R-value is a term used in the building industry to evaluate the insulating properties of construction materials. Most individuals are, at a minimum, familiar with the logic that supports an R-value, since manufacturers of some building products have allocated significant money to promote the notion that the incorporation of high R-value materials into an exterior building envelope will significantly reduce energy use.
Gypsum board has a relatively low R-value–usually listed as 0.5, but varying slightly depending on the thickness of the material–so it does not function well as a stand-alone thermal insulation material. It is not unique in this regard since many earthen materials–natural rocks, dirt, concrete, and the like–display similar low R-values. Insulation materials tend to have higher R-values as do products specifically manufactured to provide high R-values, such as vacuum insulation panels, specialized caulking and sealant products.
Using supplemental insulationGypsum board, similar to other low R-value products, has to be combined with other materials–primarily insulation–to provide an acceptable overall R-value for a specific building system. An exterior wall system, for example, sheathed with a gypsum panel product, can provide a fully acceptable R-value when the wall is properly insulated and sealed. Brick, masonry, and stone are examples of other basic materials that also usually require supplemental insulation to achieve an acceptable system R-value.
But, while an insulating material may display an excellent R-value, it might be manufactured using chemicals or products that could ignite when exposed to flame or an excessive heat source. In that instance, model building codes typically require the ignitable material to be covered by a thermal barrier–a material or system that will keep heat or flame from igniting the material of concern. In the event of a fire, the barrier prohibits or delays the ignition of the material it covers.
Foam plastic insulation materials often require the application of a thermal barrier to achieve a code-compliant application. In a symbiotic manner, gypsum board returns the favor that it gets from those materials and their high R-values by performing as a thermal barrier for them.
Building codes have long recognized the ability of gypsum board to serve as a thermal barrier for flammable or ignitable materials. Section 2603 of the International Building Code (IBC), for example, specifically requires foam-plastic insulation to be “separated from the interior of a building by an approved thermal barrier of 0.5-inch (12.7 mm) gypsum wallboard or equivalent.” Similar language occurs in Section R314 of the International Residential Code. Numerous material evaluation reports also contain identical or similar language, including ESR-1338, which is evaluated by ICC-ES and sponsored by the Gypsum Association. Other sections of the IBC permit the use of thinner gypsum board and other panel materials in specific instances.
Covering foam insulationIn a broad sense, model building codes basically prohibit the installation of exposed foam plastic in occupied areas of most buildings. As a result, code language has evolved to require that exposed foam plastic must be covered with a material that will preclude it from readily igniting. Gypsum board is such a material.
Therefore, you can have a material that doesn’t function well as a stand-alone building insulation material but that works well as a barrier against ignition. While that might seem at odds with itself, and is a possible source of some confusion, it all makes sense when you think of how gypsum board works when exposed to heat.
The noncombustible core of gypsum board contains chemically combined water which, when exposed to high heat, is slowly released as steam, effectively retarding heat transfer. Even after complete calcination, when all the water has been released, it continues to act as a heat-insulating barrier. Essentially, the board sacrifices itself to prevent the passage of heat and flame and does so in a sequential manner working back from the heat source. It melts as does a block of ice when the ice is exposed to a blowtorch placed immediately adjacent to one of its surfaces. While the flame of the blowtorch may gradually create an indentation into the ice, the surface opposite the torch remains cold and is not impacted by the heat.
Note that the quoted code language does not mandate the use of a type X gypsum board; it simply requires the board to be 0.5-inch (1/2-inch) thick. A thicker gypsum panel or a gypsum panel material other than wallboard will also suffice. That is important because, in many instances, the surface that is being covered is a surface that would be finished with a gypsum panel even if no thermal barrier were required. As a consequence, if the intended gypsum panel is veneer base or water-resistant board, for example, it can also function as a thermal barrier provided that it is at least 1/2-inch thick.
Check codes and evaluation reportsWhile gypsum board is by far the most commonly-used wall or ceiling thermal barrier material, other building materials also need thermal barriers. Some doors that are manufactured containing foam plastic, for example, are required by code to be faced with metal or wood of a specific minimum thickness. Again, this is done to prevent the foam in the doors from igniting when exposed to excess heat.
Model code language should be consulted for specific information on the need for, and the application of, a thermal barrier. In addition, since thermal barriers are most often required for materials that are manufactured from or contain foam insulation, evaluation reports for specific material applications should also be obtained and reviewed prior to the installation of the foam material. In any instance, the specific application criteria of the evaluation report would generally overrule the language contained in the model code.