Water condensation under house

Yesterday I had a conversation with an insulation contractor regarding the installation of insulation (say that three times real fast) in a client’s attic. This prompted the post yesterday about water stains on the ceiling.

Today we were involved with a different client’s insurance adjuster regarding water damage on a floor. The flooring was buckled inside and an under house inspection showed water drops beading on the wood floor joists.

This home is probably 50 to 60 years old but we have seen new homes with similar problems. This home was constructed over a ventilated crawl space, uninsulated wood floor, finished wood interior surface.

This is the fifth home we have been involved with this summer but this time there is no ductwork under the home. If you read yesterday’s post linked to above you can see a pictorial description of the thermal properties that must be understood to address and solve this problem.


The photo above shows several colonies of mold growing on the wood. Notice on the joist at left that the bottom has fewer colonies.


This photo shows the underside of the plywood subfloor. The light patches are mold.

Sweating Floors

Overview of crawl space flooring system

Here is a drawing of the floor system showing one solution that I have used in the past when there is no ductwork in the space. The resistance from contractors is usually something along the lines that you can’t seal up the crawl space; that it must ‘breathe’. This is not the case. The goal is to establish a vapor barrier between:

  • the crawl space and the exterior wall/ambient air
  • the crawl space and the earth below

Caveat: This method only works with a gas permeable flooring surface. Vinyl sheet or ceramic tile is not gas permeable. A wood floor with polyurethane finish might be a problem. Older floors or the newer prefinished wood seems to work OK. But do your homework here before starting.

Also this method is inadequate for crawl spaces that contain air conditioning ductwork. We have a different solution for those spaces.

This method uses your air conditioning system to dry your floor and the floor system dries the crawl space.The reason it won’t work with the flooring materials mentioned is that vinyl sheeting prevents migration of moisture from the crawl space to the home. This blocks the path of dehumidification. It does work with a wood floor or carpet.

Section view of hanging sheet detail at exteerior wall

Section view at perimeter pier

Description of work:

Lay a sheet of 6 mil polyethylene sheeting north-south from one wall to the opposite wall between each set of the interior piers. Lay another sheet east-west , wall to wall atop the first sheet. Perimeter vents must be sealed tight, then nail a hanging sheet (two feet longer than the perimeter wall is tall) from the floor sill beam down atop the sheet you placed on the ground. Weight this sheet with bricks every few feet. Run this hanging sheet around the entire perimeter of the house.

Let this hanging sheet cover the access door so that you must push it up out of the way to enter the crawl space. Foam the ground sheet to each pier.

You will not achieve a perfect seal but the better job you do in sealing, the drier the space will be. The path for the moisture from the crawl space to the air conditioning system is restricted by the flooring. A leaky crawl space may overwhelm the ability of the system to dry the space.

The construction methods in use in many of today’s homes with crawl spaces are flawed and intended for another part of the country. I’ve seen this in new homes (recently), I’ve seen this in old homes, I’ve seen a home change occupants then develop the problem. The colder the home interior temperature, the worse the condensation problem will be. I have seen this problem for decades and, before blogs and Facebook, I’ve had to give this speech a thousand (OK maybe fifty) times complete with hand-waving and scratches on legal pads. I’m putting this together for an insurance adjuster and I’ll just email him a link. I thought since I’m writing this anyway I would share it with you. Hope it is of some help.


About Swinson AC Co

Residential Air Conditioning and Heating Contractor in Central Baldwin County Alabama and the Eastern Shore 964-9009 990-0998
This entry was posted in Air Conditioning, Humidity, Mold, Odors. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s