Insulation Choices

The Right Insulation Choice Depends on the Job at Hand

UPDATED 11/16/2012


Batts are readily available, are inexpensive, and can be installed without special equipment.

Cost: Low
Air sealing: Poor
Best use: Above-grade walls
Other uses: Floors and roofs

Blow-in/Loose Fill

Loose-fill insulation can be blown into attics or dense-packed into framing cavities.

Cost: Low
Air sealing: Low to moderate
Best use: Above-grade walls, attic floors
Other uses: Rafter bays

Rigid Foam Board

Rigid foam is frequently used to insulate foundations and the exterior or interior of walls or roofs.

Cost: Moderate
Air sealing: Very good
Best use: Walls, slabs, sloped roofs
Other uses: Floors

Spray Foam

Spray foam seals air leaks as it provides an excellent layer of insulation.

Cost: High
Air sealing: Excellent
Best use: Air sealing difficult areas
Other uses: All insulation jobs



Insulation Overview

Insulating Roofs, Walls, and Floors

The Building Envelope

Air Barriers


Radiant barriers are shiny sheets of material—aluminum foil, for example—with a low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. (low emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces.) surface. When an air space has a low-e material on one or both sides, the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the air space or building assembly increases.

The effect of a radiant barrier on a building assembly's R-value may be significant or insignificant, depending on whether the assembly is well insulated or poorly insulated. Radiant barriers do not significantly benefit well-insulated assemblies. A poorly insulated assembly, however, will benefit from a radiant barrier. That's why radiant barriers make the most sense when installed in an uninsulated steel warehouse; they make the least sense when incorporated into residential walls or roofs, as these assemblies are already required by code to be insulated.

Radiant barrier products can be foil-faced kraft paper, foil-faced polyethylene film, or foil facings on rigid insulation or wood-fiber sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . No paints — not even aluminized paints — qualify as radiant barriers. If the radiant surface is touching another material, it won't work; an air space is required on at least one side of a radiant barrier in order for it to function as designed. Otherwise, it functions as a conductor.

If radiant barriers are installed horizontally with an air space above—for example, on an attic floor—their usefulness rapidly deteriorates due to dust accumulation.

Radiant barriers have no R-value. However, if installed adjacent to an air space, they can help raise the R-value of the air space.

For more information, see Radiant Barriers.


For more information on insulation choices, see the Insulation section of the GreenSpec Product Directory.


Insulation cuts pollution and fossil-fuel demand

Insulation is one of the most important components of any environmentally responsible building because it reduces energy consumption and therefore pollution. More insulation means less coal is burned at the power plant, and less gas or oil is burned in a furnace.

In fact, with good design and appropriate levels of insulation, you can minimize (or even eliminate) the need for central heating and cooling in many buildings. This principle is demonstrated in all superinsulated buildings, including PassivHausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. buildings.

In this sense, any insulation material is a “green” product. Green builders need to focus on choosing which insulation is greener or better for a particular application. A few considerations include:

  • What needs to be insulated
  • Raw materials used for insulation manufacture
  • The global warming potential of gases released during the manufacture or installation of the insulation
  • Health and safety concerns for installers and occupants
  • Whether scraps can be easily disposed of or recycled
  • When choosing an insulation material, consider how it will work with the rest of the wall, roof, and floor system—and also consider what additional functions, such as air sealing, the material might serve. Some types of insulation stop air movement and reduce heat flow while shedding water and allowing drying (the four functions of the building enclosure).

    Here are a few issues to consider when pairing insulation materials and structural elements for maximum efficiency:

    Life-cycle considerations
    Because different types of insulation are made from different raw materials and are manufactured using different methods, their environmental impacts vary. These life-cycle impacts should be considered along with factors such as R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. , air-sealing ability, and cost.

    Installation—a 4% mistake yields a 50% penalty
    Poorly installed insulation will not achieve the energy savings that its rated R-value would suggest. A California study concluded that a 4% void in fiberglass batts resulted in a 50% decrease in insulation effectiveness.

    Structural partners
    The type of structural framing also affects the performance of insulation. Steel studs conduct heat much more readily than wood studs, so they create thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. that can bypass insulation installed in the cavities. The U.S. Department of Energy recommends that buildings with steel framing include exterior insulative sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. in all U.S. climates.

    Strengths and Weaknesses


    Fiberglass batts are inexpensive and can be installed without special equipment. However, fiberglass batts do little to slow air leakage; to perform well, they must be installed in conjunction with a very well-detailed air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. Unaddressed air leaks in fiberglass-insulated walls can easily lead to condensation and moisture problems.

    Cotton batts are less likely to irritate the skin of insulation workers than fiberglass batts. However, they cost considerably more than fiberglass and have many of the same drawbacks that fiberglass batts do. They are also more difficult to install than fiberglass batts.

    Mineral wool batts are available as R-15 batts for 2x4 walls and R-23 batts for 2x6 walls. Variously referred to as rock wool, slag wool, and stone wool, this type of insulation was one of the first insulation materials to be widely produced commercially — starting back in 1871 in Germany. Roxul and Thermafiber are the two best-recognized brands in North America.


    Blown-in cellulose is the first choice of many green builders for insulating walls, flat ceilings, and sloped ceilings. Because cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. is made from recycled newspaper, its manufacture has little negative environmental impact (in fact, there may be a positive environmental impact). Cellulose is inexpensive and better than fiberglass batts at reducing air infiltration. One disadvantage of cellulose: it doesn't respond well to moisture. When cellulose is exposed to a roof or plumbing leak, it becomes soggy and repulsive. Because cellulose is able to absorb and hold a lot of water, detection of roof or plumbing leaks may be delayed.

    Blown-in fiberglass does a better job of filling nooks and crannies than fiberglass batts, and therefore performs better. To most green installers, however, chopped glass fibers are less environmentally benign than recycled newspaper.


    Rigid foam insulation is a more effective air barrier than batts, blankets, or blown-in insulation, especially if the seams between sheets are carefully sealed with caulk or tape.

    Expanded polystyrene (EPS) is a versatile insulation that can be used below grade, on walls, or on roofs. EPS is less expensive than XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. or polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. . If the correct density is chosen for the application, EPS is not affected by moisture.

    Extruded polystyrene (XPS) shares many of the strengths of EPS. However, it is stronger, denser, smoother, more water-resistant, and has a higher R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch (R-5 for XPS versus R-3.8 for EPS). It also costs more than EPS.

    Polyisocyanurate has the highest R-value per inch (R-6.5 to R-6.8) of any rigid insulation. Its blowing agent is environmentally benign, and it is free of the brominated flame-retardant chemicals that taint polystyrene. However, polyiso easily absorbs water, so it can't be used below grade.

    Cork insulation panels are made from the bark of Portuguese cork oak trees, a renewable resource. Cork insulation can be used on walls and roofs, just like rigid foam panels.


    Closed-cell spray polyurethane foam performs better than any other insulation. It has a high R-value per inch (R-6.2 to R-6.5), it's impervious to moisture, and it's an effective vapor retarder. It is also an excellent air barrier. Closed-cell spray polyurethane foam can be used under slabs, on below-grade or above-grade walls, in ceilings, or even as roofing. It is also the most expensive residential insulation available.

    Open-cell spray polyurethane foam has some but not all of the virtues of closed-cell foam. Like closed-cell foam, open-cell foam creates an effective air barrier. However, open-cell foam has an R-value of about R-3.6 per inch—much less than that of closed-cell. Unlike closed-cell foam, open-cell foam absorbs and holds water. It is also vapor-permeable, which can be a virtue or a drawback, depending on the application. Open-cell spray foam costs less than closed-cell foam.

    Further Resources

    Building Science Corp.
    Guide to Insulating Sheathing by Joseph Lstiburek and Peter Baker

    Fine Homebuilding
    “A Buyer's Guide to Insulation” by Martin Holladay
    “Save Energy With Rigid-Foam Insulation” by Rick Arnold

    Environmental Building News
    “Revisiting Rigid Foam Insulation and Ozone”
    “Insulation: Thermal Performance Is Just the Beginning”

    Image Credits:

    1. Bonded Logic
    2. Can-Cell Industries, Inc.
    3. CertainTeed
    Tags: , , , , , , , , , ,
    Aug 26, 2016 3:34 PM ET

    by Steve Knapp CZ 3A Georgia

    Jill. If you are sensitive to smells, you might want to avoid spray foam. As far as I know, this isn't a toxicity issue but more a question of comfort. My properly installed open cell foam has an odor, but I don't notice it unless in an area where the foam is not encapsulated behind drywall. Your best option is likely to use rigid foam on the exterior, cellulose on the interior, or these two materials in combination.

    Aug 26, 2016 2:49 PM ET

    Response to Sacie Lambertson
    by Martin Holladay

    Thanks for your excellent suggestion.

    Aug 26, 2016 2:27 PM ET

    by Sacie Lambertson

    Under 'batts" the list of known insulations need to be updated to include Roxul don't you think? Now available in the United States mostly through special order.

    Aug 3, 2011 5:14 AM ET

    Response to Jill Buffie
    by Martin Holladay

    Installers of spray polyurethane foam must wear protective equipment during foam installation, but I haven't seen any information to indicate that cured foam is toxic. Lingering odors have been reported in a small number of cases; if these cases worry you, you can always choose a different type of insulation.

    Aug 2, 2011 1:50 PM ET

    by Jill Buffie

    Does anyone know which of the spray on insulation products off-gasses the least? I hear that Icynene needs 30 days to off gas once applied.

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