mold between insulation and roof

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Old 11-16-12, 11:09 PM
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mold between insulation and roof

Hi! Doing renovation on a porch addition turned into interior and when I removed the ceiling drywall/insulation I found some mold. The roof is corrugated steel/tin on top of 3/4" OSB on top of yes... 2x4s. Doesn't look like any holes are there but maybe it's due to inside vs. outside temps rather than direct precip. When I replace the insulation and reinstall drywall I want to do it right, should I get kraft faced insulation and do a plastic vapor barrier between the OSB and the insulation? Attached is a pic of the roof without insulation.

Thank you!
 
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Old 11-17-12, 04:59 AM
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Is there roofing felt between the OSB and metal roofing?
 
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Old 11-17-12, 06:43 AM
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Good question and one I'll have to get back to you on.
 
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Old 12-03-12, 12:28 PM
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There is roofing felt between them.
 
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Old 12-03-12, 03:30 PM
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The felt should alleviate any concerns with the metal sweating and getting the wood wet. Ideally you'd have air ventilation between the underside of the roof and the ceiling but being a porch, that's probably not an option. Hopefully someone smarter than me will have a better answer for you
 
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Old 12-04-12, 12:44 PM
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Well the roof is the corrugated steel fit together sheeting that should allow for some ventilation. I'm trying to find a closed cell sprayer in town but I'm coming up empty and for 145sq.ft. it's not worth it to have a company drive here from the nearest town (~80miles away).

I'm thinking just faced batts with maybe some 6mil poly on the face of them and seal up any potential leaks in the roof from above.
 
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Old 12-04-12, 01:00 PM
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Can you buy spray foam for a DIY application? Something like the gaps & cracks cans but in larger quantities with a wider spray and higher velocity?
 
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Old 12-04-12, 03:24 PM
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There are some sites online that sell diy foam insulation kits. They come with a canister of the foam, hose and gun. They are a little pricey...... that's about all I know about them.
 
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Old 12-04-12, 03:56 PM
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My guess would be the moisture issue is coming from humidity inside the home migrating by diffusion or air leakage up through the insulation where it is finding a cold surface. If you spray in foam generously, the inside surface may (depending upon how you cover it) stay warm enough to avoid condensation. To use any type of batt insulation you would need an air gap and ventilation between the insulation and the bottom of that roof. Venting above the 3/4" OSB is a different issue.

I assume this space is heated?
Any soffits or overhang into which you could install low vents.
Then you would need roof vents at the highest possible point to provide an exit.

For this area, the cost to spray in 2 to 3" of foam would not be that bad when compared to providing proper ventilation to utilize batts.

There are several products that can be shipped directly to you and DIY applied. A Google search for DIY spray foam should find a source.

Bud
 
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Old 12-05-12, 11:48 AM
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Bud,

Spray foam guy quoted me $406 for 3.5" of spray foam or $300for 1" of spray foam + unfaced batts. The space is heated. There's a slight overhang of a few inches.

I'd rather just install faced batts and be done with it but I also don't want the OSB to get any worse and I'm afraid since it's a corrugated metal roof (north facing below the gable, receives very little sun) if I don't do it right I could be causing more mold and having to replace the OSB in the not so distant future.

Looking into it, were I to go the DIY route for spray foam the materials for 1" of foam and shipping alone would be the same price as having the guy do 1" + batts.
 
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Old 12-05-12, 12:06 PM
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With your climate (cold) one inch of foam would be marginal. Once you cover it with the batt insulation, you still run the risk that the dew point will fall below the foam, thus moisture in the insulation and on the 2x4 rafters that never get as mush foam. Flash and batt as the practice is called also has the issue of a poor fit for the batts. Spray in foam is never neat and every little gap provides a path for moist air.

AND then there is moisture diffusion where the moisture vapor in the air migrates right through the drywall and sneaks past the vapor barrier with all of the staple holes to get to the places you don't want it. $406 for 3.5" of foam looks like a good deal to me for a job you don't want to do over. Clean it up, treat the mold, and fill it with foam.

Bud
 
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Old 12-05-12, 02:25 PM
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Sigh. So what's the other option besides dropping $400 on 150sq.ft. of 3.5" foam insulation?
 
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Old 12-05-12, 04:25 PM
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Well, if you can add the proper high and low vents, then you can install baffles the full length and use batt insulation. But between the vents, the baffles, and less r-value for the end result, it may end up the same or more for cost with less desirable results.

Spray foam is generally $1 per board foot. That's $3.5 X 150 = $525 so your quoted price is very good considering it is a small job, which often cost more.

You have seen what doing it wrong can lead to so no way to repeat what was there. Even with the a full fill of foam, you will be below R-20 and in your area R-40 to R-60 would be better. Batts with an air gap would be less than R-12.

Bud
 
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Old 12-06-12, 12:09 PM
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Sigh, I know you're right and if i want to do it right foam is the way to go unless i completely redo the roofing sublayers. I'm going to spray the moldy stuff with a little bleach+water tonight so they can foam first thing in the morning and then go over the outside surface of the roof and make sure all holes and screws are properly installed and sealed.

I think your deduction that it was warm inside air going up causing the moisture is accurate and really don't want to have to mess with the roof once the foam is in. I just don't want to redo anything in the immediate future and if that means spray foam for 2-3x the cost of batts I'm ok, and it should really really really help seal that room up and add a little more support.
 
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Old 12-07-12, 04:26 AM
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Mold

Mold only grows where there is food and water to keep it alive.
In your case the food is the OSB the water is provided by condensation.
Remove one or both and the mold dies.
Mold cannot live on many things, clean the OSB with bleach and coat the OSB with gloss paint or varnish these will allow the condensation to form but it won't lead to mold.
You can leave the OSB uncovered where the warm air circulation will keep the OSB dry.
You can clean the mold off and then insulate by fitting sheet polystyrene up tight against the OSB carefully filling the spaces between the rafters, then seal the space by fitting a water vapour proof plastic sheet across the bottom of the rafters before fitting dry wall.
 
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Old 12-08-12, 11:34 PM
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It's done. Spray foam, while not DIY, was done and it's wonderful. OSB was cleaned with bleach water.
 
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