MOISTURE IN CONCRETE SLABS POURED ON GRADE IN ARIZONA
You bought a new home, ordered your options and moved in. A year or two go by and you start rearranging
the furniture. That’s when you noticed the odor. You lifted the protective mat under your desk chair and
the smell made your eyes water or maybe you smelled it when you opened a closet door or an unvented
room. Did you order a light colored vinyl flooring for your kitchen but now the floor has a darker
color coming through? Maybe it has a grey or green tint, maybe it has a purple or orange hue or maybe
it has bubbled or lifted off of the floor.
These problems and many others are caused by moisture vapor coming up through the concrete slab. This
condition is occurring more frequently here in the Phoenix area and there are a few factors that contribute
to the problem. Before we cover those factors, let’s look at the problem a little closer.
Moisture vapor coming up through a slab can cause wood floors to warp, delaminate swell and promotes mold
growth. The moisture can cause the mastic that holds tile to the floor to fail, this leads to the floor tiles sounding
hollow and lifting off the floor. The vapor frequently passes through carpet and other floorings unless there is
some type of vapor barrier. Vinyl flooring creates a barrier by itself; other examples of barriers could be a chair
mat, floor runners with a non-slip backing or anything else that prevents the moisture from escaping. The bottom
line is, if moisture can’t pass through the flooring, then it is trapped under the flooring. That leads to the next question,
where does the moisture come from?
The moisture typically comes from two sources. The water in the concrete mix slowly dries by traveling to the top
of the slab and evaporating. This process can take six months to completely dry a properly placed slab. The other
source of moisture is from the soil beneath the slab. Moisture evaporates and once concrete is placed on the ground,
the evaporation of moisture in the soil is severely restricted. The moisture in the ground is rarely viewed,
discussed or even considered an issue during construction since the slab is still in the drying process
and there is not a large temperature difference above and below the concrete. However, this changes once
the home is completed. Now the air inside the home is conditioned and the temperature change and pressure
change along with the difference in relative humidity between the air above the slab and the soil beneath the
slab causes a vapor pressure difference. Simply put, moisture travels from heat to cool. The moisture under the
slab is drawn to the cool, dry, air conditioned concrete slab. Concrete is porous, it is not water proof and the
moisture vapor rises to and passes through the slab by capillary action and vapor diffusion.
Moisture under a slab wasn’t an issue in the desert a few years ago, but today is a different story. Today’s construction
techniques are a little different, contractors are flooding building pads to address issues with the soil like
compaction and expansiveness. Homes are also being constructed on properties that used to be agricultural land.
This land may have been a dairy or a farm field that used to be irrigated frequently. Soils tests are commonly
performed before the land is developed. Recently these soils tests results indicate moisture levels of 20% to 50%
in the soil anywhere from two to 10 feet down. (It is rare that soil samples are taken below 10 feet deep)
It takes a long time for moisture to rise to the top of a concrete slab from 10 feet down, maybe even years.
Sometimes the grading and drainage of the property directs moisture into the soil below the foundation.
This, in effect, can recharge the moisture source under the slab. There is also the possibility of an underground
plumbing leak or the landscape system adding moisture to the soil under the slab. However, homes are built on
moist soils all over the world, why do we have a moisture problem in the desert?
The moisture is drawn up to and through the slab by capillary action, vapor diffusion and evaporation. The quickest
way to prevent the moisture from migrating through the slab is to stop these actions from occurring.
One of the things that need to change is the aggregate base course (ABC) that is being used in the desert. Currenty
the ABC is a sand/stone mixture, however sand promotes the capillary action and therefore makes a poor choice
as ABC in an area where there is elevated moisture content in the subsoils. Using a stone as ABC would help
eliminate the capillary action.
Another way to reduce the moisture is to provide adequate grading and drainage. This should include controlling
roof drainage and diverting it at least 8 feet away from the foundation. The landscaping plumbing and control
valves should also be located several feet away from the foundation as the control valves have a tendency to leak
and the further away that moisture is from the foundation, the less impact it will have on the foundation.
The most effective way to control the moisture is with a vapor barrier. This is a thin plastic sheet placed on top of the
ABC before the concrete is poured. Previous construction practices would place the vapor barrier below the ABC
or place a thin layer of sand on top of the vapor barrier so that moisture could escape through the top and bottom
of the slab, however the moisture still migrates up through the slab. Placing the barrier below the sand or ABC creates
a reservoir for the water that drains from the wet concrete and capillary action prevents moisture from draining by
gravity through the sand.
It may interest you to know that a vapor retarder is required by the International Residential Code (IRC) in section
506.2.3, however, it may be omitted if the local building official approves the omission due to local site conditions.
So how is the issue corrected?
Correcting the Issue
The best way to get rid of the moisture in the slab is to prevent it from getting there in the first place. Check the moisture
content of the soil, use ABC that is ½ inch or larger and install a vapor barrier above the ABC and below the concrete
If the home is already built and you own it then you need the help of professionals.
Have a calcium chloride test performed on the floor slab to determine the rate of vapor transmission through
the slab. If floor coverings are present they will need to be removed before and during the testing. Testing
commonly takes 24 to 60 hours. There are many different flooring companies that can perform this test for you,
or contact the engineer you want to assist you with this project.
Check for moisture sources around the home including grading, drainage, irrigation, roof downspouts, and swimming
pools. Have the supply and the waste plumbing checked for leaks. If you still have not corrected the issue, then
consult with professionals. Expect them to core through the slab and determine if a vapor barrier is present and
investigate the conditions of the soil and ABC under the slab.
Methods of repair are limited and should be considered fully before proceeding. The slab surface can be sealed.
This will reduce or even stop the vapor transmission temporarily. A good sealer may last ten years, inexpensive
sealers may not last a year. Before sealing, all floor coverings and furnishings will need to be removed from the slab.
There are other methods of venting or draining the slab that are available, some involve forcing air under the slab or
drawing air from under the slab. Another method is to install a drainage system that gets the moisture away from the slab.
All of these repairs should be reviewed by an engineer before they are implemented, and the work should be performed
by a contractor experienced in this area.
Scott Warga, is the Qualifying party for ACSI American Construction Specialists and Investigations LLC,(ROC216772) a
dual licensed residential and small commercial contractor. He is also a qualified home inspector certified by the Arizona
Board of Technical Registration (#38062) and was appointed to the Arizona Board of Technical Registration’s
Enforcement Advisory Committee. He has 9 years construction experience and has performed residential and
commercial property inspections for over 8 years. He has specialized in forensic inspections, investigating failed, damaged
and defective construction for over 3 years. He is a member of the American Society of Home Inspectors, (#205826)
and currently sits on their board of directors. Scott is also a member of the International Code Council (#5095644). He has
been an instructor of home inspection at Mesa Community College and Arizona Sun-Tech Home Inspection School.
He is an instructor for Inspection Training Associates, a Kaplan Professional School. He has served as District Chairman
& Vice President for the Arizona chapter of the American Society of Home Inspectors and an approved instructor for
both them and the Arizona Department of Real Estate.