Fiberglass Versus Cellulose: How the Materials Behave

Material behavior

What each material is made of

Fiberglass insulation is made from very fine strands of spun glass, formed into a light, springy mass. It is sold as batts and rolls, and also as loose fill that can be blown into place. The strands trap tiny pockets of air, and it is that trapped air that slows heat movement. Fiberglass is not organic, so it does not feed mold or pests as a food source.

Cellulose insulation is made mostly from recycled paper, such as newsprint, that has been shredded into small bits and treated so it resists fire and some pests. It is almost always sold as loose fill, though it can also be installed damp in wall cavities. The paper fibers are denser than glass fibers and tend to pack more closely together.

Both are mainstream materials and both have long track records in attics. The question for a South Florida homeowner is less about which one is good and more about which one behaves better in the particular space, given the heat, humidity and installation method involved. This page compares behavior, not brands, and it avoids numbers because they would not apply to every home.

Before comparing details, it helps to name what you want the material to do. Some homeowners want an attic that stays stable for many years with little attention. Others are working in stages around a budget and want to add to what is already there. Others are getting ready for a roof replacement or a new air conditioner and want the insulation to fit that schedule. The same material can be a good or a poor match depending on which of those stories sounds like yours.

How they are installed

Fiberglass loose fill is blown through a hose into attic spaces, where it settles into a light, fluffy layer. Fiberglass batts are cut to fit between framing members and pressed into place. Because the material is light and springy, it recovers well if it is not crushed, and it can be cut cleanly where needed.

Cellulose is blown in too, using a machine that breaks up the material and feeds it through a hose. The result is a dense, uniform blanket that packs into odd spaces and around obstacles. In wall cavities, a denser version can be installed through small openings by trained crews, and some applications use a damp spray method that helps the material stick.

In both cases the work is for trained installers. It involves equipment, attic access, heat exposure, and dust, and it takes place near wiring, ducts and roof framing. Homeowners should not attempt to handle or install either one. See the blown in insulation page for how loose fill jobs generally proceed.

Equipment and noise differ little between the two, but dust handling does. Cellulose can be dustier during installation because it is made of fine paper particles, so crews take extra care with containment, particularly around the hatch and any openings into the living space. Fiberglass releases fibers when cut or disturbed. In both cases, expect the crew to protect floors, seal the work area where they can, and clean up when they finish.

Air movement through each material

Neither material is an air barrier on its own. Air can move through a bed of loose fiberglass quite freely, which is why some homeowners are surprised to learn that the insulation can look thick and still allow leaks. Cellulose is denser and resists air movement through the layer somewhat better, though it still does not replace sealing at the ceiling plane.

The important point is that air leaks at the ceiling, around recessed lights, at wall tops and through chases carry heat, moisture and dust. Sealing those paths is a separate step whichever material you choose. The air sealing page explains the how and why.

Fiberglass can also show a pattern called wind washing when air moves across its surface, especially near eaves, and installers often use baffles or wind blocking at the edges. Cellulose handles wind movement a bit better because of its density, but it still benefits from the same edge details. Ask the installer how the perimeter will be protected.

Pressure differences in a home push on these materials too. When the air conditioner runs, ducts that leak can create pressure imbalances that pull attic air into the house or push house air into the attic. Loose material sitting in the way of that moving air picks up dust and moisture and does less than it should. This is why a plan that treats the duct system and the sealing together with the insulation tends to hold up better than a plan that adds material alone.

Moisture behavior in a humid climate

Moisture is where the two materials differ in ways that matter in Florida. Fiberglass does not absorb water into the fibers in the same way, so it can shed some moisture and dry out, but wet fiberglass still loses much of its performance while wet and can hold dust and debris. Cellulose, being paper based, can absorb and hold moisture, and a prolonged wetting event can make it heavy, matted and slow to recover.

Cellulose is treated with borate compounds that resist fire and discourage mold and some pests, but treatment does not make it immune to long term dampness. If a roof leak or a leaking duct drips on loose material, either type may need to be removed. The key step is fixing the water source first.

In a coastal, humid region the roof, duct and attic ventilation condition counts for more than the material name. Ask the installer how they would handle any stain or odor they find. The humidity and insulation in Florida guide provides background, and the wet or stained insulation page describes what to tell an installer.

Think about recovery after a storm, too. Hurricane season can bring wind driven rain through damaged roofing, and attic material below a leak can take on a lot of water. After any storm, an installer or roofer can judge whether the material is still serviceable. Do not enter a wet attic yourself, since wet material is heavy, ceilings below may be weakened, and wiring may be exposed. Photograph stains from below, if safe, and share them.

• Fiberglass is made of glass strands and is not a food source for mold or pests.
• Cellulose is paper based and treated, and it holds moisture longer than glass fibers.
• Both materials lose performance when wet and may need removal after a leak.
• Neither material is an air barrier, so air sealing is a separate step.
• The roof, ducts and ventilation matter as much as the material choice.

Settling and density over time

Loose materials can settle. Fiberglass loose fill tends to be light, so it can compress a bit under its own weight and can shift if disturbed. Cellulose is heavier and denser, and it also settles, which is why installers account for that when they plan thickness. Settling is a normal behavior to ask about, not a defect.

Density affects how a material feels in the space. A dense layer of cellulose has more weight per unit of thickness, and it tends to fill small voids more completely. A fluffy layer of fiberglass has more air in it and can leave tiny channels if it is not applied carefully. Neither is automatically better, but they respond differently to disturbance by later trades.

If you are concerned about an existing layer that has flattened or shifted, the insulation settling over time page explains how an installer thinks about it.

Fire, pests and treatment

Every insulation material you may encounter has its own fire behavior and its own treatments, and the details vary by product. Do not assume something is safe near a heat source just because it is common. Installers follow the product instructions about clearances around recessed lights, flues and other heat producing fixtures, and your city building department sets requirements for the work.

Pests are a separate concern. Rodents and insects will nest in many materials, including both of these. Treated cellulose discourages some pests, and glass fibers offer no food value, but neither one is a barrier to a determined animal. Closing entry points and cleaning up contamination matter more than the material choice.

If you have seen droppings, shredded material or nesting near the hatch, do not handle the area. Note what you observed and tell the installer. The pest damaged insulation page describes how those situations are commonly approached.

Dust, itch and household comfort

Handling either material can release fibers or dust, and people may notice skin or throat irritation during installation or when an attic has been disturbed. This is one reason the work is left to trained crews wearing protective gear, and one reason a crew will tape off the hatch and protect the living space.

In the finished home, a properly installed layer in a sealed attic should not drift dust into rooms. If you notice dust or an odor after the work, tell the installer. The cause is more often an unsealed air path than the material itself. See insulation itch and irritation for what homeowners commonly ask. This page does not give medical advice.

Household members with sensitivities should talk to the installer ahead of time about how the space will be protected and ventilated, and should ask their own health provider about any health concern. Pets and children should be kept away from the work area, as the guide on preparing for attic work with pets and kids explains.

A practical detail is the hatch itself. If the hatch is in a hallway or closet, the crew will cover nearby surfaces and may set up a path from the front door. Moving breakable items, clearing the hall and keeping pets in a separate room make the day smoother. You do not need to do anything inside the attic, and you should not.

Removal, replacement and mixing

Can you mix them? In many homes, the answer is yes, with caveats. Adding one loose fill over another is common when the existing layer is dry, clean and sound. Mixing is less sensible when the old layer is wet, contaminated or heavily settled. The installer should inspect what is there before suggesting a layering plan.

Removal is a specialized job. Crews with vacuum equipment can remove loose fill from an attic floor, but the work is messy and a job for professionals. Batts can be lifted out by section. The decision to remove usually depends on contamination, moisture damage or a need to correct underlying problems like air leaks.

See the removal and replacement page and the remove and replace versus adding on comparison for how that decision is made.

Older homes and concrete block construction

Many older South Florida homes have concrete block walls and low sloped roofs with limited attic clearance. Those conditions affect which material is practical. Where space is tight, a dense loose fill may reach corners that a batt cannot. Where the attic is spacious and framing is regular, batts or fluffy fill may work well.

The existing material also matters. An older attic may hold a mix of layers from different eras, and some layers may be compressed, stained or partially removed by earlier trades. A new layer can only do its job if it is applied over a sound, dry base, so the installer needs to see what is there before recommending anything.

The insulation for older Florida homes guide and the concrete block homes and insulation guide go deeper into those situations. Ask the installer to describe how they will handle each layer they find.

Newer homes, built with tighter framing and more regular attic trusses, are friendlier to either material. Even so, the same questions apply: how will the edges be handled, how will ventilation stay open, how will the layer be checked for even coverage, and how will air leaks be sealed first. A newer house does not remove the need for those questions. It just tends to make the answers easier to carry out.

Making the choice

Begin with the condition of the attic and the reason for the project. If moisture or leaks are suspected, solve them first. If air leaks are obvious, plan sealing. Then ask the installer which material they would use and why, in terms of your specific attic: access, existing layer, ventilation, edge details and equipment location.

The process is the same for any material. Call and talk through the home, including property type, location and the areas that concern you. Look over conditions such as existing material, access, air movement, and moisture and heat exposure. Choose a direction that matches the material and installation approach to the assembly and the goal, then complete the work with the space protected and the finished area walked through with you.

For more comparisons, see mineral wool versus fiberglass and loose fill materials compared. Ask your condo association or city building department about any requirements, since they vary by building and community.

Questions and answers