Low-Density SPF

Low-density SPF is first defined by its installed weight which is commonly 0.5 – 1.0 pound per cubic foot or rather light. The make-up of the foam is in the form of many air cells that are open to each other in their configuration—simply referred to as “open cell” insulation. This creates a resultant installed material that is semi rigid, very soft, and easily cut or manipulated. It is also recognized as a green building product because the blowing agent used during installation is typically water with zero ozone depletion potential. These products also do not contain any CFCs, HCFCs, urea-formaldehyde, asbestos, or any cellulosic material.

The make-up of low-density SPF insulation gives it several appealing characteristics. First, the cost of open cell spray foam insulation is generally very attractive and competitive when compared to labor and materials for other types of insulations. From a thermal standpoint it achieves an R-value of approximately 3.2 – 4.5 per inch of depth. This makes it comparable to or better than most other lightweight insulation materials commonly used in cavity wall construction. Its comparatively softer make-up means that it effectively air seals around the edges and perimeter of stud cavities and any penetrations, thus making it an effective air barrier in a wall assembly. It also means that it can flex and adjust to continue to provide an ongoing effective air seal even as the building may settle, expand, or contract.

Excellent sound attenuation properties have also been achieved with low-density open cell foam because it has a tendency to absorb sound waves, thus reducing typical airborne noise. Specific tested assemblies by different manufacturers offer STC ratings from 49 to 52. So, for example, a wall assembly with a 2x6 common base plate with staggered 2x4 studs and 3½ inches of open cell foam, has been shown to obtain an STC 50 rating. Similarly, a floor assembly with 2x12 joists, 3½ inches of open cell foam with resilient channels also achieves an STC 50 rating.

One thing to be aware of with low-density, open cell insulation is that, while it can serve as an air barrier, it does allow water vapor to permeate through it. This can be an advantage in that any moisture that finds its way into an assembly can migrate out. Nonetheless, in cold climates a warm side vapor retarder such as vapor retardant paint over gypsum board will be needed to control vapor diffusion in an exterior assembly. Some approved vapor retarders can be painted directly onto the insulation.

Medium-Density SPF

As would be easy to guess, medium-density foams are a bit heavier than low-density foams, typically ranging from 1.7 to 2.5 pounds per cubic foot with 2.0 pounds being the norm. They also differ from low-density foam in that medium-density SPF is produced in a “closed cell” configuration such that each air cell is isolated from the others around it. This configuration and weight makes medium-density SPF much more rigid and stronger than low-density SPF. Due to their rigid nature, these products are very popular in metal framed buildings or installations where they might be exposed during construction or even occupancy. Medium-density SPF is also recognized as environmentally friendly in that they use non-ozone depleting blowing agents (not water in this case) and can also be manufactured with agricultural oil and recycled content in some cases.

The two types of SPF insulation have different physical properties. Low-Density Open Cell SPF is suitable for interior and above-grade applications only. Medium-Density Closed Cell SPF is suitable for all interior and exterior applications. Image courtesy of Demilec (USA) LLC

Medium-density SPF has some notably different characteristics from low-density foam. Thermally it is actually superior with R-values available up to R-7.4, meaning that higher building energy performance can be achieved in thinner assemblies. This trait can help make up for the fact that, not surprisingly, denser foam costs more per cubic foot than lower-density foam. Also helping the cost performance, medium-density foam does not require a separate air barrier since it is also an effective air sealing foam. Perhaps most notably different, medium-density SPF has tested as a class II vapor retarder meaning it has very low permeance, much more so than open cell low-density spray foam. That means that the cost of installing a separate vapor barrier can be eliminated.

In considering which type of spray polyurethane foam insulation to select, there are some notable limitations on its use that need to be recognized. First, low-density foam is not a flotation foam, it should not be used as a vapor barrier and it is not designed for contact with bulk water. If any of these concepts are design parameters, then medium-density closed cell foam is the product of choice. Second, low-density foam has no inherent structural capability while medium-density SPF can actually add some rigidity and structural strength to a framed assembly.

When installing SPF, regardless of type, it should not be used on wet, dirty, or oily substrates in order to avoid potential adhesion issues. In ceiling applications, it needs to be carefully installed around recessed can lights, even those designated “IC” for insulated installations. The reality is that the excellent thermal properties of the foam compared to other insulation may not allow enough heat from the light to escape and can lead to overheating and flickering of the lamps. This is typically only an issue in vaulted ceiling applications, where fiberglass can be used in contact with the IC cans to provide the typical 3-inch gap between the can and the foam.

A few other general limitations include avoiding any spray foam from finding its way inside of electrical boxes—the boxes will have to be properly prepped or cleaned to remove all necessary foam. Finally since the maximum continuous operating temperature for most spray polyurethane foams is approximately 180 degrees Fahrenheit, this typically means no direct contact with fireplace flues, but domestic hot water pipes are usually acceptable since they are typically about 120 degrees F.