ASHRAE Journal - October 2012 - 66

building sciences
Air Space Wood-Based Subfloor Wood-Based Subfloor

Gypsum Board Cavity Insulation

Rigid Insulation

Gypsum Board Cavity Insulation

Rigid Insulation

Figure 1 (left): Hot Stuff. Best way to construct a warm floor. Continuous rigid insulation coupled with an air space above cavity insulation. Figure 2 (right): Efficient Stuff. Not as warm, but energy efficient. Filling the air space reduces heat transfer, but the floor is colder.

Bobby Darin and Thermal Performance* T
By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

his all started pretty innocuously. I just wanted a client to have a warm floor. How complicated could that be? Well, some seemingly innocu-

ous code language made things irritating, but not difficult, and I sucked it up. But then the LEED fascists made things difficult and unworkable and finally I said enough is enough. It is time for the old guy to strike back.†

When you have a bedroom over a garage in a cold climate, builders typically get complaints of cold floors. What a surprise.‡ It is easy to construct a warm floor. The best way to do it is shown in Figure 1. Continuous rigid insulation is used across the underside of the floor framing. Notice the air space above the cavity insulation. That air space makes the floor warmer than if the cavity was

completely filled as shown in Figure 2. Huh?§ Yes, the air space makes for a warmer floor. Filling the air space completely reduces heat flow and saves energy but does not make the floor warmer. Old guys taught us this. Remember back in the 60s when we had ghosting on steel framing in commercial buildings? The ghosting was due to Brownian motion. We quickly learned

that installing insulation according to Figure 3 made the interior surface of the studs warmer than if the insulation was installed as in Figure 4. Warmer interior stud surface temperature made the ghosting go away. Think 2-D heat flow and have a beer and it will make sense to you. Or play with THERM. Trust me the beer approach is better. Anyway, understanding this explains why the air space on the top of the insulation in Figure 1 makes the floor warmer. So what is the problem? We knew how to do this since I was a young guy in the Great White North. Life was good. Floors were warm, and the beer was cold. Then I fell in love. Moved south. It happens. Apparently, how to have warm floors over garages is not well understood outside of Minnesota in the Land of the Free and the Home of the Brave.

* One of my all time favorite guys is Bobby Darin, whose first hit song was “Splish Splash”…taking a bath. The bathtub connection to this column and, hence, to Bobby Darin will become obvious. Legend is that he died of a broken heart; he should have married Connie Francis, rather than Sandra Dee. He never woke up after heart surgery and died in the recovery room Christmas 1973. At the time I was on a beach in Florida, a mixed up teenager with his own problems of the heart. As a child Darin suffered from rheumatic fever that resulted in a weak heart and that is what is cited as the official reason for his death. But I believe the legend—a broken heart sometimes never heals—at least that is what I thought when I was a teenager.
†

OK, I am not striking back. I am whining. But I have it on good authority that whining is a form of therapy. Apparently, lots of folks I know are in this process of therapy. no question mark on this sentence. It is a statement, not a question. It is a rhetorical device, a literary technique.

‡ Notice

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October 2012

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