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Community and Q&A

Moisture content before Sheetrock and insulation?

Jamie_K | Posted in General Questions on
We have our slab on grade shop (we are going to live in for a bit) all sided and roofed. Framing was done w good clean kiln dried Douglas fir in late dry summer months (same for plywood sheathing used for both walls and roof). Roof got dried in w/o rain. As weather has changed with freezing overnight temps, some rain, etc without a garage door on quite yet..some of that outdoor moisture is inevitably moving inside with a pinless moisture meter reading up to 20% (ranges from 13-20 I think w one board being 28) on some framed interior boards using softwood setting. No falling rain has gotten inside. I know I can’t be alone on this as in building it takes a certain amount of time to get finishing details down with humidity etc fluctuating outdoors.
1) I believe mold growth can occur at something like 14% and rot over 19% or somewhere thereabouts.  BUT Our wood was all clean, dry and covered (roofed/house wrapped) properly this late summer in process of building so is this really a concern for mold growth?
2) Once we get garage door on hopefully in a week, we can start heating w temp heaters we will purchase new (and I’m thinking not use again because of drywall silica??) to dry out. I’m assuming leave windows cracked during that process  all day/night (and when drywall taping/spraying) even if it’s raining outside to let moisture escape from heaters trying to dry it inside?
3) Is 11% what walls are suppose to be at before insulating and drywall?

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  1. Expert Member

    The answer is a bit dependant on your walls drying potential, but unless you are using a very unusual wall assembly, the framing lumber just needs to be at 18% before being insulated and drywalled.

    1. Jamie_K | | #2

      My father in law who built a lot for tribes in Alaska thought 11% was necessary before moving forward? Any thoughts?

      Wall’s drying potential? From inside out: Sheetrock (will probably upgrade to green board everywhere but try DensArmor in bathroom) with latex paint (less barrier requirements for shop) > 2x6 framing with plywood wall sheathing > Tyvek house wrap > Hardie lap siding. Vented roof: plywood sheathing > synthetic underlayment > asphalt shingles.

      1. Expert Member
  2. GBA Editor
    Martin Holladay | | #4

    You are following normal protocols. First, get your roof on and all of your windows and doors installed. Then, install temporary heat for a short period of time. Then, insulate and hang drywall -- and don't worry.

    Remember, the moisture content (MC) of your wood on the day you insulate will not be the MC of your wood forever. Your walls will continue to lose moisture content for a year after you move in. Your walls will continue to dry.

    According to a paper by Mark Willians (“Developing Innovative Drainage and Drying Solutions for the Building Enclosure”): "Most water-induced deterioration of wood-based products and mold formation requires wood moisture content above 20% (Morris, 1998). Readings from 20% to 28% are typically considered moderate and indicate that damage may occur if moisture levels are sustained. Readings of 30% and above indicate that wood-based components are saturated and damage is likely if sustained (Morris and Winandy, 2002)."

    1. Jamie_K | | #5

      Thank you, Martin.

  3. Jamie_K | | #6

    I’ve heard dry out framing to 18%, then 16-17% and as low as 11% before insulating and drywalling:
    - Is this just to avoid trapping too much moisture in walls or for another reason as well?
    - And does it apply to ceilings w a vented attic also? If so, is that because too much moisture will be released into Sheetrock and/or through insulation added if not dried to certain %?
    - Anyone know what is a realistic dry out MC % for my area in climate zone 5/6 in N Idaho/Eastern WA?

    We are having trouble getting wood below 20% with vented attic open. Some reads as high as 29-30% & some as low as 15% (about equal of each). We have lots of fans running for ventilation but heaters don’t have enough electricity w temp power pole until strong perm power turned on in 1-2 weeks when I would think most drying would take place when heaters can produce hotter temps (but how much good will that do w vented attic space still not enclosed..maybe do quite well). However I am wondering if someone can teach me a little bit more about if attic framing (trusses) need to be dried out same or how differently than walls before sheet rocking and insulating?

    Thank you so much! We don’t want to find out after the fact we trapped to much moisture in something that we needed to learn be process better on beforehand. :)

  4. GBA Editor
    Martin Holladay | | #7

    I wouldn't worry about the moisture content of trusses in a vented attic, as long as your drywall contractor understands truss uplift, and used clips where necessary instead of an excess of screws. If you don't know about truss uplift, you can Google it.

    Otherwise, the moisture content of your roof trusses is irrelevant. They will dry quickly in a vented attic.

  5. Expert Member
    Peter Engle | | #8

    The USDA Forest Products Laboratory has done quite a bit of work on wood decay, and there are many resources there that can be reviewed for free. A Report by Carll and Highley for the Journal of Testing and Evaluation provided a relatively thorough overview of the available literature at the time (1999). They found that wood rot species do not germinate or grow at moisture contents much below the point of fiber saturation. They also found that there is a variable relationship between fiber saturation, relative humidity, wood species, and measurable moisture content of the wood. For most softwood species, fiber saturation occurs at 28%-30% MC, which equates to something over 97% RH at the wood surface. It is common and well-documented that wood can be held at MC of 23%-25% indefinitely without growing mold or rot.

    However, we like to have margins of safety in our construction, and wood that is held in service at these high moisture levels can easily exceed them with minor wetting events, leading to rot and mold growth. Therefore, Carll and Highley supported the general recommendation to maintain in-service moisture levels below 20%. Again, that is a design and test condition for in-service softwood lumber. Brief excursions above this level during construction and other relatively short-term events cause no damage, and the risk of damage decreases significantly as temperatures decrease, with zero risk below about 40 degrees.

    The reason we see some recommendations for drying to lower numbers before installing finishes is primarily cosmetics. Wood shrinks as it dries, and if drywall is applied to lumber at 25% MC and the wood subsequently dries to a more normal 10%-12% in service, there will be significant cracking, nail pops and other cosmetic issues with the drywall. These can largely be avoided by drying the frame prior to insulating and drywalling. Drying the frame also reduces the risk that unidentified construction moisture trapped in the wall actually causes an increase in MC before the walls dry naturally post-construction.

    Martin also pointed out the issues of truss uplift, which are related to differential moisture contents of the lower and upper truss chords. When the bottom chord is buried in insulation, it tends to reflect the temperature/humidity environment of the interior while the upper chords are exposed to outside conditions in a vented attic. The differential expansion of the wood causes the trusses to arch upwards, pulling it away from the interior walls of the house. This is avoided by using truss clips instead of fasteners at the tops of walls. Google will get you to lots of sites with products, graphics and other information.

    In your climate, everything will dry pretty quickly by itself now that the shell is watertight. Pinless moisture meters are OK for finding relatively wet vs dry conditions, but you should not believe the absolute numbers, as they can be off by quite a bit. Don't worry - you've done everything right and your measurements are normal. Get the doors on and sealed and proceed with interior finishes.

    You mentioned temporary heat. I hope that is not in the form of propane-fired "salamander" heaters. Those introduce more moisture (and products of combustion) to the air.

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