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Unintended vapor barrier using oil-based stain on wood?

user-958947 | Posted in General Questions on

I need to stain some new bookcases (floor-to-ceiling) located on exterior walls. I want to use oil-based stain but am concerned that may act as a vapor barrier. The literature and other posts discuss the need to avoid vinyl wall paper or oil based paint on the wall in my climate zone (2B). Does oil-based stain (probably with varnish top coat) on the book case fall into this same category? The wall-to-book case surface of the book case is unpainted. The bookcases are shoved up against the wall and screwed to the wall, but typical building imperfections are such that it’s not all in a nice snug fit (like wallpaper would be).
So, is this a problem? Should I avoid the oil-based stain?
Located in climate zone 2B. Exterior walls are brick veneer, 1″ gap, tar paper, 1/2″ plywood, 1×6 wood framing full of OC spray foam, gyp drywall, latex primer.

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  1. Expert Member
    Dana Dorsett | | #1

    In a zone 2B climate it simply doesn't matter what the vapor retardency of the stain is, especially with your stackup. The amount of exterior moisture drive you'll see would be miniscule even during an Arizona monsoon season.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana. One other point: even if you finish the bookcases with an oil-based stain and varnish, the bookcase will still be somewhat vapor-permeable. So don't worry.

  3. user-958947 | | #3

    Dana & Martin, thank you for your responses.
    Just to be clear, I'm in central Texas in a Hot-Humid climate where air conditioning runs for a good portion of the year. My wall design was based on the premise of drying primarily to the interior, using Lstiburek's book (Builder's Guide to Hot Humid Climates) and other BSC papers. I'm not saying that I'm positive that I interpreted it correctly, so help me understand if I missed the point. In any case, the following was my design criteria:
    1) A/C homes should be able to dry to the interior due to vapor diffusion from warm-to-cold, and from high to low vapor pressure (high humidity to low humidity).
    2) Avoid "vapor barriers" (don't use materials of less than 1 perm such as oil based paints, vinyl wall paper, or polyethylene film). IBC and other codes define vapor retarders as 1 perm or less d.c..
    3) HH climates are also subject to significant solar driven/wet brick moisture drive into the wall.
    4) In general, moisture drive into the interior of A/C homes in HH climates is significant and interior vapor barriers should never be used.

    So, this is where my concern about the oil-based stain comes from. I don't want to have a mold problem between the drywall and the bookcase.
    a) If I used an oil based stain topped with a coat or two of semi-gloss oil based clear varnish, would that not be greater than 1 perm? What permeability would it be?
    b) Am I overdoing the 1 perm criteria? If so, what should I target?

  4. GBA Editor
    Martin Holladay | | #4

    While my instinct says that you'll be fine, I'll admit that this is a complicated issue. Ideally, your builder would have included a layer of rigid foam between your plywood wall sheathing and the air space adjacent to your brick veneer; that's always the best approach, because it stops inward solar vapor drive. But your wall doesn't have any exterior rigid foam.

    One thing in your favor: your wall assembly will be able to dry to the exterior, because you don't have any vapor barriers.

    I'm not sure what the permeance of your wood finishes will be. If you are worried, an easy way to limit problems would be to install vertical furring strips on the interior drywall before you install your bookcase. If you could assure some air movement by having hidden vents in the kickspace of the shelves, and hidden vents near the ceiling, you could sleep like a baby at night.

  5. Expert Member
    Dana Dorsett | | #5

    Zone 2B is a hot dry climate not hot humid. Even in San Antonio (officially on the zone 2A side of the line) the mid-summer average outdoor dew point is a bit shy of 70F. Most people aren't setting their thermostats anywhere near that low.

    Got a ZIP code?

    Simply ensuring that the brick veneer is back ventilated would take the edge off moisture drives from the cladding. The worst-case corner would be the southeastern and south sides of the building, since those are the sides subject to bursty high moisture drives when sun hits dew-wetted brick. Having decent roof overhangs to limit rain wetting and reasonable surface drainage limits the amount of moisture wicking up from the ground.

  6. user-958947 | | #6

    Dana, I think I misinterpreted the GBA climate zone map. Now that I look at it again, I think I'm in Zone 2A (assuming 2A stretches along the coast from Florida to mid Texas).
    Zip code is 77805. It's on the east side of central Texas.
    My basis for choosing the HH criteria was originally based on a figure in Lstiburek's book. My location falls quite handily within the HH zone in that figure. He bases his figure on several resources (including ASHRAE, IECC, Dept of Ag, etc.).

    In any case, boots on the ground will tell you that it's hot and it's humid. It's 70 miles NW of Houston.
    Sorry for the confusion.

    Regarding back venting of the brick veneer, there is a 1"gap (brick to wall), and there are typical vents (gap between bricks every few feet) along the bottom and top courses of the brick. How open the bottom vents are is unknown (mortar drop).

    Can anyone send me to a reference for paint permeability? Manufacturers don't seem to test for this.
    What if I used a latex stain? Don't want to, but may be the best solution. Also, is there a latex version of clear varnish?

  7. Expert Member
    Dana Dorsett | | #7

    Bryan TX is definitely zone 2A. Pulling up a dew point chart and zooming over a whole summer season it looks like you're mean outdoor dew points are in the 71-72F range which is pretty sticky, but not exactly tropical humidity, and still below where most people would keep the interior temps:!dashboard;a=USA/77805/TX/Bryan

    But with a vented cavity behind the brick veneer you really don't have to sweat it too much.

    If you take a peek at BSC's material properties list, way down at the bottom you'll see some paint specifications:

    A single coat of oil paint with primer runs between 1.6- 3 perms, which is fairly vapor tight, but nowhere near as tight as vinyl or foil wallpaper, it's 50-100x more vapor open(!). And it's unlikely that an oil stain would be anywhere near as vapor tight as a single coat of oil paint + primer. You're really over thinking this. The wood itself is on the order of 1 perm @ 1" when dry- the stain isn't going to affect it's overall vapor permeance by very much.

  8. user-958947 | | #8

    Thanks, Dana. I'll check out the BSC link.
    The wood I'm worried about is only 1/4" plywood backing on the bookcases (obviously thicker at bookcase framing, but the greatest percentage is only 1/4" thick). So, I'm not worried about the raw wood. I just don't want to screw it up by unwittingly causing a problem.

  9. rocket190 | | #9

    There are some very good clear water based finishes available, and I'm usually not one to advocate low or no vocs...give me something that works!! IMO, General Finishes has some superior products. My house has a lot of wood countertops and I've been impressed with the durability. Water based finishes don't warm up the wood tones, so you may want to use some dye as toner or boiled linseed oil for bringing out the wood color before top coating.

    I can't honestly tell you if the permeance of a water based finish is better than oil based, but at least you have other options.

  10. Dana1 | | #10

    At ~10-12% moisture content 1/4" plywood is under 1 perm (and thus a class-II vapor retarder.) When it's moisture content is high enough to support mold it's over 10 perms (not even a class-III vapor retarder.)

    Finishing it with just about anything will bring it into the class-III vapor retardency range, but so what? Standard interior paints (including single coats of oil paint on oil based primer) are also class-III vapor retarders- do painted walls in that house grow mold? In general stains are more vapor permeable than paints. It's highly unlikely that you'd make the plywood SO vapor tight that it would be a problem, unless the walls already have problems.

  11. user-958947 | | #11

    Thanks, Dana. I looked at the BSC chart.
    1) I'm confused about the math.
    CDX plywood is listed at .75 "perm-in" dry cup, with note 1 that says perms are inversely proportional to thickness. It also lists 3/8" thick as a "typical relevant dimension".
    So, is 3/8 plywood .75 perms dc, or is it .75 perm-in / 3/8 in = 2 perms dc ? (calcs to 9.3 wc).
    1/4" plywood would calc to be 3 dc & 14 wc.
    I don't have CDX on the bookcases, but it's the closest thing to what's on the chart, and I would assume CDX is less permeable than cabinet grade plywood.
    2) Help me understand the relationship between the BSC dc/ wc numbers and the 10-12% moisture content that you mention.
    3) I see where oil paint ( 1 coat + primer) is listed in the chart at 1.6 to 3 perms. But in other BSC publications ( Joe's book in particular) it lists "oil based paints" at 1 perm, so it looks like I'm on the bubble.
    4) In general, how do you view the permeability of a plywood + paint assembly. Do you simply view each component separately (e.g. if you have a 1 perm plywood with a 1 perm coating, is the entire assembly considered to be 1 perm?)?

  12. GBA Editor
    Martin Holladay | | #12

    You didn't respond to my earlier suggestion: "If you are worried, an easy way to limit problems would be to install vertical furring strips on the interior drywall before you install your bookcase. If you could assure some air movement by having hidden vents in the kickspace of the shelves, and hidden vents near the ceiling, you could sleep like a baby at night."

  13. user-958947 | | #13

    Thanks for the suggestion, but I'm beyond that point. The bookcases are installed. They have crown molding at the top. They cover most of three walls. The installers spent a lot of effort getting them all lined up, so I hesitate to pull them off the wall and start over. Also, some of the crown was special order to get it in the needed lengths in one piece, and they seemed unable to salvage pieces that they had to pull off and redo during the installation. Also, I live in a rural area, so I have tried to avoid open hidden areas that can become mouse homes and bug homes.
    I was hoping that I could simply make a wise selection on the coating to avoid any mold problems.

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