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For the last couple of days we have noticed more and more mold growth on the ceiling throughout the house. Wondering what was going on, I climbed into the attic this past weekend and notice that no vapor barrier was ever put in place along with an inadequate level of insulation. I plan on blowing in the additional insulation to bring the home up to the recommended levels, but what can be done in regards to the missing vapor barrier?

Update: I live in central Wisconsin. I am seeing the mold in the living area of the house mostly around the can lights.

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Is there mold on the attic side of the ceiling or just the conditioned space side? If just the latter, you have a different problem--namely way too much moisture in your conditioned space. A VB won't help much with that. –  DA01 Mar 19 '13 at 4:18
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Vapor barriers are normally installed in walls, but rarely on the bottom side of the ceiling joists under attic insulation. Where exactly are you seeing the mold? Living space or in the attic? We may be talking a ventilation issue here. –  shirlock homes Mar 19 '13 at 10:08
    
It might help if you let us know where you live. –  Edwin Mar 19 '13 at 11:14
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2 Answers

Moist air is condensing (barring any oughtright leaks) It is most likely due to air leakage from house to attic:

  • Ceiling to attic is leaking moist air, condensing. Solution: air sealing of all penetrations (light fixtures, ducting, electrical, gaps in construction)
  • Insufficient insulation, combined with air leakage allows condensation to occur. Vapor barrier (VB) insulation (batt with paper facing) should NOT be placed OVER any existing insulation, creating a trap. If used, the VB is placed against the heated side in winter. Do not use poly for a vapor barrier. The assembly must be able to dry in either direction, should condensation occur.

Update RE can lights Recessed cans are notorious for leakage. If they are IC rated (designed for direct insulation contact) they can be air sealed with a ASTME 136 rated firecaulk directly. If they are non-IC, a XPS box can be built that meets the clearance requirements of the fixture (insulation must be a minimum of 3" around a non-IC fixture (NEC 410.66)) Many commercial products exist for covering cans.

Video: Airsealing recessed lighting by Green Homes America

I've seen this condition develop when a bath vent fan was dumping air directly into the attic and it saturated the insulation and it grew mold down through the drywall.

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I find the statement that escaping heat causes condensation to be misleading. Additional heat raises the air temperature, increasing it's moisture capacity, reducing the likelihood of condensation. No, the problem is the warm moisture laden air that is escaping must dump it's moisture load as it is cooled by the air from outside, causing condensation. You are right that poor insulation can cause condensation issues, I'm just quibbling over how you portrayed the physics. –  bcworkz Mar 19 '13 at 22:50
    
Agree RE physics, was loaded for bear on condensation. Will update. –  HerrBag Mar 20 '13 at 2:52
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This can happen around can lights. You get moisture in your attic and it will eventually drain out. The cans are your drain holes.

  1. You need to vent your attic better. Let the moisture escape. I mention this because it is cheap and a DIY job if you are up to it.

  2. Make 5 sided boxes out of xps or other rigid insulation sheets. The boxes should more than cover the can area - one box per can light. I would span the boxes across the joist the can is in between and maybe 2 feet the other direction. Caulk with silicone under the boxes to the sheetrock or joists is optional. Note on this - figure out if your cans are rated for insulation. If they are make your boxes whatever size. If they aren't your boxes will have to be a certain distance from the light housing.

  3. Add more insulation. I would stay clear of fiberglass. Spray foaming would almost be an instant cure and rock wool would work OK (not as well) too. You could even skip the boxes if spray foaming. But spray foaming attics sucks because you lose access to the hundreds of things running around up there (electrical). Just no fiberglass (and I wouldn't do recycled either).

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You can't skip the blocking around the cans unless they're IC-rated, regardless of insulation type. –  Evan Johnson Apr 19 '13 at 19:39
    
Correct. The blocking is an extra that the can installers (usually electricians helpers) around here use as an up charge. They usually charge like $15 a light which is actually something that most people should spend unless they will do it themselves. –  DMoore Apr 19 '13 at 20:53
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