GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Dealing with a vented crawl space

patrick_cox | Posted in Energy Efficiency and Durability on

Our home was built in the late 60s and sits on a vented crawl space in the Southeast. The crawl space is about 1800 square feet and actually has only 3 block size vents. So there is really not much ventilation currently. There is a vapor barrier installed on the dirt floor but there are many gaps and it is your basic 6 mil plastic.

There are signs of “wet block” in certain areas of the crawl. One issue that I may have is poor grading in certain spots of the house that is allowing water to collect.

The crawl space shows humidity in the summer in the range of 70% – 85%. There is no standing water in the crawl and no evidence this has ever occurred. We have lived in this house for over 20 years and are not aware of any standing water occurring. There is some yellow surface mold in some areas but no structural damage.

In 2008, at the recommendation of our HVAC company, we installed insulation in the ceiling of the crawl and as of today it is in pretty good shape. No real “drooping” of the insulation.

I have had a number of different contractors evaluate the situation with varying recommendations and proposals. Full encapsulation will cost over $6,000 and I am trying to approach this in a slower less aggressive manner.

I am considering the following steps for now and would appreciate your comments…

1. Fix grading around the house so water more easily drains away from the house.

2. Remove existing vapor barrier and install a new thicker 8-12 mil floor barrier that is taped at the seams and fully covers the floor. I can also run that barrier part way up the foundation wall.

2. Now to my real question. Since we only have 3 small vents, I was thinking about sealing at least one of the vents and maybe two, and then in the remaining open vent or two, install an exhaust fan that will pull air out of the crawl space to the outside. I am thinking that since there would be no open vents, I am thinking that the fans would end up pulling the makeup air from the house down into the crawl space. I am thinking the benefits of this would be as follows – if there is any radon gas in the crawl it will get sucked outside, and then secondly, allowing some conditioned air from the house to get into the crawl should help condition the crawl somewhat.

My hope is this will improve the conditions in the crawl space to a more acceptable level of humidity without a full encapsulation.

So thanks for any comments!


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that you might find helpful: Building an Unvented Crawl Space.

    Your idea of converting one of your three vents to an exhaust fan makes sense, as long as (a) the fan is correctly sized -- you don't want it to be too powerful, and (b) you install a floor grille to allow air from the upstairs to enter the crawl space.

    Most building codes require sealed crawl spaces to have this type of floor grille. They also require the crawl space to be conditioned. One conditioning option is the one you suggest -- providing “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.”

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |