What is Radon Soil Gas Mat?
Soil gas mat also called vapor mat or geotextile drainage mat is a product that allows soil gases, vapors, and radon to be channeled to a vent pipe and exhausted outside.
Vapor mat comes in a few shapes and sizes but is commonly a one-foot wide and 1-inch tall and comes in a 45-foot roll.
It has a plastic material that allows for airflow.
The geotextile mat is wrapped in filter fabric to keep dirt and concrete from blocking the soil gas movement.
Where is it Used?
Soil gas mat is primarily used in new construction.
It can be used instead of a 4-inch layer of clean rock below the concrete floor.
Vapor mat is placed directly on the soil or on a 4-inch layer of sand.
The mat is to be placed within 12 inches along the inside perimeter of the foundation walls.
It can be routed around obstacles such as plumbing.
What if There are Interior Footings?
The soil gas mat should penetrate interior footings to form a continuous loop.
Some suppliers have sleeves used to connect the mat through footings.
Does it Need to be Trenched In?
No, however, we trench it in.
Some of the manufactures may say that it can simply be rolled out and doesn’t need to be trenched in.
We don’t like the thought of the concrete only being 3-inches thick on top of the mat when the rest of the slab is 4-inches.
My dad owns a concrete company and has seen cracking where the vapor mat is simply rolled out without being dug in.
If there is going to be 2-inch foam insulation added on top of the mat and a 4-inch concrete slab, we set our subgrade at 7 and ½-inches below the top of the floor.
This allows the foam board insulation to sit flat on top of the soil gas mat and a uniform layer of concrete.
Can There be Splices?
Yes, there can be splices.
Check with your supplier to see how they recommend making connections.
Typically, the two sections are overlapped, re-covered with the filter fabric.
Then the seam is taped and stapled in place to prevent movement.
How Much do I Need?
This depends on how large the foundation size is.
Foundations larger than 2,000 square feet, but less than 4,000 square feet should have an additional section of matting bisecting the foundation in half.
If the foundation is larger than 4,000 square feet it should have additional sections of vent mat with one vent riser per 4,000 square feet.
How is it Secured?
8-inch landscape staples, 10-inch spikes or 60 penny nails are used to keep the soil gas mat in place while the concrete is being poured.
It should be secured to the ground at a minimum of every seven feet.
If a vapor barrier is not required in your area, you’ll want to secure it more often so that concrete doesn’t get underneath the vapor mat.
Use caution when working next to plumbing lines so that you don’t drive a nail or staple through a water or drain line.
What if There is Insulation Being Used Below the Concrete Floor?
The soil gas mat should be in contact with the soil, not sandwiched between insulation, poly or concrete.
What About the Collection Point?
Your vapor mat supplier will have a collection tee adapter to transition from the vapor mat to a PVC vent pipe.
This adapter should be secured so that movement doesn’t occur when the concrete floor is poured.
A 4-inch diameter section of schedule 40 PVC pipe is then sealed into the adapter with glue or a polyurethane sealant and screws.
Be sure to label the riser pipe with a radon reduction system sticker.
You will also want to place a cap on the vent pipe to prevent concrete or other debris from entering the system.
Where to Place the Collection Point?
This is typically in the mechanical room.
You should avoid placing the riser pipe where it can’t be accessed.
We’ve seen some passive radon vent pipes buried in a wall or behind a linen closet.
If the system is activated in the future, an access panel will have to be installed in order to mount the U-tube manometer and radon system alarm.
Be sure to provide access to the vent pipe at the lowest level of the home.
Is an Inspection Needed?
Yes, in the Minnesota cities we have worked in an inspection is required.
You should call your city to find out before starting the project.
Be sure to call your building official to schedule the inspection before concrete is poured.
You may also want to ask what standard they are following as there are a few different versions.
This will help ensure you are both on the same page and help avoid any delays in the schedule.
Radon Resistant New Construction Standards and Documents
- EPA Building Radon Out
- IRC Appendix F Radon Control Methods
- RRNC 2.0 Reducing Radon in New Construction of 1 & 2 Family Dwellings and Townhouses
- CC- 1000 For Multifamily New Construction of 3 or More Dwellings and Less Than 3 Stories
What’s Next?
After your inspection, you can carry on with the rest of the project.
Other components of a passive radon mitigation system include a vapor barrier, labeling, sealing, running the vent pipe, insulating the vent pipe in the attic, and adding an outlet in the attic so a radon fan can be added in the future.
A light is not required in the attic with a passive system.
It is, however, if a radon fan is added in the future.
You may want to ask your electrician to add a light during construction.
This may save you money.
A permit and electrician will be needed if a radon fan is added to activate the passive system in the future.
