Two photos depicting Rockwool insulation and spray foam insulation in one frame with the caption "versus"

Because of the diversity of homes and buildings that require insulation, there are different forms of insulation that have been developed to fit those needs. With all those options, it can be extremely difficult to choose the type of insulation you want to use. Spray foam and Rockwool insulation are the two forms we’re looking at today, and each has its own pros and cons.

Both spray foam and Rockwool insulation have many benefits. Spray foam comes in two forms and is better at keeping moisture out. It also prevents any unwanted airflow. Rockwool comes in a variety of forms as well. It’s comparable to fiberglass batts, but denser, which helps to dampen sound.

To give you a better idea of which insulation to choose, we’ll talk about the differences between the two. By understanding these differences, you should be able to tell which type will work best for you by the end of this article. If you want to explore more types of insulation, check out our Ultimate Guide to Home Insulation.

Spray Foam

Spray foam is one of the more commonly used types of insulation because of its versatility. To understand why it is becoming such a popular choice for insulation, we need to explore its two forms: open and closed cell. The different chemical proportions of the two help to enhance different qualities that are desirable for different applications.

A man in full protective gear is sitting on the attic floor while he sprays spray foam insulation on the underside of the attic ceiling.
Spray foam installation requires specialized equipment.

Chemical Makeup

Both are made up of the same chemicals (but in different quantities) to create the desired effect. So, here’s a mini chemistry lesson to help explain open and closed cell spray foam.

Spray foam is a type of polyurethane. Basically, this means that an alcohol group (a polyol in this case) reacts with an isocyanate compound in a way that creates a solid form. The different ratio combinations will determine how flexible or rigid the spray foam will be.

Because it is a chemical compound, this means that if you decide to use spray foam insulation, it may be best to have a professional come and install it. This also means that it is going to be more expensive. However, it lasts for a long time before it needs to be replaced. You could install it yourself, but it is important to do so carefully. The materials that are used to form this insulation are carcinogenic, so be sure you have accurate information and equipment if you want to try the DIY route.

Open Cell Spray Foam

The combination of chemicals in open-cell spray foam have been mixed to allow for more flexibility. The expansion of the foam is more rapid, which allows for more air holes to be present. This is one of the advantages of open-cell. Not only is it light and pliable, but it will also move with the structure of the house. This helps to prevent damage that could happen to more rigid insulation.

Another benefit to using open-cell foam is that water can move through it. Because of its open structure, any water that may leak from the roof will be able to move through the insulation. Even though it allows for water movement, it doesn’t necessarily mean that mold will grow there. Because of its chemical makeup, it will be hard for mold to take hold and grow on this type of insulation.

Closed Cell Spray Foam

Closed-cell foam is made in a chemical combination that has little expansion. This creates a much denser form of foam. Because the cell structure is closer together, it works less effectively to dampen sound. It is much more expensive to install, partly because it is much more durable.

The cost is also higher because it takes much more of it to cover the area you want to insulate. However, it does limit the amount of air leakage just like open-cell. It is much better for smaller spaces because it won’t expand at an exponential rate. Closed-cell foam does not allow for water to move through it, and it also prevents mold growth. This is especially nice when you live somewhere that is quite humid.

Benefits Of Spray Foam Insulation

Overall, the biggest difference between open-cell and closed-cell insulation is how much each one expands. Either foam helps to block any airflow from outside. Both are durable and have some sound dampening properties.

Open-CellClosed-Cell
CompositionLight (Flexible and Pliable)Dense (Durable)
Expansion Rate30-60X From Liquid VolumeLittle To No Expansion
Sound DampeningMuch HigherSome
Air SealingHighly EffectiveHighly Effective
CostLower (Rapid Expansion)High (More Insulation Needed)

Rockwool

Rockwool, or mineral wool as it is also called, is not made from chemicals. Instead, it is a rock-based mineral. The name is actually quite accurate in this case! It is made from basalt rock (which is a volcanic rock) and recycled slag (a by-product of steel and copper). It is also known as fiber wool because the earth and metal are stretched until the resulting product looks like fiber.

True to its name, it acts similarly to wool. Rockwool does not wick or absorb water directly, but it helps to keep moisture away. If enough water builds up on the wool, the water will drain off the surface. However, Rockwool is vapor permeable, which means that it can get wet. If it does, it is important that it dries thoroughly. If it doesn’t, this will counteract the ability of the Rockwool to insulate the area properly. Once it is fully dried, it functions just as it did before.

Rockwool only comes in batt, board, and pipe forms. This means that it is extremely important to know the amount of space that you need to insulate. Once bought and used, you can’t return the extra material. Because of the way it is manufactured, it isn’t available in the form of a spray. (Cellulose insulation can be sprayed–read more about Spray Foam vs. Cellulose.)

a pair of hands with white gloves on them installing rockwool mineral wool insulation in between two studs in a wall

Benefits & Drawbacks Of Rockwool

One of the biggest benefits of using Rockwool is that it is three times denser than fiberglass batts. This means that it is highly effective at absorbing sound. This may seem contradictory since closed-cell foam does not absorb sound well. Because the properties that make up these two types of insulation are different, the absorption properties are also different. The Rockwool still has air pockets combined with the number of fibers in the bundle. However, it is important to remember that insulation is meant to go inside walls or attic spaces because of the dangerous chemicals that can enter your lungs.

Another benefit to Rockwool is that you can install it yourself. It is also less expensive than spray foam, but you may need to get more Rockwool as it breaks down. You can have it installed by a professional company, which can increase the chances of it lasting longer. The efficiency of Rockwool is rather low because heat can escape quickly.

Perhaps one of the biggest drawbacks to Rockwool is that it does not create an air barrier. That makes it much easier for cold air to come in and heat to escape. Rockwool is a much more effective option in hot climates. Because of its makeup, it can withstand extremely high temperatures. Because vapors can escape from Rockwool, this is also an effective type of insulation for humid climates. It can help to lower the temperature and to minimize the amount of moisture that may gather inside.

Comparison Breakdown

To make it easier to see the differences between these two types of insulation, we’ll compare their qualities directly to each other.

RockwoolSpray Foam
Locations for InstallationUnfinished Walls
Floors and Ceilings
Ducts in Unconditioned Spaces
Floors and Ceilings
Unvented Low-Slope Roofs
New Cavities
Unfinished Attic Floors
Effectiveness in Blocking AirNo Air Barrier WhatsoeverCreates a Tight Air Barrier
LongevityLimitedMore Permanent
Extreme Weather TemperaturesRapid Escape of Heat in Cold Temperatures
Effective in Hot and Humid Climates
Effective in Both Hot and Cold Climates
CostLow Initial CostHigh Initial Cost
Installation ProcessDo-it-Yourself; Anchoring of Insulation RequiredProfessional Installation;
Do-it-Yourself With Caution
Health HazardsCarcinogenic with Long Exposure Carcinogenic with Long Exposure

Location: Spray foam is more commonly used for floors and ceilings, not vented low-slope roofs, new cavities, and unfinished attic floors. Rockwool is used for floors and ceilings, unfinished walls, and ducts in unconditioned places. These different uses for location are important to look at since that will be your biggest determining factor.

Air: Spray foams (both open-cell and closed-cell) create an air barrier. It can be used on walls, floors, and ceiling cavities. It won’t compress, sag, or settle over time, which makes the maintenance process simple. Rockwool, on the other hand, does not create any type of air barrier.

Longevity: Spray foam is much more permanent. Closed-cell foam is much more likely to need replacement because it is not as flexible. However, the open-cell foam may need to be replaced if a lot of weight is placed on it. Rockwool has a limited lifespan; it will lose its insulating properties over time. This is determined largely by how it is installed.

Extreme Weather Temperatures: Because spray foam is such an effective sealant, it is effective for extreme weather temperatures. Rockwool is more effective in places where it is hotter. Heat escapes much more quickly—using it in places that are typically cold will not be effective at keeping you warm.

Cost: Spray foam is a lot more expensive. The amount that you have to pay is determined by each square foot of space you want to cover. It is important to remember that using closed-cell foam will require more insulation material than open-cell. If you’re unsure of how much it is going to expand, talk to a contractor.

Installation

Once you decide which type of insulation you think is best for you, here’s a quick run-through of how you can install them yourself. The most important thing to remember is to put safety first. If you are doing home improvements and you have children, you’ll want to make sure that they are out of the way.

Neither installation process will take much time if you are doing a small section. It is important to add that if you are not sure about how to install your insulation properly, you should contact a local contractor. It is much better to pay the amount of money to have it done properly than to spend more money trying to undo any mistakes you’ve made.

You can install both types of insulation on your own. You can get a spray foam kit from a local home improvement store. If you aren’t sure how to use the spray foam kit, the video below is a great step-by-step guide of how to do it.

This video does a great job of explaining how to do it properly and how to keep yourself safe in the process. Because you are dealing with chemicals, you want to make sure that you are wearing a mask that will keep you safe. As we mentioned above in the table, spray foam insulation does have carcinogenic chemicals in it. This means that you can get cancer from it if you aren’t properly protected and wearing a special mask.

You’ll also get your Rockwool from a local home improvement store. Just remember that you’ll need to anchor your Rockwool insulation as you install it. Again, because there are dangerous chemicals contained in this insulation, you’ll want to wear a mask and be safe in the way you proceed to install it. Below is another good video on how to install Rockwool insulation.

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One Comment

  1. Can I do a thin layer of Rockwool and then open cell foam on top?

    I live in New Orleans and have a sloped roof that is gutted inside after hurricane Ida. I have the chance to replace my fiberglass for something more noise dampening.

    I’m leaning to open cell foam but seems that won’t help noise. Could I do both spray and wool?

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