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Bill Brodhead WPB Enterprises Inc
Radon & Vapor Intrusion |
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Bill Brodhead WPB Enterprises Inc.
Radon & Vapor Intrusion |
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20 Rules for Difficult Mitigations |
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WPB Home Page Pressure Field Testing PFE |
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20 Rules for Difficult Mitigations |
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WPB Home Page |
What are the considerations remediating a difficult homeNear my office was a new development that had the highest radon levels ever recorded in the United States. My company remediated almost all of the homes in the development. The following slides and some additional comments are from a presentation of the lessons learned remediating these homes.
Rule 1: Radon levels vary widely house to house. Even with homes over 1000 pCi/l the home next door can be more typical radon levels. Below is an example of what we found. Radon levels can vary dramatically because soil permability can be 100's of times different in a short distance.
Rule 2: Verify you have achieved negative pressure under the entire ground contact slab. You must use a micro-monometer and measure sub-slab to room pressures at the farthest distances from the suction pit or pits to know if the mitigation system is treating the entire slab. Building windows and doors need to be closed. Minimum acceptable negative pressure varies with current weather, climate zone area, soil type and house position on a hillside. Generally in the North East a minimum of - 1.0 pascal (- 0.004") in the heating months is usually adequate although any measureable negative pressure will often be enough.
Rule 3: Even with good PFE always look for other possible radon sources.
The lowest level of a building is typically under a negative pressure that can draw in
soil gas from other pathways besides the lowest slab. It is
important to always note other possible radon sources besides the lowest
slab. In this
picture the garage does not look like a likely source because it is set
below the poured wall. The front entry does not have its slab
poured yet but the soil is right at the same level as the top of the
foundation. This is an easy path for radon entry into the basement
if the sill plate does not compress the thin sill sealer insulation.
Small variations in concrete height can easily create openings under the
sill sealer allowing radon to be sucked from the front entrance
sub-slab. Crawl spaces or utility pipes or floor drains can also
be alternate radon sources.
Rule 4: Adjoining slabs that are at the top of a basement
foundation wall Garage slabs, front entrance slab and rear patio slabs are often an additional radon source if they are near the top of the basement foundation. Always note the number of steps from the first floor framing to the adjoining slab. If it is only one step like a typical front entrance slab it could likely be a radon source. If the foundation material is blocks then lower slabs can be a source if the block wall is not well sealed at the top of the foundation.
Rule 5: Look for sources of negative pressure in the basement. Most basement located air handlers for heat and AC tend to depressurize the basement because of leaks in furnace filter and the return duct system. If a basement is finished check the size and openings of the return grills versus the supply grills. In this slide the basement return grill is way over-sized and depresurizing the entire basement when it runs by 4.0 pascals.
Rule 6: Check basement to 1st floor pressure difference. Run tubing from a micromonometer under the basement to 1st floor door. Make a measurement with the HVAC off and then with the HVAC on. If the tubing is connected to the signal port and the instrument is in the basement than any reading of positive pressure indicates the basement is negative this amount. A reading close to or more than 4.0 pascals (0.016" sp) can often backdraft a natural draft gas water heater as well as draw in radon from multiple sources.
Rule 7: Use Radon Sniffers to locate radon sources. There are passive and active radon sniffers. A passive radon sniffer has no pump to draw in an air sample while an active has a pump. Passive sniffer depends on simple diffussion from the room into the detector head. One passive sniffer the EcoTracker has a fast radon response and can give a result in 5 to 10 minutes. Active radon sniffers like the GM1-2 and the CT007-R use a pump and provide results in six minutes or less. Additional time sequences are typically needed to improve the sniffers accuracy. In general radon levels less than 5 pCi/l greater than background are not easily identified by sniffers. Sniffing a radon source greater than 10 pCi/l above background is sometimes needed to have confidence that the sniff location has elevated radon. Minimizing the time a location is sampled is also important to reduce background radon counts in the instrument. The website for the three different radon sniffers are listed below. Refer to "Radon Sniffer Testing" for more explicit information about sniffers and how to use them.
Fixing a home with only 59 pCi/l. After reducing the radon levels to below 4.0 pCi/l on a home that had an initial level over 3000 pCi/l, I assumed this next one would be a piece of cake to fix.
Rule 8: A jumper pipe can be
used to transfer vacuum from Even though the radon levels started off low,a single basement suction pit only reduced radon levels to 13.7 pCi/l. The radon levels were 4.6 after adding suction pits to the garage sub-slab which was two steps down and the more likely source the front entrance sub-slab that was one step down. A sniffer test was then used to find elevated radon at the top of the foundation adjacent to the side entrance slab. A jumper pipe was used to treat this side entrance slab. A basement sub-slab depressuriztion as low as 2.0 pascals or greater can be transferred to an upper adjacent sub-slab depending upon the size of the slab and how air tight the upper sub-slab is. It is therefore important to seal any slab openings of the upper adjacent slab.
Rule 9: Carefully
inspect the perimeter and re-caulk all crack openings or
The vacuum under the basement slab was reduced to
Height of an adjacent slab Front entrance or side entrance or rear patio slabs are typically only one step down from the house. This means the top of the sub-slab gravel or dirt is at the same level as the foundation sill plate. The insulation air sealer under the sill plate is about a 1/2" thick but may have gaps or not actually be air tight allowing soil gas to move into the basement.
Always note how many steps down ever adjacent slab is from the main house. Garage slabs that are two steps or more down from the house are not able to pass soil gas directly to a basement unless the foundation is block and the block wall was not sealed at the top of the wall. There can however be elevated radon in the garage.
Rule 10: Use a sniffer in areas that have no sub-slab
depressurization system. A grab sample with a radon sniffer can be done in a slab on grade room or garage to quickly identify if it is higher or lower radon levels than the adjoining house. Keep in mind that a grab sample taken during the day may be low because of recent ventilation or limited stack effect.
The garage had higher radon levels than the basement ! In this house the garage was 2 steps down so it was assumed to not be a source. After a single basement sub-slab suction, the levels dropped to a third or 45.5 pCi/l. Even though there was - 5.5 pascals of vacuum under the basement slab it was decided to increase the fan from a 60 watt fan to a 120 watt fan and add a front slab suction. It turned out to make little difference. Once a sub-slab vacuum is achieved under the entire slab, increasing the fan size is unlikely to make a significant difference.
The radon sniffer allowed us to measure around a utility pipe opening that went through the foundation wall. Sealing this 1500 pCi/l reading reduced the basement radon levels to 19 pCi/l. At this point I was stumped and took a grab sample in the garage air. I was shocked that the garage air was 165 pCi/l which was higher than the initial radon levels in the basement. A direct suction of the garage sub-slab and sealing the garage perimeter edge reduced radon levels to 0.9 pCi/l.
Rule 11: If there is a finished full bathroom assume that
there is an opening My general rule is, if the builder installed the bathroom as part of the original construction there is very likely to be a big hole under a basement shower or bathtub. If the basement bathroom was installed at a later date by a quality remodeling contractor than it is likely but not assured that he sealed the opening around the drain pipe.
Rule 12: Builders passive radon piping often needs to be re-worked. Any radon mitigator will tell you he has had to re-work a builders installed passive radon pipe. Sometimes there is no room to install the fan in the attic or no available electrical sources. Other times the pipe is not connected to the sub-slab gravel. Occassionally the sump pit cover is sealed. Rarely is the perimeter sealed.
Rule 13: Passive radon pipes have reduced radon levels by
50% or more I capped the radon pipe in the sump pit and then a day and half later un-capped the pipe and sealed the sump. The radon levels were reduced by 80%. I then unsealed the sump cover and the radon levels climbed back half way to the original capped level. I then capped the radon pipe again and the radon levels returned to approximately the same level as the 1st time the passive pipe was capped. Passive pipes can help if the slab and sump pit are sealed.
Rule 14: Any open conduit pipes that are routed below grade
can be a source of radon. This example from a few years ago demonstrates how radon can slip into the house from a source easily over looked. I was making multiple visits to this house to try and locate the radon source after ensuring the sub-slab depressurizartion system was extending PFE under the entire slab. I noticed the electric was routed to the house below grade and used the radon sniffer to measured the electrical panel. The direct burial electrical line was the source of the elevated radon levels in this basement even though the soil gas had to travel through the outside meter base to get into the basement. A small amount of expanding foam sealed this entry source and the radon levels went below the 4.0 pCi/l in the basement.
Rule 15: Occassionally a radon source is the concrete
slab or foundation wall, Radium thus radon is present to varying degrees in all concrete. Radon can also move slowly from under a slab through concrete. We refer to this concrete source as flux. Both of these sources are typically so small that they are not even considered. As we build homes tighter these small sources become significant. The easiest way to measure flux is to place a radon monitor that measures hourly under a stainless steel bowl and seal on top of a concrete slab or against a poured foundation wall for 6 to 12 hours or more. I use an epdm gasket material on the lip of the bowl and a weight on top of the bowl to seal it. If there is increasing radon inside the bowl there is measureable radon flux coming out of or through the slab.
The calculation of flux is done the following way. Flux bucket radon level change is divided by the hours of the change to give pCi ingrowth per hour. Ingrowth per hour is multiplied by the volume of the bowl in liters minus the volume of the CRM to get pCi/Hr. This result is divided by the slab Sq. Ft. area the bowl covers to give the pCi/SF/Hr result. If the flux is assumed to be the same across the slab, the result is then multiplied by the sq. ft. area of basement slab and the same method is used for any flux from the foundation wall. Both of these added together is the total amount of radon entering the basement per hour from concrete. The total radon entry is divided by the total number of liters of ventilation per hour entering the basement to determine the concrete contrribution to the room radon levels. This paper gives a procedure for determining the approximate ventilation rate of the basement.
Rule 16: Homes and in
particular high rise condominiums are being built more air tight This South Florida condominium development of new apartments built with concrete floors, ceilings and bearing walls had no additional ventilation. Windows and doors were all huricane proof. The only opening to the outside was a dryer vent. Current ventilation standards recommend about 0.35 air changes per hour (ACH). These units had as little as a tenth of the recommended ventilation rate and a lot of concrete exposure. These two factors combined to produce elevated radon levels in almost every unit tested. The elevated radon was not in a basement but in the entire living areas and occupants often keep their windows closed all year. The highest level measured was 30.8 pCi/l.
In one unit it took only 43 CFM to reduce radon levels If you increase the ventilation by a factor of 10 you decrease the radon levels by a factor of 10. In this case the apartment had a ventilation rate of 0.02 ACH or about 4 CFM. Adding 43 CFM is equivalent to the exhaust rate of a bathroom fan. 60 CFM is needed to reach 0.35 ACH.
Rule 17: Radon flux was stopped by two coats of 2 part epoxy paint. In a previous study Bear brand concrete paint was not found to significantly stop radon flux. In this study two coates of another product called Radon Seal also did not significantly stop radon flux. A single coat of Rock Solid 2 part epoxy garage floor paint did significantly stop radon if there were no hairline cracks in the coating. A second application covered the hairline cracks provided the results in the graph, One of the tested locations still had some flux which shows how radon still manages to find a pathway if the coating is not absolutely continuous or sufficient thickness.
Rule: 18: Outdoor radon levels do not vary significantly with height. A radon monitor was place 20 inches above the ground under this deck. A second radon monitor was placed 10 feet above grade on the upper deck floor. Both radon monitors tended to record the same radon level increase. See how the radon levels at both locations did not vary significantly. This indicates when outdoor radon levels are elvated the radon can enter the house at upper floor levels.
Rule: 19: Outdoor radon levels vary depending on wind speed. Wind speed in general is higher during
the day then at night. The warming of the earth by the sun during the day
causes the day time wind speed.
Even a slight breeze can dissapate outdoor radon levels. Not all
evenings but primarily at night the wind stops moving. In the
chart below the spikes in radon happen at night. When the radon does
not rise at night there must be a night time breeze.
Once concrete flux was stopped at the home
viewed above, In the previous chart the basement slab had been sealed only with Radon Seal. The basement radon levels of about 3 to 4 pCi/l was two pCi/l higher than the first floor radon levels of 1 to 2 pCi/l. In the following chart the basement, first floor and second floor radon levels are all the same after the epoxy coating was applied to the slab. This tends to indicate the basement is no longer a significant source of radon however the HVAC operation this time of year could be also contributing to the uniform radon levels. What does appear to be a significant source is the outdoor radon levels. The house levels are now averaging below the 4.0 guideline and are actually lower than the outdoor average.
Rule 20: Even moderate radon levels can be challenging to fix. As shown in previous examples the low initial radon level homes were as challenging to fix as the higher radon level homes. Included is a summary of the primary considerations for every home requiring radon mitigation.
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