Polyurethane spray foam is a versatile insulation material that is sprayed into building cavities where it quickly expands and molds itself to its surroundings. It is available in “closed-cell” and “open-cell” varieties, each of which offers advantages and disadvantages, depending on the requirements of its application. The following guide briefly explains the differences between these insulation options.
Closed-Cell Polyurethane Foam
Closed-cell polyurethane foam (CCPF) is composed of tiny cells with solid, unbroken cell walls that resemble inflated balloons piled tightly together. The cells are inflated with a special gas selected to make the insulation value of the foam as high as possible.
Like the inflated tires that hold up an automobile, the gas-filled bubbles, when dried, create a material that is strong enough to walk on without major distortion. Wall-racking strength can by enhanced when CCPF is applied, and its strength makes it preferable for roofing applications.
The high thermal resistance of the gas gives CCPF an R-value of approximately R-7 to R-8 per inch, according to the U.S. Department of Energy (DOE), which is significantly better than its open-cell alternative. It also acts as a vapor barrier, making it the product of choice if the insulation is likely to be exposed to high levels of moisture. Its density is generally 2 lb/ft3 (32 kilograms per cubic meter [kg/m3]).
Over time, the R-value of CCPF can drop as some of the low-conductivity gas escapes and is replaced with ordinary air, a process known as thermal drift. Research performed by the DOE revealed that most thermal drift occurs within the first two years after the insulation material is applied, but then the foam remains relatively unchanged unless it is damaged.
Semi-permeable rigid foam insulation and spray foam insulation (foam plastic) on the inside of basement foundation walls is often found during an inspection of the full-basement foundation of a house. Its use could be a good strategy for a moisture-resistant finished basement. However, fire and smoke characteristics of this type of insulation require that it be covered with a fire-resistant layer, such as gypsum wallboard (drywall).
Sometimes this requirement works fine when the basement is being finished. This requirement of having spray foam insulation to be protected by a thermal barrier is found in the International Residential Code (IRC) 2015 Section R316. In most cases where spray polyurethane foam insulation is installed, the foam should be separated from the interior living spaces by an approved thermal barrier of at least 1/2-inch gypsum wallboard (drywall), 23/32-inch wood structural panel, or a material tested to meet the acceptable criteria from NFPA. There are a few exceptions to this requirement, including flame spread index ratings.
If a basement will only be insulated and not finished, a fire-rated foam panel or similar fire-rated covering needs to be used. Because the above-grade portions of the basement wall can dry to the outside, fire-rated insulation on these surfaces may be of an impermeable type. For example, it can have a foil facing. But insulating approaches that restrict the drying potential of below-grade portions of the foundation wall toward the inside should be avoided.
In attics, a thermal barrier is not required when several conditions exist. Those conditions are listed within the IRC Code 2015 Section R316, and they include the attic access is required, the attic space is entered for only maintenance and when repairs are needed, and the foam insulation has been tested or the foam insulation is protected again ignition using a listed barrier material.
Packages and containers of spray foam insulation (or foam plastic) should be labeled and identified if they are delivered to a building site.
Open-cell polyurethane foam (OCPF) is a soft, flexible, spongy insulation with broken cell walls that permit air to fill them. They typically have a density of 0.5 lb/ft3 (8 kilograms per cubic meter [kg/m3]), which is significantly less than closed-cell insulation, as well as having a reduced R-value per inch, although OCPF still has excellent thermal-insulating and air-barrier properties. The foam is weaker and less rigid than closed-cell foams, too. It will require trimming and disposal of excess material as it expands to over 100 times its initial liquid size.
Builders often choose open-cell foam for the following advantages it affords, including:
- its low cost. Where economical yield is important, open-cell foam is generally chosen over its more costly alternative;
- providing a sound barrier. OCPF forms a more effective sound barrier in normal-frequency ranges than closed-cell foam. For this reason, OCPF is well-suited for installation beneath floors and around theater rooms;
- its flexibility. Open-cell foam is more flexible than closed-cell foam, which allows it to adjust to weather-induced expansion and contraction of framing members. CCPF, by contrast, may develop hairline fractures because it cannot flex sufficiently; and
- its permeability to moisture. While often cited as a reason to avoid the use of OCPF, in certain situations, it can be helpful for moisture to pass through insulation. Open-cell foam used in roofs, for instance, will allow a roof leak to make its way to the space below where it is more likely to be discovered. Closed-cell foam used in the same application would trap the moisture, hiding the leak and potentially leading to wood decay. In most situations, however, OCPF should not be used in any place where it might become wet, as moisture will diminish its insulative value. InterNACHI inspectors may call out open-cell insulation discovered in moist areas, such as in external applications or below grade.
These are large pieces of insulation that hold together because they’re made of long, interweaving fibers with adhesive binders. The two kinds of batts you’re most likely to encounter are fiberglass and cotton. In terms of their insulating quality, they’re pretty much equivalent. Cotton batts, though, are ‘cool’ because they’re made of recycled blue jeans.
The problem with batts, however, is that they don’t work well because they don’t fill the space well. For the best performance, an insulation material needs to fill the whole space, with no gaps, voids, compression, or incompletely filled areas. Batts are about the worst you can do here.
See that photo above? Notice that you don’t see insulation filling all the spaces between the ceiling joists. In this case, it’s because they weren’t cut to fill the cavity completely. Another reason that batts don’t do so well is that the house is full of other stuff where we want the insulation to go: wires, electrical junction boxes, framing, bathroom exhaust fans, can lights… Batts don’t do well when they have to compete against all that.
A better choice is insulation that comes in smaller chunks. The installer, taking his best firefighter pose, holds a large hose and blows the chunks into the attic. A large machine outside churns the chunks and uses air to blow them up through the hose.
The two main choices here are fiberglass and cellulose, and each has its advantages and disadvantages. They both insulate about the same, though, with R-vales in the 3 to 4 per inch range. Cellulose comes from recycled newspapers. Fiberglass comes from what I’ve heard one major fiberglass insulation manufacturer call a ‘rapidly renewable’ resource – sand. Hmmmm. I don’t know about that, but it’s a common insulation material that works much better in the blown form than in batts.
The photo above shows an attic insulated with blown cellulose. Notice how you don’t see any of the ceiling framing down at the ceiling level. You also don’t see any gaps that allow you to see all the way down to the ceiling drywall. That’s because blown insulation is great at filling the gaps and giving you a good, complete layer of insulation.
The third major type of insulation is spray foam. Just as there are two types of blown insulation (fiberglass and cellulose) and two types of people (those who divide everything into two groups and those who don’t), there are two types of spray foam – open cell and closed cell. Each has its pros and cons, as well as its own set of adherents who will tell you never to use the other type. That’s an article for another day, however.
The main advantage of spray foam is that it allows you to move the building envelope – the boundary between conditioned and unconditioned space – from the attic floor to the roofline. If you’ve got your HVAC system and ducts in the stupidest place they could possibly be (the attic), then moving the envelope to the roofline can be a good thing. In a new home, spraying foam in the roofline can bring the ducts inside the envelope without having to redesign the system and house.
If you don’t have HVAC and ducts in the attic, spray foam on the roofline isn’t really necessary. I’d blow insulation on the attic floor (after air-sealing, of course). The big disadvantage with spray foam is cost. It’s generally 3 to 4 times what you’ll pay for blown cellulose or fiberglass.
Insulation R Value Chart
|Insulation R-Values for Location, Heat Type & Area*|
|Location||Heat Type||Attic||Wall||Floor||Crawl Space Wall**||Basement Wall|
|Zone 1||Natural Gas||38-49||13||13||13||11|
|Zone 2||Natural Gas||38||13||13-19||13||11|
|Zone 3||Natural Gas||30-38||13||13-19||13-25||11|
|Zone 4||Natural Gas||38-49||13||25-30||25||11|
|Zone 5||Natural Gas||38||13||25||25||11|
|Zone 6-8||Natural Gas||49||13||30||25||25|
*Ranges resulted from the slection of two different zip codes within the same zone (i.e. Dover, DE and Chattanouga,TN for zone 4)
|R-value of Materials and Depths|
|Material||R-value/in||3 1/2″||5 1/4″||10″||12″||15″|
|Fiberglass (batt)||3.1 – 3.4||10.8 – 11.9||16.3 – 17.8||31.0 – 34.0||37.2 – 40.8||46.5 – 51.0|
|Fiberglass blown (attic)||2.2 – 4.3||7.7 – 15.0||11.5 – 22.6||22.0 – 43.0||26.4 – 51.6||33.0 – 64.5|
|Fiberglass blown (wall)||3.7 – 4.3||12.9 – 15.0||19.4 – 22.6||37.0 – 43.0||44.4 – 51.6||55.5 – 64.5|
|Mineral Wool (batt)||3.1 – 3.4||10.8 – 11.9||16.3 – 17.8||31.0 – 34.0||37.2 – 40.8||46.5 – 51.0|
|Mineral Wool blown (attic)||3.1 – 4.0||10.8 – 14.0||16.3 – 21.0||31.0 – 40.0||37.2 – 48.0||46.5 – 60.0|
|Mineral Wool blown (wall)||3.1 – 4.0||10.8 – 14.0||16.3 – 21.0||31.0 – 40.0||37.2 – 48.0||46.5 – 60.0|
|Cellulose blown (attic)||3.2 – 3.7||11.2 – 12.9||16.8 – 15.0||32.0 – 37.0||38.4 – 44.4||48.0 – 55.5|
|Cellulose blown (wall)||3.8 – 3.9||13.3 – 13.6||19.9 – 20.8||38.0 – 39.0||45.6 – 46.8||57.0 – 58.5|
|Polystrene Board||3.8 – 5.0||13.3 – 17.5||19.9 – 26.2||38.0 – 50.0||45.6 – 60.0||57.0 – 75.0|
|Polyurethane Board||5.5 – 6.5||19.2 – 22.7||28.9 – 34.1||55.0 – 65.0||66.0 – 78.0||82.5 – 97.5|
|Polyisocyanurate (foil-faced)||5.6 – 8.0||18.2 – 28.0||29.4 – 42.0||56.0 – 80.0||67.2 – 96.0||84.0 – 120.0|
|Open Cell Spray Foam||3.5 – 3.6||12.2 – 12.6||18.4 – 18.9||35.0 – 36.0||42.0 – 43.2||52.5 – 54.0|
|Closed Cell Spray Foam||6.0 – 6.5||21.0 – 22.7||31.5 – 34.1||60.0 – 65.0||72.0 – 78.0||90.0 – 97.5|
**Crawl space walls that are vented or have moist problems should not be insulated.