Donald Hester
Washington State Licensed Home Inspector #647
WSDA#80050
InterNachi Member #NACHI10022401
509-670-9572
Contact Us
Washington State Licensed Home Inspector #647
WSDA#80050
InterNachi Member #NACHI10022401
509-670-9572
Contact Us
DonHester's blog
Cement, Moisture and forces of Equilibrium
Submitted by DonHester on Tue, 06/25/2013 - 18:19.Cement, Moisture and forces of Equilibrium- Wenatchee and Leavenworth Home Inspection
On a home inspection my client asked me to look at a part of the foundation that he thought may be failing.
After evaluating the issue it was obvious the problem was not a direct failure of the foundation but the failure of not properly installing a proper vapor barrier for the concrete of that foundation.
Most people think concrete is water tight. Heck we make water tanks and dams out of concrete, right! But in these systems dampness is acceptable.
Concrete can do a good job of containing liquid water (at least when there are no cracks). But water vapor moves readily through concrete. The rate of moisture transfer depends on the concrete's porosity and permeability.
Think about the concrete slab in your home as a hole in the moist ground. The soil below the concrete will almost always be damp. It nearly always has a relative humidity of 100%. That means it is a continuous source of moisture into the slab. This is why in this area all concrete below grade needs to be at a minimal damp proofed.
All natural systems will always tend to migrate towards a state of equilibrium. This is much like a hot cup of coffee transfers heat to the air in the room until they are both at the same temperature (the second law of thermodynamics).
In chemistry higher concentrations of a chemical will move towards areas of lower concentration. The same is true for areas of higher or lower relative humidity (Relative Humidity is a measure of the vapor pressure of water vapor in air and is directly effected by temprature). The movement of this moisture vapor is called diffusion.
These principles mean that if the moisture in and below the concrete slab are higher than the relative humidity of the air above the slab the moisture is going to try to move into and out of the slab. Without a vapor barrier, the relative humidity in the slab or just below is most likely will be 100%. Since the air is seldom that humid (at least here), moisture is going to move from the slab into the air and as the surface dries a bit it will draw more moisture up from the bottom (capillary action).
Here we see metals in direct contact with the concrete that has a high rate of moisture diffusion. It is causing the metal to rust and expand resulting in cracking of the concrete. This is the problem my client was seeing.
The attention to the detail of the vapor barrier prior to the concrete being poured is critical. Understanding the principles behind moisture diffusion helps with understanding why this be happening in the home.
Cement, Moisture and forces of Equilibrium- Wenatchee and Leavenworth Home Inspection
“You can't trust water, Even a straight stick turns crooked in it.”
W. C. Fields
NCW Home Inspections, LLC is a Licensed Washington State Home Inspection service located in Wenatchee Washington serving Chelan County, Douglas County, Kittitas County, Okanogan County and Grant County Washington and the cities of Wenatchee, Leavenworth, Cashmere, Oroville, Cle Elum, East Wenatchee, Quincy and many more…
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