Spray foam insulation is a spray applied foam, most commonly consisting of polyurethane. Its advantage to other types of insulation, such as rigid board or a fibrous insulation, is that handles both critical components of insulation: R-value and air leakage. R-value measures an item’s ability to limit heat conducting through the material. Air leakage is the passage of air through cracks and crevices. Air leakage can account for most of a building’s energy loss, so an airtight seal is critical to energy efficient building performance. The National Research Council of Canada (NRCC) concluded that without a proper air barrier, insulation alone cannot deliver optimal energy efficiency and comfort.1
There are many different types of spray foam, but all have the same application process. The nature of the application – a fluid material, sprayed onto surfaces that then hardens in place – lends itself to completely filling gaps and voids, thus achieving an air seal. There are two components, usually called “A” and “B,” that are heated and sent through a mixing gun. The blowing agent is different depending on what density insulation is being used. Foam can be applied to vertical and horizontal surfaces.
The cost of spray foam is higher than it’s competitors, but theoretically pays for itself with energy savings. For example, payback can occur in 3-4 years by reducing heating and cooling bills by up to 50%.2 It is a long lasting material, so will perform for the life of the building. It will make a building more valuable as energy efficient buildings are more desirable. An energy efficient building also requires less HVAC equipment, so there is also potential savings in being able to purchase less equipment; it also means less space needed for equipment.
General advantages of spray foam are:
- Thermal comfort by providing a consistent and draft-free indoor environment
- No harmful long-term emissions
- Provides insulation and air-sealing in one step, saving time and materials
- Superior moisture management by being able to dependably choose whether to allow or prevent moisture penetration, depending on the design of the assembly
- Fills hard-to-reach and oddly shaped cavities including plumbing and electrical penetrations
- Not a source of food for mold
- Improved air quality due to the prevention of allergen and pollutant infiltration
- Does not settle or sag over time
- Will not be damaged by water
- Deadens sound transfer
- Can meet LEED’s “Environmentally Preferable Product” credit
General disadvantages are:
- Petroleum-base materials
- Higher initial cost than other types of insulation – fiberglass batts, rigid board, mineral wool, cellulose, etc.
- Should be applied by a trained, experienced installer. If the chemicals are mixed with the wrong ratio, or not heated to the correct temperature, the cured foam can shrink away from the surfaces onto which it has been sprayed.
- Installation is messy. Best to only have crewmembers present during installation including someone to keep an eye on flyaway droplets.
When framing cavities, the IBC requires a thermal, i.e. protective, barrier such as ½ gypsum board.3 In vented attic spaces, the IBC requires spray foam to be covered with an ignition barrier which could be one of many different materials.4
Compared to other types of insulation, spray foam has several advantages. Long-term R-values are as good or better than batt-type insulations. Proper installation fills voids completely where batt installations allow varying densities due to shoving insulation to fit or gaps in oddly shaped cavities, and tends to sag over time. Unlike fiberglass, foam inhibits air flow by filling cracks, thus eliminating airborne moisture transfer which lowers R-value by drawing moist air into assemblies. Depending on what type of spray foam is used, moisture is either allowed to pass through or is blocked out entirely; moisture is not trapped which can lead to the degradation of other building materials such as rotting of wood or corrosion of metal. According to the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), moisture damage contributes to 90% of all building and building material failures.5
Spray foam insulation is further broken down into two categories – open-cell and closed-cell. Air cannot pass through either. The most significant performance factor between open- and closed-cell foams is open-cell is permeable to moisture. For example, at 3 inches thick, open-cell foam has a permeance of 16 perms making it relatively vapor permeable.6 This is appropriate for many conditions, such as a renovation of an existing building where the wall construction cannot be made watertight, allowing for water vapor that enters the assembly to escape. A 2-½ inches thick, closed-cell foam has a permeance of 0.9 perms making it effectively a vapor retarder.7 This could be appropriate for new residential construction where the wall construction can be designed with moisture impermeability in mind.
Open-cell spray foam is low-density at ½ pound per cubic foot, a.k.a. “1/2 pound foam,” with an R-value of about 3.5-3.7 per inch. Closed-cell foam has a density of 2 pounds per cubic foot, a.k.a. “2 pound foam,” costs roughly twice as much, and has an R-value of 6.5 per inch.8 Some high-density foam is available at approximately 3 pounds per cubic foot.9 If a certain R-value is desired, the thickness of the wall assembly could inform which type of foam to choose – if there isn’t enough depth to achieve an R-vale with open-cell, closed-cell might be the answer. Approximately 1/3 of the material is used for open-cell as the equivalent final thickness of closed-cell foams.
For comparison, less expensive insulations like cellulose and fiberglass have R-vales of 3.5-3.7 per inch.10
Because the open-cell foam lacks a moisture barrier, in cold climates (climate zones 5 and higher for example) a separate interior, a.k.a. “warm side,” vapor retarder is needed. High-humidity areas, such as indoor pools, in climate zones 1-4 need one as well. For example, vapor-retardant paint is acceptable, but only when applied on a separate surface such as a layer of gypsum board; vapor retardant paint is not effective when applied directly to cured spray foam.11
Open-cell foams have the advantage of an environmentally benign application process using water or carbon dioxide as the blowing agent. Closed-cell foams use chemicals, some with hydrofluorocarbons (HFCs) which have high Global Warming Potential (GWP) factors. Installation is also quicker with open-cell because it can be applied at any depth, whereas closed-cell is often limited to 2 inch passes to allow for curing; this also reduces labor costs. Another benefit is because of its softer make-up, it seals around edges and perimeters in a flexible manner; if a building settles or framing expands and contracts with the season, the seal between the foam and surface is maintained, unlike the harder closed-cell foam which is more likely to separate and produce hairline gaps. Good for interior walls.
Closed-cell foams have the advantage of structural strength due to its density and firmness. They also have exceptional resistance to water so are a good option in hurricane- or flood-prone areas. FEMA defines a flood resistant material as “if they can withstand direct contact with flood waters for at least 72 hours without being significantly damaged. ‘Significant damage’ means any damage that requires more than low cost, cosmetic repair (such as painting).”12 FEMA approved locations are in floors, walls, and ceilings.13
Both open- and closed- cell spray foam help with acoustical control, though given the lighter, softer composition of open-cell, it is a little better at absorbing sound.
Beware of green wash! (The marketing ploy of a company trying to make their product appear environmentally friendly when it is not) Many products will promote themselves as “bio-based” or “soy.” According to the U.S Department of Agriculture’s BioPreferred Program, a minimum of only 7% of the product needs to be biobased content.14 While that is better than nothing, the other 93% can be nasty stuff.
Each project will have it’s own set of needs and has to be evaluated individually; it is impossible to establish definitive rules for appropriate uses.
- International Building Code, 2003 edition, Section 2603.4