November 1, 2004

Abstract: 

Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior and different climates require different approaches. Ideally, building assemblies would always be built with dry materials under dry conditions, and would never get wet from imperfect design, poor workmanship or occupants. Unfortunately, these conditions do not exist.

Ideally, building assemblies would always be built with dry materials under dry conditions, and would never get wet from imperfect design, poor workmanship or occupants. Unfortunately, these conditions do not exist.

It has been accepted by the building industry that many building assemblies become wet during service, and in many cases start out wet. Furthermore, the industry has recognized that in many circumstances it may be impractical to design and build building assemblies which never get wet. This has given rise to the concept of acceptable performance. Acceptable performance implies the design and construction of building assemblies which may periodically get wet, or start out wet yet are still durable and provide a long, useful service life. Repeated wetting followed by repeated drying can provide acceptable performance if during the wet period, materials do not stay wet long enough under adverse conditions to deteriorate.

Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior. They also involve the drying of building assemblies should they become wet during service or as a result of building with wet materials or under wet conditions.

Moisture Balance

Moisture accumulates when the rate of moisture entry into an assembly exceeds the rate of moisture removal. When moisture accumulation exceeds the ability of the assembly materials to store the moisture without degrading performance or long term service life, moisture problems result.

Building assemblies can get wet from the building interior or exterior, or they can start out wet as a result of the construction process due to wet building materials or construction under wet conditions. Good design and practice address these wetting mechanisms.

Various strategies can be implemented to minimize the risk of moisture damage. The strategies fall into the following three groups:

  • control of moisture entry
  • control of moisture accumulation
  • removal of moisture

Strategies in the three groupings can be utilized in combination and have been proven to be most effective in that manner. Strategies effective in the control of moisture entry, however, are often not effective if building assemblies start out wet, and in fact can be detrimental. If a technique is effective at preventing moisture from entering an assembly, it is also likely to be effective at preventing moisture from leaving an assembly. Conversely, a technique effective at removing moisture may also allow moisture to enter. Balance between entry and removal is the key in many assemblies.

Historically successful approaches to moisture control have typically been based on the following strategy: prevent building assemblies and surfaces from getting wet from the exterior, prevent building assemblies and surfaces from getting wet from the interior, and should building assemblies or surfaces get wet, or start out wet, allow them to dry the exterior, the interior or both.

Approach

Water can come in several phases: liquid, solid, vapor and adsorbed. The liquid phase as rain and ground water has driven everyone crazy for hundreds of years but can be readily understood — drain everything and remember the humble flashing. The solid phase also drives everyone crazy when we have to shovel it or melt it, but at least most professionals understand the related building problems (ice damming, frost heave, freeze-thaw damage). But the vapor phase is in a class of craziness all by itself. We will conveniently ignore the adsorbed phase and leave it for someone else to deal with. Note that adsorbed water is different than absorbed water.

The fundamental principle of control of water in the liquid form is to drain it out if it gets in — and let us make it perfectly clear it will get in if you build where it rains or if you put your building in the ground where there is water in the ground. This is easy to understand, logical, with a long historical basis.

The fundamental principle of control of water in the solid form is to not let it get solid; and, if it does, give it space; or, if it is solid not let it get liquid; and, if it does drain it away before it can get solid again. This is a little more difficult to understand, but logical and based on solid research. Examples of this principle include the use of air entrained concrete to control freeze-thaw damage and the use of attic venting to provide cold roof decks to control ice damming.

The fundamental principle of control of water in the vapor form is to keep it out and to let it out if it gets in. Simple, right? No chance. It gets complicated because sometimes the best strategies to keep water vapor out also trap water vapor in. This can be a real problem if the assemblies start our wet because of the use of wet materials or get wet via a liquid form like rain.

It gets even more complicated because of climate. In general water vapor moves from the warm side of building assemblies to the cold side of building assemblies. This is simple to understand, except we have trouble deciding what side of a wall is the cold or warm side. Logically, this means we need different strategies for different climates. We also have to take into account differences between summer and winter.

Finally, complications arise when materials can store water. This can be both good and bad. A cladding system such as a brick veneer can act as a reservoir after a rainstorm and significantly complicate wall design. Alternatively, wood framing or masonry can act as a hygric buffer absorbing water lessening moisture shocks.

What is required is to define vapor control measures on a more regional climatic basis and to define the vapor control measures more precisely.

Building assemblies can be designed to dry to either the outside or the inside or to both sides. The rules to accomplish this depend on the climate zone. In general, assemblies should be designed to dry to the outside in cold climates, to the inside in hot-humid climates and to both sides in mixed-dry climates and hot-dry climates. In mixed-humid climates it is preferable to design assemblies to dry to the inside and to control exterior sheathing temperatures during heating periods using insulating sheathing.

Wall Assembly Design Recommendations

The following wall assembly design recommendations are climatically based and are sensitive to cladding type (vinyl siding, brick or stone veneer, stucco).

The recommendations apply to residential occupancies. The recommendations do not apply to business, assembly, educational and mercantile occupancies and to special use enclosures such as spas, pool buildings, museums, hospitals, data processing centers or other engineered enclosures such as factory, storage or utility enclosures.

The recommendations are based on the following principles:

  • Avoidance of using vapor barriers where vapor retarders will provide satisfactory performance. Avoidance of using vapor retarders where vapor permeable materials will provide satisfactory performance. Thereby encouraging drying mechanisms over wetting prevention mechanisms.
  • Avoidance of the installation of vapor barriers on both sides of assemblies — i.e. "double vapor barriers" in order to facilitate assembly drying in at least one direction.
  • Avoidance of the installation of vapor barriers such as polyethylene vapor barriers, foil-faced batt insulation and reflective radiant barrier foil insulation on the interior of air-conditioned assemblies — a practice that has been linked with moldy buildings.
  • Avoidance of the installation of vinyl wall coverings on the inside of air conditioned assemblies — a practice that has been linked with moldy buildings.

Each of the wall assembly design recommendations were evaluated using dynamic hygrothermal modeling. The moisture content of building materials that comprise the building assemblies all remained below the limiting-equilibrium moisture contents as specified in ASHRAE 160 P under this evaluation approach. Interior air conditions and exterior air conditions as specified by ASHRAE 160 P were used. WUFI was used as the modeling program.

More significantly, each of the assembly design recommendations have been found by the author to provide satisfactory performance under the limitations noted. Satisfactory performance is defined as no moisture problems reported or observed over at least a 15 year period. . .

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