COLUMN BUILDING SCIENCES FIGURE 1 Water Pathways. Rainwater enters the stucco. Rainwater is absorbed by the stucco. Rainwater also gets past the stucco into the block mass wall. Rainwater is absorbed by the block mass wall. Rainwater drains down the block wall cavities. FIGURE 2 Vapor Flow Excluding Air Transport. Vapor flow outward and inward due to temperature gradients and concentration gradients. FIGURE 3 Staying Out of Trouble. Reduce rainwater absorption by painting the stucco with a paint that is " liquid water closed " but " vapor open " -hydrophobic and vapor permeable-greater than 10 perms " wet cup. " Inward vapor flow can be " throttled " -slowed down, but not stopped-by the permeance of the interior insulation. PHOTO 6 Impermeable Paint. If the paint on the stucco is too impermeable, it bubbles, blisters and peels. The paint on the stucco needs to be greater than 10 perms " wet cup " or it will bubble, blister and peel. past the stucco into the block mass wall. Rainwater is absorbed by the block mass wall. Rainwater drains down the block wall cavities. Let's add air conditioning and solar radiation. Florida is known as the " Sunshine State. " Also lots of folks think Florida is a " cool place, " especially during " spring break. " Vapor flow, excluding air transport, is due to a temperature gradient ( " warm to cold " ) and due to a 40 ASHRAE JOURNAL ashrae.o rg J U LY 2022 concentration gradient ( " more to less " ). In a rain-wetted stucco CMU block wall, the water in the exterior stucco and exterior side of the block wall is at a higher concentration than the water vapor in the outside air. When the sun is shining on the wall, the water in the exterior stucco and exterior side of the block wall is also warmer than the water vapor in the outside air. As such there is outward vapor flow (Figure 2). Not done yet. The interior is air conditioned. Air conditioners are also dehumidifiers. So the water vapor in the interior air is colder than the water in the exterior side of the block wall. The water in the exterior side of the block wall is also at a higher concentration than the water vapor in the inside air. As such there is also inward vapor flow (also Figure 2). So how do we stay out of trouble? A couple of ways. The first is to reduce the rainwater absorption by painting the stucco (Figure 3). You need to paint stucco-or you need to add " magic " stuff like polymer modification tohttp://ashrae.org