If you are a regular follower of our blog, you already know January is Radon Action Month, so let’s talk Radon! We often get asked what is the best way to mitigate radon? As Inspectors, we routinely see depressurization mitigation systems in the field, however, there is an alternative mitigation system that we are seeing more often. We asked mitigation expert, Matt Bednarz of Connecticut Basement Systems Radon to help us understand the differences between the two most common Active Airborne Radon Mitigation Strategies, the standard Sub-Slab Depressurization, and the alternative Heat Recovery Ventilator System.
First, Active Soil Depressurization (ASD), otherwise known as Sub Slab Depressurization (SSD), is the most common approach to mitigating elevated indoor airborne radon concentrations. A typical ASD system involves running PVC pipe through the basement slab and routing it up to the roofline of the structure. An in-line sealed fan motor draws radon-contaminated soil gas through the pipe and exhausts it above the roofline. By creating a negative pressure field under the slab, it prevents radon from permeating up into the basement and entering the home.
The popularity of ASD in the radon mitigation industry is due to the following factors (listed in no particular order):
- ASD “prevents” the entry of radon-containing soil gas into the structure.
- There is no loss of conditioned air experienced.
- Radon reduction efficiency oftentimes exceeds 90%.
- These systems can oftentimes be installed indiscreetly in homes and businesses.
- ASD systems require little to no maintenance.
- Compared to other common home improvement projects, the cost to install and operate an average ASD system is generally budget-friendly.
However, there are times when ASD is not a good fit to address an elevated indoor airborne radon concentration. It is in these circumstances where we turn to Heat Recovery Ventilators (HRV), otherwise known as “air to air” heat exchangers which are the primary alternative active radon mitigation technology option to ASD.
An HRV system typically consists of a unit measuring roughly 3 ft. or 4 ft. long x 2 ft. wide x 2 ft thick, suspended from the basement ceiling. A series of insulated ducts are installed on the HRV unit and routed to various points within the structure. Two of these ducts are routed to the exterior of the home, either through basement windows or through the foundation wall itself. These two external ducts are positioned at least 12 ft. apart. One is a supply duct to draw fresh air into the basement, the other is an exhaust duct to expel radon-contaminated basement air to the exterior. The incoming fresh air passes by the exiting basement air inside the HRV. While the air masses themselves do not mix, they pass by a heat exchange coil within the HRV that transfers heat from the exiting basement air onto the incoming fresh air. Ductwork is routed off the HRV unit itself to run throughout the basement to distribute fresh air (supply), and to draw out (exhaust) basement air.
The general concept of an HRV system is to dilute the basement air with enough fresh air so that airborne radon concentrations are reduced. Another way to look at it is that an HRV system is increasing the natural ventilation rate (VR) of the basement. Every home “infiltrates” (takes in) and “exfiltrates” (exhausts) air on a continual basis. This exchange of air is referred to as the VR of the home. By speeding up or increasing the VR of a home, you can typically lower indoor airborne pollutant concentrations. However, when using HRV as an airborne radon mitigation strategy, we typically expect a radon reduction efficiency of no more than 50%. While greater reduction efficiencies have been noted in the field, this conservative reduction efficiency assumption has proven to be a reasonable average.
We reserve the implementation of HRV on a strict basis. Generally speaking, we only use HRV when there are innumerable potential radon entry points in a basement that preclude reasonable sealing processes, there are inaccessible areas in the basement that require “hands-on” attention, or there is a known elevated water table under the basement slab that would prevent the ability to create a negative sub-slab pressure field. Care must also be taken when installing HRV. If there’s any question as to the presence of “friable asbestos”, any HRV installation activities need to be suspended until this has been sorted out.
HRV is the lesser favored active radon mitigation strategy due to the following factors (listed in no particular order):
- HRV addresses radon after it has already entered the building.
- There is a loss of conditioned air as HRV operates.
- HRV has limited radon reduction capability.
- There are filters on the supply line that need to be replaced on a regular basis.
- Higher annual operating costs due to increased energy consumption based on system operation, and conditioned air loss.
- The ductwork is bulky (6″) and can be obtrusive, not practical for finished basement areas.
- Install cost can be considerably higher than ASD.
As with any capital home improvement project, consideration must be given to all aspects of the install from initial cost and site requirements to ongoing operating expenses. Successful airborne radon reduction can only be accomplished when the correct solution is applied in an appropriate manner.
For more information or questions on radon mitigation, contact us:
Connecticut Basement System Radon