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Thread: Cabin Floor Insulation.

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    Member Dirtofak's Avatar
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    Default Cabin Floor Insulation.

    What is a good way to insulate a cabin floor? I am utilizing 12" JCI/BCIs. I was thinking that I would put 2" blueboard on the bottom ledge, fill it with loose insulation, glue the flooring over it and put thin plywood on the bottom. Do I need a vapor barrier anywhere?

    Thanks,
    Mike

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    Quote Originally Posted by dirtofak View Post
    What is a good way to insulate a cabin floor? I am utilizing 12" JCI/BCIs. I was thinking that I would put 2" blueboard on the bottom ledge, fill it with loose insulation, glue the flooring over it and put thin plywood on the bottom. Do I need a vapor barrier anywhere?

    Thanks,
    Mike

    That should work OK. You didn't mention what size of cabin or wether your floor was elevated significantly. This is another one of those things where if you insulated a little more, you'd be better off. I can't remember the R-Value of 2" blueboard, but it can't be more than R-12. I would go with R-30 if you can. I know, hauling is going to be a pain.
    Make sure you get the plywood underneath on good or the critters will nest in there or worst start taking it away for their nests.

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    The hardest part of pre-insulating a floor is that you have to work fast to get the roof on. Even a small rain will start to soak the insulation. Forget about tarping it. It will spring a leak somewhere and you will be sorry. Most of them I have done I have just sucked it up and crawled under there after the roof was on. For a 12" BCI floor I would put R-38 in there then cap it with 7/16 OSB or 3/8 CDX on bottom to keep the insulation in and critters out.
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    Quote Originally Posted by dirtofak View Post
    What is a good way to insulate a cabin floor? I am utilizing 12" JCI/BCIs. I was thinking that I would put 2" blueboard on the bottom ledge, fill it with loose insulation, glue the flooring over it and put thin plywood on the bottom. Do I need a vapor barrier anywhere?

    Thanks,
    Mike
    Like AKDoug said the loose insulation is problematic. Better to do batt insulation after the fact. Then blue board and 1/4" wire mesh or 7/16" OSB.
    Yes you want a vapor retarder on top of your JCI/BCI and under your sub floor. If you have plumbing entering or exiting the building be sure the insulation wraps these. Be sure to tape any seams or joints in your vapor retarder.
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    Low tech for me. From underneath I used faced insulation and stapled through the fiberglass, facing toward the subfloor. Then I simply pulled the fiberglass back down to full thickness. The facing is the vapor barrier and holds the insulation in place. Leaving a couple of inches of air space between the bottom of the insulation and bottom of joists I enclosed the bottom with 1/4" plywood and left vent slots on both sides, then ran screen over the top and used 1x1 nailers to hold the screen in place. Simple, fast, and effective for nearly 20 years.

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    Quote Originally Posted by garnede View Post
    Like AKDoug said the loose insulation is problematic. Better to do batt insulation after the fact. Then blue board and 1/4" wire mesh or 7/16" OSB.
    Yes you want a vapor retarder on top of your JCI/BCI and under your sub floor. If you have plumbing entering or exiting the building be sure the insulation wraps these. Be sure to tape any seams or joints in your vapor retarder.
    Sorry, I meant to say unfaced fiberglass batt insulation instead of loose (like blown in) type insulation.

    I always thought that the initial flooring went directly on the top of the JCI/BCI and was glued (to prevent squeeks) and nailed down. I have never seen a floor with vapor barrier between the JCI and decking material. I was thinking that I might need some between the bottom of the JCI and the bottom covering/screen or between the decking and flooring.

    Thanks for the replies,
    Mike

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    Default Insulation installation and vapor barrier

    I would not proclaim to be an expert on the subject, however, I have always heard that if a vapor barrier is installed it should be located between the heated space and the insulation. If two types of insulation are used the higher R-value should be placed towards the heated space. Plywood by its very nature serves as a fairly good vapor barrier thus reducing the need for vapor barrier on the floor. Good luck in what ever you decide to use! I personally used multiple layers of foam cut to fit between the floor joists and am happy with the results!

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    What Akfarmer said. The plywood subfloor will act as your vapor barrier. Typically, you install something over that which will serve to seal the cracks between sheets of plywood.

    Also, cold dry air tends to come in through cracks in the floor, under doors and other low areas. Warm moist air trys to go out higher up. It's the moist air that causes the problems.

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    Sorry guys. Plywood is NOT a vapor retarder. Not even close.

    Any time you separate a warm space from a cold space the humidity in the warm air will condense on any surface that reaches the dew point. Somewhere in your walls between your 70* interior and the cold outside that temperature is reached. Where in the wall it happens changes with temperature changes. The way to prevent from having soggy, wet insulation is to install a vapor retarder on the warm side of the wall or floor. You might argue that a vinyl floor is an adequate vapor retarder. It probably is. Other flooring is not.

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    Default Kind of like installing Tyvek....

    Tyvek goes over the sheathing, but under the siding...Floor vapor over the floor underlayment, but under the finished floor.Using this meathod, I would then use a glass insulation without a vapor barrier.GR

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    Quote Originally Posted by dirtofak View Post
    Sorry, I meant to say unfaced fiberglass batt insulation instead of loose (like blown in) type insulation.

    I always thought that the initial flooring went directly on the top of the JCI/BCI and was glued (to prevent squeeks) and nailed down. I have never seen a floor with vapor barrier between the JCI and decking material. I was thinking that I might need some between the bottom of the JCI and the bottom covering/screen or between the decking and flooring.

    Thanks for the replies,
    Mike
    I was not sure what type of flooring you plan on using but you need a vapor retarder on the warm side either over or under the sub floor.

    Quote Originally Posted by Mr. Pid View Post
    Sorry guys. Plywood is NOT a vapor retarder. Not even close.

    Any time you separate a warm space from a cold space the humidity in the warm air will condense on any surface that reaches the dew point. Somewhere in your walls between your 70* interior and the cold outside that temperature is reached. Where in the wall it happens changes with temperature changes. The way to prevent from having soggy, wet insulation is to install a vapor retarder on the warm side of the wall or floor. You might argue that a vinyl floor is an adequate vapor retarder. It probably is. Other flooring is not.
    Absolutely corect. Moisture migrates through wood with ease. Inside the insulation the temp goes from above freezing to below freezing. If the air is moist then the moisture will condense and freeze. This will cause you to loose the r value of the insulation. Moisture move through the path of least resistance. If you have little or no resistance to the moisture migrating through the floor then it be prepared for wet/frozen insulation and a cold floor.

    Quote Originally Posted by Rovingarcher View Post
    Tyvek goes over the sheathing, but under the siding...Floor vapor over the floor underlayment, but under the finished floor.Using this meathod, I would then use a glass insulation without a vapor barrier.GR
    This would work better than what I first said. Joist with decking glued and screwed, then vapor barrier, then finish floor.
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    Default Heat loss

    Quote Originally Posted by akfarmer View Post
    I would not proclaim to be an expert on the subject, however, I have always heard that if a vapor barrier is installed it should be located between the heated space and the insulation. If two types of insulation are used the higher R-value should be placed towards the heated space. Plywood by its very nature serves as a fairly good vapor barrier thus reducing the need for vapor barrier on the floor. Good luck in what ever you decide to use! I personally used multiple layers of foam cut to fit between the floor joists and am happy with the results!
    With respect to heat loss, it should not matter what order you installed the insulation. R value is merely the value to define resistance to heat loss. Plywood is not a good vapor barrier.

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    Quote Originally Posted by qkayak View Post
    With respect to heat loss, it should not matter what order you installed the insulation. R value is merely the value to define resistance to heat loss. Plywood is not a good vapor barrier.
    Order maters if the insulation gets wet, which causes a loss in R value. That and the vapor retarder always needs to be on the warm side.
    It ain't about the # of pounds of meat we bring back, nor about how much we spent to go do it. Its about seeing what no one else sees.

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    Quote Originally Posted by akfarmer View Post
    I personally used multiple layers of foam cut to fit between the floor joists and am happy with the results!
    THat's just what I did, using 2" blue. Exposed on the bottom for over 12 years now and no critter problems.
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    Quote Originally Posted by bullbuster View Post
    THat's just what I did, using 2" blue. Exposed on the bottom for over 12 years now and no critter problems.
    Insulating is easy avoiding rodent damage isn't

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    Default insulation order

    Quote Originally Posted by garnede View Post
    Order maters if the insulation gets wet, which causes a loss in R value. That and the vapor retarder always needs to be on the warm side.
    I completely understand what you are saying about wet insulation having a lower R value. But my point is if your insulation is wet to the point where it loses R value, you are in a world of hurt or you just flooded the cabin.

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    Does anyone know of a book which covers these issues of proper ventalition and insulation that may be problematic when building small cabins? Or some sort of plans software program would be great?

    Thanks

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    Default Book...

    Fairbanks University has all the answers to building in Alaska.GR

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    Quote Originally Posted by AKFishOn View Post
    Does anyone know of a book which covers these issues of proper ventalition and insulation that may be problematic when building small cabins? Or some sort of plans software program would be great?

    Thanks
    Not specificaly for cabins but "Norther Building Design" is a great book. It is put out by the AIA. it is available through UAA book store or www.aiaak.org
    It ain't about the # of pounds of meat we bring back, nor about how much we spent to go do it. Its about seeing what no one else sees.

    http://wouldieatitagainfoodblog.blogspot.com/

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    Here is a link to the Alaskan Dept. of Housing finance. They put out a great manual, containing everything about building energy efficient loghomes. Been using it and it is a great resource.

    http://www.ahfc.state.ak.us/referenc...ion_manual.cfm

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