located in the midwest with cold winters... Decade old asphalt shingles on ~23 year old house. Recently noticed two minor water spots on the drywalled ceiling and when going into the attic noticed there's frost/condensation on the underside of the roof decking during the very cold weather recently.

Trying to think best how to address this... From research, was looking to ensure the ceiling is properly air sealed and seal any/all gaps. If going to be up there already, considering replacing the fiberglass insulation with stone wool insulation as well for better R value and better deal with any moisture.

Or is there any better solution? Was thinking whether adding foam baffles up there would help to at least divert any dripping condensation to the soffit?

Thanks for any advice!

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  • There are attic venting provisions in the International Residential Code: codes.iccsafe.org/content/IRC2021P2/…. It's 1ft^2 vent area per 300ft^2 attic area if you've got equal soffit and ridge vents (it's actually a little more complicated). 1ft^2 vent area per 150ft^2 if you've only got soffit vents. Do you about meet the requirements with your venting?
    – popham
    Jan 20 at 20:08
  • Is frost an indicator of a moisture problem, actually? I wouldn't expect problem moisture to condense directly above your soffit baffles like that. Problem moisture would find dead-air areas, right? I don't understand frost's mechanics, but I'm used to seeing it on outdoor surfaces like your roof sheathing. I'd invest in a moisture meter before planning solutions.
    – popham
    Jan 20 at 21:00

1 Answer 1


Deal with the moisture source. Make sure the roof isn't leaking, and if it's not, then it's warm moist air getting into the attic from the house.

Replacing insulation is usually overdoing the solution - adding to the insulation (with the air &/or water leaks solved) is generally the better idea .vs. throwing out old insulation.

Also check that there isn't something like a bathroom or kitchen fan exhausting below the soffit vents there.

  • Thanks for the feedback. My thinking with replacing the insulation was that if I'd be pulling up the existing fiberglass insulation to look for any signs of moisture damage and to air seal caulk/foam any gaps, etc, I might as well replace it with stone wool insulation for better R value, would enjoy more noise dampening, etc, besides personally hating fiberglass. Just laying some extra batts on top of the existing insulation I was thinking would just make it harder to navigate when needing to go up there and covering up any moisture issues with the existing fiberglass insulation.
    – Michael
    Jan 20 at 20:22
  • Apart from the personal hatred, you can pull it out, find/solve issues, install rockwool, and put the fiberglass on top of the rockwool going the other direction, with less handling of the fibreglass than getting it out of the attic and into a dumpster, and you get the benefit of the already-paid-for R-value it has in addition to the new rockwool, but that's entirely a personal call.
    – Ecnerwal
    Jan 20 at 20:45
  • The frost directly above the soffit baffles is bugging me. I don't understand the mechanics of frost, so I can't offer an alternative explanation. But it's normal outdoors, right? Maybe it's normal in attics under the right conditions regardless of what's going on inside the home's thermal envelope.
    – popham
    Jan 20 at 21:03
  • @Ecnerwal ah my concern with reusing fiberglass on top would primarily come down to the concern over mold / less friendly toward moisture than stone wool, but will consider that option. Thanks.
    – Michael
    Jan 21 at 2:09
  • @popham had thought about that too but given that condensation is forming and now causing ceiling drywall damage during very cold temperatures, thinking it's not too likely. From what I'm hearing a major contender is likely that the bathroom exhaust fan is vented via the soffit. Even though we haven't used the fan in years, likely enough passively making it through into the attic to cause issues. Hoping roof venting that and air sealing things in the attic while upgrading/replacing insulation will fix it.
    – Michael
    Jan 21 at 2:12

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