Ask the Guru: Is Vapor Barrier Required?

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DEAR POLE BARN GURU:  Finishing & insulating inside pole barn that was here when purchased last year. What are your thoughts on vapor barriers for ceiling between the metal and insulation?

Also in quandary over no over hangs for soffit vents it’s a Mo**on with the gable ends venting where trim meets the ribbed siding, snow blows inside currently and not sure if after I have it closed in it will still do so and get insulation wet (fiberglass). Thanks BLOWING IN, IN OHIO


DEAR BLOWING: It is essential to have a vapor barrier between the roof steel and any conditioned area below. In the event there is not a vapor barrier between the roof purlins and the roof steel, there are some options. The best choice, is also the most work.  Remove the roof steel, install a reflective insulation on top of the purlins and reinstall the roof steel, using larger diameter, longer screws than what originally held it in place. Spray foam insulation could be placed on the underside of the roof steel (may be cost prohibitive), or reflective insulation could be installed under the roof purlins, as long as all of the seams are tightly sealed.

Assuming you are placing the fiberglass insulation at ceiling level, there should not be a vapor barrier below it. You want any warm moist air from inside the building to be able to rise into the dead attic space, which must be adequately vented. Venting can be accomplished by a combination of sidewall eave vents and a vented ridge, or by gable endwall vents.

 If snow is coming in under the gable end/rake trim, the screws can be removed from the gable face of the trim, expanding closures can be placed along the edge of where the trim lies, and then reinstall the trim using metal-to-metal stitch screws. The expanding closures should be placed so the lower edge of the closure is just covered by the trim.

 In any case, you want to be certain any fiberglass insulation will not get damp, as it will lose its effectiveness, as well as possibly contributing to decay of any wood members it is in contact with.

18 thoughts on “Ask the Guru: Is Vapor Barrier Required?

    1. Greg ~

      Thank you for your question. I’ve never addressed fanfold insulation as a blog topic, because I do not see it as a viable or practical answer to preventing condensation issues from steel roofing. I will be writing an article on it soon.

  1. Hi pole barn guru, I really enjoy reading these blogs but I have a question about the above post regarding a vapor barrier.

    Currently I have a 30×32 pole barn with a vapor barrier installed between the steel roofing and purlins which seems to be a must to prevent a wet roof. The problem I am running into is finding out if I need a vapor barrier for my ceiling. I plan to install sheet steel to the bottom of the trusses and then blow in fiberglass insulation on top of the steel. In the above blog you mention that a vapor barrier is not needed so that the moist air can escape. This makes sense to me, but others keep saying I should install a vapor barrier between the sheet steel and insulation. Can you give me a definitive answer to this? Oh, Also I live in Michigan and plan to heat the building occasionally during the winter months.

    Thanks Scott,

    1. Scott ~ Thank you greatly for your kind words and for being a loyal reader. The purpose of an insulated vapor barrier is to prevent warm moist air from rising and coming in contact with a cooler surface above, where condensation could occur. Although the steel ceiling is rather impervious (other than any unintended gaps at overlaps or edges) there does exist the possibility of some quantity of warm moist air escaping into the attic area. A vapor barrier immediately above the steel ceiling, would tend to trap any of this moisture on top of the steel ceiling – which could eventually lead to deterioration issues (keep in mind most steel liner panels have a very limited galvanization coating, under very little paint).

      1. built pole building installed thermax sheathing on outside wall under tin 1/2 polyicocyanurate installed 8 inch batt insulation in wall do I need vapor barrier on inside been told outside sheathing vapor barrior thinking of installing certainteed membrain vapor barrier wich is breathable looking for answers been told I would have two barriers wich traps moisture

        1. As long as you adequately seal all of the air leakage pathways you would not need an interior vapor barrier.

  2. Have 1/4″ foam on top of truss then metal roofing,, want metal ceilling,, do I use Vapor Barrier w/celluse insulation or should I use fiberglass.. If I use fiberglass can I get away w/o vapor Barrier?

  3. I have a 40×64 pole barn with a steel roof over 7/16 osb. Roof is vented from soffit to ridge vent. I’d like to put a metal steel celing in the barn with R30 unfaced batts above with the option of blowing in more insualtion in the future. Do I need any kind of vapor/moisture barrier between the steel and the insulation? Thank you.

    Greg in Michigan

    1. You should not require a vapor barrier under the attic insulation. Be sure to not block the flow of air coming in through the soffit vents.

  4. Hi,
    I live in West Virginia and get variable weather from bitter cold in winter to hot in summer.
    I have a 60×35 building that was used as a horse barn that we are converting into a home. As it sits now it has 2 levels (ground, 1st floor) and the top floor is open from floor to peak. The peak is not vented but there are 2 gable vents. My intention is to install drywall and blow in insulation. We will be heating our space with mini split and wood burning stove. How can I ensure to prevent moisture? I am using vapor barrier on walls in between insulation and drywall, am I okay to be without one for the attic? Would installing a roof vent be recommended?

    Thank you for your time!

  5. My pole frame cabin is 24×32 in zone 7 of northern Minnesota. Soffits on warm side with ridge vent and house wrap. I have 2 questions. Ceiling will have vapor barrier. I want panel boards with a satin stain finish. Am I ok having a satin stained ceiling since there will be a vapor barrier? Question #2) I would prefer Sheetrock walls, there will be a vapor barrier on the backside of the sheetrock. I don’t like shiny walls. My question is can I use a satin paint or should I first use a high gloss primer and then 2nd coat be satin? Thank You for your time

    1. Your paint/stain questions will be best answered by whomever is providing those products.

      I will hope housewrap is installed in your walls between wall girts and siding and not between purlins and roof steel. If it is latter of these, it will not act as an effective condensation control and should be cut out, with two inches of closed cell spray foam applied directly to underside of steel. Unless your site has over 8000 heating degree days, a ceiling vapor barrier is typically not recommended by building scientists.

  6. I am plan to enclosed a 20×20 car port with metal walls and roof. The concrete slab beneath has 6 mil plastic. The area will not be conditioned. The roof is sloped 1”/12”. Will removing the roof then wrapping roof and walls in 2mm polyethylene foam laminated with a vapor barrier prevent condensation? Should I even do both the roof an walls since it will not have heating/air? Also should I use the foam enclosures at both ends of the metal on both the roof an walls that was supplied by the metal company? Located in eastern NC.

    1. TO prevent condensation under the roof steel requires a thermal break – such as two inches of closed cell spray foam insulation. 2mil poly would not do it and is so thin it would be difficult to work with. Yes, use the foam closures.

  7. Hello, is it a must to have a condensation barrier between roof steel and purlins when a steel ceiling with R38 blown in ?

    1. admin Post author

      You need to have some method of controlling condensation or it will rain in your attic. I recommend ordering your roof steel with an Integral Condensation Control factory applied.


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