Fiberglass, Cellulose or Foam

What each material is made of

Fiberglass insulation is made from fine glass fibers spun into a fluffy mat. It comes as batts and rolls, which are cut to fit between framing, and as loose-fill, which is blown into attics and closed cavities. It is the most common insulation in the country, and many older homes here have it in some form.

Cellulose is made mostly from recycled paper, shredded into small fibers and treated with borate compounds that help it resist fire and pests. It is installed as loose-fill, blown into attics, or dense-packed into closed wall cavities. In some products it is sprayed damp with a binder so it sticks to open framing.

Spray foam is a plastic made on site by mixing two liquids that react and expand. It comes in two main types, open-cell, which is soft and sponge-like, and closed-cell, which is dense and rigid. It sticks to what it is sprayed on and can fill an irregular space completely.

These origins explain the behaviors we describe below. Fibers hold air and pass vapor. Paper absorbs water. Foam forms a continuous layer. In a humid climate each of those traits matters.

Moisture: how each behaves when it gets damp

Moisture is the stress test in South Florida. Fiberglass fibers do not absorb water into themselves, but water can sit between the fibers and in the paper or foil facing on batts. Loose-fill fiberglass can mat down when wet and lose loft, but it dries if the source is fixed and air can move. In many cases brief wetting does not ruin it, though staining and odor are signs it should be replaced.

Cellulose is more sensitive. Paper fibers take up water and hold it, and wet cellulose clumps, sags and stays damp longer. Because it is organic, it can support mold if it stays wet. Borate treatment helps resist growth, but it does not make the material immune. After a significant leak, wet cellulose is usually removed.

Closed-cell foam resists water and does not absorb it in any meaningful way, which is why it is used where flooding is a concern, such as rim joists and some walls. Open-cell foam absorbs water like a sponge and often needs replacement after wetting. Foam also forms a barrier that restricts air movement, which keeps humid air from reaching cool surfaces.

Fire behavior and safety

Fiberglass is made from glass and does not burn. Its paper or foil facing, when present, can burn, which is why facings are kept away from heat sources and exposed surfaces in some applications. Loose-fill fiberglass has no facing and is generally considered the most fire-resistant of the three in typical attic use.

Cellulose is paper, and untreated paper burns. The borate treatment makes it resist ignition and slows flame spread, and it tends to char rather than flame. Even so, it needs the right clearance around flues, chimneys and fixtures that run hot. It should never be packed against recessed lights that are not rated for insulation contact.

Spray foam is combustible and gives off smoke when burned. In most applications it must be covered with an approved thermal or ignition barrier in spaces people use or enter, such as attics with storage or garages. The requirement depends on the product and the location. A reputable installer knows the rules and follows them.

None of this makes any material unsafe when installed properly. It does mean that clearances, coverings and product choice matter, and that you should ask about the fire rating of what is going into your home.

Settling, loft and long-term performance

Loose-fill materials settle. Over time they compact under their own weight and from vibration and humidity. Cellulose tends to settle more than fiberglass, and installers account for it by installing to a depth that allows for settling. In practice, a very old attic may have noticeably less depth than when it was installed.

Batts do not settle in the same way but can sag, slip, bunch up or leave gaps along the framing. Poorly cut batts that are squeezed into small spaces lose their effect, because compression reduces the trapped air. Batts that get wet and dry can stay compressed.

Foam stays where it is put. Closed-cell foam keeps its shape and R-value well over long periods when installed correctly. Open-cell foam is more fragile and can change with moisture exposure. Poor mixing, wrong temperatures or poor surface preparation can leave voids, shrinkage or poor adhesion, which is why the installer matters as much as the product.

Pests, odor and indoor air

Rodents and insects interact with each material differently. Mice and rats burrow into loose-fill and batts, leaving tunnels, droppings and a smell that can be hard to remove. Cellulose, because of its borate treatment, can discourage some insects, but it is not a pest control product. Foam is harder for most pests to nest in, though rats can chew it.

Odor and indoor air quality are mostly tied to installation and condition. Properly installed loose-fill stays in the attic and does not enter the living space, provided air leaks into the home are sealed. Dust can come down through gaps in the ceiling in some houses. Foam may have an odor during curing and for a short time afterward, and the home should be vacated and ventilated per the installer's instructions.

People who are sensitive to dust, fibers or chemicals should say so before the work starts. A technician can then explain how to minimize exposure, including containment, protective gear and timing.

Air sealing and thermal performance per inch

R-value per inch differs among the three. Loose-fill fiberglass is a little over two per inch, cellulose is about three and a half, open-cell foam is a little under four, and closed-cell foam is roughly six or more. These are estimates, and they vary by product and installation. The point is that, for a given depth, foam and cellulose resist more heat than fiberglass.

That matters when space is limited. A low-clearance attic may only allow a certain depth, and foam can reach a higher R-value there. In a roomy attic, loose-fill gets to the same level with more depth and less cost in effort, since the material can be blown in deep.

Air sealing is a separate matter. Foam seals as it goes. Cellulose, when dense-packed, resists air movement more than fluffy fiberglass does, which is one reason it is used in closed walls. Loose-fill fiberglass resists conduction but lets air through, so the sealing has to be done first. The page on air sealing covers that sequence.

Where each material tends to fit best

Loose-fill fiberglass is a common choice for attic floors because it is not combustible, does not hold water, goes in quickly and can be added to over time. It works well with ventilated attics and when ceiling leaks have been sealed first. The page on blown-in insulation describes how it is installed.

Cellulose works well for attic floors where a denser, quieter layer is wanted, and it is a standard material for dense-pack in existing walls through small holes. It needs a dry environment and a plan for moisture, which in this climate means attention to roof leaks and humidity control.

Spray foam suits roof decks in unvented attics, rim joists, hard-to-reach cavities and places where air sealing is as important as insulation. The page on spray foam insulation explains the process. Batts suit open framing in new work or additions where the cavity is regular, and they need careful cutting and fitting to perform.

Concrete-block homes, wood-frame homes and garages

Many South Florida homes are concrete block on the first floor with a wood-framed roof and sometimes a wood-framed second story. The attic is where most of the insulation goes in all of them. Block walls are usually insulated differently, using furring strips with foam board, foam in cores or spray foam on the interior. Walls of wood framing can be filled with batts, dense-packed loose-fill or foam.

In a garage, rules about fire separation shape the choices. Walls and ceilings shared with the house may need specific materials and coverings. A garage door with rigid foam inserts or a reflective kit is a separate product from wall insulation. The page on garage insulation goes through the options.

The right mix for one house may differ from the next. A technician looks at construction type, age, previous work and any moisture history before suggesting materials for each area.

Hurricane season and storm behavior

Storms test insulation in ways a calm summer does not. When wind lifts shingles or tiles and rain reaches the attic, loose-fill on the floor is the first thing to get wet. Fiberglass loose-fill usually comes through a short soaking better than cellulose, and it is easier to dry or remove. Cellulose may clump and hold water against the ceiling, adding weight and staining.

Foam on the roof deck changes the picture. Closed-cell foam adds stiffness to the deck and does not soak up water. Some owners feel it adds a margin of protection when the roof covering is damaged. Others worry about the leaks it can hide. Both points have merit, and the answer depends on roof type and the condition of the decking.

Wind-driven rain can also push water into wall cavities. Dense-packed cellulose in a wall that gets wet is hard to dry and may need to be opened and replaced. Closed-cell foam in a wall handles it better. After a storm, any material that has been wet should be assessed rather than assumed to be fine. The page on wet insulation after a hurricane explains how that assessment goes.

What the choice does not change

No material will fix a leaking roof, a failed duct or an oversized air conditioner. Insulation reduces heat flow, and each product does that job within its limits. If other problems are present, the best insulation in the world will not make the house comfortable.

No product is permanent either. Dry, undisturbed insulation can serve for many years, but every material needs a reasonable environment. Attics that leak, vent into the wrong places or collect moisture from exhaust fans will shorten the life of any layer. The page on how long insulation lasts goes through what extends and shortens life.

And no material is a substitute for sound installation. Gaps, voids, compressed batts and blocked vents cost performance regardless of what is in the bag. When you compare proposals, compare the preparation and the installation details as closely as the product name.

Removal, replacement and mixing materials

Existing insulation is often left in place and added to. Loose-fill can go over loose-fill, and fiberglass and cellulose can be layered when the older layer is dry and sound. Foam over existing loose-fill is not usually done, because foam needs to stick to a clean surface. In that case old material is removed first.

Removal is sensible when the material is wet, moldy, contaminated by pests, badly compacted or full of debris. The page on insulation removal and replacement explains how a vacuum system pulls material out and how the attic is cleaned and sealed before new insulation goes in.

When mixing materials in one assembly, keep moisture in mind. A vapor-facing layer on top of another layer can trap moisture between them. In this climate the safest pattern is simple layers with nothing that traps moisture, and installation by someone who understands how the parts behave together.

How to decide and what to ask

Start from the problem. If the attic floor is thin and the main issue is cooling load, loose-fill is often a direct answer. If the main issue is air leaks and a hot, humid attic with ducts in it, foam on the deck may fit. If the main issue is cold or noisy walls, dense-pack may be the answer.

Then ask about the product: what is it, what is its rating, how does it handle moisture, what will it do near the fixtures and flues in my attic, and how will you handle roof repairs or leaks later? Ask what is being removed and what is being kept, and what air sealing is part of the work.

Bring photographs and notes to the call. Quotes are given only by phone, and complicated jobs need an on-site visit first. A service call charge applies and is explained before dispatch. Call the number on this page to talk through the choice for your home.

It also helps to know what you already have. If you can identify the material in your attic, whether white or yellow fluff, gray-green shredded paper, or paper-faced batts, tell the technician. A photograph from the hatch is usually enough. Knowing the starting point lets the conversation focus on what to add, what to remove and what to leave alone, instead of guessing from a general description of the house.

Questions people ask

Related pages