ASHRAE Journal - May 2012 - 66

building sciences

Joe south Assemblies
S
by Joseph W. lstiburek, Ph.d., P.eng., Fellow ASHRAE

ometimes, to do the right thing, you have to do “a workaround.” I love the “I-Codes.” But they can drive you crazy. I love them be-

cause most of the time they are right. In fact, almost always they are right. But, every now and then…. When they are wrong, a workaround is called for. I call it “playing the game.”
It is a dirty little secret that the codes display a cold climate bias. It is because the south lost the Civil War. Now, it happens that lots of buildings get built in the south. Just for yuks, let’s pretend we are going to build a building in Houston. It is pretty obvious that we are dealing with a hot and humid climate. No one has to go to ASHRAE Fundamentals and look at the design conditions. Or even worse go to NOAA.* Let’s say that we want to construct a wood framed building—say an apartment building with a flat roof that is vented. Very common. Not my first choice for a roof assembly. I am an “unvented compact roof built up membrane over rigid insulation over a roof deck” kind of guy. But, the folks down there sure like their vented flat roofs, and they work pretty well; so that is what I typically have to deal with. What does the code have to say about this type of roof? Check it out: 2012 International Building Code 1203.2 Attic spaces. …shall have cross ventilation….The net free ventilating
Oh the games people play now ev’ry night and ev’ry day now Never meaning what they say, yeah never saying what they mean.
© 1968 Written, composed and performed by Joe South.

area shall not be less than 1/150th of the area of the space ventilated. Exceptions: 2. The net free cross-ventilation area shall be permitted to be reduced to 1/300 where a Class I or II vapor barrier is installed on the warm-in-winter side of the ceiling. Yes, you read that right. If you install a vapor barrier you can reduce the vent area. In Houston? A vapor barrier? In a vented attic? It is dumb enough to install a vapor barrier on the inside of a wall in Houston, but in a roof? Now this is clearly wrong. The language was intended to apply only to cold climates. But amazing as it seems, the language has been in the

code this way from the very beginning. So fix the code, right? Easy to say, tough to do. Not going to happen soon. Well, why not just live with the higher vent area? It gets expensive, that’s why. Do you have any idea how many holes you now need? And big ones at that. Frustrating, especially since you don’t need them from the physics perspective. So we play some games.** What is the intent of the ventilation? To control moisture. It is not to reduce heat gain. The dominant heat transfer mechanism in the roof assembly is radiation and venting the assembly has a negligible effect on the radiation load. Use a light-colored membrane with lots and lots of insulation to handle the radiation piece. Increasing vent area to reduce heat gain is not an effective strategy. However, reducing vent area to save costs is a good strategy as long as you don’t reduce it to the point where you now have a moisture problem. What is that lower limit? Ah, a debate for another time. For the sake of brevity and laziness, I am going to stick with the “consensus” value of 1:300. Plus the code allows it. Ok, we want a roof design with a vent ratio of 1:300. What does the code want? I think Figure 1 pretty much says what the code wants, but with the code you are never quite always sure. Let’s stick with Figure 1. Note where the “vapor barrier” needs to be. This should make you a little nervous with hot, humid roof assemblies.

* Originally, a “conglomeration” of the Weather Bureau (formed in 1807) and the United States Coast and Geodetic Survey (formed in 1807) and the Bureau of Commercial Fisheries (formed in 1871). Useful agencies doing useful stuff. Ah, the good old days. Richard Nixon unified the three agencies under the NOAA banner in 1970. He is responsible for the EPA as well. When they were first formed, both the EPA and NOAA were welcomed with open arms and were sorely needed, and did good, timely work. They did clean up the environment; and I, for one, am grateful. ** I am also playing some games in this column—mostly to make a point and to get across a principle dealing with material characteristics. There is another “exception” in this code section that allows the 1:300 ratio to be used without the installation of a “vapor barrier” if the vents are more or less split equally between “high” and “low” vents. I hate this “exception” for vented flat roofs because it means putting holes in your roof since that’s the only location that meets the “upper” vent location.

ASHRAE Journal

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May 2012

ASHRAE Journal - May 2012

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