Drying potential. If I have a soapbox to stand on when it comes to building science, this is it.
Years ago, it was common for builders to say that houses need to breathe. Their concern was to allow the wood components of the house to dry. This worked well for drying potential but not for energy efficiency. Today builders are building houses tighter to address new energy codes and to make a home more energy efficient. This is good for energy efficiency but not good for providing drying potential of the wood components in the house structure. The houses we build today may not need to breathe, requiring a respirator of sorts to do this, but the wood in the structure still does. Seem impossible? It’s not. Actually, it is quite easy if you think about it and not that expensive. So why don’t builders do it. My guess is that it’s a habit that hard to break or they just don’t know any better.
Borrowing a phrase from Joe Lstiburek, a well-known and respected wood scientist: Wood is the best building material we have and it is the worst. Why make such an oxymoron statement. Because wood is easily fashioned and attached to structures making building in the field easier; however, if it gets wet and cannot dry, it molds, mildews, rots and allows for bugs to build their homes in the soft wood.
So when we built homes to breathe, there was no insulation in the cavity, sheathing boards had air spaces between them and the moisture-laden air was free to move in and out. Today, we use sheets of sheathing that are placed tightly together. In some cases, the seams of the sheathing may be taped. By code, we at least have to cover the sheathing with house wrap. To make house wrap an air barrier, the top and bottom of the wall must be sealed and all seams taped (bad idea by the way). Then we fill the wall cavity with insulation.
Some insulations are blown in expandable foams that adhere to the inside face of the sheathing and around the studs (again, a bad idea). The problem is, the moisture-laden air still wants to move in and out through the wall system, but now it is slowed down and sometimes trapped in the wall system. What building science likes to see is a sheathing with enough permeability to allow the water vapor to pass through. With that said, there are sheathings being used that range in permeability from semi-impermeable to semi-permeable. So you have to ask yourself, does this mean that if my wall sheathing is semi-impermeable, will it fail? To me, it is not so much the permeability, but does the sheathing have any drying potential?
More and more I hear people talking about the permeability of sheathings, house wraps and other components of a wall system. While vapor permeability is important, it is my belief that more than vapor permeability, drying potential is more important. As a builder, you cannot trust what manufacturers tell you about permeability. There are several ways to spin it and they do. Therefore, the best thing that a builder can do is to make sure that all wood components in the wall system have good drying potential. But that is not what they do.
Most builders refuse to add a drainage plane between the cladding and sheathing jamming cladding, house-wrap and the like right up against the sheathing then go inside a fill the wall cavity full of insulation…..leaving absolutely NO drying potential for the sheathing or the studs for that matter.
It seems to me, that this is not a hard or a costly problem to fix. barriers, moisture barriers, insulation be it blown pushed in or sprayed in all are helpful to the protection of the home and those living in it, but they are less important than creating space to give the building materials drying potential. Homes today are built tight, wrapped in plastic and sealed up tight. Up North, they put plastic on everything to prevent moisture from entering the home. Where have you seen a home that stays completely dry inside? Of course here in the South, we have humidity in the air. Open a window or door and it will come in. Hopefully, we have air conditioning to help wick the moisture from the air but if the building materials are not allowed to breath and air to circulate, they will mold and rot. House wraps and products put straight up against the actual OSB traps moisture and if not allowed to dry, the moisture has nowhere to go.
Sick homes are popping up across the country and much of the problem is wet wood. If the wood gets wet and not allowed to dry, all kinds of molds will grow. So if you think it too expensive to add in good drying potential or not worth the effort, go ask someone who lives in a sick house and their children are having medical issues due to a tight but moldy house.
This is one of those problems that has a simple fix. If builders would simply apply firing strips between the walls and the siding, much of the building envelope will survive and could be mold free. Some builders do this on roofs and this is required of some sidings(?), but it needs to become a standard practice. As much as we want to stop water from getting into a house, it most likely getting in somehow. What we have to do is allow it to dry.
If given drying potential, the wood materials in your home will last and last.
