Spray Foam Insulation vs. Fiberglass Insulation

What's the difference between spray foam and fiberglass insulation? Is one insulation more affordable than the other? Can spray foam and fiberglass be installed in the same places? Which insulation will deliver the best performance for your project?

South Central Services has performed hundreds of insulation installs, including both spray foam and fiberglass. We work in Greencastle, Pennsylvania, and in many surrounding towns. Insulation and how it supports the comfort and longevity of a building is our bread and butter.

Every day, we help customers determine which insulation best fits the needs of their project. This article will answer all of your questions about the differences between spray foam insulation and fiberglass.

By the time you finish this article, you’ll know:

• What spray foam insulation and fiberglass insulation are
• Why spray foam is expensive, and fiberglass is not
• What features spray foam and fiberglass offer
• Which type of insulation is best for your needs

What Are Spray Foam And Fiberglass Made Of?

Spray foam and fiberglass are composed of different materials. The clues to their materials are in their names.

Spray Foam Insulation Is A Polyurethane Plastic.

Spray polyurethane foam (SPF) is a two-component polyurethane foam insulation. Two chemical components are heated, mixed, and sprayed.

Their chemical reaction creates a plastic foam that sticks to whatever surface is sprayed.

Fiberglass Insulation Is Made Of Glass Fibers.

Fiberglass insulation is composed of fine glass fibers. It's most commonly sold in batts, but is also available in blankets and loose fill.

If you’ve ever seen what looks like pink cotton candy in someone’s attic or wall, you’ve seen fiberglass insulation.

How Do Spray Foam And Fiberglass Work?

There are two main ways to measure the usefulness of insulation: R-value and air-sealing. Let's examine these in more detail.

Both Fiberglass And Spray Foam Have High R-Values.

When temperature changes happen outside, excellent insulation also prevents those changes from happening inside. Materials that do not conduct heat well generally make for effective insulation.

What spray foam and fiberglass have in common can be measured by R-value or resistance value. R-value measures how well materials resist the thermal transfer of heat. The higher the R-value, the better the material can resist conductive heat transfer.

Both fiberglass and spray foam are made of materials that don’t conduct heat well. Approximately the R-values of these insulations are as follows.

• Fiberglass batts: R-4.3 per inch
• Closed cell spray foam: R-7 per inch
• Open cell spray foam: R-3.7 per inch

Only Spray Foam Can Create An Airtight Seal.

Spray foam insulation offers an airtight seal. Rather than slowing air and its heat as it travels through, spray foam blocks the air altogether. This prevents air leakage and any heat from traveling through convection or radiation.

Fiberglass has countless tiny air pockets that slow heat transfer but do not keep out air. Fiberglass insulation allows air to pass through. That air leakage allows heat to leak as well.

Where Can Spray Foam And Fiberglass Be Installed?

Spray foam and fiberglass insulation have very different application options. While spray foam is a rather versatile insulation, fiberglass has only a few places where it performs reliably.

Spray Foam Insulation Can Be Installed Anywhere.

There are two kinds of spray foam: open cell and closed cell. Between these two variations, spray foam can be installed anywhere in a building. 

Open cell spray foam is the less versatile of the two types of spray foam. Open cell foam can be installed in above-grade exterior walls and attics. Open cell is also helpful in open cavities and as a sound barrier between floors.

Closed cell spray foam is the most versatile type of spray foam. Closed cell offers a built-in vapor barrier, which makes it effective in below-grade applications.

Closed cell foam can be installed anywhere open cell can be installed, as well as in below-grade areas like basements, crawl spaces, and underslab.

Fiberglass Insulation Is Best For Interior Walls.

Fiberglass insulation can be installed anywhere that open cell foam can be installed. Interior walls are an excellent use for fiberglass batt insulation. There is no need for an airtight seal in your interior walls, so fiberglass can perform well as a sound dampening insulation.

Blown-in fiberglass is another variation of fiberglass. Blown-in is most commonly used for insulation on an attic floor. Fiberglass batts are also installed in attics.

Like open cell foam, fiberglass does not have a vapor barrier, so it should not be installed where it can absorb moisture. Despite this, many homes have fiberglass batts installed in the crawl space ceiling.

Since fiberglass has many open air pockets, it can collect moisture and allergens as air passes through. Fiberglass performs poorly in below-grade areas like basements and crawl spaces.

What Features Do Spray Foam And Fiberglass Insulation Offer?

Fiberglass and spray foam are made of different materials and are best used in different areas of a building. However, both are still insulation products. Let’s compare spray foam and fiberglass by the following features.

1. Fire resistance
2. Mold resistance
3. Ease of installation
4. Product lifetime

1. Both Spray Foam And Fiberglass Have Class 1 Fire Ratings.

While spray foam and fiberglass are made of different materials, both insulations have the fire resistance to meet building codes.

Spray foam is a combustible material, but it still has a Class 1 fire rating. Flammable materials can receive a Class 1 rating if fire testing indicates low smoke development and flame spread. Other building materials that are technically combustible but meet building codes include wood and cellulose.

To prevent smoke development and flame spread, spray foam insulation has fire retardant added to its formula. This retardant allows spray foam to not continue burning if the fire source disappears.

As a noncombustible material, fiberglass insulation has an automatic Class 1 fire rating. Noncombustible materials do not burn if exposed to fire. Fiberglass, stone, and concrete are all examples of noncombustible building materials.

Some fiberglass batts do have combustible paper facings. In the case of a fire, fiberglass insulation facings may burn, but the fiberglass itself will not burn.

2. Both Spray Foam And Fiberglass Are Mold Resistant When Installed Correctly.

Insulation is not supposed to grow mold. Mold growth on insulation indicates either a faulty installation or other problems going on in a home. As long as these insulations are installed with their properties in mind, mold growth should not be a concern.

Closed cell spray foam has a built-in vapor barrier, making it moisture- and mold-resistant. Moisture cannot pass through closed cell spray foam. Because of its vapor barrier, closed cell can be used successfully even in damp locations in a home, like the basement and crawl space.

Open cell spray foam does not have a built-in vapor barrier. Installers can add additional coatings to open cell foam after installation. However, even with coatings, open cell foam is not approved for installation in places like basements and crawl spaces. This would be a faulty application and could result in mold growth on the insulation or its substrate.

As for fiberglass, the insulation is mold-resistant on paper. Fiberglass insulation is made from mold-resistant materials like plastic and glass. However, in real-world applications, fiberglass insulation can grow mold when installed properly. An example would be an interior wall in a kitchen or bathroom. These rooms are exposed to more moisture than most rooms in a home, which means the insulation is at a greater risk of facilitating mold growth.

3. Only Fiberglass Insulation Can Be Installed As A DIY.

If you search online for DIY information for fiberglass or spray foam, you will find information for both. Fiberglass is commonly installed in attics by homeowners, whether in batt or blown-in form. DIY spray foam kits are also available to purchase.

However, only spray foam insulation is created through a complex chemical reaction. Installing fiberglass on your own has significantly fewer risks than attempting spray foam as a DIY.

Spray foam is an insulation that requires professional contractors to install. These professionals monitor and control variables like temperature, pressure, and ratio. Spray foam installed by a qualified professional should be stable and on-ratio. Hiring a professional also protects you from off-gassing fumes, which contractors protect themselves from with respirators.

Fiberglass can be installed by homeowners or professional contractors. While fiberglass requires some precision to install correctly, homeowners can save money by handling fiberglass insulation themselves.

However, you must wear personal protective equipment to install fiberglass insulation. Safety glasses, gloves, and masks are necessary to protect you during installation. Fiberglass is made of tiny fibers of glass, which can become lodged in your skin, lungs, and eyes.

4. Spray Foam Insulation Should Last Longer Than Fiberglass.

The lifetime of undamaged fiberglass insulation is between 80 and 100 years. However, fiberglass insulation may require upkeep or replacement. For example, blown-in fiberglass installed in an attic will settle over time and need to be topped off.

If fiberglass does experience mold or water damage, it should be removed and replaced. Otherwise, it will cause damage to the home. Even without moisture concerns, fiberglass batts sag over time. Any gaps in the insulation can compromise its performance.

Gaps in interior walls insulation are less concerning than exterior wall insulation. However, fiberglass insulation performance is likely to deteriorate after 15 to 20 years.

As for spray foam insulation, opinions on the product lifetime vary. Spray foam insulation should last for the lifetime of the building. Unlike batt and blown-in insulation, spray foam does not sag, settle, or create gaps. Also, plastic foam insulation will not break down unless exposed to UV rays.

For internal applications, spray foam will keep insulating consistently without any upkeep. For exterior applications, contractors can extend the foam's lifetime by adding coatings to protect the foam from UV rays.

How Much Do Fiberglass And Spray Foam Insulation Cost?

One of the biggest differences between fiberglass and spray foam insulation is price. Spray foam is known as a premium insulation product with a price tag to match. On the other hand, fiberglass is called a budget insulation. Both products deliver the performance you pay for.

Spray Foam Insulation Is Expensive.

Since spray foam is created through a chemical reaction on-site, it tends to be more expensive than most other insulation products. There are a number of reasons for the high cost, but equipment and installation complexity are definitely among them.

Most spray foam contractor calculate cost by board foot, instead of cost per square foot. We have many guides that walk through how to estimate the cost of spray foam, but you can expect a cost between $4 to $9 per square foot.

At South Central Services, we charge the following for spray foam insulation.

• Closed cell spray foam: $1.15 to $1.75 per board foot
• Open cell spray foam: $0.25 to $0.75 per board foot

Fiberglass Insulation Is Inexpensive.

Fiberglass insulation is one of the least costly insulation products. Fiberglass is not as expensive to manufacture as spray foam insulation is. As far as performance, fiberglass also does not offer as many features and benefits and spray foam.

You can expect to pay between $1 to $4 per square foot for fiberglass insulation.

The Bottom Line About Fiberglass And Spray Foam Insulation

Depending on what your project is, either fiberglass or spray foam could be an effective insulation option. Spray foam insulation has more performance and features to offer, but comes at a steeper cost. Fiberglass insulation is much less expensive than spray foam, but it isn't effective in many building applications.

Now that you know the differences between these two insulations, your next step is to:

• Compare open cell and closed cell spray foam in more detail
• Compare blown-in fiberglass and fiberglass batt insulation
• Estimate the cost of spray foam insulation for your project
• Discover how spray foam insulation can save money over time