Opening the Cell: Polyurethane on Probation
In our line of business, which tries to focus on the conservation of older, mostly adobe buildings, we are frequently called upon to assess the warning signs of distress. Those signs most often come in the form of moisture damage. We have been getting a lot of calls lately about newer homes, those built in the last fifteen or twenty years, that have the same indications. More often than not, the problems are not the result of structural failure (as in the settlement of a footing) but of the unforeseen consequence of using materials of convenience.
Regular readers of this column know that at the core of my philosophy of both preservation and new construction are two principles: (1) not to seal houses hermetically, and (2) to avoid the modern stratigraphy of incompatible materials exemplified in Santa Fe by asphalt-emulsified adobes, Portland cement, elastomerics and, the subject today's specific grievance, sprayed-on polyurethane foam.
Most architects, almost all code enforcers, and essentially every builder and homeowner operates under the mistaken notion that sprayed-on insulation over adobe is required by code. It is not. The New Mexico Energy Code will allow single wythe adobe walls to remain un-insulated provided the design compensates for the low R-value of earth by paying special attention to the floor, placement of windows and, particularly, the roof. The code acknowledges that correctly oriented windows will allow for gain, and insulating the floor and roof will help offset loss (this is enlightened policy and allows for the use of adobe in its purest form, including the un-lathed application of mud plasters inside and out -- but that is not today's lesson).
Since about 1980, the state-of-the-art roof in the region has been sprayed-on polyurethane. In theory, it is acceptable because it is quick, easy and relatively inexpensive to apply. In fact, it is a scourge. When my clients allow me to be imperious, I always remove sprayed-on roofs and here is why.
Polyurethane is a marvelous insulator, no question. It does its job for the same reason that pumice floats, and that is because it is a solid with a majority of its volume being entrained air. In other words, the solid serves only to encase little pockets of atmosphere -- the ultimate insulator. The real issue is not so much with the product as with the manner in which it is installed. After shooting the stuff onto the roof and parapets, it is commonly covered with a membrane that is also either sprayed or painted on. In some instances, the system is covered with gravel.
In the twenty-five years that I have been observing sprayed-on roofs, I have yet to see one that by my standards is a success. Not a single one -- out of hundreds. The reason is because the "protective" membrane fails; birds peck it, hail perforates it and the cable guy wearing waffle-stompers leaves it compromised. Moreover, the gravel that you sometimes see as a redundant sheathing is, often as not, crushed rock -- sharp and small, perfect for punching small holes in a membrane that is measured in thousandths of an inch.
Once the protective membrane is pierced, the system begins to deteriorate. Polyurethane is extremely vulnerable to ultraviolet rays. At this latitude and altitude we have lots of those and they go to work insidiously to break down the walls of the little cells that isolate the entrained air. Once that happens, the roof becomes a sponge. I needn't get into the spectacular structural results.
There are plenty of newer materials that I like on flat roofs, including torch-down materials, fiberglass reinforced asphalted sheeting, and a number of others that can be applied over polyurethane. My preference for insulation is fiberglass batts in a cavity roof overlain with a deck and a built-up roof, flooded with asphalt and embedded with rounded gravel.