How much do oil-based stains affect vapor permability?
MichaelBa| Posted inEnergy Efficiency and Durabilityon
Hi guys,
I stained my siding recently (bevel siding) with oil based stain.
Since my house is built fairly vapor open, I am curious how much oil stains will affect the vapor permability of the building’s envelope and ability to breath?
I am guessing it leaves the wood still semi vapor permeable because almost every house has paint/stain.
How do you guys approach this when building a vapor open house? Even if it makes it vapor closed, I do have 1/2″ furring strips behind my siding, so would that be suffecient to allow vapor to escape from the house?
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Replies
Michaelba,
The short answer is that there is no way that you can make siding "vapor closed" with stain. Stain is not polyethylene. While the stain may reduce the vapor permeance of your wood siding, the sheathing and siding can still dry to the exterior (albeit at a slower rate).
In addition, you should realize that the drop in permeance is temporary. After a couple of years of weathering, the reduction in permeance will be mostly gone.
I'm not sure why your goal is to build a "vapor open house." What you want is a functional house, not a vapor-open house. (I sure hope that the concrete slab in your basement, if you have one, isn't vapor open.)
Finally, if you have 1/2-inch furring strips behind your siding, the vapor permeance of your siding (and the effect of the stain) is almost irrelevant. Most of the outward drying that happens on this type of wall assembly happens by ventilation drying, not diffusion. A rainscreen gap (that's what we call the gap created by the furring strips) encourages ventilation drying, which is good. Ventilation drying is far more effective at lowering the moisture content of your sheathing than depending on diffusion.
Martin, thanks for clearing this up. Just was something I was thinking about after the fact and was curious about it. This all makes sense.