Does a Vapor Barrier Stop Radon?


Living without worrying about radon poisoning is a fundamental factor to consider when building a home, but it’s one we rarely think about.

However, any home can experience a radon problem and vapor barriers are commonly touted as an easy solution to prevent radon from entering the home.

But can a vapor barrier really stop radon from coming into your house? Yes, it technically can, but the vapor barrier must be incorporated into your slab and walls during the original construction of the home. Otherwise, if an existing home has been found to have high radon levels, it will have to be mitigated using a retrofitted active soil depressurization (ASD) system.

However, as is the case with most homeowners, people give little to no attention to radon build-up in their homes until it’s too late sadly. Also, most people do not know how vapor barriers work and how they help mitigate the spread and build-up of radon gas in their homes after construction.

So, to help you understand, we have compiled everything you need to know about radon gas, vapor barriers, and how the two work to minimize your chances of radon poisoning.


Get to Know Your Vapor Barrier

What is a Vapor Barrier?

The dictionary definition of a vapor barrier is a thin layer of material, usually polyethylene, used in construction. This impermeable material locks out moisture from your building, which often compromises the integrity of your structures.

A vapor barrier differs from an air barrier.

While the vapor barrier reduces the diffusion of water into your slab and walls from the ground, the air barrier blocks the flow of air and moisture into the building. 

So, while an air barrier will be 100% effective in locking radon out of your home, a vapor barrier is a little short in its effectiveness.


Types of Vapor Barriers

Vapor barriers can often go by different names, and those are typically just nuances on the same general idea. They may be also known as soil-gas retarders or radon gas mats.

But the idea is the same – they are part of moisture control techniques employed when building structures that can have the deliberate purpose of mitigating radon and other gases as well. These materials do not eliminate vapor 100% but rather slow down the rate of diffusion and divert the gases and moisture towards the exterior of the home overall.

The ability of a material to block the flow of water into your structure through diffusion is a measurable aspect, with units called ‘perms’ representing the material’s permeability.

There are three classes of vapor retarders:

  • Class I

These have less than 0.1 perms. The materials in this class include glass, polyethylene sheets, rubber membranes, and metal sheets.

  • Class II

These materials register between 0.1 and 1.0 perms. Materials in this class include plywood, bitumen, coated Kraft papers, and unfaced extruded polystyrene.

  • Class III

This class comprises materials such as gypsum, fiberglass, and cellulose insulation. This class of materials has more than 1.0 perms but less than ten perms.

Other materials include concrete blocks and board lumber.



What is Radon?

Radon is an element in the periodic table that exists in gaseous form. This colorless and odorless gas exists as a by-product of decaying uranium, a radioactive substance.

In small amounts, radon is harmless. However, with time, even a small a build-up of radon has been shown to cause lung cancer. It’s a lethal substance and it’s hard to detect without specific tools.

Radon is also present in almost all soils, and the air we breathe contains traces of the same.


What Causes Radon Build-Up?

Like mentioned before, low levels of radon are harmless. However, it becomes a problem after a considerable build-up. And after prolonged exposure to the trapped gas, you can develop lung cancer.

Radon builds up when the gas penetrates into your home through crevices in the slab, concrete, or walls, without anything to mitigate, or remove it simultaneously.

It is a common occurrence in crawlspaces and basements, where there is little to no vapor barrier.


How Does Radon Enter a Home?

Almost all homes have their foundation in soil. And like we’ve seen before, all soils contain traces of radon. So, it is normal to have negligible radon levels in the air or your living space. However, without any barrier between the soil and your slab, the radon gas penetrates the pores of concrete and wall into your home.

Any opening with an exposed space to outdoor elements is an entry point to radon. The most infamous cases tend to be that of a vented crawl space with no vapor barrier.

But despite these issues, there are ways to mitigate radon penetration in your home.

And today, we will be looking at vapor barriers as a radon mitigation technique when building a home.


How Vapor Barriers Work to Mitigate Radon Poisoning

Most homeowners do not care for the threat of radon poisoning in their homes. And yet, this radioactive gas is a cancer risk, especially to smokers.

Therefore, when buying homes or moving into a new house, they rarely ask about radon mitigation techniques employed or whether the house has a vapor barrier or not.

As an engineer or architect, you need to do your due diligence during design and construction to protect future residents of the home.

One way to mitigate radon poisoning during construction is to install an Active Soil Depressurization (ASD) system.


What is an Active Soil Depressurization (ASD) System?

An ASD is a well-designed system used to create a low-pressure zone under your home to collect radon emanating from the soil.

The system comprises fans, pipes, and yes, a vapor barrier is considered to be one part of the ASD system.


How ASD Works

There is at least a 6 mm thick vapor barrier installed during construction before pouring in the slab. This thickness is a standard recommended by the Environmental Protection Agency (EPA), although it isn’t unusual for builders to use thicker vapor barriers that are generally 8 mm, 10 mm, or thicker.

Fans, installed strategically, then draw any gases collecting under the vapor barrier towards the attic and roof. The fans then expel the radon gas through an intricate system of connected pipes.

This system is cost-effective when building a new home and is most suitable for crawlspaces, basements, or slabs.

But what happens if you’ve already moved into a home and your radon tests exceed the federal limit?

The best way to handle this radon build-up is to install an ASD system in your crawlspace. Besides helping to mitigate radon, an ASD system has been shown to also act as a quasi dehumidifier – great for those damp basements and crawlspaces!  

An ASD system install will typically run about $2,000 per average home, but considering how severe of a problem radon is, it’s well worth it.


What About Existing Homes?

A vapor barrier obviously should be installed during new construction or major renovations. But once the home is already built, what should you do if you start measuring alarming radon levels?

The short answer is that you get it mitigated by professionals.

While they cannot turn back the clock and add a vapor barrier to an already existing home, they can still do other parts of the ASD system install.

They can still run pipes through unused closets up the walls, and they can still add powerful fans to draw the air from the soil up to be expelled through the attic. Essentially, they’d be retrofitting the home.

Also, the key with ASD is to simply depressurize the soil so that any radon gas can be diluted or dispersed throughout instead of entering the home. While vapor barriers installed on the slab are a way to depressurize the ground immediately under the home, there are also other opportunities for depressurization.

For existing homes specifically, this may require getting creative by running the system through sump pumps and drain tiles, or adding extra suction pipes, for example.


Example of one type of ASD configuration for existing home. Source: EPA Technical Guidance Third Edition



  • Radon is a colorless, odorless, radioactive gas found in all types of soil. When building a home, you must employ relevant radon-resistant techniques to protect you and future occupants from the risk of lung cancer.
  • One of the best ways to significantly decrease the rate of radon and water vapor penetration into your home is to use a vapor barrier.
  • For homeowners, installing a radon mitigation system during construction is cost-effective to keep your clients happy and healthy. Start with the right step to avoid the high costs of radon mitigation in the future.