The High R-Value Foundation Assemblies are summaries of the results of BSC’s ongoing High R-Value Enclosure research — a study that BSC has undertaken for the US DOE’s Building America research program to identify and evaluate residential assemblies that cost-effectively provide 50 percent improvement in thermal resistance.
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Standard 8-in Concrete with R10 interior insulation results in moisture buildup behind the wall.
According to the IECC, new residential construction of conditioned basements in DOE climate zones 4 and greater must be constructed with continuous R-10 insulation or R-13 in a framed wall. Continuous R-10 is typically installed by applying a roll batt directly to the foundation wall which consist of fiberglass batt. In some areas, the roll batt is covered with a polyethylene vapor barrier, as was simulated in the hygrothermal analysis. In the IRC, there have been improvements to the building code which do not recommend a Class I or II vapor control layers in the basement or on the below grade portion of any wall. Commonly a perforated facer is used which is vapor and air permeable.
The installation of R-10 continuous insulation, even as a roll batt, has significant energy improvements over uninuslated foundations, with savings of approximately 31 MBtus (more than half of an uninsulated basement) according to simulations. Roll batt is used because it is very inexpensive and meets code, although there are other alternatives that peform better, as shown in some of the following cases. These alternatives are more expensive for the contractor, and homeowners are unaware of the benefits.
There are moisture issues with this insulation strategy that are evident both in field investigations and simulations. Fiberglass batt is air and vapor permeable, so moisture and air can move through the insulation.
As can be seen in Figure 2, the relative humidity against the concrete foundation wall is elevated through the entire year. If there is air leakage
Uncontrolled and/or unintended airflow through a building enclosure or between units of occupancy. Leakage from indoors to outdoors is known as exfiltration and leakage from outdoors to indoors is known as infiltration. Air leakage can cause indoor air quality problems, condensation, excess energy use, comfort complaints, and smoke transport.
Related resource: Air Leaks—How They Waste Energy and Rot Houses, RR-0303: Ventilation and Air Leakage
Constructability and Cost
This is the most inexpensive alternative in terms of initial capital cost, which is the reason it is chosen. Continuous roll batt makes finishing the basement with gypsum board difficult, unless the roll batt is removed.
This wall is not recommended based on this analysis, other reports, and field investigations of moldy basements.