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Drying to interior and paint

GBA Editor | Posted in General Questions on

Hello all,
I’ve been learning all the Building science I can lately, and of course, that leads to questions! Hope folks here can entertain a few.
Here is one question I’d love some feedback on.
Consider a New England home that is to get outsulation, or exterior foam insulation in retrofit, or new construction. Almost all outsulation details create a vapor impermeable surface, and thus the wall is then expected to be able to dry to the interior.
How many coats of paint can a gypsum board wall take before the perm rating gets so low as to be risky?
People like to repaint, either to change color, or just “spruce the place up”, particularly when a house changes hands.
Is this method just insuring a future for sheet rockers?


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  1. GBA Editor
    Martin Holladay | | #1

    As long as you have no interior polyethylene, and you avoid vinyl wallpaper, you should be fine.

    Here's some permeance information:

    Vinyl-acrylic primer ... 8.6 perms
    Primer-sealer ... 6.3 perms
    2 coats of oil paint on plaster ... 1.6 to 3.0 perms
    Vapor-retarder paint ... 0.4 to 0.9 perm
    3 coats of oil-based paint ... 0.3 to 1.0 perm

    As a point of comparison, 6 mil poly has a permeance of only 0.06 perm.

    Even if you end up with 0.5 to 1.0 perm, some drying will occur.

  2. Nathaniel | | #2

    Thanks Martin,
    Do you have any data for 2 or 4 coats of regular interior Latex? Wives can be fickle! Heck, I once put on a color that turned out to be terrible, and repainted immediately - that's 2 coats right there.

  3. GBA Editor
    Martin Holladay | | #3

    Different sources are not always consistent, but here's what I've found:

    One source reports that "Three coats of standard latex wall paint have a permeance of 10 perms."

    One source shows a graph of the permeance of 1/2-inch gypsum wallboard painted with one coat of primer plus two coats of latex paint: the permeance varies from about 1 perm (when the RH is 10%) to 10 perms (when the RH is about 75% or 80%).

  4. Riversong | | #4


    Oil paints with metal oxide pigments are less permeable than acrylic paints, and enamel paints are less permeable than flat paints.

    But you raise an important point. The ideal thermal envelope will be able to dry in both directions. Limiting drying to one direction by design creates the potential for changes over time - due both to additional interior and exterior finishes and to wear and tear or limited maintenance and the likelihood of future leaks - that will either further restrict drying or require greater drying potential than the wall has.

    There are pluses and minuses to all design decisions or building methodologies. While outsulation theoretically limits the condensation potential and perhaps the wetting potential of the structural frame, if it also restricts the drying potential then there may be no net gain and a possibility of future net catastrophe.

  5. Nathaniel | | #5

    Thanks y'all.
    Robert, a more forgiving and durable wall, from a vapor control standpoint, is preferred. I've yet to see alternate designs for retrofit situations that seem to meet those goals without interior remodeling and loss of interior space. I think I'll start another tread on that.

  6. Riversong | | #6


    The modified Larsen Truss wall system I use for new construction is based on John Larsen's parallel chord truss that was invented for retrofitting existing buildings into superinsulated structures without any interior modification. They can be filled with dense-pack cellulose, allowing drying to the exterior.

  7. Nathaniel | | #7

    Interesting. I've read your article on Build it solar, but apparently glazed over the initial paragraph on the history! And I always scold my wife for skimming... Oh well.
    Do you have a link, or document detailing the original method? I see that it was described in Fine home building, but I can't find a copy.

  8. Riversong | | #8


    There is almost nothing either in print or on the web about John Larsen's original design. There was the FHB article "High Efficiency at Low Cost" by Jim Young, Fine Homebuilding, Spring, 1994 Edition, pp. 79-81, and a mention in Ted Benson's timberframing book, but little else.

    I'm the only one I know who has actually used Larsen's original design in new construction or retrofitting, and my BuildItSolar webpages are probably the most extensive discussion you'll find.

    Larsen's technique is described in a patent application for a factory-fabricated wall truss similar to John's at

  9. GBA Editor
    Martin Holladay | | #9

    John Larsen lives in Edmonton, Alberta.
    More information on Larsen trusses can be found:

    Fine Homebuilding #20, April/May 1984, "Retrofit Superinsulation."

    The Superinsulated Home Book by J. Ned Nisson and Gautam Dutt.

    The Timber-Frame Home: Design, Construction, Finishing by Tedd Benson.

  10. Nathaniel | | #10

    Thanks Martin and Robert.
    I'll see if I can get some of those references.
    I'm going to start another thread to ask a few questions on the Larsen truss.

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