Photos: Cavity Spray Foam and Exterior Rigid Foam Yield R-34 in Thin-Stone Veneer Home
Originally published by: Journal of Light Construction — May 12, 2016
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This is a story about my wanting to build something that I liked but that others said couldn’t be done. The stone supplier and the stone mason said it couldn’t be done, and I couldn’t find any published construction details for doing it. But if nothing else, I’m persistent and have come to learn that there’s always a way to reach your goals.
The project, located in the Hudson Valley in New York, called for a low-maintenance, energy-efficient house. We were aiming for a durable, maintenance-free exterior using Nichiha fiber-cement lap siding on the second-story and real stone on the first floor and walk-out side of the basement. Everyone involved wanted real stone, not “cultured” or “cast” stone. Touching and seeing the real thing provides a visual, sensory, and emotional experience that comes only from authentic materials.
To make the house as energy efficient as possible, we used closed-cell spray foam to fully fill the 3 1/2-inch stud cavities, and 1 1/2 -inch-thick foil-faced polyisocyanurate foam on the exterior to create a thermal break. This gave us a total R-value of around 34 in a wall only 6 inches thick, allowing us to use windows and doors with standard-depth jambs.
In my search for information, I made contact with an engineering firm that wrote a white paper for the Foam Sheathing Coalition on installing claddings over rigid foam. (This report eventually became the basis for the New York State Building Commission’s approval of generic fastener requirements for installing claddings over exterior foam.) The process outlined in that article started with determining the weight of the cladding. The real stone we specified weighs 13.4 pounds per square foot. Adding the weight of all the wall components—including the foam, rainscreen, expanded metal lath, and mortar scratch coat—gave us a total weight of 21 pounds per square foot. Based on that, we were able to use the 25-psf category in published design specs—namely, the guidelines in the Evaluation Service Report for FastenMaster HeadLok screws, the fasteners we eventually chose for securing the cladding assembly to the framing. The process for building this assembly is described in the photos and captions.