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The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability

03.08.17   Thomas L. Bane, PE | More by this Author

The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability

Welcome to the third part of a four-part series on “The Four D’s of Building Enclosures: Deflection, Drainage, Drying, and Durability.” When applied correctly, the four “Ds” increase the longevity of buildings and decrease maintenance costs.

Water that isn’t deflected or drained away from a building is going to stick around until it evaporates. By encouraging rapid drying in a building’s design, you reduce the chance of condensation, mold growth, and freeze-thaw deterioration.

Things dry in two ways: water evaporates into the air and the air is blown away and replaced by drier air, or water evaporates and diffuses through materials until the liquid water is gone. The first way is the only quick way, as discussed in an April 2016 blog post entitled “Understanding the Mysteries of Moisture Migration.”

A good example of this principal is your home’s unconditioned attic. Most attics in the Midwest have ridge vents and soffit vents which let outside air in at the bottom, and vent out the top. This happens because the attic is usually warmer than the outside air, gaining heat from your home in the winter and from the sun hitting the asphalt shingles in the summer. As the hot air rises and escapes the attic, it pulls excess moisture out of the attic with it, helping to prevent condensation and mold.

If you work in construction, you are familiar with weep holes and vents that are installed at the top and bottom of a section of a brick veneer wall to create a stack effect similar to an attic. As new air floods the cavity from the bottom, older, wetter air is exhausted through the top. This helps the brick masonry dry out more quickly lessening the chance of damage to the mortar during the freeze-thaw cycle.


Air flow through the attic and behind brick veneers helps remove moisture accumulation.

Problems occur when drying requirements are ignored. For example, single-wythe masonry walls have real trouble recovering from wetness because the CMU is often coated on both sides with a paint that resists moisture vapor diffusion. Once water gets in, it can’t get out quickly and causes the coatings on both sides to blister and fail. This means that small leaks over time become BIG problems.


Moisture in a single-wythe CMU wall causes the paint to blister and fail.

Other times, well-meaning vents can cause more trouble than they are worth. In commercial low-slope roofs, vents are sometimes installed in the surface to let moisture out. However, without a stack effect or other method to create air flow within the roof, the hole can actually let water in.


Roof vent in a low slope roof assembly.

In the example below, the vents allowed air flow, but instead of drawing in the relatively dry outside air, it drew in the very hot and humid air from the natatorium below, which caused detrimental condensation issues.

Warm pool air pushes into the building enclosure and exits through the roof vents creating condensation.

These types of scenarios are all too prevalent. It is important to think about how wet things in your building are going to dry out and to determine where the moisture is coming from so you can prevent it from getting there in the first place (see previous blog posts on deflection and drainage for additional help there).

If you’re have trouble with water in your building, or you are planning to construct a building and want to keep things dry, SME can help. Contact Tom Bane for more information.



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