Radon Resistant Construction
Building radon resistance
into a new house is far less costly than radon mitigation after
construction. The passive system outlined below costs only about $300
for a modest house and has no operating costs. A retrofit sub-slab
depressurization system costs about $1,000 to install and has operating
costs.
Radon enters basements by pressure driven flow through cracks and
openings to soil. Basement air tends to be at slightly lower pressure
than air with radon in the soil, due to warm indoor air rising in cold
weather (the stack effect) and wind on the house and soil.
Methods:
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In new construction, adequate
depressurization under the basement slab to prevent flow of radon-laden
air through small cracks can be created with a passive stack. Before the
basement slab is poured, a four-inch diameter PVC stack is placed
vertically from under slab, up through an interior heated partition of the
house and through the roof. When outdoor air is cold, warmer air in the
stack rising creates a slight depressurization under the slab if the
sub-slab region is well sealed from the basement. There should be gravel
so that the depressurized region can extend completely under the slab.
The bottom of the passive stack connects to a PVC tee with about ten feet
of perforated pipe on both ends of the tee, in the gravel. Dirt-floor crawl spaces are treated similarly with depressurization under
a membrane adhered to the walls. Studies show that passive stacks in
properly built and sealed new construction typically reduce the radon in
indoor air by 50% compared to the radon measured with the stacks capped. |
It still can happen that the radon in a home is elevated despite these
precautions, though it is significantly less likely. In that case
installing a fan in the stack, in space originally provided in the attic,
ensures adequate control of the radon at low additional cost.
Costs:
If radon resistant construction is not used and radon is a problem, the
slab cannot be as well sealed as during construction. Moreover, hidden
openings through the slab behind finish walls and elsewhere may permit
withdrawal of conditioned air by a fan-powered system, which is the only
thing that will work post construction. The energy for the fan and the
makeup heat for air withdrawn through hidden openings can cost in excess
of $100 per year. This cost can be avoided by building radon resistance in
from the start.
The extra cost to build radon-resistance into new construction, where
sub slab gravel and a vapor barrier are standard, is just
for the PVC stack (with room for a future fan in the attic) and sealing cracks and
openings to soil during construction. This typically totals a few hundred
dollars.
A house built to resist radon entry must be tested for radon when
completed, because there is still some chance the radon will be higher
than 4 pCi/L, in which case a fan needs to be installed in the stack where
room and nearby power were provided for it in the attic.
Useful detailed information for radon-resistant construction, with a set
of schematics and explanations of details, is given at the State of Colorado's radon website
(exit DHS). |
