Appeal 35030

1812 Radon Control Methods – Group R-2 and R-3 Occupancies

The intent of this code section is to protect residential occupants from exposure to radon gas that may be in the soil under a residential building. The section describes radon resistant new construction (RRNC) techniques. The Oregon Health Authority recommends radon testing after construction to determine if occupants are safe from radon exposure or if radon exhaust ducts must be powered or if additional mitigation measures are required.

OSSC 1812.2.1 Subfloor preparation. “A layer of gas-permeable material shall be placed under all concrete slabs and other floor systems that directly contact the ground and are within the walls of the living spaces of the building to facilitate future installation of a sub-slab depressurization system, if needed…”

1812.2.2.2 Soil-gas retarder. “A minimum 6-mil polyethylene or equivalent flexible sheeting material shall be placed on top of the gas-permeable layer prior to casting the slab or placing the floor assembly to serve as a soil-gas-retarder by bridging any cracks that develop in the slab or floor assembly and to prevent concrete from entering the void spaces in the aggregate base material…”

1812.2.5 Passive sub-slab depressurization system (basement or slab-on-grade). “In basement or slab-on-grade buildings, sub slab soil exhaust system ducts complying with Section 1812.2.6 shall be installed during construction…”

1812.2.6 Subslab soil exhaust system ducts (SSESD). “SSESDs shall be provided in accordance with this section and shall run continuously from below the soil-gas retarder to the termination point described in Section 812.2.6.5. 1812.2.6.5 Termination. SSESDs shall be extended up through the building floors, and terminate not less than 12 inches above the surface of the roof…”

For Building C only, maintain soil-gas retarder (radon barrier) but exclude sub-slab soil exhaust system ducts (SSESDs) which will fill with ground water and not function. Install SSESD exhaust through roof if needed for radon mitigation after post construction testing. See below. Test building after construction to verify radon levels and if levels require mitigation, then mitigate with measures listed below.

Original Text:
Building C, which is below the water table, will have a radon barrier that is incorporated into the waterproofing membrane. At Building C, the low levels of radon found on the site will remain in the water underground and will
be prevented from entering the building by the radon barrier. Radon will be greatly reduced from the tested current low levels where no radon barrier is present. We have specified two competitive waterproofing membranes, both of which have been tested for radon control. I have included the product data for the two products, with the radon test results highlighted for your review.

Reconsideration Text:
The specified radon barrier will be installed against gravel under the entire concrete slab and against soil at the west building/retaining wall. The radon barrier will be either 20 mils thick or 70 mils thick, depending on which is selected by the General Contractor for the project. The radon barrier membrane also provides waterproofing (see attached literature). The membrane will be installed vapor-tight, air-tight, and water-tight to form a bathtub around the base of the structure. Both of the radon barriers are thicker than the 10 mil membrane required by code and both are certified for radon control. Ventilation in the apartment units provides 75 CFM of fresh tempered air. A hidden switch in the fan can increase the ventilation rate to 125 CFM.

Original Text:
Due to the high water table, any radon in the soil will enter the water and it won’t dissipate into the atmosphere until that water is exposed to air. So, to mitigate the radon, we have to create “air” under our building. This means we have to dewater under the building in perpetuity. [Note: The site development reviewer, Ericka Koss, concurred that it was not possible for the radon system to perform if it was full of water.]

To accomplish this mitigation, the radon collection pipes under the slab would need to be sized to collect water and air (each pipe would be sized to collect water but be at least half full of air). We can’t collect air without collecting water so that means we would probably end up delivering thousands of gallons of water/hour into the storm sewer. That water would result in a huge stormwater charge. It could also lower the water table in the neighborhood, which could affect adjacent properties including Safeway.

Reconsideration Text:
The code exempts counties from mitigation when they are known to have low levels of radon. There is a low radon level on this site, as verified by testing. The project team consulted with a radon expert in the Geology Department at Portland State University (see attachments). He knows of this site from personal experience and states this locale is known to have low radon levels. He reviewed the test reports provided by the third-party testing company that recorded an average of 0.5 pCil/L in the existing basement. It is already lower than the 4.0 pCil/L code maximum allowed, and this testing was done with no radon barrier in place. He confirmed the testing protocol was appropriate. He complimented the project team on testing the site for radon prior to construction but recommended constructing the building and testing after construction. He doubts that high radon levels will be found, particularly with the new robust radon barrier in place. He said he would be happy to discuss the project conditions with City reviewers (see attachments for contact information). He complimented the City on its diligence in protecting the public from harm from radon.

The team has added a requirement in the specifications (see attached) for special inspection of the building for radon. A 48 hour radon test is required by a third party tester after completion of the radon barrier and air barrier. It will be performed from Friday evening until Monday morning when construction crews will not be on the site. The results will be provided to the building inspector.

The team has a mitigation strategy, if high levels of radon are found after testing. The apartment units in Building C are ventilated with ERV units (Energy Recovery Ventilators) that supply outside air that has been preheated by a heat exchanger taking heat from the exhaust air. Additional ventilation capacity is available because the ERV fans have an internal dipswitch that can increase the ventilation rate from 75 cubic feet per minute to 1.25 cubic feet per minute (see attachments). This 66% increase in capacity has been found in past projects to resolve radon found. Note, this dipswitch is not adjustable by the occupant, so ventilation rates cannot be reduced by the tenant..
If, for some reason, the initial mitigation is not adequate, the testing will be on record and further mitigation measures will be obligatory, until testing shows compliance with code.

In the event that radon levels are still high, after increasing the ventilation rates, then the second mitigation backstop will be constructed. A collection plenum will be constructed above the lowest level using pressure treated 2x4 sleepers with subfloor sheathing over the slab on grade. The sleepers will be notched to create a continuous air plenum under the entire ground floor that will be connected to the vertical SSESD piping, already shown in the documents.

The project team believes that neither of these mitigation measures will be necessary, based on the low levels of radon found on the site and the opinion of our expert. However, we want the City to know that if radon is found after construction, we have viable mitigation strategies. We prefer to not reduce the ceiling height of the lowest level units by installing a plenum under the floor, but we will do that if necessary for this project.

Regarding the code recommended method of mitigation, installing collection pipes under the building to collect both radon gas and water is expected to not perform because the pipes will be constantly full of water. Were the project to drain this water into the storm sewer at a rate faster then it infiltrates, there might be room in the pipes for radon gas to be collected, but there is a possibility that the local water table would be disrupted, potentially impacting surrounding construction including the large Safeway building across the street. The storm sewer charges would be waived to this project because it is affordable housing, so the cost of dewatering is not the issue. We just don’t think it is the best alternative to ensure occupant safety.

The building Owner and project team take radon control very seriously. This is an affordable housing project and is greatly needed by its intended population. We want it to be safe for occupancy. In the proposed alternative the ground water stays in the ground, the water table is not disturbed, the radon stays in the soil, and the building occupants are protected.

If you still have concerns, please don’t hesitate to contact our expert as he requested (see attachments).