Case Study: Atlanta Basements
Moisture Explanations and Solutions
Purpose:
To identify cause and solutions for moisture build up in basement wall cavities during winter conditions. I walked single family detached dwellings in the Johns Creek area at foundation, insulation, and finished phases of construction. The finished homes have both finished and unfinished basement conditions. The appearance of organic discoloration is reported to be present on the lower 24” of gypsum in the finished basement condition, and on the lower 24” of OSB in the unfinished condition. Condensation on the OSB itself has been found when the fiberglass is removed. Most problems are reported in spring. I did not observe these conditions during the field walks.
What is causing condensation
In order for moisture to condense on a surface, the surface temperature has to be below the dew point. Think beer! Open an ice cold beer in August inside your home (typical dew-point inside is around 50-60 degrees), and voila, condensation! Go outside to enjoy your frosty beverage on your back deck (typical dew-point around 75-80 degrees), and water is raining off your beverage. Now let’s relocate your beverage to Scottsdale, AZ in June for a relaxing round of golf (typical dew-point around 35 degrees) and your beverage is cold and dry. Another tried and true strategy for dry would be a beer coozy. The outside surface is dry because there is just enough insulation to heat the exterior surface above 75 degrees. The beer is now dry, even on your back deck. Your hand is also warm.
Key contributors
There are 4 key contributors to moisture build up. Relative Humidity must be above 60% for organic discoloration to occur. Relative Humidity is likely 70-80% or higher over a prolonged period of time for organic discoloration to occur. The key contributors are:
- Wall assembly
- Concrete moisture content
- Heating and Cooling
- Ventilation
Wall assembly
The wall assembly has enough insulation that the interior surface of the OSB is below the dew-point inside the basement. If enough air impermeable insulation (foam sheathing or Sprayed Polyurethane Foam) is installed to increase the temperature of the interior surface above the dew-point, then condensation will not occur. Think beer coozy!
We have completed WUFI hygrothermal analyses for several above grade and foundation wall assemblies that represent the base, good, better and best scenarios. The primary approach is to model the wall assemblies to validate what the indoor RH is when the assembly starts to fail. The best assemblies tolerate up to 70% RH, and the current assembly tolerates around 50% RH. The assemblies were modeled in Atlanta on a cold year. The North orientation was selected to model the wall assembly with limited drying potential.
Concrete moisture content
The reported warranty records indicate the moisture concerns are primarily an issue in the first year. We reviewed the concrete placement practices, and water is typically added on site. As concrete sets excess moisture is released as vapor to the permeable side. The setting of concrete in the first year is a probable contributor to elevated moisture levels. We have identified concrete technologies that can reduce moisture levels in 2 different ways.
- Water reducers – water reducing ad mixtures can potentially reduce the moisture content while increasing workability
- Integral water proofing agents – integral water proofing agents can reduce the permeability of concrete reducing the amount of moisture released into air during initial setting period (commonly used in water treatment facilities)
Heating and cooling
In the finished condition the HVAC is 1.5 tons, which is 3 to 5 times the capacity needed to condition the area. In the unfinished condition the HVAC ducting (around 1/4 ton) comes very close to matching the actual load. Because there is condensation potential, conventional HVAC sizing alone is likely not adequate to dehumidify the space. Reduced sizing will, however, reduce cost. The condensation occurs during heating mode so there is no dehumidification capacity during the heating season. Auxiliary dehumidification systems can be added to create a space dry enough to prevent condensation. Expensive mechanical systems such as mini-splits will have no impact in the winter conditions identified.
Ventilation
Ventilation systems are commonly used in cold climates to prevent condensation. A continuous Heat Recovery Ventilator can be installed to introduce outside air during heating conditions only. If it is 35 degrees and raining (100% RH) the RH will be around 25% once heated to 70 degrees. This may be a much more cost effective strategy than dehumidification with a similar result.
Conclusion
A systems approach utilizing best practices in each of the key contributors is the most cost effective, reliable solution. Expensive mechanical systems such as dehumidification systems could solve this by themselves, but there is a huge penalty in operational expense. Analysis of each assembly and the potential condensation points can give us the information needed to refine the approach to minimize the condensation potential during winter conditions.
About the author. Aaron Davenport is SES Foam’s Director of Building Science and has more than 20 years of experience providing solutions in our industry. For more information please contact SES at AIAinfo@sesfoam.com
