I found mold on the underside of my plywood attic flooring. The attic has rollout fiberglass insulation with a paper vapor barrier in the attic joists with no rafter insulation. I have installed plywood flooring to allow for storage which might have caused the problem. This problem is throughout the attic. I have noticed that Moisture is condensing on the underside of the floor when it is very cold in the attic. It appears that the vapor barrier is inadequate. Looking for any thoughts and the best product to re-insulate with. FYI, I live in a cold area. Thanks much.

1 Answer 1


How old is the house? What you really need is the attic to be air sealed from the house. paper backed vapor barrier on one side of the insulation as the only vapor barrier isn't going to be effective.

The typically standard is 6mil poly installed before the drywall. Retrofit is going to be difficult - remove all the insulation and install continuous poly in all the joist bays. Tape all seams.

Moisture coming out of the house will condense on the first cold surface so putting the plywood right after the insulation allows it to collect there. Possibly removing the plywood would allow it to be vented out but moisture getting into the attic is still bad and ultimately the air bringing the moisture is costing you in your heating bill as well.

  • Thank you for your thoughts. Much appreciated. The house is 26 years old. Your comments make sense. I think I’ll reach out to an insulation contractor to seal the joist bays as you note and reinstall new insulation. It is above my ability. Again thank you and any other thoughts would be appreciated. Dec 27, 2020 at 1:45
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