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How do I select the right vapor encapsulation barrier?


How do I select the right vapor encapsulation barrier?

Crawlspace radon systems are some of the toughest jobs, and yet they’re some of the most affordable for homeowners. Why is that? In short, marketing and consumer demand. A “vapor encapsulation system” often costs upwards of $5 per square foot. A “radon mitigation system” in that same crawlspace will often be closer to $1. In this post, I hope to show you why these two systems are more equal than you might think, and why many buyers are misinformed due to dated information.

“I need a 20 mil barrier”.  I hear this from contractors—at the behest of homeowners—all the time.  Why is the mil thickness the only deciding factor for the buyer?  Unless your crawlspace is a landfill or brownfield site (what most 20 mil barriers are made for), then you probably don’t need more than 10.  And if you are covering that: may I suggest moving instead??

Using MIL (millimeter thickness) to select a crawlspace encapsulation barrier is a lot like using “number of cylinders” to select a new car.  It’s dated. What you really want to focus on is technical data sheet points like: puncture resistance; tensile strength; and permeance—permeance being the most important because after all, aren’t you trying to keep radon and water vapor out of your home?  Modern 6 mil barriers perform better than many 20 mil barriers from just a few decades ago.  See links at the bottom of this page for more.

a moisture and radon encapsulation barrier

Why is a properly installed radon system better than a vapor encapsulation system?  Radon systems exhaust radon gas and in the process they exhaust gallons of water vapor each day.  An encapsulated crawlspace does just that: it captures the moisture and tries to block it out.  This is often ineffective (as we’ve seen time and time again: radon cannot be blocked out; it must be provided a new pathway outside the home).  A radon system, while moving moisture and radon gas, often gets rid of “musty smells” too.  Try it: next time you power up a radon fan, do a smell test 10-15 minutes later and see the difference in the lowest levels of your home!

So, what should homeowners really be asking?

What is the permanence rating of your barrier?  What is its ASTM classification (use CLASS A)?  Do you mechanically fasten the barrier to the walls?

radon vapor barrier comparison chart

Getting back to my original premise.  Many radon contractors skip a crucial step–which is why homeowners get such a good price on their systems.  Lots of radon pros do not mechanically fasten the barrier to the wall.  Mechanical fastening can be done with “ramset”, with wooden 2x4s, or with christmas tree pins–to name a few methods.  Mechanical fastening not only makes your system look better, it makes it hold up over time.  Simply slapping a barrier up the wall with caulking doesn’t look very appealing and has the potential to be torn off over time.

I would choose a mechanically fastened CLASS A vapor barrier with a radon exhaust, over a 20 mil “encapsulation” every, single time.  

Please visit the links on this page as well as my how to guides page for more information and demo videos.  Choose a certified radon pro that follows AARST/ANSI guidelines and you’ll find you get a better system at a fraction of the cost!

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How can I use an air exchanger to reduce radon?

Soil depressurization is by far the most economical and energy-efficient way to reduce and prevent high concentrations of indoor radon, but what do you do when soil depressurization is not achievable?

Scenarios include: inaccessible crawlspaces (2’ high or less that cannot be sealed and covered); extremely porous soils like karst or old mine tailing fields (which let air flow freely into the home sometimes by the hundreds of cfm); extremely abnormal radon levels (100s of pCi/L).  Certain homes– “Houses from Heck” as we call them in the industry—just don’t respond to the traditional methods of soil remediation.

Sometimes the ‘solution to indoor air pollution is dilution’!  Air-to-air exchangers, HRVs or ERVs depending on your climate, are typically hooked up to your HVAC system.  Air exchangers take the “dirty” air from your home and send it outside, while simultaneously take fresh, clean outdoor air and bring it inside.  These two air channels pass through the air exchanger thermal matrix (from now on I’ll call them HRVs, since that’s what we use in Colorado).  The HRV (heat recovery ventilator) takes the thermal energy from the outgoing air and transfers it to the inbound air.  So in winter conditions, 32-degree air can be warmed as much as thirty or forty degrees before it enters your home!  These units typically only transfer thermal energy (sometimes moisture), but they’ll rarely transfer radon and other contaminants.  


Properly-sized HRVs can prevent radon indoors by replacing all the air inside a home several times per day!!  Outdoor radon concentrations average 0.3 pCi/L in most of the US, so your air exchanger can drastically reduce and prevent indoor radon with these systems running balanced. In extreme cases, you can damper the exhaust (“dirty”) air, which will positively pressurize your home.  If you’ve read my other articles, you know that “your house sucks”.  It sucks in air due to the stack effect and with that brings soil gasses like radon.  

Now think of your house as a leaky balloon.  Instead of sucking air in, with an imbalanced HRV you’re pushing air out through every crack and crevice.  Ex. 200 cfm in, but 180 out, that’s 20 basketballs of air per minute (cfm) that have to find their way out of the thermal envelope of your home.

Conceptually, this is very simple but in practice you need to take into account many other variables so you don’t backdraft appliances or cause other nasty problems.  It is always recommended you use a licensed radon professional and HVAC pro to perform set-up and installation of an HRV as a radon reduction measure.

For energy efficient HRVs, for radon reduction, see the Whole House Indoor Air Quality section of our website here.  Modern HRVs like Fantech’s HERO Series filter out other air contaminants like mold, pet dander, outdoor allergens, and more!

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How much CFM do I need for my radon fan?

We get this question a lot. The true answer is 1+. If you’ve depressurized your soil under your home, you’ve typically solved your radon problem. However, that’s not really what people are asking when they call. They want to know, “what radon fan should I buy?” Without doing PFE testing using professional equipment (pressure field extension), you can’t size a radon fan properly, and radon fan sizing is a complicated process. 

You see, in other industries that use in-line fans–like HVAC–fans work with known inputs and outputs. Duct size, wattage, amperage; all these things can determine a fixed CFM that a fan will run at to do its job. In radon mitigation, there are too many variables to post a consistent CFM. What is your radon level? What are your soils like? Are you using 3” PVC or 4” PVC? Are you connected to a perimeter drain, a crawlspace, a suction pit, or something else? The same fan in any one of these configurations will pull a different CFM every single time. This is why in radon we talk about the fan curve and pressure field.

A fan curve is a chart of airflow versus pressure. The greater the pressure (resistance) the lower the airflow. I like to use the “milk shake versus beer bong” example. A milkshake is thick and hard to pull (high resistance), whereas us college grads know the beer bong is fluid and easy to pull (low resistance). That’s kinda how the fan curve works. Using a manometer like the U tube, or digital professional grade micromanometers, you can estimate your fan’s current CFM by plotting it on a fan curve chart provided by the manufacturer. These charts are also listed on our How to Guides and Data Sheets page.

However, even if you know the CFM, you’ll still need to know how far your pressure field extends to know if you’ve gotten rid of the radon problem. Many radon pros can provide pressure field extension testing these days, however few provide it in every job due to the cost-competitive nature of the business.  It’s cheaper to “poke and hope” and provide a radon-reduction guarantee than it is to do extensive PFE testing on every home.  Ask your NRPP licensed mitigator if they can do PFE testing for effective radon fan sizing that will get the most economical energy use.  You may pay more up front, but having a more energy efficient radon fan will save you in the long run.  You can dive into the specifics of PFE testing by watching the instructional videos at