Moist OSB under addition built on sonotubes


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Old 03-29-16, 09:49 AM
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Moist OSB under addition built on sonotubes

I live in New England. We built a 500 sq ft square addition on Sonotubes.

The floor joists are insulated with R-19. The R-19 is supported with OSB sheets hanging in the joist bays on strapping secured to joist bottoms.

I enclosed the 1 ft crawl space below the addition with painted PT plywood (with vents) to keep critters and drafts out and for looks (stupid). I now have soaking wet OSB board that is disintegrating and transferring moisture to the joists.

I opened up the crawlspace re-enclosed with lattice backed with bug screen.
I now have greater airflow.

In terms of replacing the wet OSB, I need some advice

note:I can't do any work from below as the crawlspace is not high enough

I plan to:
1. rip up the subfloor, remove the insulation, remove the wet OSB and strapping.

2. Roll the underside of the joists with primer to account for any moisture damage (not sure if necessary).

3. Rip down sheets of foam insulation board to hang in the joist bays (support with small angle brackets) to replace the OSB that was supporting the insulation.

4. Replace R-19 insulation and subfloor.

Since I only plan to do this once in my life, I want to make sure I do it right.

Does anyone see any faults in my plan or better suggestions? Should I cut the foam board to fit snug into the joist bays to prevent airflow or leave a small gap to allow air flow?

I could take full sheets of the foam board and attach to underside of joists rather than ripping to hang between the joists.
That would eliminate and ability for air flow though...

Please share your thoughts.
Thanks in advance!
 
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Old 03-29-16, 10:56 AM
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Do you have a vapor barrier on the ground in the crawlspace?
 
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Old 03-29-16, 04:43 PM
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No vapor barrier in place, just dirt. I should have included that I do intend to cover the ground with poly sheeting
 
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Old 03-29-16, 04:46 PM
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Yeah, I think the lack of that is your biggest problem.

In northern climates, it's usually better to seal a crawlspace from the outside and open it to the conditioned house air. From your description, that sounds like too much work so you'll want to make sure you keep good air flow. Just how much space is there between ground and the structure?
 
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Old 03-29-16, 04:53 PM
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Why, that's the first thing that should have been done before the sub floor even when in.
Paintings not going to do anything.
If it's that moist then fungus or mold is sure to follow if it has not already.
Any floor joist closer then 16" to grade should have been pressure treated.
Now your stuck with having to treat what's there with a product like Boracare with mold care.
BORACARE with MOLD-CARE
 
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Old 03-30-16, 01:52 AM
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Only about 12 to 14 inches from ground to joist
 
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Old 03-30-16, 01:56 AM
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About 12 inches from ground to joist
 
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Old 03-30-16, 05:06 AM
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Understood I need a vapor barrier on the ground. Any thoughts on closing the joist bays from below? With foam board sheets or sheathing or anything? I remember reading that sealing the joist bays from below will lead to moisture problems within the bays since no air flow from above or below....Thoughts?
 
 

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