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Thread: insulate floor?

  1. #1
    Member akriverunner's Avatar
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    Default insulate floor?

    I am using BCI's floor joists and wondering how I should go about insulating the floor. I was thinking of ripping osb to lay between the joints then use fiberglass bats but should I vapor barrier before the subfloor?

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    Member Dirtofak's Avatar
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    I used 12" JCIs on 12 in centers. R34 fiberglass between them, then put 2"blueboard on the ledge. The whole bottom is sheeted with OSB using sheetrock screws. It made a HUGE difference.
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
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    You should have a vapor barrier above the insulation, but I didnt and it hasnt been a problem so far. It's very dry here tho.

    I also insulated full depth of my floor joists with fibreglass batts, then covered over from the bottom with OSB. I just nailed it on.

    It made a huge difference in my place also. Didn't have the bottom skirted, and the dog water froze in the bowl in the house in some 20 below weather. I thought the floors felt a bit cold,....

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    Member AKFishOn's Avatar
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    I would use a vapor barrier if you are in a wet area. I did not where my cabin is (not to wet). I also later skirted the cabin as that is what State Farm wanted me to do in order to insure it.

  5. #5

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    Vapor barrier should be on the side of a wall, floor, or ceiling that places it as close to the heated living area as possible. This means that when you are inside the cabin, the vapor barrier is the first thing beneath the subfloor, or behind the dry wall/panelling. For cabin floors, it's best to put vapor barrier down on top of the floor joists before installing the sub-floor.

    The idea is that the living space is where moisture accumulates from cooking, breathing, bathing, etc. If there were no vapor barrier, the moisture in the air (water in a gaseous state) would migrate outward through the walls, floor, and ceiling. Eventually, as it moves outward through the insulation, it will cool until its temperature reaches the dew point. At the dew point, you would get water condensing in the insulation, or if the dew point were below freezing, you would get frost formation in the insulation. The vapor barrier prevents this migration of the humid interior air.

    Fortunately, winter air in south central and interior Alaska is usually very dry to begin with. This means the air in a cabin will start out very dry and become only moderately humid while you stay there. So, in a cabin without vapor barrier, there may be little or no problem depending on the habits of the occupants. If you were to install a high capacity camp shower, and take a lot of long hot showers, your insulation with no vapor barrier protection could end up dripping wet!

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    Moving outside air through your insulation toward the heat will make condensation, too. That's the likely scenario with your floor. Absolutely yes you should insulate. Since you're doing that you need to add a vapor barrier. Foil faced insulation installed with the foil facing up is a decent way of doing it easily. Adding a sheet vinyl floor on the top side helps, too. Critters will love your insulated floor if you don't take measures to keep them out.

  7. #7

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    What are your BCI sitting on now? Is there a foundation or is your subfloor resting on railroad ties, pilings etc.? Did you already put down the plywood on top of your BCIs? I think the obvious answer is yes you need to insulate and vapor barrier, the question is how should you best go about doing it. Can you offer a little more information first?

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    Member Music Man's Avatar
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    rifleman's description is right on. I have 1/4' square mesh wire under mine and have had no problems with critters. It is easy to access to add wiring, propane pipe. fresh air for stove or what ever might need changing in the future. OSB would be another couple trips with the freight sled or boat.
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    DO NOT put poly on your floor joist, unless you like a squeaky floor. Glue down sub floor to joist. You can start with putting in wire mesh on underside of joist, then insulate. or do it from below. When done like this it allows any moisture to evaporate. None of the cabins I've worked on has had any problems with rot. It is a good idea to skirt the building.

  10. #10
    Pilot1995
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    I have built several small cabins and have had good luck with what I've done. First, lay out the floor joists & rim joists as normal, square it, sheet it from the bottom with 1/2" CDX (or OSB) using nails. Fill the joists with fiberglass, lay down a sheet of viqueen for a vapor barrier then dowscrewn the 3/4" flooring. I can't say I've got squeaky floors from doing this.

  11. #11

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    Quote Originally Posted by Pilot1995 View Post
    I have built several small cabins and have had good luck with what I've done. First, lay out the floor joists & rim joists as normal, square it, sheet it from the bottom with 1/2" CDX (or OSB) using nails. Fill the joists with fiberglass, lay down a sheet of viqueen for a vapor barrier then dowscrewn the 3/4" flooring. I can't say I've got squeaky floors from doing this.
    That is a real good idea Pilot1995. I going to have to remember that idea. A question though. Why not just vapor barrier the bottom instead and then nail the CDX or OSB with 8" rimshanks? Then lay down your fiberglass and screw and glue the 3/4 on the top? I always lay a bead of glue down first before I lay down the 3/4 floor. It might ever be a good idea to run a bead of black death around the rim first too. Before you nail the 1/2 CDX or OSB to the bottom. Good idea. Thanks for sharing with us.

  12. #12
    Pilot1995
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    Attachment 47617Attachment 47616

    Here's a couple pictures of how I have done it - many ways to do it & its interesting to know how others have done things.

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    Pilot1995
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    Quote Originally Posted by Bushwhack Jack View Post
    That is a real good idea Pilot1995. I going to have to remember that idea. A question though. Why not just vapor barrier the bottom instead and then nail the CDX or OSB with 8" rimshanks? Then lay down your fiberglass and screw and glue the 3/4 on the top? I always lay a bead of glue down first before I lay down the 3/4 floor. It might ever be a good idea to run a bead of black death around the rim first too. Before you nail the 1/2 CDX or OSB to the bottom. Good idea. Thanks for sharing with us.
    In cold cimates you want to keep the moisture inside the building away (out of) the insulation. That's why you vapor barrier on the inside of the building. Putting the vapor barrier on the bottom of the floor will allow moisture into the floor & keep the moisture in the insulation.

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    Quote Originally Posted by rifleman View Post
    Vapor barrier should be on the side of a wall, floor, or ceiling that places it as close to the heated living area as possible.
    Wanted to mention that placing the vapor retarder "as close to the heated area as possible" is not necessary.

    Yes, it needs to be on the warm side, however, it does not need to be absolutely on the inside. There is a formula for figuring out what percentage of the insulation needs to be outside versus inside the vapor retarder, I could find it in a CCHRC publication I have somewhere, it is based on the coldest outside temperatures expected.

    In southcentral AK, for all places except perhaps the really cold spots in the willow area (and even there probably ok), it's acceptable to have up to 1/3 of the wall's r-value inside the vapor retarder. This is assuming your conditioned air has a reasonable humidity, and since we tend to be too dry rather than too humid, there's no problem really. I believe in the interior of the state such as fbks and delta, the general rule of thumb is max of 1/4 of the wall's r-value inside the vapor retarder.

    What happens is that as the air travels through the wall, the dew point is never reached in the inner third of the wall.

    Placing the vapor retarder on the inside pf the studs of an r-19 or r-21 wall, and then firring with 2" rigid foam insulation inside of the vapor retarder before hanging drywall allows for a 2 or 2 1/2 inch conduit to run all wiring, plumbing, etc inside the vapor retarder, and you end up with almost zero penetrations.

    This method is much superior to putting the vapor retarder on the very inside and then proceeding to poke dozens of holes as you plumb and wire the place, as sealing up those holes is time consuming, and as a result often done poorly.

  15. #15

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    Quote Originally Posted by Pilot1995 View Post
    In cold cimates you want to keep the moisture inside the building away (out of) the insulation. That's why you vapor barrier on the inside of the building. Putting the vapor barrier on the bottom of the floor will allow moisture into the floor & keep the moisture in the insulation.
    Good point. Thanks for the tip.

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    Member Gerberman's Avatar
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    This is good reading, by accident I did the right thing, I got the insulation up in the roof, but did not have the time to put up the tongue and grove, so I put up Tyvek to hold the insulation, then next trip I put up the tongue and grove right over the Tyvek, there is 10" R34 insulation above the Tyvek 2" airspace, then 1/2" plywood, tar paper and then roofing. It should keep the moisture in the right place.

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    Member Music Man's Avatar
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    Quote Originally Posted by gerberman View Post
    This is good reading, by accident I did the right thing, I got the insulation up in the roof, but did not have the time to put up the tongue and grove, so I put up Tyvek to hold the insulation, then next trip I put up the tongue and grove right over the Tyvek, there is 10" R34 insulation above the Tyvek 2" airspace, then 1/2" plywood, tar paper and then roofing. It should keep the moisture in the right place.
    I don't think tyvek is a vapor barrier. It is used to wrap the outside of houses to keep out air infiltration but visqueen is still needed for a moisture barrier. Some one correct me if I'm wrong.
    When seconds count, the cops are just minutes away.
    '08 24' HCM Granite HD "River Dog"

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    Member Gerberman's Avatar
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    Default Tyvek

    This is what DuPont says about it
    DuPont™ Tyvek® HomeWrap®


    DuPont™ Tyvek® HomeWrap® delivers the optimum balance of properties for superior performance against the elements. Our house wraps help prevent outside water from entering the walls and help to seal the home to keep outside air where it belongs. DuPont™ Tyvek® HomeWrap® helps reduce homeowner energy bill because homes stay cool in the summer and warm in the winter and dry all year round.

    I hope it works as they say it should, Thank You for making me research it.

  19. #19
    Member power drifter's Avatar
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    Nope Tyvef is not a vapor barrier. Its like gortex for your house. Its made to keep the wind from blowing in but let any moister there may be pass through. Plastic sheeting works and aluminum is best. Thats why some foam sheets have a foil side.

  20. #20
    Member Dirtofak's Avatar
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    Quote Originally Posted by power drifter View Post
    Nope Tyvef is not a vapor barrier. Its like gortex for your house. Its made to keep the wind from blowing in but let any moister there may be pass through. Plastic sheeting works and aluminum is best. Thats why some foam sheets have a foil side.
    Agreed! And it is also good to tape the staple lines and seams of the visqueen.
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
    Bill Hicks

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