When we think about the movement of water, we usually think of it flowing downhill. Rivers flow from high to low. Raindrops roll down the window, never up.
But there are three other ways that water can move, and it's important to be aware of them when evaluating risks for moisture intrusion in a home.
In addition to gravity, water moves as a result of surface tension, also known as capillary action.
It's the same force that trees use to pull water up from the earth. Lumber still possesses this capacity to draw water up from the ground. This is why inspectors frown upon wood/soil or wood/slab contact. The wood will have a tendency to continue drawing water, leading to premature rot and structural failure.
Furthermore, tiny cracks and fissures in masonry can draw water into the building through the same principle of capillary action.
Water also moves in the form of vapor. So it is possible for water to move "up" in your house. If bathroom vent fans terminate in the attic space, problems with mold and moisture can result. This is also true for unsealed recessed lighting fixtures that open into a roof cavity. Not only will the unsealed fixture allow significant energy loss through the attic, it can also cause problems with moist, warm air condensing on the underside of the colder roof structure.
Finally, water moves across temperature gradients. This is a key concept when discussing split-faced block. This porous, cementitious material is able to absorb great quantities of water if it hasn't been sealed. When the sun hits a saturated wall, it can force the water IN - across the temperature gradient from hot to cold.
Because of water's tendency to move across temperature gradients, the proper drainage planes and air gaps must be built into structures using split-faced block. Otherwise, rot, mold, and significant structural damage may result.
So the next time you're thinking about water and houses, remember:
Water doesn't just flow downhill. It can move in any direction through a variety of materials via capillary action.
It can rise up and pass through openings and permeable surfaces through evaporation and condensation.
And it can move in any direction through a porous material like split-faced block, following the temperature gradient from hot to cold.
Tom Jansson - Acuity Home Inspection Service
State Licensed - InterNACHI Certified