Insulating sill plate

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  #1  
Old 03-22-16, 07:00 PM
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Insulating sill plate

The inspector in our parts does not like xps used in the sill as he wants it covered with fire proofing. Currenlty it is fiberglass there stuffed in the cavities with a drop ceiling. I could install the xps after inspection however I rather go with a safer insulation as recommended by the inspector. What are the best options other than xps and spray? Rockwool? I want to prevent mold...at least I can inspect up there yearly for growth.

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Old 03-22-16, 07:18 PM
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Roxul would be an ideal product. Waterproof, vermin proof, mold proof, fire resistant, better R value.
 
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Old 03-22-16, 08:07 PM
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I can under stand your inspectors logic but once the drywall's installed what's the danger?
By the time the XPS is going to be an issue there's going to be bigger fish to fry and the house would be totally involved.
 
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Old 03-22-16, 08:31 PM
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He is old school...His logic is the sill plate section is not covered with drywall. Yes the walls are enclosed and firestop at the top of the walls, but if you lift the ceiling tile you can see it. How it would ever ignite is beyond me as there is no electrical etc in the sill....and the rest of the ceiling between joist is stuffed with FG.
I thought of just leaving the fiber then putting in the foam "down the road" although this would be harder going through the ceiling tiles.
 
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Old 03-23-16, 12:01 PM
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Just like against the wall itself, foam on the outside followed by something fireproof like Roxul sounds like a good idea to me. Foam to retard moisture infiltration and decrease the likelihood of condensation and then mineral wool for more insulation and fire block.
 
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Old 04-03-16, 03:13 PM
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I have read that fiberglass can cause mold when stuffed into the cavities. Can you use a "hybrid method" using both foam and fiberglass. If I first used spray foam such as great stuff around the perimeter ONLY of each bay (outline all the corners like a picture frame) would this stop down on the air infiltration? Would I then be able to stuff with the FG baths and not have to worry about the possible mold issue?
 
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Old 04-03-16, 04:11 PM
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Just foaming the perimeter won't solve the issue. The potential for mold comes from warm moist air diffusing through the fiberglass and condensing on the cold rim joist. Two inches of spray foam or 2 inches of rigid foam with the edges sealed with can foam are enough insulation that the inside surface of the foam won't be below the dew point of the inside air.

The concern with exposed foam (meaning foam not covered with a thermal barrier, not foam you can see) is that the stuff gives off noxious toxic vapors when exposed to high temperatures. This is why the code mandates that foam be behind a thermal barrier.

The combination of 2 inches of foam and then stuffing the remainder of the area with Roxul is a great way to go. Since Roxul is mineral wool, it serves both as a fire stop and a thermal barrier.
 
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Old 04-03-16, 04:21 PM
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Use Thermax, tape edges with foil tape and show him the class A fire rating. If your area requires something more, then you have to comply. He is right that ordinary XPS needs to be covered. All it takes is putting a piece of drywall over the xps.
 
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Old 04-03-16, 04:50 PM
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Great. Thanks for the input everyone.
 
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