We have an older home with a separate high efficiency furnace for the second floor. It sits on a floored area of the attic. We have virtually no insulation and I've gotten various recommendations from contractors. The concern is that the furnace need to be in a heated area. Right now, without insulation, the attic is somewhat heated. One contractor suggests walling off the furnace area and insulating the rafters with foam to make a warmer area for the furnace. But I'm concerned about blocking air circulation if we do this. Is there a good solution?

  • how much snow do you get in winter? ... if you heat an un-insulated attic, then you will melt some of the snow on the roof. ... the resulting runoff will re-freeze at the edges of the roof. ... an ice dam can form with water pooling behind it and the water can leak into your house.
    – jsotola
    Mar 6, 2018 at 3:13
  • Yeah, what are your ground snow loads like? Also, is over-roofing an option? Mar 6, 2018 at 12:37
  • We're in the Chicago area so we do get snow. The attic has been fairly un-insulated for the past 30 years and haven't has a problem with water leaking into the house. We'd be adding more insulation, not heating the attic but I need to protect the furnace from extreme cold. I don't know what over-roofing is? Is that adding insulation from the roof side?
    – Deb Brown
    Mar 7, 2018 at 16:40
  • @DebBrown -- yes, over-roofing would be building insulation + a new roof deck atop the existing deck Sep 15, 2019 at 21:04
  • This is an old question with not even close to the amount of info needed to answer it right... voting to close.
    – DMoore
    Feb 12, 2020 at 19:18

2 Answers 2


How big is the attic? One solution is to take 2, 3 or 4" insulation board (R13 to R25) and affix it to the rafters. This would basically make your attic conditioned space (and thus the air between the foamboard and your ceiling will also add R-value) while also protecting your ventilation pathways. A board with a radiant barrier on each side would be a plus and be sure to seal between the boards. If you Google "Superattic", you'll find a better explanation. NOTE: On a DIY basis, it's not very expensive. But if you hire someone and your attic is big, the cost can add up.

  • Note that you cannot do this in a high snow load climate (ice dam zone) or else you'll get bit hard by ice dams due to the R-value of snow causing the temperature gradient to shift so the roof surface is above freezing Feb 13, 2020 at 0:04

so high efficiency furnace you have pvc piping for venting and a condensate drain for furnace? if this is correct. i would frame and insulate walls for the furnace in leaving a good access, then have a opening in the plenum cut in so you can keep heat in. otherwise the condensate or worse yet the secondary heat exchanger freeze up.

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