Vapor Barrier question

topic posted Tue, March 27, 2007 - 2:07 PM by  Witchy
I'm almost done with the insulation on the addition. Everything I've read says to put up a vapor barrier between the insulation and the drywall. They mention plastic sheeting, but nothing tells what mil to use.

Any ideas?


posted by:
  • Unsu...

    Re: Vapor Barrier question

    Wed, March 28, 2007 - 12:06 PM
    Any plastic sheeting will provide a substantial barrier. The cheaper the better.

    I'd not use it all. A Tyvek layer under the siding outside the sheathing is my preference. Trapping moisture from either side of insulation may cause it to remain damp longer.
    • Re: Vapor Barrier question

      Thu, March 29, 2007 - 10:49 PM
      It really depends on the climate. According to the texts in my Cold Weather Climates Bulding Class, in northern and colder climates, you want a 6 mil poly (six mil stands up to staples and installation) on the warm side (interior) of the house. The barrier keeps the moisture in and keeps it from condesning in the walls as it cools. In southern humid climates, the poly goes on the exterior.

      Tyvec is a semi permiable surface that blocks water but not air. That is why it is used on the exterior. It allows the moist air that managed to get throught the wall out, while preventing hard winds and driving water out.

      remeber to make sure that you have a continuos barrier (tape over seams and staple holes. the smallest holes can still let through pints of water a day.
      • Vapor Barrier

        Thu, March 29, 2007 - 11:29 PM
        Use 6ml poly - Pronounced six mill polly.

        Go to the local hardware store and ask for a roll of this, get the 10 foot wide roll, and borrow or rent a staple hammer. You will also need a caulking gun and tube or tubes of caulking enough to caulk all along the outside edges of the poly that you put on the wall. You will also need insulation tape made specifically for this application - It is usually red and see-through. You will find this tape at the hardware store too. It is used for connecting the edges of the poly together. You will also need a utility knife to cut the poly as needed.

        You have to cut pieces of poly and staple them to the studs, there will be seams between one sheet of poly and the next. Start stapling sheets of poly to the wall, starting from top to bottom. So start by stapling the poly all along the "top plate" which is the horizontal 2X4's that run along the top of your wall. Staple the poly 1/4 from the bottom of the 2x4 top plate and leave at least an inch of poly below the staples.

        This is important! Leave at least an inch of poly on the inside of the stapled edge. The reason for this is simple: Once you have finished stapling the poly to the top plate of the wall, you will have to run your caulking gun around every opening - all the window and door openings. You will have to seal all those edges with a "bead" of caulking running along the the entire inside and outside edge of the poly. This seals the poly to the wall and prevents moisture from getting into the wall.

        When you have a free hanging sheet of poly running the length of the wall, go along with your exacto knife or utility knife and cut holes in the poly that fit the windows, doorways, electrical outlets, etc. When cutting holes in the poly for light switches, cable outlets and electrical outlets, always make the hole smaller than the size of the outlet box that the cable, wiring etc is coming from.

        For windows and doors, cut the poly along the inside of the opening being carefull not to shave any pieces of the finished door or window frame wood when cutting the poly.

        DO NOT CAULK THE WALL UNTIL AFTER YOU CUT OUT THE OPENINGS FOR DOORS, WINDOWS, SOCKETS, ETC. if you caulk the wall before you cut the openings in the poly, it WILL get messy before your finished.

        Whenever you have 2 pieces of poly coming together to form a seam, allow enough poly to overlap at the seam to at least 2 inches. Caulk the sheet of poly that is on the inside - caulk at least 1 &1/2 inches of the overlap so that one piece of poly is stuck to the other by caulking. Now use your insulation tape to seal the overlapping poly at it's edge being sure to use one stripe of tape equally along the seam so that the tape covers both pieces of poly equally along the seam.

        Always seal telephone outlets, electrical sockets, cable outles, light switch boxes and any other holes as well as you can. Use the caulking. Be sure the face plates of these sockets and outlets will cover the poly. do not let the poly cover over these outlets.

        Any questions?

        • Re: Vapor Barrier

          Fri, March 30, 2007 - 8:10 AM
          I'm with Cliff on this one. I would not put a vapor barrier on the inside of a house wall if you already have house wrap on the outside. Wooden houses have to do a certain amount of breathing. I asked this very question of my building inspector when I was building my house and he said if I sealed the inside wall, I would be asking for mold problems because the moisture would not be able to escape.
          • I have an idea Witchy. Go ask someone 'else' who has built houses, and is currently in the business of building houses - Someone with a "good" reputation. Especially if this is YOUR house that you are talking about. I spent 6 years in the construction industry and have known house inspectors - and have had my VB work inspected by them and approved.

            It is my opinion that if you are putting drywall on the in-side of an outside wall, you need a vapour barrier. The only buildings I have seen without a vapour barrier are thatched houses in Jamaica. And yes, climate is a factor - Do you live in a desert that experiences zero precipitation?

            You will get many opinions in online forums such as this one, but in the end; who are these people? And what qualifications do they have Witchy? Please be careful regarding something as serious as home renovations and online advice. Go ask someone who lives in the "Real World" and does this sort of thing for a living.

            And please, do not let unknowns in this forum making unfinished statements and giving partial information that may not be contextual to your situation color your decisions. Also, cheaper is not better.

            • I am certified in energy efficient building techniques. I can sign off on 5+ star rated homes. Dive is absolutely right. You need to talk to somebody localally. One place you could go is the home builders association or go on-line the the forums at Fine Homebuilding through tauton press. A lot of the longer time builder out there can't seem to wrap their brains around the newer building technology and will nay-say it to death. They do have experience, but that experience is often at odds with new materials and technology.

              That being said....

              Energy effeciency doesn't trump health when it comes to problems like mold. The house is a series of inter-working systems that need to be balanced. Yes, a house needs to breath, but you can control the breathing through passive and mechanical means. In fact you can actually recover heat loss through air exchange by installing heat covery ventillators.

              You can also have a mold problem if you don't put up a vapor barrier. As the hot air passes through the wall, the cold air causes the water to condense on the inside of the exterior wall. This soaks the wood and insulation. The fiberglass insulation is rendered usesless and becomes a wet sponge sitting on wood. (Wood rot and black mold.) The other effect is that you would now have a cold spot along the wall. The wall will now condense in the interior space at the cold spot also causeing mold. Well installed VB helps eliminate mold problems.

              • Unsu...
                i agree andy. the vapor seal always goes on the inside. the vapor seal is NOT the same as tyvec. which you are also correct about. one more thing. you don't caulk vapor barrier seams you TAPE them. and another thing.... mold needs moisture, darkness, warmth and STILL AIR to thrive. deny these and theres no mold problem.
                • Yes, again an important distinction between a VAPOR barrier, a MOISTURE barrier and permeability.
                  Tyvek is a semi permeable MOISTURE barrier but NOT a VAPOR barrier meaning water cannot go through but water vapor CAN.
                  Used on the outside of the house, think of it as goretex for your house.... it can sweat out but not get drenched.

                  A VAPOR barrier in not permeable, meaning its plastic... it holds water, air, everything from passing through it. This goes on the inside. behind the drywall.

                  The simple common rule I have always heard is the VAPOR barrier almost always alway goes on the warm side of the wall. a moisture barrier may go on the outside.
            • Unsu...
              ******I have an idea Witchy. Go ask someone 'else' who has built houses, *************

              You can be telling yourself that the people here have no building experience.
              There are several of us here who have backgrounds as tradesmen in the building industry.
              I'm one of them.
      • Re: Vapor Barrier question

        Thu, September 8, 2011 - 9:14 AM
        Just purchased a 1950 home with hydronic heating in the floor and vented drop down boxes under all the windows on the South and West walls. We pulled out old sheeted insulation in May (when we moved) to get the benefits of the air circulation (and whole house fan), but now that fall/winter is coming on, I'm not sure what to put back into those boxes. There are 9 boxes approx. 9 1/2" x 6' long (all slightly different of course): There may be approx 5 to 6" of depth space inside of each box ( by 6' length) They are all screened. The windows above them are all fixed double paned glass that do not open (amazing views).

        Thought about block foam and attach some type of handle to pull them out yearly. Don't want to put back the attic style insulation that they had in there cause of the mess. We live in OH

        Any ideas?
  • Re: Vapor Barrier question

    Sun, July 18, 2010 - 9:08 PM
    I stumbled on this thread via a google search. I was appalled enough that I went through the hassle to create an account just to post this:

    Vapour barrier, or more accurately, vapour retarder is used to stop moisture inside your home from migrating out towards the cold side and condensing on the way. This is especially true in harsh winter climates, where you have double digit negative temperatures on the outside (i.e. think about how dry the outside air gets at -30˚C).

    Weather barrier such as tyvek, which goes *outside*, has the sole purpose of keeping water (not moisture) and wind out of the house.

    The caulking some are talking about is useful too, stuff like Acoustiseal by Lepage, and is essentially a step beyond using tuck tape.

    If you live in Canada, get the "CMHC book" for something like 35 bucks and it details every best practice about wooden stick frame construction. You'll be happy you did.

    Btw, a lack of vapour barrier is the kind of thing that you won't notice for 40 years until you open your dry wall for some renovation and notice its moldy and rancid in there from decades of slow moisture seepage.
    • Rob
      offline 0

      Re: Vapor Barrier question

      Sun, January 16, 2011 - 6:07 AM
      Builders and bureaucrats will always have their 'company line' when they provide advise because they are accustomed to being held liable if something fails -- they always fall back on the manufacturer's installation instructions.

      ANSWER: A vapor barrier stops air humidity from travelling past a line.
      --->Think of putting your shower stall next to an exterior wall, and steam the place up a couple times a day when it's cold outside. Obviously, you want a vapor barrier on the inside to keep the steam from collecting on any cold surface.
      ---> Now think of installing a walk-in freezer where you keep your nutty buddys and sno-cones fresh. The moisture moves in from the outside. This is where you want your vapor barrier.
      --->But what about a normal house that has heat in the winter and air conditioning in the summer? Do you need to rip out and rebuild your walls every 6 months to prevent mold?

      The notion that 'the wall must breathe' seems incredably stupid to me. The use of rolled or batted fiberglass insulation is a necessary evil to the DIYer. This is an excellent insulator, as long as it's kept dry. The only solution for fiberglass is to install vapor barriers on BOTH sides so no vapor moves at all. Any insulation is useless if there is air flow through it anyway.

      Professionals are now using FOAM insulation. Foam is a good vapor barrier all by itself, and it gives no opportunity for any moisture to move from either side. Indeed, this is the perfect solution. I've seem foam used directly under prefab roof sections without an air gap... and it worked, and it's warranteed.

      I see a day decades from now when people will scoff at fiberglass insulation as if it's knob/tube wiring. But for now, that fiberglass is the only thing a DIYer can afford to use in both cost and set-up time. Let's see.. foam needs $10,000 worth of equipment, fiberglass needs a $20 stapler.
      • Re: Vapor Barrier question

        Sun, January 16, 2011 - 3:55 PM
        We finished an addition this year and used 3mil (?) plastic as a vapor barrier. We also have house wrap. I live in Maine.
        • Unsu...

          Re: Vapor Barrier question

          Mon, January 17, 2011 - 6:51 PM
          I used to live in Maine. North of Bangor. Had horses a huge property lots of woods immediately behind stretching up to Canada
  • Re: Vapor Barrier question

    Wed, October 19, 2011 - 3:18 PM
    If you live in a cold climate, you might want to check my website to see how we did it, step by step. It's at
    • Re: Vapor Barrier question

      Thu, October 20, 2011 - 2:29 AM
      In all my previous experience the moisture barrier only goes on the outside of the insulation, it doesn't matter about sealing it as long as any moisture that condenses can only run away from the insulation and not into it......ventilation is important in timber framed buildings as
      wood will expand and contract with humidity. The house needs to breathe
  • Re: Vapor Barrier question

    Tue, February 7, 2012 - 6:29 PM
    I would agree that how much plastic sheeting you should use depends on what kind of climate that you live in. another thing to consider is where there would be a lot of humidity in the house. If you apply the plastic sheeting inside the house as well, you might lock in the moisture in the house, resulting in mold problems. Your house needs room to breathe as well.

    Mark -
  • Re: Vapor Barrier question

    Wed, March 14, 2012 - 9:30 AM

    I am building a Timber Frame in the Spring and I would like it to be dried in asap. I am asking an opinion on if anyone thinks this will work.
    1. Erect the timber frame
    a. Fur wall out 5/8 for drywall installation
    2. Wrap the house in 6 mil poly to dry it in
    3. Buils out the wall and secure with sip screws in each bay
    4. Run all electrical in out side wall as not to disrupt the vapor barrier
    5. Install Batt insulation from the out side and cover is a rigid foam insulation, or plywood then ridgid foam but hopefully just foam. 2-4 inches.

    6. Do I need tyveck over this or can I just tape the seams?
    7. Strap the insulation for a nailer for the siding.

    This allow for a space between the insulation and the wood siding.

    I am wondering if the Foam insulation would cause too much moisture to get caugh within the batt insulation.
    so from inside out vapar barrier, batt insulation, foam,(tyvek?) siding

    Thank you

    • Unsu...

      Re: Vapor Barrier question

      Thu, March 15, 2012 - 1:41 PM
      I am unfamiliar with the phrase "dry in" I'm guessing you are working green wood?
      Green shrinks 6% calculate that in your engineering.
      You may make a hash of it if you set things up so that one side of the frame members are drying substantially faster than the opposite or adjacent side. Wood warps (sometimes dramatically) when dried unevenly.

      SIP screws because you want them to act like stand offs?

      putting your wiring on the outside side of the walls may be an issue later because nobody will anticipate it and may end up driving fasteners or cutting tools into it when renovating.

  • Re: Vapor Barrier question

    Tue, March 27, 2012 - 12:39 AM
    It actually depends on how humid and wet your climate can get. Some others might also consider installing screen doors to help with vapour protection. If you do not know how much to use, I might suggest putting slightly more. I think more would be a lot safer than putting less.

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