Robust Walls Brunswick GA

Robust Walls

At the right thickness and with the right details, rigid foam on walls provides a strong defense against moisture

by Ted Cushman

Because of its versatility and convenience, rigid foam sheathing has become more and more common throughout the U.S. Contributing anywhere from R-3 to R-7.5 per inch, sheet foam is a handy way to boost the wall's total R-value without adding too much thickness. But foam sheathing does more than simply improve the thermal performance of a building: Located just inboard of the wall cladding, sheet foam can also function well as an augmented drainage plane to help keep wind-blown water out of walls, and it can act as both an air barrier and as a vapor barrier to defend against the intrusion of air and water vapor. These properties make it a good choice in most coastal climates — but only if you get the details right. A foam-sheathed wall system has to be designed and detailed with all functions in mind, taking into account the site's climate and weather exposure.

Insulating Value
The various foam products on the market have different R-values:

Expanded polystyrene (EPS), an open-cell "thermoplastic" foam that melts at high temperatures, is made by expanding polystyrene beads with steam inside a mold. Its R-value varies from about R-3.2 to R-4.4 per inch, depending on the density of the plastic and the size of the cracks between the expanded beads (a typical value would be R-3.9).

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