Spray Foam vs Fiberglass Insulation: Cost & Performance

Spray foam insulation vs fiberglass insulation is a debate that many homeowners have.

In truth, both materials make excellent insulation – and each has its place in the home.

Why is insulation so important? According to the U.S. Department of Energy, your heating and air conditioning equipment in the average home accounts for as much as 49% of your total energy bill.

Unless you live in an area of the world that enjoys an ideal year-round climate, your house is surrounded by air that is warmer or cooler than what is comfortable, or perhaps even healthy for you and your family much of the time. In parts of the U.S., air temperatures can vary by 120 degrees F or more as the seasons change.

If you are building a new house or addition, there are insulation options available today that can help keep your home comfortable throughout the year, regardless of the weather.

This chart summarizes the information in this guide.

Type	R-Value per Inch	Cost per Inch per Sq. Ft.
Fiberglass Batt	3.5	$0.21 – $0.25
Fiberglass Loose	2.5-4.0	$0.30 – $0.42
Open Cell Foam	3.5	$0.45 – $0.80
Closed Cell Foam	6.5-7.0	$1.00 – $2.00

* Installation is Extra – Add about $1.00 per square foot for installation

Types of Insulation: Spray Foam vs Fiberglass

While there are several types of insulation available today, two of the most commonly used are spray foam and fiberglass. In this fiberglass vs spray foam insulation guide, each is explained along with pros, cons and a head-to-head comparison.

Spray Foam Insulation aka Spray Polyurethane Foam (SPF)

SPF is composed of two different chemicals that when combined through a sprayer under pressure, form a foam-like substance. It quickly expands to fill in even the smallest gaps and cracks on the surface to which it has been applied.

SPF “skins over” in just a few seconds and completely cures in a few minutes filling the spaces between the framing of your house. It can be used for roof and wall junctions, wall studs, attic floors and ceilings (roof deck) and rim joists. When properly applied it will also seal electrical, mechanical and plumbing penetrations in the exterior of the house.

It will bond to many surfaces, including wood, rigid foam, masonry, metal and drywall, and can be applied above or below grade.

SPF can be purchased at most home improvement stores in amounts that will cover between 12 and 650 board feet. A board foot is 1 square foot of surface 1 inch thick.

Since the application of this product involves the spraying of chemicals, it is recommended that protective wear is used, including a full paint suit, gloves, goggles and respirator.

But you don’t need to do it yourself. There are no doubt professionals in your area who specialize in insulating with SPF, and they have the know-how and experience to get the job done right in much less time than the average homeowner can. Plus, it is essential to get the application correct to be sure you get the air sealing benefit, which is a major reason to use spray foam insulation.

As we explained in detail in our Spray Foam Cost and Ultimate Guide, there are two main types of SPF: Open Cell and Closed Cell.

The are several differences between them:

Open Cell Spray Foam (also called ½ lb. foam or Low-density SPF)

Open cell SPF is made up of cells (bubbles) that aren’t completely surrounded by the polyurethane material of which it is made. This results in an insulating material that is softer and more flexible than closed cell foam. Open cell foam is lighter and it will “breathe” more than closed cell foam, but it will also allow water to pass through.

Pros

• Costs less than closed cell foam
• I particularly effective in hot climates where cooling the home – and preventing outside heat from penetrating back into the house – is the main objective
• Provides a better sound barrier than closed cell foam

Cons

• Lower R-value per inch of thickness
• Does not provide a vapor barrier
• Cannot be used where water is present or below grade

Closed Cell Spray Foam (also called 2 lb. foam or Medium-density SPF)

Closed cell foam is comprised of cells that are completely surrounded by the polyurethane material. They are completely closed. Because they are pressed together, air and moisture cannot get inside the foam as they can with open cell foam. This makes closed cell foam usable in places where its open cell counterpart is not advisable.

Closed cell SPF is also denser and heavier, giving it the ability to provide some added strength to the structure.

Pros

• Provides the highest R-value per inch of all insulating materials
• Creates a vapor barrier
• Makes the structure stronger and more stable
• Provides a better air seal to stop drafts
• Can be used in below-grade applications

Cons

• Costs more than twice as much as open cell SPF foam insulation
• Overspray is more difficult to clean up

Fiberglass Insulation

There are also two main types of fiberglass insulation: Loose Fill (Blown) and “Batts.” Both consist of plastic filaments fortified with recycled glass spun into very thin fibers.

Loose Fill fiberglass comes very tightly packed in bags. Application is generally a two-person job. It is accomplished by one person opening the bag and feeding the loose filling into a special blower. These blowers can be rented, or sometimes are provided rent-free by the supplier if enough insulation is purchased. The other person directs a hose from the blower to broadcast the fiberglass filling into whatever spaces and cavities that need to be insulated. This method works best when filling horizontal spaces such as attic floors, but can also fill vertical spaces between wall studs with proper preparation and special techniques.

The process of blowing fiberglass results in large amounts of airborne fiberglass particles that are very irritating to the eyes and respiratory system. Good goggles and respirators are recommended for anyone participating in installing this product.

Fiberglass batts are like blankets that come in either rolls or pre-cut lengths. They are available in different widths (16 or 24 inches on-center), and thicknesses (3 1/2 inches thick for 2×4 walls and 5 1/2 inches thick for 2×6 walls). These sizes are designed to fit typical stud wall spacing and framing for the various areas of your house. Where a vapor barrier is desired, batts can be purchased that have a specially treated paper that will accomplish this. Where no vapor barrier is desired, unfaced batts or rolls can be used.

Installing fiberglass insulation is something that many homeowners do themselves, but there are also professional installers that can do the job for you.

Should I Use Spray Foam or Fiberglass?

The decision concerning which type of insulation you should use is perhaps the most important; it will depend on knowing which types work best in different situations, as well as what best fits your budget.

Spray Foam Insulation – Pros and Cons

Here are reasons to use SPF and issues you’ll want to consider before investing in it.

Pros

• Has a higher R-value than fiberglass per inch of thickness. This means that if you are insulating walls that have been framed with 2×4 studs, for example, you only have a space that is 3½” thick. By using SPF you can achieve an R-value of between R-21 to R-24, as opposed to a maximum of about R-13 with fiberglass in the same space. 
• When closed cell foam is installed in new construction, it will eliminate the need for a separate vapor barrier, which is important in colder climates
• When closed cell foam is installed on the roof of an attic space, it can eliminate the need for attic ventilation, since it will stop moisture from entering the attic through the roof – Not that all gable, soffit and ridge vents will need to be sealed to create a sealed attic in this manner
• Is more easily applied to hard-to-reach areas (crawl spaces, odd-shaped cavities, etc.)
• Provides a barrier against insects and rodents, especially in below grade applications
• Does not provide a food source for mold
• Provides a much tighter seal to prevent air leaks around door and window frames, electrical receptacles and anything that penetrates the exterior sheathing

Cons

• Is more expensive than fiberglass
• Requires more preparation of surrounding areas to prevent overspray, etc.
• Can be applied by the homeowner, but in many cases is best done by professionals. This will result in additional cost
• Should not be applied over existing attic insulation; it can lead to accumulation of moisture between the ceiling and itself

Fiberglass Insulation Pros and Cons

Here’s the other side of the fiberglass vs spray foam insulation debate – The pros and cons of considering fiberglass instead of foam.

Pros

• Is less expensive than either type of spray foam insulation
• Both loose fill and batts are more easily installed by the homeowner than spray foam
• Can be installed over existing attic insulation to quickly increase the R-value there

Cons

• Provides a lower R-value per inch than spray foam
• Does not seal air leaks as well as SPF, especially in hard-to-reach areas

R-Values

We have mentioned R-values several times in this article. The R-value is a measure of how well the material resists the transfer of heat through itself.

The higher the number, the greater resistance to heat transfer. In other words, higher R equals better insulating ability. 

• Open Cell spray foam has an R-value of about 3.5 per inch of thickness.
• 2 lb. Closed Cell SPF has an R-value of about 6.5 per inch of thickness, while 3 lb. Closed Cell (High-density – usually used in commercial applications) will be about 7.0 per inch.
• Fiberglass batts and rolls have an R-value of about 3.5 per inch of thickness.
• Loose fiberglass insulation has an R-value of about 2.5 per inch of thickness.

Cost

Someone once said, “The bottom line is usually the ‘bottom line’.” The ‘bottom line’, of course, is a common way of referring to the total cost.

When thinking about the total cost of any insulating job, whether in a new house or an existing one, several factors should be considered.

When Fiberglass Makes Sense: For example, looking at the chart above, you can see that both fiberglass batts and open cell foam have about the same R-value per inch. This means that with equal thicknesses of both, they will both provide about the same amount of insulation, and consequently, the same savings on your energy bill. But since fiberglass is considerably less expensive than the foam, it might be your best choice, as long as it is in an area that will allow you to easily apply the fiberglass batts.

When Foam is Worth Considering: On the other hand, if you are building a new home and planning to live in it for many years, it might well serve you best to use closed cell foam wherever possible, since it will be a much better insulator and air sealer than the other options. Over the years, your savings in energy would probably more than pay for the additional cost of using this more expensive product.

Additionally, you might decide to use one type of insulation in one area of your house and a different type in another area. You might also choose to install insulation in the areas of your house that are readily accessible to you and hire a professional to insulate the more difficult areas. This is completely acceptable. The main thing is to use the products and the applications that work best for you and your house.