Moisture Intrusion And Building Science

After three hurricanes and buckets of rain, many of us in central Florida may have personal experience with moisture intrusion through the walls of our homes and aren't lookin' to repeat.

So how does water get in? Answer, science. 1. Gravity is pretty reliable, up to down. 2. Greater pressure to lower pressure. 3. Temperature, warmer to cooler. 4. More to less, higher concentration to lower concentration. Now there is always the human factor (poor construction) but there's not much you and I can do with regards to time travel, so..

Let's look at a typical block wall. There are some interesting dynamics here. Different materials all put together that breath, shrink and move...hello cracks. Now ask this system to perform like a submarine and you get just as silly an answer...hello moisture intrusion. OK, foundation settlement can cause cracks however this source is not the most prevalent and not the focus of this article. Most cracks in block walls come from the materials and application of these materials in the wall.

A recent UCF Constructability Lab study focused on block wall performance with regards to moisture intrusion right here in central Florida! I can paraphrase the points most applicable to us...block leaks like a sieve...stucco provides the majority of the "water proofing"paint just helps. If building codes and ASTM Standards were followed, we could reduce moisture intrusion issues almost entirely.

Most of our homes have a stucco type veneer whether it's on wood frame or concrete block. Stucco is made up largely of Portland cement and is porous by nature. When stucco is applied it 'cures'. As part of the curing process, stucco gives up moisture to the atmosphere. As it dries, it cracks like our skin. There is an ASTM C926 Standard that requires multiple applications, thicknesses and curing times, but according to the UCF study, proper application is rare.

A minimum thickness of 4/8-7/8" is required for block and frame construction respectively per Florida Building Code Residential 703.11. Instead, an average of 3/8-5/8" is what is commonly found in the field. Secondly, if stucco were installed in a 2 or 3 coat process with proper curing time in between, we simply would not have the moisture intrusion issues we currently encounter. The science thing takes over from here...gravity, pressure and temperature gradients and capillary action. A porous material with cracks subjected to wind driven rain...picture a napkin dipped in water. The water races right up the napkin via capillary action.

Common sense says the more paths provided and the longer those paths are exposed to inclement weather, the more vulnerable a wall system is to moisture intrusion and subsequent adverse ramifications. Now during our normal 15 minute Florida thunderstorms, we just don't have the quantity of rain and hurricane type winds to exacerbate an already poor condition wall assembly. The UCF study however, found that cracks don't need to be wide to allow moisture penetration. Cracks 1/64 of an inch allowed moisture penetration and accumulation at the base of test walls.

Besides the stucco and block shrinkage we also have to deal with mortar joints shrinkage and other problems with the initial installation of vapor barriers and transitions between the framed second floor and concrete block first floor.

Discontinuous membranes, under-lapped membranes and the lack Z flashing allow water to run behind the stucco veneer until it runs into an electrical outlet or your baseboard. Other common moisture intrusion areas are around poorly sealed windows and inconsistent/improper window installation and other wall penetrations. The one I really love is when I find bare block around and behind electrical panels. Bare block sucks, water that is.

Next, let's look at remedies and actions we should take to help minimize moisture intrusion for stucco veneered walls. A line I have coined is "the better we caulk, seal and paint, the better the house performs" works for me. Most agree that a good 'breathable' acrylic paint be applied. This will help keep the water out while still allowing the wall to breathe (Allows water vapor transmission or has a high perm rating). Just a token piece of advice...don't buy cheap caulk or paint.