Construction Moisture Control—Dry It Out!

The recommendations for vapor control in Section 4.3.3 provide solutions that allow walls to seasonally accumulate tolerable amounts of moisture and then release that moisture during seasonal periods conducive to drying. This is the way moisture control is supposed to work after a building is completed.

But what happens if the building starts out wet? The answer to this question depends on a number of factors such as construction sequence, timing of completion, properties of materials, and the amount of moisture present in materials when assemblies are “closed in.” Typical problems associated with an inadequate moisture control strategy during construction are as follows:

• Wet wood framing materials may shrink or bow excessively as they dry after construction. This can cause nail pops, squeaky floors, cracked drywall, bowed walls, and unlevel surfaces. Significant shrinkage after construction due to excessive moisture during construction can cause damage to plumbing, mechanical systems, and windows and sealants, compromising the first line of defense against moisture intrusion (Figure 4–19).
• Formation of mold within building assemblies and on construction materials.
• Excessive indoor relative humidity during early operation of the building resulting in various moisture problems such as attic condensation, mold on interior surfaces, and window condensation.
• Wetting of insulation materials and degradation of moisture-sensitive material properties due to premature installation prior to completion of house dry-in (roofing, windows, water-resistive barrier, and flashing installed) (Figure 4–20).
• Use of wet-blown insulations (e.g., some types of fiberglass or cellulose) and not allowing sufficient drying time prior to close-in of the assembly, or drying only minimally to industry recommended 25% maximum moisture content prior to close-in.

The generation or release of moisture from construction materials and practices can be substantial. For example, a 4” thick concrete slab covering 1,000 square feet can release as much as 1 ton of water vapor over the course of its curing process (several months to a year or more). Wood materials that are enclosed in construction when still too moist can release as much as 2 tons of water vapor as it dries over the course of many months after the building is completed. The common practice of using fuel-fired heaters to “dry” buildings can create as much as 1 gallon of water (in the form of water vapor) for each gallon of fuel burned (water is a by-product of the combustion of fossil fuels). When moisture is not controlled during construction, the potential for moisture-related problems during the first year of building operation is significantly increased.

Fortunately, problems with construction moisture or “built-in” moisture can be minimized with the following actions:

• When supplemental drying is desired to accelerate construction, avoid the use of fuel-fired heaters. Instead, use ventilation when conditions are correct, use heaters that do not generate moisture (e.g., electric or forced air with combustion products vented to the outdoors), and after the building is enclosed and dried-in, use dehumidifiers.
• Follow a construction sequence that achieves a “dried-in” status as soon as possible, and prior to installation of moisture sensitive materials other than necessary for structure and dry-in.
• Store construction materials elevated above ground and with water-shedding cover (e.g., tarp or under roof) that is adequately vented to avoid trapping moisture. Refer to Figure 4–21 for an example of what not to do! Materials can also be scheduled for “just-in-time” delivery to avoid exposure during jobsite storage.
• Use a moisture meter to ensure materials are adequately dry when received and especially prior to enclosure within a building assembly (e.g., installation of insulation, vapor retarders, drywall, flooring, etc.). For wood framing, a moisture content of 15% or less should be attained prior to enclosure for shrinkage control purposes (e.g., prevention of drywall nail pops). For installation of wood floors, a much lower moisture content is advisable as well as equilibration of the moisture content of floor framing, sheathing, and wood flooring.