ASHRAE Journal - November 2010 - 36

building sciences

Figure 1: Perfect Wall Configuration for SPF. Spray-applied closed-cell high-density foam is the water control layer, the air control layer, the vapor control layer and the thermal control layer. Photo 1: The Perfect Wall?

exterior spray Foam
By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

pray polyurethane foam (SPF), the high density stuff,* is the only

product (so far) that can perform all of the functions of the principal

control layers of the “Perfect Wall.”† The functions are water control, air control, vapor control, and thermal control.

Figure 1 and Photo 1 illustrate the configuration. Pretty simple and straightforward, eh? Wrong.‡ Simple and straightforward only if you have no windows, no doors, no penetrations, only use brick cladding and only use brick cladding on one-story buildings, and if it is not too cold when you apply it, and if it is not too

windy when you spray it.‡ In any event, I still like the stuff despite some of this stuff because the stuff can do stuff no other stuff can do. As with most materials and systems, you must work your way through the weaknesses and maximize the strengths. Let’s start with the easy stuff first and

work our way through to the more difficult stuff. Windows first. Absolutely, positively do not, and I repeat, do not spray the SPF to the windows. Are you crazy? How are you ever going to get them out? You will have to hack them out. Ask me how I know. Think windows don’t break? Think they don’t leak? Think we can reduce the deficit without reducing spending and controlling entitlements? Dream on. The best play is to provide a transition assembly between the glazing systems and the wall assembly. Use the concept of an extended “buck” (Figures 2a and 2b for masonry backup walls and Figures 3a and 3b for steel stud backup walls). Notice the plywood “lip” and the use of a liquid-applied flashing

* We are talking the 2 lb/ft3 (32 kg/m3) stuff, not the low-density 0.5 lb/ft3 (8 kg/m3) stuff or the “new” medium density 1 lb/ft3 (16 kg/m3) stuff. In reality, the 2 lb/ft3 (32 kg/m3) high-density stuff is not really high-density stuff at all. There is much higher density SPF used in industrial applications, but we folks in the construction industry do not care about industrial applications, so the 2 lb/ft3 (32 kg/m3) stuff is high density to us. It is not high-density stuff to chemical engineers and to the chemical companies who make the chemicals that comprise the stuff and provide the formulations to blow the stuff, so confusion reigns. And, that is good because obviously the world needs more confusion. It gets even better. We also call the 2 lb/ft3 (32 kg/m3) stuff “closed-cell” spray-applied foam and the 0.5 lb/ft3 (8 kg/m3) stuff “open-cell.” We don’t quite have a “cell” name for the medium-density stuff yet, and if I had my way we would never get one. In any event, the “cell” stuff relates to vapor control. The 2 lb/ft3 (32 kg/m3) stuff is around 3 perms per inch of thickness, and the 0.5 lb/ft3 (8 kg/m3) stuff is around 50 perms per inch of thickness. The difference is a big deal. The 1 lb/ft3 (16 kg/m3) stuff is around 15 perms per inch of thickness. There is more. The 2 lb/ft3 (32 kg/m3) stuff has a thermal resistance (R-value) of 6.5 per inch, the 0.5 lb/ft3 (8 kg/m3) stuff is around 3.5 per inch, and the 1 lb/ft3 (16 kg/m3) stuff is around 4 per inch. † Building Sciences from the May 2007 ASHRAE Journal. ‡ Do you have any idea how difficult it is to get this “stuff” off a car parked downwind? The word “stuff” is not the word I wanted to use in this footnote rant. For the first job I specified SPF on in 1979, I made the mistake of driving my nice new shiny red Camero to the site. Lucky for me the new wax job on my car made it only irritating to me. But not for the framer’s car. Guess who had to clean his car?

ASHRAE Journal

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November 2010

ASHRAE Journal - November 2010

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