rated batts.

  6. charlie_sullivan | | #6

    I like Martin's suggestion of drywall on the interior--you get air sealing and some extra fire protection. And housewrap on the outside is good too.

    For the top of the room, I'd install a ceiling as low as you can in order to fit as much insulation on top of it as you can, and still have open vent space between the insulation and the roof.

  7. sbkim | | #7

    Quick question. I have similar issues as the original poster - furnace in the attic but in an insulated room. Interior of this room is finished with drywall but outside has R19 exposed batts.

    Martin mentioned that perhaps house wrap outside would provide additional air sealing. I am curious if adding house wrap causes any condensation issues? I believe the answer is no given breathability but just wanted to confirm given that my builder used faced batts (facing warm side).

    Secondly, due to design of our roof, there is no good ventilation in this area which is further worsening ice dam issues. Is it OK to do closed cell around the rafter in this area only? I have attic floor insulation (batt/blown in) of R60.


  8. Expert Member
    Dana Dorsett | | #8

    Housewrap is vapor permeable, and will not create condensation issues.

    If you still have ice dam issues with R60 on the attic floor, there has to be a heat leak at the top of the wall or something. Do you have R60 all the way out to over the top of the wall, or does it get thinner to keep proper clearance from the roof deck? Chasing this one with an infra-red camera may be useful.

  9. sbkim | | #9

    Dana - thank you for the quick reply! I believe you are absolutely correct. I thought it was perhaps sun melting the snow and freezing on the northern side of my house. It's been in the low 20's here in Chicago with no sun, and I still see water dripping/icicles beginning to form.

    I believe it's a combination of complex roof structure with poor ventilation (I believe there is ridge vent on the roof but I have no soffit vents nearby. I will double check that ridge vent is not blocked.

    The biggest culprit, I believe, is this furnace room in the attic. It has R19 around the perimeter but something must be emitting heat to the roof. I have 98% efficiency furnace that has two pipes going out of the room and into the roof (intake and exhaust). I definitely feel the heat near the exhaust pipe.

  10. sbkim | | #10

    To answer your other question - the insulation definitely gets less as it nears the top plate (corners)...

    Please see picture below. Ice dam appeared at the bottom where the shovel / laddar is located.

  11. GBA Editor
    Martin Holladay | | #11

    More on ice dam solutions: Prevent Ice Dams With Air Sealing and Insulation.

  12. sbkim | | #12

    As always, thank you for that great article! I wish I had read many of your other articles advising against having furnace in the attic before my builder finished my house...argh.

    I am hiring HVAC contractor this week to mastic seal all joints in the furnace room and possibly lower the supply line away from the ceiling. The room is warmer than the 2nd floor and I feel like there is much to gain from sealing up the furnace.

    The room itself has drywall inside and batt outside. Is there any benefit to taking down the batt (R19) and doing close cell to ensure no air leakage and perhaps better R value?

    Finally, do you recommend doing closed cell on the roof rafter area (just around the attic furnace room) where it's most prone to heat transfer? As mentioned, i have R60 on at the attic floor...


  13. sbkim | | #13

    Perhaps picture is worth 1000 words, please find attached pict of the furnace room. I took some temp measurements and the furnace room is running 10F higher than 2nd floor space. Towards the top of the ceiling (where the supply ducts are), temp is 100F! I am hoping that there is decent insulation above it and will double check.

    I presume this has to be causing the ice dam? Also found a irregular hole that the builder either missed or intentionally left it.

    Are there any advantages to venting this attic room to 2nd floor to drop temperature?

  14. GBA Editor
    Martin Holladay | | #14

    Q. "Are there any advantages to venting this attic room to 2nd floor to drop temperature?"

    A. I've not sure what you mean by "venting this room." Probably you mean installing a grille or installing a duct, with or without a fan -- but unless you explain what you have in mind, I'm not sure.

    If you have a hot room in your attic, and you want to limit ice damming, you want: (a) to make sure that the room is as airtight as possible, and (b) to make sure that the room is surrounded by a thick layer of effective insulation that has a high R-value.

  15. sbkim | | #15

    Hi Martin - my apologies for not being clear. I meant adding vent in the attic access to allow air from 2nd floor. However, I realized that this may not work well given that heat rises.

    I am doing exactly what you are suggesting and focusing on air sealing. I am told by my builder that ceiling of this room is R38 but walls are R15.

  16. sbkim | | #16

    I hate to resurrect an old thread but had a quick follow up question about having two air barriers in attic furnace room. I will have drywall inside and about R21 fiberglass. I am seriously considering the following for the wall facing unconditioned attic:

    1. House wrap
    2. Tyvek Thermawrap R5.0 (would this application work to limit air barrier?
    3. Spray foam (installers recommendation)

    I am trying to minimize warm air entering the unconditioned attic space.


  17. CraigCoffaro | | #17

    Doesn't look like anyone has posted on this in over 2 years, however I'll give it a shot. I live in Chicago (cold winters), I have 2 HVAC systems in the house, basement (sealed) and attic (atmospheric). The one I am concerned with is the atmospheric unit (Carrier) in the attic....I know now that this is a no no...too late now. We get ice damning in the winter and looking to stop this. We've been told that building a room around the HVAC unit and insulating will help with this and I know we would need to have venting for combustion as low in this room as possible. Is this the best idea or are there other options I should be exploring? Also, I do not have a ridge vent, but do have 2 gable vents, soffit vents, 4 vents in the roof and adding R-60 blown in cellulose on the attic floor with baffles to increase air flow. Thoughts?

  18. user-2310254 | | #18


    How old is the attic unit? What is its efficiency?

  19. Expert Member
    Dana Dorsett | | #19

    Craig : Is replacing the attic unit with a (right-sized) heat pump something you're considering? It's not unusual for gas furnaces to be more than 3x oversized for their loads, and for air conditioners to be more than 2x oversized.

    Atmospheric drafted gas-burners have much higher stack temps than condensing gas, and more likely to start ice damming by melt-out around the stack. Heat pumps don't have that point-source hot-spot, and run generally lower duct temps as well. With ducts buried in cellulose and a reasonably tight & insulated air handler unit the stray heating losses to the attic will be much lower than you have been experiencing.

    Run the the heating & cooling load numbers on a room-by-room basis for the zone served by the attic unit,using an online load tool such as, using the tightest duct & construction assumptions the tool offers. Even with most-aggressive assumptions the freebie tools will still overestimate reality by a bit, but not by 2x. Then compare that to the specifications for the current HVAC equipment and report back. If it's less than 22,000 BTU/hr heating, 18,000 BTU/hr cooling it's within range of a cool-climate mini-ducted mini split it may be cheaper / better / more comfortable to retire the existing equipment now rather than to build an air tight insulated crypt around oversized equipment that's half way through it's normal service life. A mini-duct cassette doesn't take much of an enclosure to insulate over, and could even be installed under the ceiling (in a closet or soffit) completely inside of conditioned space.

  20. GBA Editor
    Martin Holladay | | #20

    Q. "Is this the best idea [building an insulated mechanical room around my attic furnace] or are there other options I should be exploring?"

    A. I'm not a fan of the idea of building an insulated mechanical room around an attic furnace, for a variety of reasons -- and in fact I recently wrote a blog on the topic (to be published sometime in the next few weeks). The cost to build a decent insulated mechanicial room is high, and the money is better invested elsewhere. Moreover, attics have bad access, and are therefore difficult for maintenance workers. Atmospherically vented furnaces don't belong in a small mechanical room in an attic.

    In many cases, it's cheaper to abandon the attic furnace and attic ductwork and replace the equipment with one or two ductless minisplits for the second floor of your house.

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