Spray Foam – Frequently Asked Questions

What are the differences between open-cell vs. closed-cell SPF insulation?

There are two general categories of SPF insulation materials; open-cell, low-density (a.k.a. ‘half-pound foam’) and closed-cell, medium-density (a.k.a. ‘two-pound foam’). Both foam categories provide excellent insulation and air sealing. Although both are made using almost identical chemical reactions, there are some inherent physical property differences that often determine which product is chosen for a particular project.

Open-cell spray foam (ocSPF) has an open cell structure where the cells are filled with air. The open-cell structure renders soft, flexible foam, with a density of about 0.5-0.8 pounds per cubic foot (pcf). Still air is the primary insulation medium in ocSPF, fiberglass and cellulose. These insulations work by reducing the natural air movement within these materials thereby reducing the ability of the material to conduct heat. The R-value per inch of open-cell foam typically ranges from R3.6 to R4.5 per inch. Unlike fiberglass and cellulose, the fine cell structure of ocSPF makes it air-impermeable at certain thicknesses. The air-impermeability of ocSPF qualifies it as an air-barrier material, dramatically reducing air leakage through the building envelope, significantly lowering the building’s heating and cooling costs.

ocSPF, like fiberglass and cellulose insulations, is moisture-permeable, and may require the installation of a vapor retarder in colder climates. Closed-cell spray foam (ccSPF) has a closed cell structure which yields a rigid, hard foam, with a density of 1.8-2.3 pound per cubic foot (pcf), and has been demonstrated to provide structural enhancement in certain framed buildings. These smaller cells trap an insulating gas, called a blowing agent. This blowing agent has a lower thermal conductivity than still air, and increases the R-value. Typical R-value per inch of closed-cell foam ranges from R5.8 to R6.9* per inch, makiing it a great choice in applications where clearance is limited.

Like ocSPF, ccSPF is also air impermeable at certain thicknesses and and can qualify as an air-barrier material. The closed-cell structure of ccSPF makes it water-resistant, and is the only spray foam that can be used where contact with water is likely (e.g., below-grade concrete walls, in contact with the ground, or on exterior side of the building envelope). At a thickness of 1.5 inches, ccSPF has a moisture permeance typically less than1.0 perms and no additional vapor retarder is required for most applications.

*Consult product data sheets

Does SPF absorb water?

Closed-cell foams, by nature, are resistant to water absorption, and are approved by FEMA as a flood-resistant material. Open-cell foams can absorb and retain liquid water at varying rates. It is important to consider the different properties for each foam type for each application.

What fire protection measures (thermal or ignition barriers) are required for SPF?

SPF, like many construction materials, is combustible, and can ignite when subjected to heat or flame. For this reason, model building codes require that SPF materials (with some exceptions) must be separated from interior (occupied) spaces by a 15-minute thermal barrier, such as ½” gypsum board. In limited access areas like crawlspaces and attics, an ignition barrier may be permitted in place of a thermal barrier. Prescriptive thermal and ignition barriers are defined in the model building codes, and alternative coatings,coverings and assemblies may be used. For more information on thermal and ignition barriers, please see SPFA guideline AY-126 from our Technical Documents section on www.sprayfoam.org

What are the structural benefits of closed-cell SPF?

Because of its rigid nature and ability to adhere to many materials, closed-cell SPF (ccSPF) can provide structural enhancement to framed buildings. Racking strength of certain framed walls, as well as uplift strength of framed roof decks can be significantly increased with the addition of just 2-3 inches of SPF. For more information, please contact SPFA to obtain detailed reports.

Is SPF suitable for residential retrofit insulation applications?

SPF is an ideal product for insulating and air-sealing existing homes. SPF can be used to create energy-saving unvented attics and crawlspaces that seal against air leakage and bring under-insulated and leaky HVAC ducts inside the conditioned space of the building. In addition SPF, can be used to insulate and air-seal band and rim joist areas where the framing meets the home’s foundation.

Should access to the work area be restricted during and immediately after spray foam installation?

During and immediately following spray foam applications, fumes and mists are generated that can be hazardous to your health. Access to the work area during this time should be restricted to personnel wearing appropriate personal protective equipment (PPE), including respirators, and whose job responsibilities require them to be in the area.

Can SPF be used with Other Insulations (Hybrid Insulation Systems)?

Closed-cell SPF can be used in combination with other insulation materials such as fiberglass, cellulose and foam board products. These cost-effective hybrid systems use SPF to insulate and air seal, and use other insulations to provide assembly R-values that meet energy codes. In colder climates, special design considerations are needed to address potential moisture condensation issues. For more information on using SPF in hybrid insulation pplications, see SPFA’s guidelines on hybrid systems:

AY-146 Spray Polyurethane Foam for Hybrid Insulation Systems – Climate Zones 1-3 and AY-147 Spray Polyurethane Foam for Hybrid Insulation Systems – Climate Zones 4-7


Problems with SPF installation should be resolved between the building owner (customer) and SPF contractor that installed the foam. If these issues cannot be resolved at this level, the building owner should contact the supplier of the foam and/or coating systems. Below is a list of typical installation and service problems.

What causes delamination and adhesive failures with SPF?

Much like painted coatings, SPF should strongly adhere to nearly all construction materials that are clean, dry and free of oils and grease. If the substrates are free of oil, dust, dirt and moisture, and applied according to manufacturer’s instructions, there should be no problems with SPF adhesion. Adhesion should be occasionally checked by tearing a small area of foam from the substrate. When properly adhered, the foam itself should tear, leaving a thin but visible residue of foam on the substrate. However, SPF may not adhere well to some construction materials, such as polyethylene sheeting, under-cured concrete (containing excessive moisture or surface contaminants) and certain metals. These materials may need special surface treatments, such as primers or coatings, before SPF can be applied.

Is SPF susceptible to mold and mildew?

All building materials, when installed to create a building envelope assembly, work interactively as system, to control the movement of heat, air and moisture. When not properly designed or installed, moisture can move through the building envelope and condense on cold surfaces that are below the dewpoint temperature or create high levels of moisture. This moisture, at certain temperatures, in the presence of organic food sources (paper, wood, bacterial dust, etc.) can provide the conditions necessary to promote the growth of mold and mildew.

While SPF is not a source of food for mold, mildew and bacteria, organic dusts can collect on the surface of the foam. In combination with moisture at the right temperatures, these organic dusts can result in mold and mildew. SPF, like all insulation products, can result in mold and mildew problems in building envelopes that are poorly designed or constructed. Proper air sealing, as well as use and placement of vapor retarders, and sufficient levels of insulation are key to proper building envelope design.