If your upstairs rooms feel drafty in January and stuffy in July, your attic is the first place to look. Homes in Southeastern Pennsylvania deal with cold winters and hot, humid summers, and a poorly insulated attic loses heat in one season and soaks it up in the other. Loose fill insulation is one of the most effective ways to fix that, but not all loose fill insulation materials perform the same way. Below, we compare fiberglass, cellulose, and mineral wool so you can choose the right fit for your home.
The attic matters most because warm air rises. In winter, heated air moves up through ceilings and escapes through the attic, so your furnace runs longer to keep up. That same escaping heat can warm the roof deck and contribute to ice dams along the eaves. In summer, the sun can turn an attic into an oven, and without enough insulation, that heat radiates down into your living space and makes your air conditioner work harder.
What Is Blown-In (Loose Fill) Attic Insulation?
Loose fill insulation is made of small fibers or particles that come compressed in bags. Installers feed the material into a blowing machine, which breaks it apart and sends it through a long hose into the attic. The result is an even, fluffy blanket that covers the attic floor to a set depth. That’s why the terms “loose fill” and “blown-in” are used almost interchangeably.
Loose fill is one of three main types of attic insulation, alongside batts and spray foam. For most existing homes with an open, unfinished attic, it’s the most practical and cost-effective option.
Why Blown-In Insulation Works So Well in Attics
Attics are rarely a clean, open space. Joists, wiring, plumbing vents, recessed light housings, and bracing all get in the way. Batts have to be cut and fitted around every obstacle, and each gap left behind becomes a spot where heat slips through. Loose fill insulation flows around those obstructions and settles into corners and irregular spaces, creating a more continuous layer.
It’s also the easiest way to upgrade an attic that already has some insulation. If your existing insulation is dry and in decent shape, new loose fill can usually be blown right over it to raise the total R-value, without tearing anything out. And because the material travels through a hose, installers can reach low-clearance areas near the eaves that would be hard to work in by hand.
When the existing layer is wet, moldy, or damaged by pests, it should be removed first. A professional attic insulation inspection will tell you which situation you’re in.
Comparing Loose Fill Insulation Materials
The three most common loose fill insulation materials are fiberglass, cellulose, and mineral wool. Each one is measured by its R-value per inch, which describes how well it resists heat flow. The higher the R-value, the better the insulation performs, and the more total inches you install, the higher your attic’s overall R-value climbs.
Blown-In Fiberglass Insulation
Fiberglass loose fill is made from molten glass spun into fine fibers, often with recycled glass content. It has an R-value of about 2.2 to 2.7 per inch, the lowest of the three. Its big advantage is stability: it settles only about 2 to 4 percent over time, so the depth you pay for stays close to the depth you keep.
Because each inch does less work, you need more of it. Reaching R-38, for example, takes roughly 14 to 17 inches of fiberglass. That makes it best suited to attics with enough room at the eaves for a very deep layer. Fiberglass is lightweight and doesn’t absorb water readily, though any insulation loses effectiveness when it’s wet. It also needs to be installed at the manufacturer’s recommended density, since “fluffed” fiberglass won’t deliver its rated R-value.
Blown-In Cellulose Insulation
Blown-in cellulose insulation is made mostly from recycled paper, treated with fire retardants to improve fire resistance. It packs more performance into each inch, with an R-value of about 3.2 to 3.8 per inch. That means it reaches R-38 in roughly 10 to 12 inches, a real benefit in attics with tight space near the eaves. Its denser makeup also slows air movement through the insulation layer better than fiberglass.
The trade-off is settling. Loose fill cellulose settles about 10 to 20 percent over time, so experienced installers blow in extra depth to make sure the attic still meets its target R-value after it settles.
Moisture is the other factor to take seriously. Cellulose absorbs water, so a roof leak or ongoing condensation can make it clump, compress, and lose much of its insulating value. Damp insulation that stays wet can also lead to mold. Cellulose performs very well in a dry, properly ventilated attic, which is why any active leaks should be handled with roof repair before new insulation goes in.
Loose Fill Mineral Wool (Rock Wool)
Mineral wool is made from natural rock (rock wool) or from slag, a byproduct of steel production (slag wool). It has an R-value of about 3.0 to 3.3 per inch and settles very little. Its standout quality is fire resistance: mineral wool is naturally noncombustible and doesn’t need added fire retardants. It also repels water rather than soaking it up.
The drawbacks are cost and availability. Mineral wool is usually more expensive than fiberglass or cellulose, and it’s less commonly used as a loose fill product, so fewer installers work with it.
Loose Fill Insulation Comparison
Material | R-value per inch | Settling over time | Fire resistance | Moisture tolerance | Relative cost |
Fiberglass | 2.2 to 2.7 | Low (about 2 to 4%) | Noncombustible | Good; doesn’t absorb water readily | Low |
Cellulose | 3.2 to 3.8 | Higher (about 10 to 20%) | Good; treated with fire retardants | Fair; absorbs water if the attic gets wet | Low |
Mineral wool | 3.0 to 3.3 | Low | Excellent; naturally noncombustible | Good; repels water | Higher |
How Much Blown-In Insulation Does Your Attic Need?
The right amount depends on the R-value your attic needs, what’s already up there, and which material you choose. Because each material insulates differently per inch, the same target R-value can mean a 10-inch layer of cellulose or a 16-inch layer of fiberglass. Before choosing a material, it helps to know how to measure attic insulation.
Attic Insulation Installation in Montgomery County, Bucks County & the Main Line
Choosing between loose fill insulation materials is easier with someone who has seen your attic firsthand. At JL Roofing & Siding, we look at your attic’s current insulation depth and condition, check for roof leaks and ventilation issues, and recommend the material that fits your home and budget. Because we work on roofs every day, we can spot the moisture problems that ruin insulation before they cost you money.
From our location at 1966A Pioneer Rd, Huntingdon Valley, PA 19006, we serve homeowners throughout Montgomery County, Bucks County, the Main Line, and New Jersey. Call us at 215-443-5955 or request a quote to schedule your attic insulation assessment.