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Understanding Vapor Drive in Reservoir Claddings (Stone & Brick)

This topic is one that I’ve only recently understood fully how it affects houses over the decades, and it just so happens that I’m remodeling a house that’s about 50 years old with a stone veneer front!  So, in this video I’m going to demonstrate how porous stone veneer is to water transport, and I’ll show you the damage that the house suffered due to both liquid and vapor water issues.

Now that you’ve seen the video let’s talk about vapor barriers as they relate to this reservoir cladding.  The second law of Thermodynamics says (in basic terms):
-Moisture moves from More to Less
-Moisture moves from Warm to Cold

When that stone front gets wet, the sun comes out and heats the stone.  That warm stone drives the moisture out of the stone (both to the inside of the house and to the outside), because the walls behind the stone are cold due to the air conditioner running that vapor is moving to the cold side of the wall.  The air conditioned house is also less humid so moisture is tending to move into the house (both points of moisture movement are at play)  Air leakage also is a part of this equation.  As air leaking into the house (both warm & moist) hits a cold surface it condenses into liquid water and that’s where the troubles really begin for houses in our Hot/Humid climate zone.  Houses with this issue will first have mold issues, but if that water continually wets the inside of those walls then that’s when the termite issue will surface over time.  That wet environment is enticing termites to come find their dinner.

So, a few lessons to learn from this failure.
#1 We want a vapor impermeable surface behind the stone cladding.
#2 We want to air seal exterior walls so that air infiltration won’t bring moisture with it to condense inside the wall cavity.
#3  We never want to use impermeable surfaces on the inside of the house.  That includes vinyl wall paper, some types of paint films, and certainly not any plastic sheeting behind the sheetrock.

In summary, Reservoir Claddings must be allowed to dry to the front & back of the veneer.  A 1″ air gap must be maintained and we need air flow at the top and bottom of the walls.  Behind the veneer we want a vapor impermeable surface that also acts as an air barrier.  Behind that we want materials that are vapor permeable so they will dry to the inside where the air conditioner is keeping the humidity in the house low.

For some more in-depth geeky building science info on this issue, check out buildingscienc.com
Here’s a great article on the second law of Thermodynamics.  It’s talking about space shuttle failure & FEMA trailer issues post-Katrina.  Fascinating article.
Here’s another article on Reservoir Claddings that got me started down this path.  Thanks to Joe Lstiburek and all the geeks at Building Science Corporation for all your research and sharing of knowledge.  I am proud to call myself a building science geek!  -Matt Risinger  Risinger Homes in Austin TX