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Thread: Floor Vapor Barrier Question

  1. #1

    Default Floor Vapor Barrier Question

    Ok - this is probably a stupid question but I figured I would ask it anyway as I do not know the answer.

    I am building a cabin, the foundation is on peers and beams. The joists sit on top of the beams. I will put hardfoam board in the spaces between the joists and then fill with insulation. On top of that will go the floor. I had plans of putting 6mil plastic on top of the joists under the plywood floor. A friend asked if I was going to glue the floor down and I said no just nail it. But that got me to thinking - I could not use liquid nails if I had a vapor barrier there and since I know some people do use liquid nails, am I about to make a mistake?

  2. #2
    Member Dirtofak's Avatar
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    IMO - You would be making a mistake.

    Vapor barrier goes toward the warm side. In Alaska that would be the top side of your floor. I glued the floor sheeting down to add rigidness and eliminate bounce and squeaks. I plan to add a sheet of cheap linoleum to act as the vapor barrier. I don't have it down yet and my floors aren't bad at all. R38 in the floor with OSB sheeting the bottom.

    A cold floor really stinks in a cabin. Be sure to check the bottoms of your walls along the sill plates for air leaks too. A ceiling fan REALLY helps with cold floors too.
    I don't mean to sound bitter, cold, or cruel, but I am, so that's how it comes out.
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    Moderator bkmail's Avatar
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    We laid plywood over the floor joists, no vapor barrier. Plenty of insulation below. Eventually we will lay a 2nd layer of "better" grade plywood over the "construction" flooring/plywood and paint it. No issues, floor is warm.
    Older cabin has same setup and been fine for 20+ years.
    BK

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    You can do what you said, vapor barrier over the joists and then the plywood subfloor. You are right in that you can't glue the plywood to the joist so I would use screws, not nails, to fasten it down. If the joists and the plywood are sized right, and you use screws, it shouldn't squeak. The vapor barrier is still on the warm side of the insulation.

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    One method i have seen used with sucess is to frame and sheet the floor and before putting down your finished flooring is to paint the sheeting with 2-3 layers of concrete-type water sealing paint and let it dry well.

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    Moderator bkmail's Avatar
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    I would not want the vapor barrier UNDER my floor. If you spill something or get water on the floor it will soak into the flooring and be trapped there till it evaporates. We also plan to paint the flooring.
    BK

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    I have had success with putting plastic on the joists then screwing the 3/4" plywood down on top of that. I used fiberglass to fill the joists and 1/2" plywood nailed to the bottom of the joists to close up the bottom.

    As mentioned it might collect water between the plywood and plastic but I decided to do this since I thought it would keep the moisture out of the fiberglass insulation better. That is what is important to me. I wanted to avoid having frost build up in the insulation. We just try to keep up on wiping up the water on the floor.

    With reguards to gluing the plywood down: I have been happy with several floors which I have not glued and only screwed the plywood down. Just don't be shy with the # of screws. They don't squeak or I at least don't notice that they do.
    As always there are many ways to skin this cat! Good luck with your research and cabin progress.

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    Member Rock_skipper's Avatar
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    Your house needs to be envoleped with a vapor barrier. Contray to all beliefs, there is a glue that sticks to both plastic and wood.

    When we were building in the villages we did glue the vapor to the joist and then screwed and glued the subfloor to it according to the Corps recomendition which was about every 3". That was about 10 years ago, so I don't know if its changed since then.

    The finish floor was mostly vinyl tiles.

    Its basically like if you have a crawl space you need to cover the ground and the walls all the way to the roof.

    If you build on pilings you need to envolpe the interior part of the building.

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    I have the excact same set up but with no vapor barrier. I also sealed up the bottem of the floor with cheap 1/4" plywood and boy did it make a difference holding heat. I screwed my floor down but if I had to do over again I would glue the %$^$ out of it before screwing. I have squeeks but they are MUCH better since I just put in laminate flooring. Although the floor is EXTREMLY slippery right now but been told that will go away with ware.

    Dirt- DON'T DO IT!! You seen what linoleum did inside my cabin. It's way to cold up our way for it. And I didn't use the cheap stuff either...
    '
    Right now Home Depot has laminate for .68 sq foot!

  10. #10

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    You do not want to use a hardfoam for the under side of the floor. You want wood or metal . The Squirrels will have a field day once they get a hole started. One thing that works good and is cheep is the window cutouts from metal doors. They are very ridged and easy to fasten to the bottem of the floor joist. The door shops are glad to get rid of it or pay a dump fee.

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    Moderator bkmail's Avatar
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    To hold the insulation in place under the floor our cabin has chincken wire stapled to the bottom of the floor joist and the insulation sits on top. Never been cold or had issues with critters (mice, squirrels, etc...).
    I initially wanted to enclose it with plywood to trap heat but the neighbors told me to leave it as is. No complaints from me after 5 years now.
    Not saying it is the correct method or best, but it's easy and works well.
    BK

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    Member Boone's Avatar
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    Partial thread hijack:

    My cabin is already constructed without a vapor barrier. I have between 3' and 6" space between the bottom of the structure and the ground.

    Would it be advantageous to place a vapor barrier On the ground under the cabin?

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    Default Floor Vapor Barrier Question

    There are some factors to consider to have a more complete picture of the building such as is the cabin skirted, is the skirting insulated, is the floor insulated and how much, etc etc.

    But I feel pretty safe saying no unless your floor is uninsulated and your crawlspace is thus heated which i doubt.

    Vapor retarder belongs on the warm side of the insulation. Put it outside your insulation and you just trap moisture were you don't want it, reducing the R value of your insulation as it fills with moisture and increasing the rate at which your building rots. Hence I am not impressed with house wrap despite its moderate permeability i believe it is too tight to let a wood building breathe enough.

    There are a lot of climates in Alaska where a vapor retarder is far from essential so I would not lose much sleep over your cabin not having one.

    Condensation in a crawl space is normal if that is your concern, best bet is to ensure it's ventilated and don't worry too much about it.

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    It seems to me that alot of ppl don't understant the whole purpose of the vapor barrier. The entire point of a vapor barrier is to prevent humid inside air form condensing inside you insulation and running on to your framing and rotting it from the inside of the walls, that is all, it NOT there to stop air leaks, thats what the insulation is for. It is also not there to stop outside moisture from getting in. If it is warm enough for the air outside to be moist, then it is too warm for it to condence inside your insulation. If it is so wet where you are that airborn moisture is causing rot, you need to use treated or marine grade lumber or ventelate the area to outside air to vent the excess moisture as in the case of enclosed crawl spaces or unfinished basements.

    The topic of floor vapor barriers has always had an almost even split between ppl who say you NEED them and those that think you don't. i'm not going to say either "side" is right. I have seen many cabins that are many years old and have none in the floor and have never had problems however those that don't always have a good source of outside air to the insulation and also allowing gravity to drain any liquid water that does build up. I would say if you are going to sheet off the outside of your joists with plywood or metal, then you would need a floor barrier. A 45-year old cabin i had in Maine was built with no barrier, however it was not closed off, it was covered from the bottom with 1/4 inch wire screen to keep out the squirrels, mice, ect. it worked great and never had any signs of water stains, rot, molds or critters.

    I would also agree with the use of basement water sealers. they work great against liquid water and lower the humidity comming from walls, they will definately seal a floor, however be warned, they are not designed for wear, so i wouldn't use it for a finish layer, but under a lamenate/finish paint it should work great. my suggestion would be to find one that doesn't cure hard and stays plyable (spelling??)

    and to Boone: you NEVER want a vapor barrier on the cold side of insulation. All this will do is keep the moisture IN your insulation and against your joists. What you are trying to avoid is the wet air from making it to the layer if insulation that is at the dew point temp. If it is on the outside, it will still condence and actually make things worse cause now it can't vent to outside air, lays against the joists and rots/molds them.

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    Member Boone's Avatar
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    Good info BK, thanks.

    I should have been more descriptive of my cabin. It's currently unskirted so air passes freely under the structure. I'm considering skirting to keep debris, snow, and kritters out. The floor has R-30.

    After the skirting dampens (or eliminates) the airflow under the cabin, would a VB placed over the ground be worth the cost of the plastic? Is there much moisture entering a craw space from the ground below?

    It would make for a cleaner storage space though.

  16. #16

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    I am going to insulate the bottom of the place soon. I was told by the framer to put batts first then R-tec on the bottom. Seems that the R-tec will act as a VB. Now with the options at hand which would be the best route to go.
    1. R-tec against the floor then batts and roofing felt stapled th the bottom of the joist.

    2. Batts against the floor then R-tec on the bottom of the joist.

    3. Batts then roofing felt.

    The roofing felt is what i have on hand, R-21 is the batts, the R-tec is R-10. screen or plywood for the finished job. The later will have to be brought in this winter.

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    Quote Originally Posted by Boone View Post
    Good info BK, thanks.

    I should have been more descriptive of my cabin. It's currently unskirted so air passes freely under the structure. I'm considering skirting to keep debris, snow, and kritters out. The floor has R-30.

    After the skirting dampens (or eliminates) the airflow under the cabin, would a VB placed over the ground be worth the cost of the plastic? Is there much moisture entering a craw space from the ground below?

    It would make for a cleaner storage space though.
    If you are going to skirt the cabin I would put plastic down on the GROUND first. It wil significantly reduce the moisture that wil come up from the earth and get trapped in the newly created crawl space.

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    Member Boone's Avatar
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    Quote Originally Posted by NRick View Post
    If you are going to skirt the cabin I would put plastic down on the GROUND first. It wil significantly reduce the moisture that wil come up from the earth and get trapped in the newly created crawl space.
    Roger that. My thoughts as well - just needed a little confirmation.

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    A lot depends on how high off the ground your cabin is, if it is skirted, etc. Having an insulated floor (with osb, ply, wire, or something like that) is fine if it is open to airflow. If a cabin is close to the ground, if it is skirted, you want at least a six mil v.b. under it with the seams all taped up and lapped over a good 24 inches or so. If you put the vapor barrier on top of the joists and insulation, and the cabin is built close to the ground and skirted, moisture leaving the ground will condensate on the v.b. and get your insulation wet and then it will fall. Condensation occurs at dew point. So anywhere the temp allows at a given humidity, you will have it.

    Having ventilation is important while maintaining the envelope because it allows humidity to escape.

    R tech is good stuff. It says R10, but when you factor in that there is zero chance of compression or cold voids (as there is in batt insulation) and full coverage (if you seal all joints with spray foam) which makes a tighter seal I think you will like it better. With batt insulation, any gaps or tight spots reduce the R value significantly. Unless every joist is laid out exactly right on center, the insulation batts will either have too small an area (sag, droop, bunched, squished, etc) or too large an area (batt won't fit, falls out, won't go from joist to joist). In either case, the effective R value is much less than advertised.

    Now if your joists are pretty flat across the bottom, the prefect system imo is to hang the batts in place and then lay the Rtech perpendicular to the joists on the bottom side of the joists. This BREAKS the thermal bridge of the joists (that's right, wood conducts heat/cold just as sure as you can't hold a tin wire in the fire without being burned). Frame and insulate the walls in the traditional manner and then attach the Rtech to the outside of the house as well and accomplish the same thing. Break the thermal bridge and combat thremal conduction. Dense wood easily conducts heat/cold.

    Tyvek is good for DuPont. Not much good for anything else. It aint even code to use it for houses, they just do a good job of convincing people to use it. You are better off with 15lb felt paper.

  20. #20

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    Thanks for your info.

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