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Thread: Under Cabin Floor Joist Insulation Covering?

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    Default Under Cabin Floor Joist Insulation Covering?

    Need some advice and knowledge.

    What should be used under the floor joists to cover the insulation between the joists. I've heard some say that plastic sheeting shouldn't be used because that won't allow the insulation to dry out if it becomes wet from the top thru the subfloor. I read one place that recommended using 4x8 sheets of peg board because the holes in it allow air to get at the insulation for drying but still keep out critters.

    Seems like plastic sheeting is popular to seal up the underside of a cabin. Is that wrong? Need to do something with that area this summer and want to do it right. Thanks for any advice.

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    Member cdubbin's Avatar
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    Generally, you want more permeable materials on the "warm" side of your insulation, and less permeable on the "cold" side. I wouldn't recommend visqueen for this application. Plywood or OSB with 1/4" expansion gaps would be best, IMO. Some people use chicken wire, I don't like that either.
    " Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    One of the main things that you want to do with your covering is to keep the critters out and keep the insulation in place. I have the bottem side of my cabin and my shop covered with the window cut outs from metal doors.They can be nailed on with 16p nails and do not have to fall on a floor joist in order to fasten them on. They are ridged enough to span a great distance on the nailing. Two things that you get with these is another 1.5 insulation and a steel covering under the floor that no rodent is getting through. Then the best part of using the door cutouts is that they are free. The door shops are glad to get rid of them otherwise they pay to dump them. If one wanted to they could chaulk the joints to completly seal tham.

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    Member thewhop2000's Avatar
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    What we used out in the bush was tyvec stapled on underneath and then applied a layer of 1/2 inch osb on top of that. This was about 15 years ago so science could have changed things now. Even if insulation does get wet from above, in time it will dryout. I would myself put plastic down underneath the floor sheeting. Just pretend the floor is like a wall in new constuction. You have your vapor barrier, insulation, tyvec and sheeting. Been working pretty well since I have been building houses but to each, their own
    If a dipnetter dips a fish and there is no one around to see/hear it, Did he really dip?

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    Member cdubbin's Avatar
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    Quote Originally Posted by Big Bend View Post
    One of the main things that you want to do with your covering is to keep the critters out and keep the insulation in place. I have the bottem side of my cabin and my shop covered with the window cut outs from metal doors.They can be nailed on with 16p nails and do not have to fall on a floor joist in order to fasten them on. They are ridged enough to span a great distance on the nailing. Two things that you get with these is another 1.5 insulation and a steel covering under the floor that no rodent is getting through. Then the best part of using the door cutouts is that they are free. The door shops are glad to get rid of them otherwise they pay to dump them. If one wanted to they could chaulk the joints to completly seal tham.
    I love that! Great idea, man, rep sent.
    " Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    Someone recently recommended the soy-based spray foam, polyseal, for under the cabin. They said it holds together longer and sticks to the under-side of the cabin great. Has anyone else (who's road accessible) used polyseal?

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    Visqueen alone will age and break up in a few years, exposed to the elements. I put visqueen first, then the least expensive CDX or OSB plywood over that. Just to keep the critters out, the breeze from finding cracks and sealed off.
    2003 220 Hewescraft Sea Runner 115 Yam'y, Soft Top "Schmidt Happens"

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    Quote Originally Posted by cdubbin View Post
    Generally, you want more permeable materials on the "warm" side of your insulation, and less permeable on the "cold" side. I wouldn't recommend visqueen for this application. Plywood or OSB with 1/4" expansion gaps would be best, IMO. Some people use chicken wire, I don't like that either.
    That's backwards. Less permeable on the warm side to keep the warm, moist air from getting into the wall/floor/ceiling cavity. More permeable on the cold side to let moisture that made it through the barrier to get out of the cavity. If it is a seasonal cabin that will only be used during summer, I wouldn't worry about a vapor barrier under a 3/4" plywood floor. I've got foamboard insulation under my cabin floor and so far nothing has messed with it.

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    Member dkwarthog's Avatar
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    Quote Originally Posted by Big Bend View Post
    I have the bottem side of my cabin and my shop covered with the window cut outs from metal doors.

    Another great use for those insulated door window cutouts is for under slab insulation. They provide insulation, a hard surface to chair up rebar and walk on, and they are my favorite price

    I second what NRick said, permeable on the exterior, and sealed on the interior. That is why we use vapor barrier under the drywall.

    As far as the exposed, underfloor insulation covering goes....I'm in the let it breath camp. You will NEVER stop moisture from getting into the insulation, it needs to breathe to dry out or you will have rotten joists...

    I had the polysoy insulation sprayed in my 30x40 metal building. It is nice stuff, but I dont think I would use it for under floor insulation unless I knew it would pass the moisture out the bottom and it would not retain moisture. Otherwise you will have rotten floor joists.

    I would prefer the isocyanurate type of poly spray insulation (in general...walls, roof, etc), it is much more rigid than the polysoy stuff, has more R value, and stiffens your framing up incredibly, but it is much more expensive...

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    Member cdubbin's Avatar
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    Quote Originally Posted by NRick View Post
    That's backwards. Less permeable on the warm side to keep the warm, moist air from getting into the wall/floor/ceiling cavity. More permeable on the cold side to let moisture that made it through the barrier to get out of the cavity. If it is a seasonal cabin that will only be used during summer, I wouldn't worry about a vapor barrier under a 3/4" plywood floor. I've got foamboard insulation under my cabin floor and so far nothing has messed with it.
    Ha! that was backwards, I had my perms mixed up, good catch
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    Well, seems the answers are varied. Seal it off or let it breathe? What about the peg board idea? There must be a firm, correct structural engineering answer?

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    in our cabin we have vapor barrier under the plywood and fiberglass insulation between the joists held in place by a plastic netting. we insulated the floor because we do use the cabin in the winter. the cabin sits up 2-3 feet from the ground and we have skirting around the cabin to keep critters out. right after snowmelt it get so damp under the cabin that the insulation gets very damp/wet. it takes most of the rest of the summer to dry out. of course this makes me worried about the joists. one recommendation to us was to put in more vents to increase air flow in addition to the few we have now. it seems like there is a lot of experience on this forum. anyone else have advice for this?

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    My cabin floor will have:
    3/4" glued and screwed ply wood to engineered floor joists.
    R30 friction fit insulation between the joists
    1" foam insulation for a thermal break on joist bottoms
    1/2" plywood cover on joist bottoms, through foam
    skirted crawl space w/Tyvik wrap under the treated plywood-this crawl space will be vented seasonally

    I have seen plastic both under sub-floor or on joist bottoms be the cause of floors failing. Using Tyvik on joist bottoms allows the moisture to escape but prevents air (warmed and cold) from mixing. The opposite of Gor-tex.

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    You need to vent your crawl space better.


    Quote Originally Posted by lakecreek View Post
    in our cabin we have vapor barrier under the plywood and fiberglass insulation between the joists held in place by a plastic netting. we insulated the floor because we do use the cabin in the winter. the cabin sits up 2-3 feet from the ground and we have skirting around the cabin to keep critters out. right after snowmelt it get so damp under the cabin that the insulation gets very damp/wet. it takes most of the rest of the summer to dry out. of course this makes me worried about the joists. one recommendation to us was to put in more vents to increase air flow in addition to the few we have now. it seems like there is a lot of experience on this forum. anyone else have advice for this?

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    A thermal break UNDER the joists makes no sense.

    I used 5/16" OSB nailed to the bottom of the joists to enclose the insulation. I have 4" metal screen venting full length at each side of the cabin. 20+ years later it's in perfect condition and has seen no condensation in the insulation. I replaced the vinyl flooring last year and opened up a couple of places to check.

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    The best thing you can do with an enclosed crawl space-"skirting" situation is to waterproof the outside with bituthene, insulate the sides with foam board, put the plastic down on grade and sealed to the foam, no insulation between joists and no vents. I've done quite a few of these type of upgrades under the state energy rebate program, this is how the engineers always called it out. Keeps things quite dry and cozy, plus cool storage for materials, food.
    " Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

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    That would only apply to a structure that's heated 24/7.

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    where can i pick up som insulated door window cutouts?
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    Quote Originally Posted by greythorn3 View Post
    where can i pick up som insulated door window cutouts?
    Try any door shop that fabs doors to the frames like Spenards in Wasilla or Robinson Millwork.

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