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by • January 22, 2013 • Building Science, Framing & Decks, UncategorizedComments (3)17920

Pre-Drywall Moisture Meter Check

Q: What happens to a 2×4 stud when it gets wet?   
A: It dries.  Wood can absorb moisture and release it!  WOW

Q: What happens if that 2×4 is wrapped in plastic after it gets wet?
A: It doesn’t dry. 

Bundle of studs that was delivered to my job site a few years ago.  I’m assuming this bundle was wrapped in plastic at the lumberyard. 

Q: What happens to a wet 2×4 that can’t dry?
A:  mold & rot

Q: What happens to the typical American house that’s built in a 4-6 month cycle?
A: The moisture in the framing lumber dries after a few months of heating/cooling and the house “Settles”.   This is indeed “Normal” but it doesn’t need to be so pronounced.

First, let’s talk about the property of wood called Hygroscopy.  Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically ‘changed’ somewhat, by an increase in volume, stickiness, or other physical characteristic of the material, as water molecules become ‘suspended’ between the material’s molecules in the process. (Source Wikipedia.org)

Most of the lumber used in my homes is Kiln Dried meaning it should arrive to the job site at a moisture content of 12-14%.  Non KD lumber or Pressure Treated lumber generally is 15-19% moisture content.  That 19% is a critical number as it’s general consensus among the Building Science experts that above 19% MC is the point where mold spores are activated and the rot processes start.  Wood can absorb water up till 28-30% MC and is saturated at that point. 

So, if the house lumber is “wet” and construction continues through sheetrock to the finish stage what problems can arise?
Sheetrock cracks, nail “pops”, cracks in the sheetrock above windows/doors
-Floor squeaks, Stair squeaks
-In a worst case scenario plumbing stacks can be affected which can lead to water leaks

Now that you have a brief lesson on Wood Moisture Content let’s talk about what to look for in regards to Moisture Content for a home under construction.  First, you NEED to own a Moisture Meter.  I like pin based meters and he’s my current Extech model I use.  Next, we want to ensure there is ZERO lumber with a MC above 19%.  This is a hard rule, if you are getting readings on your Moisture Meter above 19% then cancel your sheetrock hang crew till it’s all below that 19% threshold. 

Pressure Treated Bottom Plate on a Concrete Slab reading 14.6% MC
I like to note the date & MC reading.  That way if you have an elevated reading you can re-check as it comes down in MC.
My Extech M0220 Pin Based Moisture Meter reading this stud at 11.7% MC.  Perfect. 

 This testing is critical to really knowing what moisture content your wood is reading prior to hanging rock.  As I said before you must get all readings less than 19%, but MY standard is all readings below 13-15% for my KD lumber and less than 17% for any green Pressure treated lumber.  Once your home is completed and the heat/cooling equipment runs for a year it’s typical to see those studs reach equilibrium MC around 10-12%.  That’s why I really want to see MC %’s below 15%.  A small degree of drying is expected but when you get more than 5% MC loss the shrinkage of that lumber will show greater flaws in your finishes. 
  I’ll show you how to dry a wet house in a future post!

Best, Matt Risinger
Risinger Homes in Austin TX
Risinger Homes is a custom builder and whole house remodeling contractor that specializes in Architect driven and fine craftsmanship work. We utilize an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy and durable homes.
Be sure to check out my video blog on YouTube

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  • Great Blog Matt
    Will you do the drying blog soon, I am in the U.K and building in March and you can guarantee we will get lots of rain! WE DID ALL THROUGH 2012!

    So your advice would be most welcome soon.

    • Thanks! I’ll get that post up soon. I need to get a few more jobsite shots of the drying process. Best to you in the UK, we could use some of that rain here in dry Texas. Matt

  • Dan Henderer

    Good info Matt, appreciate it as a fellow builder of 20 years. I noticed you don’t mention testing the moisture content on the inside of the sheathing. Is this because it will likely dry out through the vapor permeable house wrap if it has a higher moisture content in it than you’re aiming for on the studs?