Another alternative to radiant barrier foil are the various types of foam insulation.
What is foam insulation?
Foam insulation most commonly comes in two forms: rigid and spray. Foam insulation rigid boards are 4'x8' sheets, usually cast of closed-cell polystyrene, designed to fit directly inside walls against the interior or exterior of plywood sheathing, as well as on top of sub-floors. The sheets come in different thicknesses -- 0.5" to 5" with correlating higher R-values -- and can be cut to size where needed. Spray foam is usually a two-part mix of polyurethane and resin, designed to be sprayed through a spray gun directly onto plywood sheathing to fill wall cavities, as well as onto concrete slabs and other structural substrate surfaces. After sticking to the surface, spray foam then expands and is cut flush with studs.
What are the pros and cons of foam?
Foam insulation manufacturers, both rigid and spray, tout the ease of installation of their products as a quick and efficient insulating measure. Polystyrene and polyurethane are also both naturally moisture resistant, which, like fiberglass insulation, helps to prevent the build-up of mold and mildew in walls. Additionally, both forms of foam hold higher R-values (the measurement of the capacity to resist heat flow) than wood and gypsum board. R-values of foam are usually less than fiberglass batting, but can be high enough to significantly decrease air flow, and both heat loss and transfer.
While foam insulation, both rigid and spray, is easy to cut to fit, doing so is also a messy process. (Imagine cutting Styrofoam into a million tiny little pieces.) Since both consist of plastic cores, they are also combustible and, to at least some degree, toxic. For homeowners looking to spray foam insulation themselves there is a very good reason why you must not only have the spraying equipment, but the full-body protective clothing and masking as well. Depending on what material spray foam is expanding on, it is also important to keep in mind there can be over-expansion that may deform the exterior of the material.
How does it stack up to radiant barrier foil?
While both forms of foam insulation can offer decent R-value properties, neither have anywhere near decent thermal emissivity properties. This means that -- especially in sunny climates -- having a radiant barrier foil, with thermal emissivity of less than .05 (or 95% heat reflected) against plywood sheathing, is the best way to reflect radiant heat transfer away from roofing to head off both conductive and convective heat transfer -- where R-value as opposed to thermal emissivity begins to add actual value.