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FishPCreek
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# Posted: 25 Sep 2012 09:37pm
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Our cabin in central PA has "hardboard" screwed to the bottom of the joists to protect the floor insulation and keep critters out. This was easier to work with a cheaper than cement boards. We are getting water stains on the seams at the perimeter where water runs down the side of the cabin, underneath the deck band board but outside of the Tyvek (we took off the lowest piece of cypress siding and added the decking after it was mostly framed). I took a piece off last weekend and the insulation is dry as it is above the hardboard, but it makes me think we might want to ventilate the space some - like small round soffit vents. Or should we ignore it, keep a watch, and if need be in the future replace it and fix the water - which would require some major adjustments in my opinion? It can be cold and windy there, and we do have gravel over plastic on the ground underneath, which ranges from 18" to 4' off the ground. I will add a picture if I can find it.
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FishPCreek
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# Posted: 25 Sep 2012 09:39pm
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The other picture did not make it - here is the hardboard shot.
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Anonymous
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# Posted: 23 Oct 2012 09:06pm
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I can't really tell what the problem is from your description; it's just not clear, for example, what seams you're referring to as being water stained.
Where would you be putting the vents you propose? And what would be their purpose? A soffit vent allows air to move between ceiling insulation and roof sheathing. Where are you proposing air be circulated in your floor assembly?
See if thinking these things through helps you find an answer, and post again if you can be more specific wrt the specific problem you're wanting to address. As long as you minimize the penetration of water and water vapor into your floor assembly, and the assembly allows the excess moisture to dry to the outside, you should be fine. If you're concerned, though, try attaching one or two or your hardboard sheets with screws and take a peek inside once a year after the wet season passes.
Certainly, minimizing the amount of moisture in your open crawlspace makes eminent good sense. Plastic sheeting is often the first line of defense, and making sure water does not accumulate on that sheeting is smart.
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Jack Strawb
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# Posted: 23 Oct 2012 09:15pm
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If it helps you get a handle on the issue, think of the floor assembly as a wall assembly laid flat. A raised floor acts much more like a wall than a roof.
We don't vent walls in the sense of putting actual vents in them. A sound approach is to minimize the amount of moisture that gets into a wall in the first place. In many climates that means putting a vapor barrier on the inside face of the stud wall assembly, because it's typically the case that indoor air is more humid that outdoor air.
We then build the wall so that any moisture that gets in can dry to the outside. A key point is that the wall's insulation can shed moisture, can get rid of it so that moisture doesn't reduce the insulation's effectiveness, and so that mold doesn't have the chance to grow.
Think of your floor assembly in light of the above. It's likely that you'll want your floor assembly to dry to the exterior, meaning to your open crawlspace. I'm not sure you would have wanted to have used cement board. Some boards act as vapor barriers, and that's not what you want. You don't want moisture that gets into the floor assembly to have nowhere to go.
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FishPCreek
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# Posted: 23 Oct 2012 09:56pm
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Thanks for the comments. I did pull back a sheet of the hardboard at one of the most stained parts and the insulation was dry underneath, and in fact was about 2 inches above the hardboard, so that any water that may have been sitting on the top would not have touched it. The water seems to be running down the outer band board and then "rolling"from the bottom of this to the hardboard which is mounted just inside this band board. I guess with the airflow under the cabin, as long as the insulation above this hardboard stays dry, maybe I will not worry about it.
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