Fiberglass Insulation vs. Spray Foam: Which Is Better?

Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.

When considering an insulation project, settling on the material to use often takes up a good part of the decision-making process. Fiberglass and spray foam are two of the most common types of insulation used by many homeowners today. But which one is better?

Spray foam insulation offers the best type of insulation, with R-values reaching up to 7.0 in most cases. However, its cost and installation process make fiberglass more attractive for many homeowners. Fiberglass is easier to install and provides excellent insulation in relation to the cost.

This article will cover everything you need to know about fiberglass and spray foam insulation, including which of them you should use for specific use cases.

What Is Fiberglass Insulation Material?

The fiberglass insulation material is made from recycled glass and other raw materials, melted down, and processed into fibers. The fiberglass fibers are typically held together with an adhesive backing like aluminum foil or paper, which works as a vapor barrier. This practical and affordable insulation solution makes it the go-to option for 85% of homeowners.

Fiberglass insulation comes in rolls, pre-cut batts, or loose-fill variants. Batt insulation is the most popular and is easily distinguished by its large blanket-like appearance, while loose fill is less prevalent. Below is a closer look at the two types.

Batt Fiberglass Insulation

This is a popular choice among homeowners because it is light enough to be picked up at the store and carried up a flight of stairs. It is also large enough to cover more ground quickly. Once you unroll the batt insulation, you can easily cover a lot of space. An additional perk is that they are typically designed to have a similar size to the standard spacing studs you’ll find on attic walls.

However, fiberglass batt has to be cut to an exact size in attics with problematic obstructions on the wall or the ground and rooms with irregular spacing. If you cut the batt too short, you’ll leave a lot of gaps in the coverage, making it easy for air to flow through. On the other hand, leaving it too long will lead to irregular compression. Both scenarios will defeat the purpose of insulation in the first place.

Also, the extremely light nature of fiberglass means that it is tough to customize batt. The process also can’t be done haphazardly as the correction can be costly. This is why most homeowners choose to call in professional installers to set up the batt, especially if the attic has irregular space between joists and many obstructions.

By avoiding mistakes, you’ll save money during the installation and ensure you get the benefits of proper insulation.

Loose-Fill Fiberglass Insulation

This type of fiberglass material comes in large chunks packaged in bags. You’ll need a professional installer’s blowing machine to install this type of insulation material. You can either hire a professional to complete the job or pay for the machine if you can do the job. The machine is essential for blowing the fiberglass onto the areas where it’s needed.

This fiberglass type is most useful if you live in an older building with some insulation in place and many obstructions. It’s also helpful if you want to fill out tight corners containing pipes and wiring – which batt insulation typically can’t cover.

This type of fiberglass is also a good option for attics with a low ceiling and a tight space because the blowing machine used for the job doesn’t require lots of maneuvering around the room during the installation process.

What Is Spray Foam Insulation Material?

This is a chemical-based insulation material generally mixed on-site by the contractors. The foam created by the mixture is applied with a sprayer. Spray foam can be produced as an open or closed-cell variant:

Open-cell spray foam contains cells that are not completely enveloped, making the product soft and flexible.
The cells in closed-cell spray foam are (as you’ve guessed) completely enveloped. The cells press up tightly to cut off the passage of air and moisture. The chemical makeup of this variant of spray foam means it’s more stable, rigid, and dense. It’s also invariably more expensive to use.

Between both types of spray foam, closed-cell spray foam is the better insulator. The moisture-resistant nature of this product means it’s a very strong barrier against water damage. However, open-cell spray foam isn’t without its benefits.

Apart from being cheaper, open-cell spray foam is a good option for attics. Due to its expanding ability, you can count on it to reach all corners of the space. Also, when one application of foam is required to fill up the space between studs, you can count on open-cell spray foam to do a better job – especially if you’ve got soundproofing in mind.

Fiberglass Insulation vs. Spray Foam: What’s the Difference?

As we’ve touched on briefly, fiberglass insulation can be installed easily with very little training. Most DIY enthusiasts complete the process on their own. If you’d like to give it a shot, you need to wear protective material such as gloves, long sleeves, protective eyewear, and face masks.

The installation process for fiberglass also doesn’t require any curing or drying time, and other projects in the building can continue safely without any health risks to the workers.

On the other hand, spray foam insulation requires professionalism and precision. It can’t be done after reading a few DIY tutorials on the subject. As you’ve seen earlier, it involves mixing two hazardous chemicals on site.

Spray foam installers typically undergo extensive training under programs through the. Spray Polyurethane Foam Alliance (SPFA) or Occupational Safety and Health Administration (OSHA). They also wear more extensive protection, including air respirators and anti-chemical clothing.

You will have to move away during spray foam application for a while. Other artisans can’t work at the same time either. The downtime required could be up to 48 hours as you have to wait for the spray to cure and for the gas to dissipate.

Another reason why spray foam is dangerous is that there’s a risk of fire from spontaneous combustion if it isn’t applied correctly. The gas from the chemical reaction may also cause health issues long after application if it isn’t properly mixed and applied.

So, fiberglass wins when it comes to easy installation. Installing batts is easy in spaces that are wide open with no obstructions on the ground and the wall. Even in scenarios where loose-fill fiberglass has to be used—such as in spaces with old insulation or many obstructions—the application process with a blowing machine is straightforward and inexpensive.

Spray foam is only an option when you can afford to hire qualified professionals to guarantee that you won’t expose yourself or your family to serious health risks from incorrect spray foam use.

Energy Efficiency

The makeup of fiberglass means it’s almost impossible to achieve 100% insulation. Around 30% of your cool air or heating will generally escape from the joints where fiberglass is installed – even in professional installations. Poor installations increase the percentage of heat or cool air escaping.

On the other hand, going with spray foam as the insulation material ensures better space-filling. Closed-cell spray foam insulations are especially air-tight and more energy-efficient than fiberglass. Therefore, the R-value (discussed below) is a lot higher with spray foam.

R-Value

To choose the best type of insulation to go with for any space, you need a good understanding of the R-value. It’s the metric used to rate the effectiveness of an insulator in terms of airflow resistance. A high R-value means that the material is better for insulation.

Spray foam has a higher R-value than fiberglass, with the former scoring as high as 7.0. Part of the reason for that high figure is that spray foam doesn’t settle or sag after a while, as you’d see with most DIY fiberglass insulations.

This doesn’t mean that fiberglass isn’t great at blocking airflow. Most high-density and medium fiberglass have R-values that are as high as 4.3. So, the material packs a decent punch as well in this regard. These numbers lose significance a bit when you consider the fact that you can stack up many sheets of batt together for more insulation, while the number of spray foam applications you can use is limited.

So, stacking up a few sheets of batt insulation together can get you closer to the R-value you’d get with a spray foam setup or even better.

Moisture, Fire, and Sound Resistance

When it comes to moisture resistance, spray foam is the better performer of the two insulation types. You can count on spray foam to resist moisture fairly effectively. While it’s not common to find mold and mildew growing on batt insulation, it’s a good idea to check every few years for moisture and replace it if necessary.

Both materials are generally non-combustible except in specific situations. If you’d like to make them completely fire-resistant, you’ll need to look at a few special applications. Talk to your installer about the proper treatment for your particular case. For spray foam, they often recommend applying intumescent paint to increase the fire resistance of the insulation by up to half an hour.

For soundproofing, both fiberglass and spray foam can do a fantastic job keeping the noise out (or in). However, many professionals will encourage fiberglass insulation if soundproofing is very important to you (more on this later).

Durability and Longevity

Fiberglass and spray foam insulation are durable and can last long. If the conditions are ideal, you can count on fiberglass insulation to last up to five decades or more. Spray foam can also post similar longevity numbers.

Over time, however, both of them may gradually lose efficacy. Spray foam will shrink as the decades pass, while fiberglass can sag or settle, depending on the initial installation. The durability and longevity scales will tip significantly in favor of spray foam if there’s a lot of humidity in the space.

You may need to replace your fiberglass installation after every decade to fight the effect of humidity on it. Spray foam, on the other hand, can hold up against moisture a lot better. This makes it a more attractive option for people looking for something close to one-time installation in a humid environment.

Cost-Effectiveness

When choosing the insulation materials to go for, the cost-effectiveness should be high on your list – probably only below the R-value. Which of the materials will deliver the best value for money? Spray foam delivers excellent insulation, but it is expensive. Fiberglass can come close to the level of insulation provided by spray foam with proper installation, and it’s a lot more affordable.

Both will also save you lots of money over time by reducing your energy bill. The exact amount of money saved will come down to the air-tightness of the installation. You can expect both of them to slash your utility bill by up to 40% per year.

Generally, the cost of using spray foam for insulation will be around three times what you’ll spend on fiberglass. The latter will set you back around $0.40 for a square foot, while the former will cost up to $4 for the same dimensions, with a thickness of around 3 inches (7.6 cm). When you add the installation costs, the expenses for spray foam insulation will increase even higher due to the difficulty of the overall process.

Still, you have to look beyond the raw numbers. If you need to stack multiple fiberglass batts across the same surface to achieve near air-tight insulation, the line between the costs gets blurred a bit.

Also, spray foam installation requires almost no maintenance and will pay for itself within a few years through utility bill savings due to the 100% air-tightness. Fiberglass will also pay for itself within a similar time range due to the lower installation costs. Still, you may need to spend some more money in the future to maintain the insulation – further eroding any savings made on choosing it instead of spray foam.

Health Effects and Risks

As we mentioned, spray foam is made from chemicals that can be dangerous when improperly handled. The chemicals contain compounds known as isocyanates, which can cause severe irritation to the stomach, lungs, and eyes. Skin contact with the foam may cause severe inflammation and rashes. This is why spray foam can’t be applied without adequate personal protective equipment.

Excessive exposure to isocyanates can lead to asthma attacks in people exposed to spray foam in the future. Prolonged respiratory irritation from exposure to isocyanates can eventually lead to chemical bronchitis. When the spray foam application is properly cured, it becomes non-toxic and inert.

However, while curing, it emits a gas that can cause blurred vision and respiratory discomfort. It can also continue to emit this gas permanently after it’s cured if the foam components are not mixed in the right proportions – another reason a professional should handle spray foam insulation.

Fiberglass also poses some health risks. It contains glass wool fibers that are believed to be carcinogenic. Some of the products contain warnings on how inhalation of the fiber during installation is a cancer hazard. Fiberglass can also irritate your respiratory system, skin, nose, throat, and eyes. It can also cause dyspnea.

However, all the health risks associated with it are connected to the handling of the product during the installation phase. There are no known health risks associated with fiberglass once the installation is complete.

Which One Should You Use?

To understand which insulation material you should go for between these two, we’ll look at different applications and use cases:

Around Windows

Spray foam is often the better insulation material to go with around windows. Fiberglass can only work if the cavity is air-sealed tightly on all sides of the windows. This is achievable but requires a lot of work, making the fiberglass installation much more expensive.

On the other hand, spray foam works as insulation and air barrier as well, without any other special work required. This makes it a better option to go with for insulation around your windows.

Fiberglass only works around windows if the cavity is wholly air-sealed on all sides. While that is possible, it’s often difficult to accomplish and maintain. On the other hand, spray foam insulation itself is an air barrier and an insulator.

In Basements

Fiberglass can work in basements, but you need to exercise some caution with it. You shouldn’t apply it directly to a concrete wall without leaving some space between the insulation and the wall. Failing to do this, mold and mildew will likely form around the insulation. You should keep in mind that fiberglass doesn’t resist moisture. This is why it’s not recommended for use in your basement.

Some contractors will recommend hybrid insulation made of batt fiberglass and closed-cell spray foam to reduce the risk of moisture-related problems. This approach will require installing a wall inside the foam insulation, with the fiberglass batts installed inside the stud bays. This is a bit more expensive and will take longer, but it ensures you won’t apply fiberglass directly to concrete.

However, you should go with closed-cell spray foam for complete moisture-free insulation for your basement. Closed-cell spray foam works well in basements because it has a higher R-value. Most importantly, it offers a level of moisture resistance that fiberglass can’t provide – without needing a very thick application.

This option fills gaps in the spaces properly, giving you the best possible protection against air and moisture infiltration. It is more expensive (as we’ve discussed earlier), but the cost pays for itself when considering the energy efficiency and the protection from mold and mildew – which can cause severe damage.

For Soundproofing

When soundproofing a wall or ceiling, it’s always best to introduce absorption in the hollow cavities. Hollow or sealed cavities will trigger a resonance you don’t want. This is why spray foam insulation isn’t a good option here – contrary to what many people might think.

Fiberglass is a good option for dampening the air cavity resonance. It’s as effective as cotton, mineral fiber, polyester, and cellulose. These materials are considered airy and open enough to interact appropriately with the sound waves in a ceiling cavity.

Spray foam doesn’t have the same properties. It’s not open enough. An additional negative with spray foam is that it can couple both sides of a floor or wall. This makes surface vibration conduction easier.

If you must use spray foam for insulation, it’s best to use open-cell spray foam instead of closed-cell spray foam. However, the results will not be close to the performance of fiberglass.

In Crawl Spaces

Closed-cell spray foam beats fiberglass when it comes to crawl-space insulation. Using fiberglass batt insulation between floor joists when insulating crawl spaces in your home delivers subpar results for some of the same reasons we’ve covered above for this material.

Firstly, fiberglass isn’t effective at ensuring an air-tight seal. This means that hot or cold air from the crawl space will get through the insulation easily and into your living areas through any spaces in the overall floor setup.

Secondly, fiberglass can absorb moisture, which makes it compress easily and detach from between joists. This doesn’t just make for ineffective insulation, as the crawl space will also become riddled with material scrapes and generally become a messy surface supporting mold growth.

On the other hand, closed-cell spray foam effectively stops air leakage in crawl spaces while also delivering a very high level of insulation.

Since it can expand to fill gaps and cracks, spray foam is effective for air-sealing and insulation of the rim joist, stretching around your house over the top of the foundation. This area is crucial because it’s where the most energy-wasting air leakage occurs in your crawl space.

Closed-cell spray foam insulation deals with the problem. It guarantees comfort and better energy efficiency. Additionally, you won’t have to worry about the insulation moving, losing its insulation value, or compressing over time as you’d get from fiberglass batts.

Open-cell spray foam is not advisable for crawl spaces because it doesn’t create a vapor barrier during installation. Also, it only has an R-value of 3.7 per inch compared to the 6 per inch delivered by closed-cell spray foam.

Factors to Consider When Choosing Between Spray Foam and Fiberglass for Insulation

We’ve seen how each material holds up in the comparisons above, but what factors should you consider for your final decision?

The Condition of the Space

If you have a lot of space to cover and the surface area isn’t too complicated, you may be better served by choosing fiberglass. Of course, this excludes crawl spaces and other similar use cases where spray foam is always the better alternative. For insulating the walls of living areas, fiberglass will almost always deliver decent results.

The Climate

If your climate gets very humid occasionally, spray foam will withstand the excess moisture much more than fiberglass can handle. However, if you don’t have to worry about excess moisture, you may get better value with fiberglass.

Your Overall Budget

How much are you willing to spend on insulation? This is a subjective point, so you should consider the insulation’s overall cost and compare it with the amount of money you’re willing to spend on the project. Don’t forget to factor in installation fees and any other knock-on renovations you may have to engage in due to the insulation.

You should also consider factors like how much you’ll save by choosing one form of insulation over the other regarding energy bill reductions. If you’re thinking about insulating a space that doesn’t add to your heating or cooling bill, like the basement, you’ll need to spend a bit more time with your calculations on cost-effectiveness.

Ownership of the Property

If you are insulating a property you own, you may be more willing to spend money on the costlier but more robust insulation material, that is, closed-cell spray foam, where necessary.

However, if you’re renting the space, that investment may be too much – especially if you may move within a couple of years. Also, your landlord may not be open to the idea of any large-scale insulation projects.

Conclusion

Based on what we’ve discussed so far, it’s clear that spray foam delivers the best results for insulation when you’re only considering R-values. However, making the best decision on the insulation choice goes beyond picking the highest R-value you can find. Review the various talking points we’ve covered and the selection criteria to make the best decision possible. Spray foam will win most duels on paper, but fiberglass is no pushover – especially for use cases where spray foam is overkill or impractical. Fiberglass will also win the cost-effectiveness battle for most average homeowners.