You know your attic needs insulation.

The harder question is figuring out where the insulation should go and which material makes the most sense for your home.

Should you add insulation to the attic floor? Would insulating the attic roof be better? Can you simply add more insulation over what is already there?

The best way to insulate an attic depends on how the space is designed, whether it is vented, the condition of the existing insulation, and what you want to accomplish. Your budget also plays a significant role.

RetroFoam of Michigan has insulated thousands of homes and attics across Michigan's Lower Peninsula. In this article, we will explain how to properly insulate an attic using blown-in cellulose, fiberglass, or spray foam insulation. We will also look at where each material is installed and how to choose the best option for your home.

Key Points: How to Insulate an Attic

 • A vented attic is typically insulated along the attic floor while maintaining airflow through the space.
• Spray foam is commonly installed along the attic roof deck to create an unvented, conditioned attic, or on the attic flat depending on how the space will be used.
• Blown-in cellulose can be a budget-friendly way to improve the insulation on an attic floor.
• Blown-in insulation slows heat transfer, but it does not create the same air seal as spray foam.
• Existing insulation should be inspected for moisture, mold, pests, damage, and air leakage before more material is added.
• You should not insulate the attic floor and roof deck without first determining how the attic’s ventilation and building envelope will be managed.
• The best attic insulation depends on your home, comfort goals, budget, and the condition of the attic.

How Do You Insulate an Attic?

There are two primary ways to insulate an attic.

The first is to install insulation on the attic floor. This separates the living space below from the vented attic above.

The second is to apply insulation to the underside of the attic roof deck. This moves the home's thermal and air-control boundaries to the roofline, bringing the attic into the conditioned building envelope.

Choosing between the attic floor and roof deck is just as important as choosing the insulation material. The two approaches create very different attic systems.

Insulating the Attic Floor

In a traditional vented attic, insulation is installed between and over the attic floor joists.

The goal is to slow heat transfer between the living space and the attic while allowing outside air to move through the attic as intended.

Blown-in cellulose and loose-fill fiberglass are commonly used as insulation for an attic floor. Fiberglass batts may also be installed between the joists.

When insulating an attic floor, ventilation pathways should remain open. Soffit vents should not be buried or blocked by insulation, and baffles may be needed to keep air moving from the soffits toward the upper roof vents.

Air leaks between the home and attic should also be addressed before new blown-in insulation is installed. Common leakage points can include:

• Plumbing and electrical penetrations
• Attic access hatches
• Recessed lights
• Bathroom exhaust fan openings
• Chimney and flue penetrations
• Interior wall top plates

Blown-in attic insulation can cover these areas, but covering a gap is not the same as sealing it. The material slows conductive heat transfer, but air can still move through or around it.

If you want to create an air seal, then spray foam insulation on the attic flat is also an option. This will stop air leaks coming in through the attic from reach other areas of your home.

Insulating the Attic Roof Deck

Insulation can also be installed along the underside of the roof deck.

Spray foam is often used for this approach because it can insulate and create an air seal at the roofline.

Insulating the attic roof deck generally creates an unvented attic assembly. Because the attic becomes part of the home’s conditioned envelope, existing roof ventilation must be evaluated and managed as part of the project.

This approach can be especially helpful when HVAC equipment or ductwork is located in the attic. Keeping that equipment inside the insulated envelope may reduce its exposure to Michigan’s temperature extremes.

However, insulating an attic roof is not simply a matter of spraying foam between the rafters. The roof assembly, moisture management, ventilation strategy, combustion appliances, and local building requirements must all be considered.

Another option for the roof deck is fiberglass batts. While it won't create an air seal, it will slow down heat transfer.

Can You Add Insulation to an Existing Attic?

In some cases, you can add insulation to an attic without removing all the existing material.

This is commonly called topping off the attic insulation.

Adding blown-in cellulose over existing insulation may be an affordable way to increase thermal resistance when the current material is dry, clean, and in reasonably good condition.

Before adding insulation, the attic should be inspected for:

• Water leaks or staining
• Mold or mildew
• Pest activity
• Compressed or disturbed insulation
• Blocked soffit vents
• Unsafe electrical conditions
• Exhaust fans venting into the attic
• Unsealed air leaks
• Insufficient clearance around heat-producing fixtures
• Damaged or disconnected ductwork

Adding more material will not correct an underlying moisture, ventilation, or air leakage problem. In fact, it can make those problems harder to find later.

If the old insulation is wet, contaminated, severely damaged, or preventing the new system from being installed correctly, removal may be recommended.

How to Insulate an Attic with Blown-In Cellulose

Blown-in cellulose is made primarily from recycled paper fibers treated for fire and pest resistance.

It is installed using a machine that breaks apart the compressed material and blows it through a hose.

For an attic floor, the cellulose is distributed across the space until the planned depth and coverage are reached. Depth markers can be installed throughout the attic to help confirm that the material is applied evenly.

Benefits of Blown-In Cellulose Attic Insulation

Blown-in cellulose may be a good fit for homeowners who want to improve their attic insulation without making the larger investment required for spray foam.

Benefits can include:

• Lower upfront cost than spray foam
• Good coverage across attic floors
• The ability to cover irregular areas and existing insulation
• Improved resistance to heat transfer
• A relatively quick professional installation
• Use of recycled materials

Because cellulose is blown into place, it can cover attic floor joists more evenly than batts that are only placed between them.

Limitations of Blown-In Cellulose

Blown-in cellulose is not an air-sealing material.

It can reduce heat transfer, but it does not stop uncontrolled air movement the way properly installed spray foam can.

Cellulose can also settle over time, potentially reducing its installed depth. If it becomes wet because of a roof leak or condensation problem, its performance may be affected, and the damaged material may need to be removed.

Loose fibers and dust can also be generated during installation. The work area should be properly contained, and installers should use appropriate protective equipment.

How Blown-In Cellulose Is Installed

A professional blown-in cellulose installation generally includes:

1. Inspecting the attic and existing insulation
2. Identifying roof leaks, moisture, pests, and safety concerns
3. Sealing accessible air leaks when included in the project
4. Installing or adjusting ventilation baffles
5. Protecting attic accesses and required clearances
6. Placing depth markers throughout the attic
7. Blowing cellulose evenly across the attic floor
8. Confirming the final depth and coverage

Some homeowners rent equipment and install cellulose themselves. However, reaching the correct depth without blocking ventilation or creating unsafe conditions can be more complicated than it appears.

How to Insulate an Attic with Fiberglass

Fiberglass insulation is made from fine glass fibers and is available as batts, rolls, and loose-fill material.

Fiberglass batts are installed between attic floor joists. Loose-fill fiberglass is blown across the attic floor using equipment similar to that used for cellulose.

Benefits of Fiberglass Attic Insulation

Fiberglass remains a common choice for house attic insulation because it is widely available and can have a relatively low upfront cost.

Potential benefits include:

• Availability at most home improvement stores
• Options for DIY installation
• Batts in standard framing sizes
• Loose-fill products for broader attic coverage
• Resistance to absorbing water into the glass fibers themselves

Limitations of Fiberglass

Fiberglass does not create an air seal.

Air can move through and around the material, especially when batts are poorly fitted or gaps are left around framing and penetrations.

Batts must be cut carefully to fit around wiring, pipes, and other obstructions. Compression, gaps, and misalignment can reduce their effectiveness.

Fiberglass fibers can also irritate the skin, eyes, and respiratory system during installation. Gloves, long sleeves, eye protection, and appropriate respiratory protection should be used according to the product manufacturer’s instructions.

Although the glass fibers are not a food source for mold, dust, organic debris, and facing materials on or around the insulation can support mold growth when moisture is present.

How Fiberglass Is Installed in an Attic

Fiberglass batts are fitted between attic floor joists without being compressed. Any vapor-retarder facing must be positioned according to the attic assembly, climate, local requirements, and manufacturer instructions.

Loose-fill fiberglass is blown across the attic floor to a specified depth. As with cellulose, soffit vents and other ventilation pathways must remain open.

Air sealing should be completed separately before the fiberglass is installed.

How to Insulate an Attic with Spray Foam

Spray polyurethane foam is applied as a liquid that expands and adheres to the surface where it is installed.

Open cell spray foam is commonly used in residential attic applications, but closed cell spray foam is also an option in some instances.

Foam insulation in an attic can be applied to the roof deck or, in some situations, the attic floor. The correct location depends on how the attic will be ventilated and where the home’s building envelope needs to be established.

Benefits of Attic Spray Foam Insulation

The biggest difference between spray foam and blown-in insulation is air sealing.

Properly installed spray foam can create a continuous air barrier that limits uncontrolled air movement through the attic assembly. This can help improve comfort and reduce the workload placed on the heating and cooling system.

Potential benefits include:

• Insulation and air sealing in one application
• Coverage around irregular framing and small gaps
• Reduced uncontrolled air movement
• The option to bring the attic into the conditioned envelope
• Better protection for attic HVAC equipment and ductwork
• Long-lasting performance when properly installed

Limitations of Attic Spray Foam

Spray foam has a higher upfront cost than blown-in cellulose or fiberglass.

It must also be installed by trained professionals using specialized equipment.

Existing insulation on the attic floor may need to be removed when foam is installed along the roof deck. Removing that material helps connect the attic to the conditioned space below and allows the installer to inspect the attic floor.

The attic and roof assembly must be evaluated before installation. Ventilation, moisture control, roof leaks, combustion appliances, and required fire-protection measures can all influence the project.

Homeowners should also ask which spray foam product will be used, how the attic will be ventilated or conditioned, and what reentry or reoccupancy instructions must be followed after installation.

How Spray Foam Is Installed in an Attic

While every project is different, the process may include:

1. Inspecting the attic, roof deck, ventilation, and mechanical equipment
2. Determining whether the attic floor or roof deck should be insulated
3. Removing old insulation when required
4. Preparing and protecting the work area
5. Addressing ventilation openings based on the planned attic assembly
6. Applying spray foam to the specified thickness
7. Inspecting the foam for consistent coverage
8. Installing required protective coatings or barriers when applicable

Spray foam should not be treated as a DIY attic insulation project. Proper chemical ratios, temperatures, thickness, ventilation, and application techniques are essential to its performance.

What Is the Best Way to Insulate an Attic?

There is no single insulation material that is best for every attic.

The best way to insulate your attic depends on what you want the project to accomplish.

Blown-In Cellulose May Be Best If You Want:

• A more budget-friendly attic insulation upgrade
• Additional insulation over suitable existing material
• Improved thermal resistance on a vented attic floor
• An alternative to the higher upfront cost of spray foam

Fiberglass May Be Best If You Want:

• A widely available insulation material
• A possible DIY option
• Batts for open and accessible floor joists
• Loose-fill insulation for a vented attic floor

Spray Foam May Be Best If You Want:

• Insulation that also creates an air seal
• To insulate the attic roof deck
• An unvented, conditioned attic
• To bring attic ductwork or HVAC equipment inside the insulated envelope
• A longer-term insulation solution with less settling or shifting

The best material on paper can still perform poorly if it is installed in the wrong location or without addressing air leakage, ventilation, and moisture.

What Should You Consider Before Insulating an Attic?

Before choosing insulation for your attic, ask the following questions.

Is the Attic Vented or Unvented?

A vented attic and an unvented attic use different insulation strategies.

Determine which assembly you have and whether you plan to change it before selecting the material.

Where Is the Existing Insulation?

Insulation on the attic floor typically separates the living space from a vented attic.

Insulation along the roof deck brings the attic closer to the home’s conditioned environment.

Is the Existing Insulation in Good Condition?

Dry, clean material may be suitable to leave in place when adding blown-in insulation.

Wet, contaminated, or damaged insulation may require removal.

Are There Air Leaks?

Air leakage can contribute to drafts, uneven temperatures, energy loss, moisture movement, and ice dam formation.

Blown-in cellulose and fiberglass do not replace dedicated air sealing.

Is HVAC Equipment Located in the Attic?

Ductwork and mechanical equipment located outside the building envelope are exposed to extreme attic temperatures.

Insulating the roof deck with spray foam may help bring that equipment into a more controlled environment.

What Is Your Budget?

Blown-in cellulose generally offers a lower upfront cost, while spray foam requires a larger initial investment because it combines insulation with air sealing and may change the attic assembly.

What Are Your Long-Term Goals?

Think about whether you are primarily trying to add thermal resistance, solve comfort problems, reduce air leakage, protect attic ductwork, or create a conditioned attic.

Those goals can lead to different recommendations.

Attic Insulation Cost Factors

The cost of home attic insulation depends on more than the price of the material.

Factors that can affect the total cost include:

• Attic size
• Insulation material
• Required insulation depth or foam thickness
• Condition of the existing insulation
• Insulation removal and disposal
• Air-sealing work
• Ventilation modifications
• Attic accessibility
• Roof pitch and framing complexity
• Electrical or mechanical obstructions
• Required protective barriers
• Labor and equipment

Adding blown-in insulation over suitable existing material generally costs less than removing everything and creating a spray foam roof-deck system. However, price should be considered alongside the performance and goals of each option.

Choosing the Right Insulation for Your Michigan Attic

Michigan homes experience hot summers, cold winters, and plenty of temperature swings in between.

Your attic insulation must work as part of a complete system that manages heat, air, and moisture throughout the year.

Blown-in cellulose or blown-in fiberglass can be a practical, budget-conscious option for improving insulation on the floor of a vented attic. Spray foam may be a better fit when air sealing is a priority or when you want to move the building envelope to the roof deck.

The right answer depends on the house.

A professional attic inspection can help identify the condition of the existing material, air leakage, ventilation needs, moisture concerns, and the best location for the new insulation.

If you live in Michigan’s Lower Peninsula, RetroFoam of Michigan can evaluate your attic and help you compare blown-in cellulose, blown-in fiberglass, and spray foam insulation based on your home, goals, and budget. Call 866-900-3626 or fill out the form on our website to request a free consultation.

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Frequently Asked Questions About Insulating an Attic

How do you properly insulate an attic?

First, determine whether the attic should remain vented or become an unvented, conditioned space.

A vented attic is commonly insulated along the attic floor, while an unvented attic may be insulated at the roof deck. Air leaks, moisture problems, and ventilation issues should be addressed before installing new insulation.

Should you insulate the attic floor or roof?

Insulate the attic floor when you want to separate the living space from a vented attic.

Insulating the roof deck may be more appropriate when creating an unvented attic or bringing HVAC equipment and ductwork inside the insulated envelope.

What is the best insulation for an attic floor?

Blown-in cellulose, loose-fill fiberglass, and fiberglass batts can all be used as insulation for an attic floor.

Blown-in cellulose offers broad coverage and a budget-friendly option, while fiberglass is widely available. Neither material creates a complete air seal on its own.

What is the best way to insulate an attic ceiling?

When homeowners refer to the attic ceiling, they may mean the attic floor above the home’s ceiling or the underside of the roof.

Blown-in insulation is commonly used on the attic floor, while spray foam may be installed on the underside of the roof deck.

Can you put new attic insulation over old insulation?

New blown-in insulation can sometimes be installed over old insulation if the existing material is dry, clean, and in good condition.

Wet, moldy, pest-contaminated, or severely damaged insulation should be addressed before adding more.

Does blown-in attic insulation air seal the attic?

No.

Blown-in cellulose and fiberglass slow heat transfer but do not create the same air barrier as spray foam. Accessible gaps and penetrations should be sealed separately before blown-in material is installed.

Does old insulation need to be removed before installing spray foam?

When spray foam is applied to the roof deck, insulation on the attic floor is commonly removed.

This allows the attic to connect with the conditioned space below and gives the installer access to inspect the attic floor. The exact recommendation depends on the planned assembly.

Can I insulate an attic myself?

Some homeowners install fiberglass or blown-in insulation themselves.

However, attic work can involve electrical hazards, heat-producing fixtures, difficult access, ventilation requirements, and respiratory protection. Spray foam should be installed by trained professionals.

How much attic insulation does a Michigan home need?

The appropriate amount depends on the insulation material, attic assembly, existing conditions, and applicable building requirements.

More insulation is not automatically better if air leaks, moisture problems, or blocked ventilation have not been corrected.

How long does attic insulation last?

Its lifespan depends on the material and attic conditions.

Fiberglass and cellulose may lose performance if they settle, become compressed, get wet, or are disturbed. Properly installed spray foam is designed to remain in place for the life of the building, provided it is not damaged or altered.