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  1. #1
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    Apr 2005
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    Default Open cell foam and vapor retarder

    I've been trying to read up on the use of vapor barriers and retarders in cold climates. My project is in a very cold climate, over 9000 hdd. Walls have about 4 or 5" of open cell sealection500 and roof has about 10" between 2x12's. Install was a little rough , leaving an egg carton like surface after the garden hoe surfacing technique, and a few odd shaped air pockets where the insulation folded over itself during the rapid expansion. Our installer says we are fine to skip any kind of vapor retarder, Even the district rep says we are ok to skip it. Joe Lstubereck and Building Science say our climate zone needs one. Is the risk of moisture and condensation enough to warrant a vapor retarder paint? are we better off letting the assembly dry quickly to the interior. What does the IRC say about it? Is there just as much science, charts and graphs to support skipping the vapor retarder/barrier as there is to include it? Is it possible we could do more harm to the walls/roof by adding the vapor retarder?
    Rob

  2. #2
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    Northern Vermont
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    Default Re: Open cell foam and vapor retarder

    Rob,
    You need a vapor retarder -- according to building scientists and the manufactureres of open-cell foam. Leave it out at your peril. I wrote an article for Energy Design Update about a house in Vermont that was insulated with open-cell Icynene with no vapor barrier. The interior moisture condensed against the wall and roof sheathing. Solution: all the walls and roofs had to be demo'd and put in a dumpster -- sheathing and insulation. (The work was performed from the outside.) Once the demo was complete, the homeowner decided to go with closed-cell foam.

  3. #3
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    Default Re: Open cell foam and vapor retarder

    CLOSED cell foams work as both a vapor and air retarder.
    Open cell foams worj as an air retarder but NOT as vapor retarders

  4. #4
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    Jun 2004
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    Default Re: Open cell foam and vapor retarder

    I don't mean to hijack this thread, but how does one determine which type of foam is appropriate for one's climate? Does the closed-cell foam constitute an interior vapor retarder or exterior?

  5. #5
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    Jan 2007
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    Brooklyn, Fire Island
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    Default Re: Open cell foam and vapor retarder

    Oh sh*t. Martin?

    I'll give it a shot, but that's a really wide question.

    Closed-cell is basically a vapor barrier from either side. So it depends on where you put it... The danger is when you trap something permeable between it & another vapor barrier.

    It also depends on how much you've got - 1/4" foam isn't a vapor retarder, but 1" is, and 1 1/2" is a vapor barrier.

    The fact that sprayed foam (open or closed), cuts off the airflow means it's preventing something like 95% of the vapor transfer to begin with.

    The idea, when closed-cell is sprayed right up against the sheathing, is that the two become one solid mass, and I guess you'd say it's a vapor-barrier from both sides.

    Hopefully Martin will be along, pretty soon, to clarify & correct this.

  6. #6
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    Default Re: Open cell foam and vapor retarder

    Charles,
    Frenchie has it right. I would only add a few points: the whole emphasis on "interior vapor retarder" and "exterior vapor retarder" has always been somewhat simplistic and therefore is, to a certain extent, an obsolete distinction.
    Building scientists like to talk about a "wall assembly." A wall assembly is made up of a great many layers; for example, siding, Cedar Breather, water-resistant barrier, OSB, studs filled with cellulose, MemBrain smart retarder, drywall. Any material falls somewhere on a scale indicating vapor permeance; so, rather than speaking of just vapor retarders and permeable materials, we're faced with materials like plywood and OSB, that can be vapor retarders but which change their properties as they take on moisture.
    So where, in this thick wall sandwich, is the "interior" and where is the "exterior"? Which component is going to be singled out and called the "vapor retarder"? Every layer can retard vapor to some extent. To really understand a wall assembly, it must be either modeled on a computer or tested in as climate chamber. If you test it in a lab, all kinds of new questions arise: how much water will you inject into the wall or spray on the wall to simulate rain? How high will the relative humidity be on the interior? Do you need to simulate sunshine? (But north walls get relatively little sunshine.)
    If you're not a building scientist, it can be helpful to ask two questions about any wall assembly:
    1. Question one is, what will make the wall assembly wet? (Usually, the answer is driving rain from the exterior, and moisture-laden air from the interior, but each of these problems varies -- if the wall is facing a covered porch, there is not much rain; if the wall is adjacent to a shower stall, it's much wetter than most living rooms.)
    2. Question two is, if this wall assembly gets wet, how will it dry?

    By now, builders have learned from experience which wall assemblies work and which don't, and have learned a few basic principles to help design walls that work.
    A rainscreen siding application helps walls dry to the exterior. Polyethylene vapor retarders can make it difficult for walls to dry to the interior, but can be useful in very cold climates. Open-cell spray foams need a vapor retarder when installed in cold climates.

  7. #7
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    Portland, OR
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    Default Re: Open cell foam and vapor retarder

    All the science we now have has pretty much supported the old theories:
    Hot humid climate, vapor retarder on the exterior
    Cold climate, vapor retarder on the interior
    VERY cold climate, vapor barrier on the interior
    Everywhere else: "When in doubt, leave it out"

    We went astray when all the codes tried to nationalize themselves and started demanding vapor retarders, usually on the inside. PLUS plastic was cheap and everybody started using poly sheets.

    Now we know better, but the word is still getting around

  8. #8
    Join Date
    Aug 2004
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    64

    Default Re: Open cell foam and vapor retarder

    So Martin what do you think of Icynene's claim of not recommending a vapor barrier in climates with less than 7500 heating degree days (the walls - attic is different story in unvented applications). That's what they are doing down here in Boston and inspectors are fine with it. I would guess Northern VT is probably 8000+ so you would fall under their requirement for needing a VB....guess these borderline climates are a great case for Membrain.

  9. #9
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    Default Re: Open cell foam and vapor retarder

    Steven,
    Icynene's recommendations represent the company's decision on the most logical place to draw the line on the climate map, from their liability perspective. Actual installations vary, of course, with many factors affecting condensation risk. The higher the indoor humidity level, the higher the risk. Since most Icynene is installed behind painted drywall, the risk in most walls is relatively low in the Boston area. (Primer plus even one coat of latex paint tends to retard vapor flow -- not a vapor barrier, but a vapor retarder.) Houses that have gotten into trouble (like the one in Vermont) have unusual interior finishes; in the case in Vermont, it was tongue-and-groove 1x6 paneling, with no drywall underneath. One building science expert I trust, who prefers not to be identified because the issue is sensitive to Icynene Inc., has measured elevated RH levels in roof sheathing under Icynene in the Boston area; to the best of my understanding, the house had a conditioned attic with Icynene sprayed on the underside of the roof sheathing, with no drywall. The solution is relatively simple: spray the cured Icynene with paint. (The paint acts as a vapor retarder.) A lot of Icynene installers don't want to come back and spray paint on the Icynene, though. Boston-area installers usually don't have to worry, unless (1) they know the indoor humidity will be high, (2) the Icynene will be exposed, (3) the interior finish is air permeable (like 1x6 paneling), or (4) they don't sleep well at night worrying about condensation on cold sheathing. Cautious installers in the Boston area (or similar climates) will certainly consider a vapor retarder on Icynene. The cost is low compared to the cost of correcting a problem.

  10. #10
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    Aug 2004
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    64

    Default Re: Open cell foam and vapor retarder

    That makes sense Martin, I was reading up on VB's again last night on building science's site after I wrote you and it did seem that latex paint would probably give you that margin of safety as a class III vapor retarder...

    Just curious with that house in Boston area, was the ceiling below the attic painted or did they leave it plain plastered which is somewhat common around here with spec homes as you can't really tell the difference unless you look closely. It would explain higher humidity levels I found in one house where icynene was used on the floor of the attic and the ceiling wasn't painted. Now I did see also that even Icynene recommends that their insulation be painted in climates with even as few as 4000 heating degree days when it is used in an unvented attic/cathedral ceiling application. Guess painting the ceiling should take care of the problem in this case which so far hasn't proved to be a problem because it does vent the moisture, albeit slowly.

  11. #11
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    Georgia
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    Default Re: Open cell foam and vapor retarder

    Quote Originally Posted by Martin Holladay View Post
    By now, builders have learned from experience which wall assemblies work and which don't, and have learned a few basic principles to help design walls that work.
    A rainscreen siding application helps walls dry to the exterior. Polyethylene vapor retarders can make it difficult for walls to dry to the interior, but can be useful in very cold climates. Open-cell spray foams need a vapor retarder when installed in cold climates.
    And what have you heard about my climate? Have any builders down here gotten into trouble by simply substituting open or closed cell foam for fiberglass and cellulose, without changing any other aspect of wall, floor and ceiling assemblies?

  12. #12
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    Default Re: Open cell foam and vapor retarder

    Charles,
    "Have any builders down here [Georgia] gotten into trouble by simply substituting open or closed cell foam for fiberglass and cellulose, without changing any other aspect of wall, floor and ceiling assemblies?" No, not that I know of. However, use of interior polyethylene in Georgia has contributed to wall problems.

  13. #13

    Default Re: Open cell foam and vapor retarder

    Martin: I have been reading your replies and I am still confused as to what to do with this situation: Open or closed cell in a metal pole barn that will have a workshop with a wood heater in it. Location is in northern Ohio. The outside of course is metal but the inside will be drywall. Depending upon who you talk to they will tell you that either is good but which is better?

  14. #14
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    Northern Vermont
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    Default Re: Open cell foam and vapor retarder

    Gregg,
    The two types of insulation have different properties. It's not a question of "better" or "worse."
    1. Open-cell spray foam insulation has a lower R-value per inch. It's cheaper than closed-cell per inch or per cubic foot. It's vapor permeable. It's softer and does not provide any structural strength. It's easier to trim.
    2. Closed-cell foam has a higher R-value per inch. It's more expensive per inch than open-cell, but not always more expensive for a given R-value. It's a vapor retarder. It's denser than open-cell, and provides structural rigidity to a wall assembly. It's difficult to trim.

    Prices vary widely by region. Talk to your local installers; talk to others in the area who have installed spray foam, and ask their opinions of the local spray-foam contractors. Get prices. Read up on foam specifications on manufacturers' Web sites.

  15. #15
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    Jan 2007
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    Brooklyn, Fire Island
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    Default Re: Open cell foam and vapor retarder

    Surprised me, Martin - wouldn't a steel building want closed-cell? Since the steel is basically an exterior vapor-barrier, otherwise... ?

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