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    Is Vapor Barrier needed when insulating garage?

    I have an attached garage and the exterior wall is uninsulated. I have some R15 Roxul Comfortbatt. However it is unfaced.

    Does anyone know if I need a vapor barrier? I'm having trouble finding it in the building code.

    Also from what I've read in other places, how you orient the barrier depends on if you heat more or cool more. I will not install A/C in the garage, but I do plan to get a wall mount electric heater. So I assume the vapor barrier will face inwards towards the garage if it is needed.

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    Re: Is Vapor Barrier needed when insulating garage?

    I think you want a vapor barrier and it should be on the warm side of the insulation. As the warm air moves towards the cold air, condensation forms and you want the vapor barrier to stop the condensation before it reaches the insulation.

    But I'll certainly defer to a pro.

    Tom S.

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    Re: Is Vapor Barrier needed when insulating garage?

    Assuming you plan to use the garage as a workspace and want to heat it, then I would put the vapor barrier on the inside. Even if you were running AC, the Triangle has more heating days than cooling days (hard to believe in summer) so the average homeowner heats more than they cool, on average.

    BTW, if you are planning a wall-mount electric heater, what about a heat-pump ?

    -Mark

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    Re: Is Vapor Barrier needed when insulating garage?

    Yes it is needed is you want to insulate and will be using some form of heat / ac. The vapor barrier only needs to cover the insulated area.
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    Re: Is Vapor Barrier needed when insulating garage?

    The advantage of using a vapor barrier is a common discussion in the southern building industry. I just had a l-o-n-g discussion about vapor barriers with my building inspector when he was at my new shop doing the framing inspection. I mentioned that I was going to use unfaced insulation and then cover the heat side with plastic as a vapor retarder, he said I should not as the use of vapor barriers may cause more problems than they solve. Right now I am insulating the walls, ceiling and floor with fiberglass insulation with no vapor barrier. I am doing a lot of reading on the subject and I am going to take a two day class on the new energy code in a week or so. I am holding off the installation of the drywall until I am almost certain I am going the right way

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    Re: Is Vapor Barrier needed when insulating garage?

    To round out the picture, there's also what I'll call the Carolina way of installing a vapor barrier: do a mediocre job of installing a vapor barrier in the walls and put none in the ceiling.

    Admittedly I've only owned two houses in NC - one tract-built, one not - and both vapor-barriers were installed in the same hit-and-miss way.

    My dad about fell over when he saw no vapor barrier in the ceiling (he comes from the Great White North).

    -Mark

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    Re: Is Vapor Barrier needed when insulating garage?

    You guys have me concerned. I've been planning to insulate my garage in the Spring but they were already drywalled when I bought the home. I was planning to open holes near the top of the walls, below the joists, and blow in insulation. Now I realize this will leave me without a vapor barrier. Any ideas?

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    Re: Is Vapor Barrier needed when insulating garage?

    Quote Originally Posted by Mr. Bill View Post
    You guys have me concerned. I've been planning to insulate my garage in the Spring but they were already drywalled when I bought the home. I was planning to open holes near the top of the walls, below the joists, and blow in insulation. Now I realize this will leave me without a vapor barrier. Any ideas?
    I am not sure but I have been told the lack of a vapor barrier will not cause any problems. The jury is still out

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    Re: Is Vapor Barrier needed when insulating garage?

    Quote Originally Posted by Mr. Bill View Post
    You guys have me concerned. I've been planning to insulate my garage in the Spring but they were already drywalled when I bought the home. I was planning to open holes near the top of the walls, below the joists, and blow in insulation. Now I realize this will leave me without a vapor barrier. Any ideas?
    You don't plan to generate large volumes of water vapor (Showers, Cooking, Laundry,etc.) in your attached Garage/shop do you? I don't have a vapor barrier in mine and don't think it's needed. You can apply a good coat of spackling in any crack and a good paint on the walls and ceiling and you'll be fine. I think you want a little ventilation to vent smoke and vapors you create in the shop.
    ‎"Good things happen to people who underestimate their setbacks."-Jason Isbell

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    Re: Is Vapor Barrier needed when insulating garage?

    I can't speak specifically about the need for vapor barriers in this application, but I will relate what happened to me on a project I just completed:

    We had an upfit to do in an existing shell of steel frame and metal perimeter stud construction. The adjacent space was already occupied by a fitness franchise, and as such, fiberglass batting had been installed in the demising wall between the spaces. No vapor barrier was installed on the conditioned side, as this was meant primarily as sound attenuation in the design, although it did provide a thermal barrier to the then unconditioned vacant space. As the finished space was used by clients from 6:00 AM until 10:00 Pm for exercise, the temperature was kept at 68 degrees during the cooling period of the year. The unfinished space we started with had a 6mil vapor barrier over the dirt and no floor except for a 4' wide perimeter slab. We had to remove the poly barrier to install plumbing and electrical in the floor and replaced it prior to pouring the slab.
    The day we poured the slab (with a concrete pump) it rained all day. The finishers had to work into the night because of the excess moisture and the inability of the humidity to dissipate. A couple days later the fitness center called me over to look at a problem that had developed. One of their rooms had an exercise area in it with workout bars attached to the wall. Between the attachment points they had installed 4' x 8' Lexan sheets to prevent someone from kicking the wall and scuffing it. The excess moisture from the concrete had condensed on the backside of the drywall and migrated to the inside. When it got to the paint and Lexan inside it had no place to go, and caused the wall to begin to mildew.
    Did the Lexan cause the mildew? Probably. There was no problem anywhere else even though the backside of the drywall showed condensation on the insulation which had to be removed.
    Would a vapor barrier have prevented it? Probably also. But as it was not considered to be a permanent exposure to the unconditioned environment, the original builder didn't use one.
    Since the problem was ours to address, I removed the Lexan, resulting in the paint and part of the drywall paper coming off as well. The paint had really stuck to the plastic, and I feel fortunate that the dreaded "M" word wasn't used before we finished.
    The point in all this is that if you don't install a vapor barrier, please allow the conditioned space to have a breathable finish.
    WHAT BOX?

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  13. #11
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    Re: Is Vapor Barrier needed when insulating garage?

    Code doesn't require a VB on ceiling insulation, at least it used not to. Mine has VB though. Latex paint is sometimes considered a vapor barrier.

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    Re: Is Vapor Barrier needed when insulating garage?

    Definitely go with a vapor barrier, polyethylene sheet is cheap and installing it before the drywall goes on is simple. Some info from my day job...

    Warm air is more humid and tends to "drive" to cold, less humid air. Drive direction in summer is exterior humid air migrating toward the air-conditioned, cool interior. In winter, the opposite occurs. (This despite us saying that interior heat dries air, and cold winter fronts are moist. Drive in winter is still inside to outside.)

    The primary point of a vapor barrier is that it keeps warm humid air from migrating half way through the insulation and condensing when it gets to a cooler temperature at the dew point. Over time, this can destroy the insulation (when the dew mattes it together or when it freezes) and create a fabulous place for mold to grow. In NC, this can happen in either direction, summer or winter.

    A secondary purpose of a vapor barrier is to prevent air infiltration into air conditioned spaces. The process of air conditioning is actually the same as de-humidification. Cooling by blowing air over cold coils dries the air, condensation on the coils is the result. (Which is why you want your air conditioner on when you are de-humidifying your car windshield in the winter, with the heat on.) This takes a LOT of energy. Without a vapor barrier, all the de-humidification (and resulting cooling) is wasted, humidity from outside simply migrates, or drives, right back into the space.

    These reasons are why the vapor barrier should go just under the drywall in NC. A light weight polyethylene sheet does the job, with plenty of attention to seals at laps.

    Now for the caveats.

    The reality is that sealing a home is nearly impossible. It is necessary to vapor barrier every square inch of wall, floor, and ceiling/roof or little benefit is gained. And there are those corner conditions (like at rim joists) that usually have no barrier. How many drywall nails/screws go through the barrier? Plus craftsmanship issues where a portion is forgotten or poorly constructed. Then if your windows are like mine, you can pass paper between the cracks at the jambs, heads and sills. Doors are equally notorious after a few years settling. And if you are like our household, the doors open and close so often in the summer I feel like I'm cooling the neighborhood! Opening windows on summer nights completely negates all the work the AC did during the day time, the humidity rushes in and infiltrates drywall, wood, cloth, carpet, etc. (It takes more than a day to re-extract it all.)

    Knowing the general issue with leakage, many building science people now recommend vapor permeable barriers on commercial buildings. These are complex chemical coatings that keep water out but let drive be unrestrained to allow pockets to dry out occasionally and resist mold growth. But we're talking about a completely different type of construction, from windows, roof, envelope, HVAC systems, etc., to the barrier itself. So I don't think these apply to residential type construction.

    If it was me, I'd go with the vapor barrier. Even if it is only on a portion of the wall, it will slow down drive in the immediate vicinity of the insulation and buy at least some slowdown of migration for energy savings to the degree that it can be installed.

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    Re: Is Vapor Barrier needed when insulating garage?

    Quote Originally Posted by SteveHall View Post
    Definitely go with a vapor barrier, polyethylene sheet is cheap and installing it before the drywall goes on is simple. Some info from my day job...

    Warm air is more humid and tends to "drive" to cold, less humid air. Drive direction in summer is exterior humid air migrating toward the air-conditioned, cool interior. In winter, the opposite occurs. (This despite us saying that interior heat dries air, and cold winter fronts are moist. Drive in winter is still inside to outside.)

    The primary point of a vapor barrier is that it keeps warm humid air from migrating half way through the insulation and condensing when it gets to a cooler temperature at the dew point. Over time, this can destroy the insulation (when the dew mattes it together or when it freezes) and create a fabulous place for mold to grow. In NC, this can happen in either direction, summer or winter.

    A secondary purpose of a vapor barrier is to prevent air infiltration into air conditioned spaces. The process of air conditioning is actually the same as de-humidification. Cooling by blowing air over cold coils dries the air, condensation on the coils is the result. (Which is why you want your air conditioner on when you are de-humidifying your car windshield in the winter, with the heat on.) This takes a LOT of energy. Without a vapor barrier, all the de-humidification (and resulting cooling) is wasted, humidity from outside simply migrates, or drives, right back into the space.

    These reasons are why the vapor barrier should go just under the drywall in NC. A light weight polyethylene sheet does the job, with plenty of attention to seals at laps.

    Now for the caveats.

    The reality is that sealing a home is nearly impossible. It is necessary to vapor barrier every square inch of wall, floor, and ceiling/roof or little benefit is gained. And there are those corner conditions (like at rim joists) that usually have no barrier. How many drywall nails/screws go through the barrier? Plus craftsmanship issues where a portion is forgotten or poorly constructed. Then if your windows are like mine, you can pass paper between the cracks at the jambs, heads and sills. Doors are equally notorious after a few years settling. And if you are like our household, the doors open and close so often in the summer I feel like I'm cooling the neighborhood! Opening windows on summer nights completely negates all the work the AC did during the day time, the humidity rushes in and infiltrates drywall, wood, cloth, carpet, etc. (It takes more than a day to re-extract it all.)

    Knowing the general issue with leakage, many building science people now recommend vapor permeable barriers on commercial buildings. These are complex chemical coatings that keep water out but let drive be unrestrained to allow pockets to dry out occasionally and resist mold growth. But we're talking about a completely different type of construction, from windows, roof, envelope, HVAC systems, etc., to the barrier itself. So I don't think these apply to residential type construction.

    If it was me, I'd go with the vapor barrier. Even if it is only on a portion of the wall, it will slow down drive in the immediate vicinity of the insulation and buy at least some slowdown of migration for energy savings to the degree that it can be installed.
    Steve, great job of explaining how the vapor drives from hot to cold and that it drives out in the winter and in during the summer. When you have a vapor barrier installed behind the drywall and the moist vapor is driving in during the summer, this moisture stops at the vapor barrier and wets the framing and insulation. During the winter the vapor barrier works correctly by keeping the moist vapor away from the framing and insulation, but during the summer it is trapping this moisture. There is alot more moisture in the air during the summer months around here, so I think that is why the the use of a vapor barrier in the south is being rethought.

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    Re: Is Vapor Barrier needed when insulating garage?

    Can you provide a sketch of your garage and its relationship to the rest of your home? Is your "exterior wall" the one containing the garage doors or a different wall like this? Is there an insulated attic or bonus room above the garage?



    Not intending to trivialize your vapor barrier question, but it may not be a hill worth dying on in the overall scheme. It's only a garage/shop with some occasional heat when desired so maybe you should just leave the space breathe with changes in temperature and humidity.



    Quote Originally Posted by kommon_sense View Post
    I have an attached garage and the exterior wall is uninsulated. I have some R15 Roxul Comfortbatt. However it is unfaced.

    Does anyone know if I need a vapor barrier? I'm having trouble finding it in the building code.

    Also from what I've read in other places, how you orient the barrier depends on if you heat more or cool more. I will not install A/C in the garage, but I do plan to get a wall mount electric heater. So I assume the vapor barrier will face inwards towards the garage if it is needed.

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    Re: Is Vapor Barrier needed when insulating garage?

    Quote Originally Posted by Jeff View Post


    Not intending to trivialize your vapor barrier question, but it may not be a hill worth dying on in the overall scheme. It's only a garage/shop with some occasional heat when desired so maybe you should just leave the space breathe with changes in temperature and humidity.
    Amen
    The water vapor load will be significanty less than inside your house. If you apply an Epoxy garage floor covering it will be minimal as the slab is the primary source of water vapor in a garage. Thats where I would put my money and effort.
    ‎"Good things happen to people who underestimate their setbacks."-Jason Isbell

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