Radon Mitigation Energy Cost Penalty Research Project

This study performed an investigation of the energy penalty of active soil depressurization (ASD) radon mitigation systems. Two analysis procedures were used to directly measure the energy penalty in houses located in the Twin Cities Metropolitan area: a. a fuel bill analysis of previously mitigated houses and b. detailed monitoring of installed systems and their impact on energy use. The first set consisted of six houses with installed mitigation systems that had been in operation for at least one year. Changes in energy use were determined through a pre/post-mitigation fuel bill analysis employing the Princeton Scorekeeping Method (PRISM). For the second set, five houses were selected from a preliminary sample of houses in which a long-term alpha track detector (ATD) test had measured radon levels greater than 5 picoCuries per liter (pCi/l) in the lower level of the house. Mitigation systems were installed in these houses during the fall of 1990. Fuel use was monitored over the subsequent heating season as the ASD systems were alternately enabled and disabled.

The results of the two sets of houses showed a large variation in the impact of the radon mitigation systems on energy use. From the fuel bill analysis of the six previously mitigated houses, the highest increase in the whole house energy use was 24.6% and the lowest was a statistically non-significant decrease of 2.5%. Three of the six houses had increases that were significant at the 95% level and the average increase for the group was significant at the 97% level. The average whole house energy penalty was 85 ccf or 7% greater than the pre-mitigation level. At a cost of 50¢ per ccf, this represents an average energy penalty of $43 per year. The whole house use includes space and water heating use and for some houses, cooking use and clothes drying. The result of the heating-only energy penalty for this set of houses was 65 ccf but this result is only statistically significant at the 72% level. This lower significance level is not entirely unexpected since the heating-only use is not estimated as precisely as whole house use can be.

For the second set of the houses, the limited amount of data obtained over the partial heating season resulted in lower confidence in the measured heating energy penalty. Of the four houses that could be analyzed, only one had a statistically significant increase in energy use of 18.4%. The average energy penalty for the four houses was 62 ccf or an average increase of 5.3% but this result is only statistically significant at the 66% level.

An average energy penalty for the two sets of houses was also calculated. The analysis of variance of the two subsets of houses revealed that the two sets did not differ significantly. A paired t test of all ten houses showed a statistically significant overall energy penalty of 76 ccf or a penalty of about $38 per year.

The monitoring of the mitigation system exhaust fans in the second set of five houses measured an average fan power of 73 Watts. Assuming continuous operation, this constitutes an annual use of 645 kWh or an annual electrical consumption penalty of $37, assuming a cost of 5.75¢ per kWh. The average ratio of measured to rated power was 79%.

Monitoring of radon levels in the five houses showed the expected reduction due to mitigation system operation, as well as effects of heating system and building dynamics on radon levels in the houses. The effect of mixing by forced air systems manifested in higher ratio of first floor living space to basement radon levels than in houses with hydronic systems, while the houses with hydronic systems showed a greater separation in radon levels between the basement and first floor. Appreciable attic radon levels in houses 1, 2, and 3 were in keeping with the observed attic bypasses in the three houses.
The discrete time humidity measurements in the five monitored houses showed that for houses 2, 3, and 4, human activities are probably an equal or greater source of moisture in the living space than the soil. The soil, however, is a significant source of house moisture at sites 1 and 5 and basement moisture levels were dramatically reduced in these houses when the mitigation systems were operating. In general, the humidity ratios measured in the houses were lower when the mitigation system was operating, were lower in the basement than the first floor, and increased steadily from January to June of the monitored period.

Perfluorocarbon tracer gas (PFT) measurements of the percentage of living space air entering the mitigation system were performed at both sets of houses. Measurements showed an average of 47% entrained basement air in the mitigation exhaust, ranging from 10 to 83%. Measurement precision is expected to be 2%. The variation in the floor area of the houses explained about 40% of the variation in the measurements of the percent entrained air. 90% of the variation in total flow is due to the variation in the entrained basement air flow. This indicates that most of the variability in the amount of entrained air is due to basement tightness rather than soil permeability. The mitigation flow through the basement sub-slab ventilation systems ranged from 9 to 62 cfm and averaged 31 cfm.

Measurements in three houses that also had crawl space depressurization systems showed a high percentage of air entrained from the crawl space (96%) in two of the houses and results from the third house indicated that all the air was coming from the outside. The high percentage of crawl space air is expected because of the inherent leakiness of the floor air barrier. For the third house, large leaks to the outside from a staircase adjoining the crawl space most likely explain the resulting measurements. The mitigation flow for the systems with crawl space depressurization averaged 39 cfm. The flow is probably higher for these systems because of air barrier leakiness.

Secondary estimates of the heating energy penalty were performed using the anemometer pipe velocity and tracer gas measurements  performed during site visits. These values were combined with information on the local heating degree days and space heating system efficiency to compute the added heating system energy use. Unfortunately, the paucity of data coupled with the large errors of the gas use estimates make it difficult to attribute the actual impact of the induced air infiltration on the mitigation system energy penalties. Uncertainties in the actual induced-infiltration impacts, changes in energy use factors such as occupant behavior or changes in the building/soil dynamics, and assumptions in the analytical methods will manifest themselves in the large scatter observed within and around the bounds defined by the two infiltration models employed in the analysis.

Moisture flow in the exhaust pipes was calculated using the results of the air flow measurements with the ambient air and mitigation exhaust dry and wet bulb temperature measurements also performed during the site visits. The humidity ratios in the mitigation system exhaust pipe varied from 0.85 to 1.25% and averaged 1.04%. With this high level of moisture content, the gross moisture flow out of the mitigation pipe averaged 42.6 pounds per day (i.e., about 5 gallons of moisture per day travel through the mitigation fan). Typically, the systems resulted in a net removal of moisture, with an average total moisture removal of 8.3 pounds of water per day (or about a gallon a day). The net moisture removal rate from the below grade areas is a function of the induced air infiltration and the difference between the indoor and outdoor humidity ratios.

Full report (PDF)
Radon Mitigation Energy Cost Penalty Research Project