Vapor barriers

Discussion in 'Homesteading Questions' started by Hoop, Dec 8, 2003.

  1. Hoop

    Hoop Well-Known Member

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    I'm about to start hanging drywall in the ceiling of my house. Its your not so typical log home, with a very typical store bought truss-roof system. The attic will be well vented from the soffitts. I'll be using blown-in insulation.

    I'm getting mixed messages regarding vapor barriers. A young guy I know, who makes his living selling building supplies to contractors, tells me I have to staple a vapor barrier to the trusses before I start hanging drywall. He is very well versed in the latest building techniques......and being in the biz......up to date.

    An old timer I know, who is long since retired from a career in the building trades, tells me vapor barriers are essentially a waste of time. He tells me a vapor barrier over drywall is not needed by any stretch of the imagination.

    Who is right?
     
  2. agmantoo

    agmantoo agmantoo Supporter

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    this is a copy and paste answer to your question
    One of the most important vapor barriers in the home is the one between the wall insulation and the interior side of exterior walls. Because wall cavities are sealed and have no means of allowing accumulated moisture to escape, an effective moisture barrier is crucial in preventing warm, wet air from entering the walls.

    For wall insulation, foil- or kraft paper-faced insulation is typically used. The insulation is installed between the studs with the vapor barrier facing in toward the house -- the one thing to remember about vapor barriers is that they are always installed between the insulation and heated portion of the house. The "wings" along the edges of the insulation are stapled to the studs to hold the insulation in place -- stapling to the face of the studs instead of the sides creates a more effective vapor barrier.

    Another alternative for walls -- which is also commonly used in the enclosed cavities of vaulted ceilings where the pressure of warm air trying to escape is even greater -- is to place unfaced insulation into the cavities between framing members and then cover the insulation on the inside with plastic sheeting. The sheeting is stapled to the face of the studs or joists, and overlapped at all seams to prevent moisture penetration.

    In today's construction, separate vapor barriers are typically not used between the home's interior and a ventilated attic. Most attics are insulated with blown fiberglass, which is relatively porous in comparison to the denser batt insulation used in the walls. The attic also has a continuous flow of air through it that is created by the roof and soffit vents, so moisture from inside the house passes through the insulation and is removed from the attic by the air currents. In areas of high humidity where moisture in the attic could still be a problem, a vapor barrier paint can be applied over the inside ceilings.
     

  3. Ovibos

    Ovibos Member

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    Yes, you should use the barrier absolutely.
    What kind of blown insulation will you be using?
    Ever heard of rockwool? great for blown applications. Highly, highly fire resistant.
    Actually it really can't burn just melt, I forget but its melting point is over a thousand degrees. Perfect for unused attic spaces as the rockwool will not spread fire like lesser insulations. You can use it in board form to insulate walls/rooms and help control/stop the spread of fire from room to room.
    It is an industrial level product and has been in use for some time.
     
  4. swamptiger

    swamptiger Active Member

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    I would install the vapor barrier in the ceiling (on the underside of the rafters)after installing unfaced roll insulation between the rafters. Then I would use several inches of blow-in fiberglass over the top of the rafters to seal the leaks and add further insulation. And make sure the attic is very well ventilated. Ventilation helps both summer and winter.

    The old-timer might be right about the moisture moving out of the house through the attic vents, but I would rather not put any moisture there to begin with. Moisture is not a good thing to have in your insulation. And in colder climates where you are heating the house at least half of the year, I don't see the harm in having the moisture in the living area, because things tend to dry out anyway.

    If you have soffit vents - buy the molded foam inserts to place above the wall plates to keep the air moving in from the soffits - "proper vents" - I think they are called..
     
  5. rutter

    rutter Member

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    The use of a vapor barrier was is related to creating highly Energy efficient house.

    The problem with a house that is built tight as a a problem with stagnation etc from no air turn over. thus you must bring in fresh air exchange and combustion air.

    A vapor barrier such as visqueen does nothing more then keep the gypsum board from touching the wood truss. holes penetrate the vapor barrier from light fixtures, nails etc so it looses its purpose.

    Vapor barries in walls can be achieved with kraft backed fiberglass batts.

    Vapor barriers cause condensation to occur at the meeting of warm air to the cooler air hence the back of your gyp.bd. will will have moisture. further in warm months when humidity is high it is trapped inside the cavity in walls.

    If you use wood heat or a dry type heat source, you will need to add humidity rather then trying to keep it out of your stucture. Most houses built extremely tight with high insulation suffer from mold.

    But if you feel better about it put it in Home Depot will be happier.
     
  6. Alex

    Alex Well-Known Member

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    If we're going to vote here, then I vote YES for a vapour barrier. Its just a few dollars - and relatively easy to install.

    I know you didn't ask about this, I am sure you already know to tape the VB real good with that red "Tuck" tape, and install plastic electric covers around all electric outlets (receptacles), fixture and junction boxes, and tape them to the VB.

    I guess you know, when ever moisture from the house (people, cooking, showers, etc ) comes in contact with a cold surface, then it will condense. If you do not let the moisture from the house get to the cold surface, then, it can not condense. This also means that you must install enough insulation above or outside of the VB so that the VB itself does not get cold enough to condense moisture, like "rutter" just mentioned in the previous post, don't let that happen. If you install proper insulation that won't happen.

    IMHO that is a good reason to coat the logs/chinking also, with a varnish, etc. (though I didn't until just recently - thirty years later - and I had no problems at the logs). Though that is not much of a problem, because its hard for the moisture to penetrate deep enough into the logs ( if they are large diameter) to meet a surface cold enough to condense moisture. Still better to seal the logs - then they won't have as much chance to absorb moisture.

    The above only applies to "cold climates", i.e.; in cold climates put the VB on the inside of the insulation. In "hot-humid" climates, the VB goes on the outside of the insulation, to keep the hot-humid outside air from condensing on the cool surfaces created by air-conditioning.

    Any moisture in a building can be very bad. In cold climates, it can condense and then freeze within walls and other structure and cause expansion damage in the winter and the residual effects can be the start of mold growth. In hot climates, then condensing moisture which is required for mold and mildew growth on food sources like drywall paper, wood, etc.. can be a big problem. It is a big health question.

    Hope you win the VB debate.


    Alex

    Growth of Mold picture from a course I took this year, from the Amercian Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), the picture is from Lew Harriman's excellent book, "Humidity Control Design Guide", published by ASHRAE, picture used in part only according to the limited permission for reviewers of this work, as a part of this review, page102,

    [​IMG]

    In general it seems like a good idea to keep moisture away from mold food sources.
     
  7. CK

    CK Guest

    I would also agree with using the Vapor Barrier, I just finished hanging some drywall myself and was told that the VB is important, so I fixed the rips and put up new plastic. It's easy to do and the cost is low.

    Good Luck with your project!

    CK
     
  8. Hoop

    Hoop Well-Known Member

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    Further research has convinced me the vapor barrier is a must. Our typical heating season here is 9000 degree days.....which certainly qualifies as a "cold" climate.
    One "expert" on vapor barriers says to leave absolutely no gaps/holes/etc......and compares it to having holes in a condum. Of course, light fixtures and electrical wires are going to leave slight gaps.....doctored up with tape.

    Much as I would like to heat with wood, the insurance companies around here have taken a very negative stance on wood heat. An outside wood burner is likely the only way one could get affordable homeowners insurance. I'll probably go with some sort of LP powered radiant heat for the basement......with electric baseboard as supplemental.


    My logs have been caulked thoroughly. Most of the logs are approx 10" x 10"....they are a 3 sided "D" log. Any cracks over 1/8" inch have had a high quality log jam caulk applied. I started to apply stain (High Sierra) but cold weather prevented completion. It will be completed as soon as the weather is conductive for staining.
    I am aware of the resiliency of logs. I have seen logs abused in the worst ways.....and they still seem to stay together. I have seen logs with cracks you could fit a pencil in......logs laying directly on the ground......logs without a bit of stain or preservative.....and they seem to be none the worse for wear.
     
  9. Alex

    Alex Well-Known Member

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    Hoop,

    Yes, me too. I have seen the most remarkable durability of even our Poplar logs that have been neglected at times.

    Coating on the inside is the most important - for a vapour barrier - even though as I mentioned - I have not varnished inside for thirty years, and still in great shape. Outside the darkening of the logs is a sort of portectant for the exterior.

    Alex
     
  10. swamptiger

    swamptiger Active Member

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    I thought I would renew this thread because of a story I heard yesterday. There is a new home being built in our neighborhood whose builder can now really relate to the importance of installing a vapor barrier in the walls after installing insulation.

    Well, this particular builder neglected to install the vapor barrier promptly after installing fiberglass batts in the walls a month or so ago. The home is being heated on a daily basis by portable heaters as the work progresses. The outside temperature here has been hovering in the teens & twenties, and dipping down to zero occasionally at night. A couple of days ago it was discovered that moisture had been accumulating on the inside of the outside sheeting all over in the house, and had actually started to mold in places already! So now, all the wall insulation has been removed, and they are trying to dry up the moisture. :no: :no:
     
  11. http://www.buildingscience.com/topten/default.htm

    This guy is THE EXPERT on humidity, moisture, insulation and many other building science concerns. I have attended one of his conferences and bought his book appropriate for my area, which is the humid south.

    Here is an excerpt from the above link on wood and water...

    Things get wet * let them dry.

    Things get wet from the inside, the outside and they start out wet.

    When the rate of wetting exceeds the rate of drying accumulation occurs. When the quantity of accumulated moisture exceeds the storage capacity of the material problems occur.

    The storage capacity of a material depends on time and temperature.

    The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow.

    As such, energy conservation has the potential to destroy more buildings than architects.


    This is not to say he is not pro-energy conservation, just that there are issues that need to be addressed in air tight buildings, moisture being a particular problem. Mold was huge concern when I built our rather unconventional house due to allergies one of the reasons I bought his book.
    Empress
     
  12. I forgot to add...
    One of the reasons I like the info from this organization is the research they do on building assemblies( like a complete wall assembly inside to outside) . Rather than building a house and seeing if it fails, they test each design in a facility made to stimulate the time and abuse it would go through in real use. Often what seems a good idea has a fatal flaw that is unrealized until it is put to the test.
    Empress
     
  13. A little more site searching found this in the FAQ's

    Q. I am confused about when and where to use polyethylene film as a vapor retarder-can you help?
    A. Yes, ONLY use poly on interior surfaces in Severe Cold Climates OR, as Nathan Yost says, "To cover your building materials when exposed on the job site!" The movement of moisture into and through envelope assemblies is complex and you should consult the BSC Builder Guide for your climate OR click here (http://www.buildingscience.com/resources/resources.htm#Mechanical) to go to this free BSC information resource on polyethylene as a vapor retarder.