©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 8, August 2013.
By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE
About the Author
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Somerville, Mass.
“I’ve looked at life from both sides now” ~Joni Mitchell
Walls can get wet from the inside, walls can get wet from the outside. Walls can start out wet. Wet happens. Because wet happens, walls need to be designed to dry. We can and should do our best to keep them from getting wet. But no matter what we do we have to accept the fact they are going to get wet. All of them are going to be wet at some point.
For most walls that point is at the very beginning. For many wetting happens all the time. In any place that there is a winter almost every wall gets wet. Not usually a lot and most of the time not wet enough for long enough to matter. Every wall that sees winter and every wall that sees rain and every wall that is in a humid place that is air conditioned gets wet. And if you do stupid stuff like irrigate your plants and garden next to your house and irrigate the wall at the same time, even walls that don’t see winter, rain or are not in a humid place that is air conditioned get wet.
Some of you might not be aware of this, but we build outside. Who knew? Before there is an inside we start out with a piece of the outside and make that piece into the inside. Outside can be pretty wet. Not all the time and not in all places, but we don’t have the luxury to build in places that are dry all the time. Besides, building in dry places does not save us much grief anyway. We build out of wet stuff. Concrete comes in a big truck and we pour it. What do we put in the joints of gypsum board? Mud. Stucco is a wet slurry. Bricks are stuck together with wet gooey stuff. We build out of wood. Wood grows on trees. There is almost more water in the wood than there is wood in the wood at first. Then, it is not unusual to fill the walls with wet ground up newspapers and cardboard that we season with salts.
Most people don’t understand that we build much of our stuff out of dead wet plants and wet rocks and then we regularly wet the stuff. With me so far? The only things missing are bows and flows of angel hair and feather canyons everywhere. So here is the deal. Walls and the other stuff we build have to be able to dry, and there are only two possible directions drying can happen—to the outside or to the inside.†
So which direction do we pick? Now it gets interesting. Walls can be designed to dry to the outside or to the inside or to both sides. Lots of choices, lots of possibilities. What you do depends on where you are and what you are building with.
What makes this even more interesting is that as important as drying is we also have to try to make sure the walls don’t get too wet and stay too wet for too long. You can never prevent all wetting but you sure can limit wetting or control wetting so that the assemblies can handle the wetting that occurs. The key with the prevention of wetting thing is to make sure that the prevention of wetting strategies don’t mess up the drying strategies. And here is another kicker—the more insulated the wall assemblies the more difficult this becomes.
Insulation reduces heat flow and reduced heat flow reduces drying. Highly insulated wall assemblies have much lower drying potentials than poorly insulated wall assemblies. It gets worse. Newer materials don’t handle the water as well as older materials. We have been here before —check out “Material View of Mold,” August 2007 and “5 Fundamental Changes in the Last 50 Years,” July 2009.
Let’s look at the most important thing first. More genius level stuff here, starting with the most important thing. Rain. The wall has to handle rain and has to handle it well. More wetting to walls happens with rain than any other mechanism. The good news is handling rain is easy — flash everything and control hydrostatic pressure. We have been here already as well (“Mind the Gap, Eh?”, January 2010 and “Hockey Pucks & Hydrostatic Pressure,” January 2012). What this really means is installing a “water control layer,” integrating gravity rain shedding (“drain the rain on the plane…”) and providing an airspace between the cladding and water control layer. Let’s look at how this is done by looking at yesterday’s “old classic” wall (Figure 1) and comparing it to today’s “new classic” wall (Figure 2).
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