Will wet wood framing dry out on its own?

If you’ve had experience with mold in your home, you know that once it starts forming, it can be very difficult to get rid of all of it.  This is one of the unknowns of building a new home...the wood framing can get wet before the home is closed in, or it can come wet from the lumber supplier, either from storing it outdoors or in the case of treated wood, with the water-based pesticide treatment.  The question is, will it dry out if you continue building and enclosing the walls around it?  According to Matt Risinger, expert builder and host of The Build Show, the answer depends on what kind of WRB (weather-resistant barrier) you use and where you place it.  He made an excellent video testing which wraps work the best (and didn’t work). 

Wood is an amazing construction material because it can absorb water and release it.  But we want to avoid preventing wood from releasing the water.  Matt showed how he tests wood for moisture content: using a pin moisture meter.  Kiln dried wood has 8-12% moisture content, which is very close to equilibrium with ambient (not too humid) air.  Saturated wood has 26-30% moisture, and anything above 15% can lead to mold growth over time.

What happens to saturated wood over time when just left to dry naturally? Saturated wood left to dry in a mild climate (Austin TX in winter) can do so, provided it’s not exposed to direct rain, etc.  Permeable exterior weather resistant barriers (WRB’s) do allow drying in winter, even some cheap ones, and this is recommended especially in northern climates. 

Matt ran an experiment where he cut up a kiln-dry 2x4 into several pieces for testing.  He left one piece dry, and soaked all the others overnight in water. He measured the moisture content of all of them before sealing them in different wraps (ziploc bag, Prosoco coating, hardware-store WRB, and Henry’s Blueskin)  and leaving all outside under an overhang for 13 days in December, to simulate winter drying conditions. Here are the befores and afters:

Moisture content experiment: Before and After 13 days

The ziploc bag was meant to simulate polyethylene barriers that were used in the past, which are typically 0 perms, allowing no drying.  Matt himself was very “transparent” in disclosing that he actually had some lawsuits from mold growing in Pacific NW houses he’d built in the late 90’s/early 2000’s.  He was using a low perm housewrap on the outside and visqueen barrier inside, causing some of the lumber to still be wet a year later.

In the south, most homes are dehumidifying towards the inside most of the year with air conditioning systems.  Therefore we don’t want ANY barrier preventing  drying to the inside.  On the outside, he says that a low or zero-permeable WRB is good (as long as you keep your home conditioned for mid to low humidity). 

What are the results of this experiment?  Correctly-installed, weather-resistant barriers are essential to keeping out direct moisture (rain) while allowing anything that’s wet inside the wall to dry.  If you’re building in a cold climate, high perms are better, meaning that it has more ability to dry out.  If you’re building in a warmer climate where the home is air conditioned much of the year, low perms are fine because the air conditioning will act as a dehumidifier and dry out the wall towards the inside.  The other takeaway is to get a moisture meter and use it!  Matt was using an Extech model (probably the Extech MO50), only $62 on Amazon.  We talk about moisture meters and how to use them in this article.

Photo by Taylor Heery on Unsplash