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How many coats of paint for a vapor retarder

Noah Demarest | Posted in Building Code Questions on

When the code requires/allows a Class II or III vapor retarder, and we want the wall to “breathe” in both directions, it seems that using a paint or primer with the appropriate perm rating will only work for a while.  I assume that the Class III paint, if the wall is repainted every few years (or if we call for 4 coats, f. ex.) may turn it into a Class I vapor barrier. True? If the Class II product is expensive, would additional coats of Class III paint accomplish the same permeance?

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Replies

  1. Andrew C | | #1

    Excellent question - what happens to a wall's drying capacity when people inevitably repaint?
    I ask myself this question every time someone designs a wall that can't dry to the exterior. Seems like an argument in favor of mineral wool if you use exterior insulation. Better have good flashing details, large overhangs, and all sides facing south if your wall can't dry in at least one direction.
    That doesn't answer your code question, but I'd be interested to know if the code makers have considered vapor barriers changing during the life of a wall.

  2. Expert Member
    Dana Dorsett | | #2

    >"I assume that the Class III paint, if the wall is repainted every few years (or if we call for 4 coats, f. ex.) may turn it into a Class I vapor barrier. True?"

    A typical application of standard interior latex primer + 2 coats of color comes in at about 3-5 perms. If it's then over-painted with another couple coats of color it'll still be at least 2 perms for the 5 total layers of paint. Repainting it with another couple other colors, 2 coats each it'll still be at least 1 perm for the now 9 layers of paint, near the very high edge of Class-II vapor retardency.

    By the time you get to 99 layers of paint (one primer with 49 applications of color, 2 coats each) you might start encroaching on Class-I vapor retardency territory.

    Vapor barrier latex primer starts at about 0.5 perms give or take, but it still takes MANY layers of color-coat to bring that down to 0.1 perms & lower. Repainting 2 coats every 5 years it'll still take over a century. If you're re-painting with 2 coats every year it's only about 25 years.

    >"That doesn't answer your code question, but I'd be interested to know if the code makers have considered vapor barriers changing during the life of a wall."

    There are a few built in presumptions about he lifecycle of a house before full-gut rehabs, which is 50-100 years. In 50-100 years we'll all be in different climate zones given the current rate of climate change and even the most optimistic projections. :-( So far I don't think the climate drift has been factored into codes, but as the 25 year weather data averages change, the county by county US climate zones will change. I'm, currently in the middle of US climate zone 5, but it looks like I'll be at the warm edge of climate zone 4 by mid-century. YMMV. See:

    https://cdn.vox-cdn.com/thumbor/_JBKVjyITIhHrBe-OKti2V3ugRY=/800x0/filters:no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/13590787/US_MAP_LOGO.gif

    1. Andrew C | | #4

      Thanks Dana and Jon for informative replies. I thought the permeability of the paint would change a lot faster with additional layers.

  3. Jon R | | #3

    Consider page 62 here (latex paint is too permeable anyway). And that a Class II is in most cases an improvement over Class III. And that it's always wise to use conservative values for exterior perms (neither too high or too low) and design (eg rainscreen, overhangs) - this will provide a safety margin if interior side perms ever do get low enough to be a concern.

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