? possible reasons for crawlspace mitigation

At an inspection yesterday, I found a crawlspace mitigation system in place. Fresh air intake duct at one end of the crawlspace, power fan exhaust duct at the other end running constantly, black plastic covering the soil. To me, it looks like a fairly typical mitigation system for radon gas. But there’s no mention of it in the disclosure. There are also two sump pumps, with a pretty constant flow of ground water through them. So my initial thoughts are either this system is a way to control humidity/moisture in the crawlspace, or else it’s for radon gas. What are the other possiblities? Any other reasons to have this type of system in a crawlspace?

Some people have ventilation fans installed around here for humidity/moisture control in crawls. With the sump you mentioned, I would suspect it is to help moisture control.

Seems as though it may serve a dual purpose – humidity and radon. Good idea.

From your description it is for ***humidity ***/ moisture control.

From your description, this is not a radon mitigation system.

As stated already, this fan system is there to assist in controlling humidity & moisture build-up.

I agree. I have run into a few of these systems. They are usually controlled by a humidistat, turns on when the humidity in the space reaches a certain level.

I have two fans in my crawl to control humidity, no radon gas.

There are two thoughts to controlling moisture in a crawl space.

1- The method that you explained above. Which is not the best method

2- The method that in my opinion works better is to; Completely seal off the crawl space. Which means sealing off vents, windows and access doors and to install a dehumidifier and a crawl space heater. The dehumidifier will maintain a healthy moisture level approx. 50% year round. The crawl space heater set on a thermostat will help from pipes freezing during winter months.

The Black plastic on the soil is called a Vapor Barrier, which is fine,I don’t have a problem with that. The sump pumps are also good to maintain a lower water and moisture issues. The problem I’ve found with sump pumps are; they are not installed correctly, Typically they do not install them to the correct depths. The fan has nothing to do with a Radon mitigation system, it’s their way of trying to control moisture or water issues. Paper backed fiber glass is not good for a crawl space, It’s better to use foam or plastic sealed fiber glass.

I hope this helps you…


I’ve been in a few crawlspaces that needed to be mitigated…

I’m weighing in on the side of humidity. I’ve seen similar set ups quite a few times here in the Pocono mountains.

sub_membrane_depressurization.jpgMost Radon systems will have a membrane of some sort… cross ply or 6 mil and collection points.

Static fans alone are not the norm for Radon. Here’s a typical pic of an active system

In humid climates or where there is active moisture in the crawl space these fans reduce the relitive humidity (RH) in the crawl space in an attempt to keep the lumber moisture content below 20%. Did you check for moisture content of the floor joists? I know that goes beyond the scope but it is likely why the fans are in place. Reducing the RH in the crawl space will reduce the moisture content of the lumber, in theory anyway.