Compare insulation facts before you choose

There was a time when “insulation” simply referred to fiberglass blankets, and the biggest choice a contractor had to make was color and manufacturer.

With advances in building science and a more comprehensive approach to insulating, a successful approach includes a variety of insulation facts, including air infiltration, moisture transport, thermal bridging and heat transfer. It usually means combining two or more insulation products for maximum effectiveness.

Other advances in insulation products address environmental concerns. Green builders and some homeowners are searching for more environmentally friendly insulation products.

While some products have replaced certain components with a green alternative (such as cotton batts instead of fiberglass or soy-based raw materials in place of a portion of the petrochemicals), the most important green aspect to consider when comparing insulation facts is the insulation’s performance and suitability for the application.

R-value measures a material’s resistance to heat flow, and typically materials with higher R-values per inch are preferable to those with lower R-values per inch.

Air-permeable insulation is usually less desirable than products that resist airflow, but air leakage concerns can be addressed during other stages of the building process – making air-permeable insulation a cost-effective and perfectly acceptable solution.

Blanket insulation

Typically in the form of batts or rolls, blanket insulation can be made from fiberglass or rock wool fibers, with green options that include cotton (mostly recycled denim), formaldehyde-free components, and even hemp. It’s available un-faced or faced with a variety of materials, most commonly Kraft paper.

The facing could act as a vapor barrier or a vapor retarder; however, interior vapor barriers are discouraged except in very cold climates. Blanket insulation does very little to slow air leakage, so careful installation in conjunction with a well-detailed air barrier is essential for maximum performance.

Blanket insulation facts

R-value per inch: R-3.2 – R-3.8 (fiberglass), R-3.5- R-3.7 (cotton), R-3.8 to R-4.3 (mineral wool)

Installed cost: $0.49 to $1 per sq. ft. (fiberglass), $1.27 to $1.40 per sq. ft. (cotton), about $1.10 per sq. ft. (mineral wool).

Suitable for: Above-grade wall cavities, ceilings and vented roofs. Mineral wool is particularly resistant to higher temperatures, making it the best insulation to use near a chimney or metal flue.

Green factors: A variety of environmentally friendly options are available, from natural or recycled ingredients to chemical-free formulations.

Blown-in and loose-fill insulation in an attic space requires a thick layer because of its low density.  When installed in a closed cavity, higher densities result in higher R-value per inch.

Blow-in insulation

Loose-fill and blown-in materials usually consist of fiberglass, cellulose and sometimes mineral wool. Because blown-in products fill all the nooks and crannies of the framing bays better, their performance is typically much better than blanket insulation.

Usually installed behind air-permeable netting stapled to studs or rafters, it can be blown into open cavities if it contains an adhesive to make it resistant to settling.

Fiberglass has a low R-value per inch, so its best application is in attics that allow for a very deep layer (up to 26 inches in some cases). Cellulose can absorb and hold water, so roof or plumbing leaks may go unnoticed longer than with other products.

Blow in insulation facts

R-value per inch: R-2 to R-4.2 (fiberglass), R-3.2 to R-3.8 (cellulose)

Installed cost: $0.60 to $1.20 per sq. ft. (fiberglass), $.40 to $1.80 per sq. ft. (cellulose), depending on installation and density.

Suitable for: Above-grade wall cavities, ceilings and vented roofs.

Green factors: Cellulose has a high level of recycled content.

Spray foam insulation fills in all irregular cavities, voids, nooks and crannies – making it the best insulating product for reducing air leakage.

Spray foam insulation

Typically made from polyurethane, spray foam products reduce air leakage better than any other insulating material. It easily fills and expands to seal off all hard-to reach and irregularly shaped cavities and voids.

The two main types are open-cell and closed-cell, with open-cell being far less dense. Open-cell foam is highly vapor-permeable. Many contractors spray cured open-cell foam with a vapor-retarding paint to create a vapor barrier when needed.

Closed-cell foam is the most expensive insulating option, but when installed well it is by far the most effective. It’s impervious to moisture and an effective vapor retarder.

Spray foam insulation facts

R-value per inch: R-3.6 (open-cell), R-6.5 (closed-cell).

Installed cost: $1 to $1.20 per sq. ft. (open-cell), $1.75 to $3 per sq. ft. (closed cell), can vary widely based on application and location.

Suitable for: Walls, ceilings and roofs (open-cell), under slabs, below- and above-grade walls, ceilings and roofs.

Green factors: Many open-cell types of foam are made from bio-based raw materials (such as soy beans) and use water and carbon dioxide as blowing agents.

Rigid foam board has many uses above and below grade. Of the three formulations, extruded polystyrene (XPS) is the preferred product for insulating slabs and foundation walls because of its high compressive strength and water resistance.

Rigid foam insulation

Sold in thicknesses from ¼ inch to 6 inches (usually special order), foam boards are an effective air barrier when installed properly and seams are sealed with tape, caulk or spray foam.

All are made from petroleum-based materials, but the three main types are expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso).

EPS is the most vapor-permeable of the three, XPS has a high compressive strength and is impervious to water (making it great for below-grade applications), and polyiso typically has one or both faces covered with foil. Polyiso is more fire resistant, but absorbs water easily.

Rigid foam insulation facts

R-value per inch: R-3.6 to R-4.2 (EPS), R-5 (XPS), R-6.5 (polyiso)

Material cost for 1-inch thick board: About $0.35 per sq. ft. (EPS), about $0.50 per sq. ft. (XPS), about $0.60 per sq. ft. (polyiso). Installation costs vary widely by application.

Suitable for: Under slabs, below- and above-grade walls, ceilings and roofs (except polyiso, which should not be used below grade).

Green factors: Blowing agents for EPS and polyiso contain non ozone-depleting chemicals.

Radiant barriers

Radiant barriers are usually fabricated from aluminum foil with a variety of backings, including Kraft paper, polyethylene bubbles or even cardboard. Resistance to heat flow depends on the heat flow direction, and these products are most effective in reducing downward heat flow.

Typically used between roof rafters and floor joists, many products are designed for easy retrofitting. For new construction, products such as foil-faced roof sheathing (where the foil faces the attic interior) are available.

R-value per inch: Since radiant barriers are intended to work differently than other insulation products, and come in a variety of materials (from simple foil rolls to foil-faced plywood sheathing or foam board) the important numbers to know are a low emittance (0.1 or less) and high reflectance (0.9 or more).

Radiant barrier facts

Installed cost: Varies widely by material chosen and installation location.

Suitable for: Most effective when used in attic applications, but can be used between floors. Radiant barriers must have an air space that faces at least one of its reflective surfaces in order to function properly (otherwise it can become a heat conductor, not a barrier). Not recommended for installation on top of other insulation on attic floors.

Green factors: Most foil-related products are easily recyclable and contain a large percentage of recycled material.

—By Rob Fanjoy