GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Q&A Spotlight

Tackling a Basement Water and Radon Issue

In Colorado, a homeowner looks for comments on a plan to upgrade perimeter drainage

Image 1 of 2
To improve drainage around the perimeter of the house, a Colorado reader proposes to alter the grade and install a new drain line in a bed of crushed stone. The reader also would like to incorporate a radon mitigation system. The lower level is 4 feet below grade.
Image Credit: All images: Rossn
To improve drainage around the perimeter of the house, a Colorado reader proposes to alter the grade and install a new drain line in a bed of crushed stone. The reader also would like to incorporate a radon mitigation system. The lower level is 4 feet below grade.
Image Credit: All images: Rossn
A site plan showing a drainage and radon mitigation system.

Rossn has been remodeling the lower level of his Colorado home and in the process discovered evidence of some moisture and bulk water issues. Although the problems are not severe enough to make the lower level a “wet basement,” Rossn has developed a plan to better manage water near the house, and solve a radon problem at the same time.

In a Q&A post, Rossn outlines his approach in the drawing that appears on the right.

His plan is to connect the radon mitigation system to the sump, and thus the perimeter footer drain, but he’s concerned that the dimple mat on the outside of the foundation will interfere by causing the system to lose suction.

“I’m not sure the trench/French drain approach will work for me near the surface, given my soil is 60% sand and drains easily,” Rossn says. “I added a liner, though am not sure if that will work, and ultimately I’d tie it into the rest of the gutter system.”

The perimeter drain is virtually flat and is as much as 110 feet from the sump. (A plan view of the project is reproduced as Image #2, below).

Will this plan work? That’s the topic for this Q&A Spotlight.

Some changes to consider

GBA Editor Martin Holladay finds Rossn’s approach sound, but he suggests two modifications.

“Your underground roof is a little close to the surface,” he says. “Either erosion or a curious dog digging a hole might expose the underground roof — so if site conditions allow, you might lower the underground roof.”

Holladay also questions Rossn’s intent to connect radon piping to his perimeter drainage system.

“In general, you don’t want to connect a radon mitigation system to any pipe that leads to daylight or is connected to the open air unless the ‘daylight’ pipe has a check valve,” he says. “Check valves can fail, so that’s sub-optimal. I’m not sure about the connection in your case — but separate radon piping connected to the sub-slab crushed stone layer is best, without sharing drainage piping.”

Radon risks

Radon is a real issue in the area where Rossn lives.

“In Colorado, you almost don’t even need to measure it to estimate that the test will be above the EPA’s recommended remediation level,” writes Dana Dorsett, referring Rossn to a state map showing where radon levels are likely to be high. “Pick a county, any county.”

Radon is a colorless, odorless gas that, according to the Environmental Protection Agency, causes some 21,000 lung cancer deaths per year in the U.S. In high risk areas, pipes are often installed below the basement slab where they collect radon before it gets into the house. An inline fan installed on the main stack vents the radon to the outside.

In Rossn’s case, footing drain lines on the south and west side of the house also serve as part of the radon mitigation system. The fan for the system has been installed in the shed (see Image #2 below). But now that Rossn is using dimple mat to help control water on the outside of the foundation wall he worries the radon collection system won’t work, or at least won’t work very well.

The house dates from the 1960s, so there is no radon piping or vapor barrier underneath the slab.

“Dana’s about right,” Rossn adds. “Radon was around 7-8 picocuries per liter originally [about double the EPA’s recommended action limit], and around 4 picocuries per liter after remediation, but now I’m tightening up the house.”

Addressing radon in older houses

For retrofits, Rossn says the usual approach in his area is to punch a hole in the slab, insert a pipe and install a fan on the pipe. The sump is often the first choice for a point of entry. If that doesn’t lower radon levels sufficiently, a mitigation specialist might add a second point from which to draw sub-slab air, seal seams and cracks, or add a more powerful fan. He refers GBA readers to an EPA brochure on reduction strategies.

“In my case, I do have about 20 feet cut in my slab for relocation of a bathroom and am extending some gravel from it to the sump,” Rossn says. “My challenge is that I’m adding the dimple mat, which would come close to the surface, and serve as a potential leak point.”

But that shouldn’t necessarily be an issue, Holladay replies.

“Dimple mat is usually terminated at the top with a horizontal termination strip or flashing (as my recent article explains), so it shouldn’t be too hard to perform air sealing work at the top of the dimple mat before the termination strip is installed,” Holladay says. “I think you’ll be OK. Of course, you’ll need to measure the radon level in the first floor of your house after the work is complete. In the unlikely event that the radon level is still high, you’ll need to call in a certified radon abatement contractor for advice.”

Jon R agrees with Holladay, noting that if the dimple mat is sealed tightly to the foundation wall with EPDM, there isn’t much of an air leakage path, other than through lots of soil.

Best spot for foundation drains?

Rossn adds that a member of the excavation crew he’s working with says it’s typical for engineers to specify that holes in the foundation drain lines be located below the bottom of the footing, but not in compressed soil. Soil would be sloped down and away from the bottom of the footing, with at least 60% of the fill being sand.

“My own opinion is that the drainage holes on the footing drain pipes have to be below the level of the slab, but the drainage holes don’t have to be below the bottom of the footing,” Holladay says. “You’ll hear lots of opinions on this issue. GBA published an article on the topic a while back called “How to Install a Foundation Drain.”

Our expert’s opinion

Peter Yost, GBA’s technical director, had this to add:

I just want to weigh in on the issue of airtightness and radon levels. New information has become available on this topic since I wrote this blog.

In a 2014 study conducted for the Oak Ridge National Laboratory, researchers evaluated more than 500 homes in 35 states, measuring radon levels in foundation spaces and first-floor living spaces before and after the houses were weatherized.

The report makes several findings:

  • Radon tests conducted after the houses were weatherized — that is, tightened up — found a “small, statistically significant” increase in radon levels. Nationally, the study suggests an average increase of less than 1 picocurie per liter (pCi/L).
  • In houses that started with low levels of radon, the impact of weatherization was relatively small. In houses with higher levels of radon before weatherization the impact was greater.
  • Higher levels of radon are relatively rare in mobile homes and in site-built homes in counties the EPA has identified as having low radon potential.
  • Colder weather increases the stack-effect depressurization of basements and crawlspaces, which in turn sucks more radon into living spaces.
  • On average, the overall effect of air sealing is a reduction in natural ventilation and proportionately higher indoor radon levels.
  • The installation of a continuous mechanical ventilation system reduces radon levels.

Keep in mind that the only way to know the impact of weatherization work on radon levels is to perform a radon test after the work is complete. Statistics only indicate probability — a specific home may be a statistical outlier.

One other thought: Rossn (the homeowner who posted the question) is concerned that the perimeter drain has a long run. I would recommend adding clean-outs to the perimeter drainage system.


  1. User avater
    Walter Ahlgrim | | #1

    Exterior radon system?
    To me it seems adding a radon system outside of the foundation is unlikely to make much if any change to interior radon levels. Are there any studies comparing interior and exterior radon systems?


  2. user-7033459 | | #2

    Basement water and radon
    When we build a house we put in double chamber drain tile, we use the top for Radon and the bottom for water. We also put cross over pipes every 6' so the water never gets pluged, water can find its way out. We vent the Radon up through the roof, around the center of the house.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |