Fiberglass Insulation: Batts vs. Blown-In

Fiberglass insulation comes in two forms: blankets (batts and rolls) and loose fibers that are blown in with a machine. It's the same material either way. The difference is how it's installed and where each form fits best.

Batts and rolls are precut blankets sized to fit between studs, joists, and rafters, and each one is labeled with its R-value. Blown-in fiberglass is loose material blown to a specific depth, and its R-value depends on how deep and how dense it's installed.

For most existing attics, blown fiberglass insulation is the easier way to get even coverage because it flows around wiring, pipes, and odd framing. Batts and rolls are the usual choice for open walls, garages, crawl spaces, and simple attic layering where the framing is regular and easy to reach.

Batts, Rolls, and Blown-In: What's the Difference?

Batts are precut pieces, commonly 4 feet or just under 8 feet long, sized to fit between framing spaced at standard widths.

Rolls are the same blanket material in one long piece that you cut to length on site. Attic insulation rolls are handy for long, uninterrupted joist runs.

Blown-in fiberglass is unbonded loose fiber, installed with a blowing machine. In an open attic it's blown over the floor. In closed walls it can be packed tightly into each cavity.

Our fiberglass insulation service installs all three forms, depending on where the insulation is going.

Both forms are sold in faced and unfaced versions:

• Faced batts and rolls have a kraft paper or foil layer on one side, which acts as a vapor retarder.
• Unfaced products have no facing. They're what you want when adding a second layer over existing insulation.

Fiberglass Batt R-Values and Thickness

Batts are labeled with a total R-value at a given thickness. Owens Corning's EcoTouch batts, for example, are listed at:

There are also cathedral ceiling batts, made denser to fit tighter spaces. Owens Corning lists R-30C at 8¼ inches and R-38C at 10¼ inches. These are useful for ceiling insulation batts in rafter bays that can't hold a full-thickness batt.

Why compression matters

A batt's R-value assumes it fills its full thickness. Squeezing it into a shallower space lowers the result. Owens Corning notes that its R-19 batt, which is 6¼ inches thick, delivers R-18 when installed in a 5½ inch cavity. The same idea applies to stuffing batts behind wiring or pipes, or packing them too tightly. Batts should be cut to fit around obstructions, not crushed around them.

Blown Fiberglass Insulation R-Values

Blown-in fiberglass doesn't come with a fixed R-value on each piece. Its R-value comes from the manufacturer's coverage chart, which lists how deep the insulation must be installed and how many bags per 1,000 square feet it takes to reach each R-value.

For Owens Corning ProPink L77, reaching R-38 in an open attic requires a minimum of 13.25 inches. That works out to roughly R-2.9 per inch.

Density changes the numbers a lot. When the same product is packed tightly into a closed wall, Owens Corning lists it at R-15 in a 3½ inch 2x4 cavity at 1.5 pounds per cubic foot. That's far more R-value per inch than it delivers loose on an attic floor.

Loose fiberglass settles very little. DOE data from the Building America Solution Center puts settling at 2 to 4 percent over time.

Fiberglass Batt Insulation for Attics vs. Blown-In Attic Insulation

When batts or rolls work well in an attic

• The joists are evenly spaced, and there's little wiring, ductwork, or blocking in the way.
• You're adding a layer over existing insulation that's already level with the joists.
• The attic is small or partly floored, and only some areas need insulation.

ENERGY STAR notes that laying fiberglass rolls is the easiest option for a DIY attic job. It recommends laying the second layer across (perpendicular to) the first to cover the tops of the joists. Any added layer should be unfaced.

When blown-in fiberglass is the better fit

• The attic has lots of obstructions, uneven framing, or low clearance near the eaves.
• The existing insulation is uneven and needs to be leveled out to a consistent depth.
• You're covering a large area and want a uniform layer without seams.

For most attic top-offs, blown fiberglass or cellulose is the common choice because it goes in fast and even. Our attic insulation service covers how we assess and level an existing attic before adding more.

Fiberglass in Walls and Ceilings

Open walls (during a remodel, new addition, or garage conversion) are where batts are most common. R-13 or R-15 batts fit 2x4 walls, and R-19 or R-21 fit 2x6 walls, depending on the product. They should be cut to fit around outlets and pipes without gaps or compression.

Closed walls can't take batts without removing drywall. Blown fiberglass, packed tightly through small holes, is one way to insulate existing walls without opening them up. Our wall insulation page explains how we evaluate closed-wall projects.

Ceilings with no attic above, such as cathedral ceilings, often use the denser "C" batts mentioned above. These spaces usually need a ventilation plan too, so they should be evaluated before insulating.

Safety Notes

Fiberglass fibers irritate skin, eyes, and lungs. ENERGY STAR recommends a disposable coverall, gloves, eye protection, and an OSHA-approved particulate respirator or double-strap dust mask for attic insulation work.

San Antonio attics get very hot. Work early in the day and drink plenty of water.

Walk only on joists or truss chords. Ceiling drywall won't support your weight.

Keep insulation at least 3 inches from recessed "can" lights unless the fixture is rated IC (insulation contact).

Owens Corning notes its unfaced batts shouldn't be left exposed in living areas because of the potential for skin irritation.

Common Mistakes

1. Compressing batts to fit a shallower space, which lowers the R-value.
2. Leaving gaps at the ends of batts or around obstructions. Heat takes the path with no insulation.
3. Using a faced batt as the top layer over existing insulation. Added layers should be unfaced.
4. Blocking soffit vents at the eaves. Rafter vents (baffles) keep air moving past the insulation.
5. Blowing fiberglass to the wrong depth. Check the depth against the coverage chart on the bag.

Frequently Asked Questions

Can you put fiberglass batts over blown-in insulation?

Yes. ENERGY STAR notes you can add loose fill over batts or batts over loose fill. If you're adding fiberglass batts or rolls on top of existing insulation, use unfaced products.

What's the difference between insulation batting and rolls?

They're the same material. Batts are precut lengths. Rolls are one continuous blanket you cut to length. Rolls save time on long joist runs, and batts are easier to handle in tight spaces.

How thick is R-30 fiberglass batt insulation?

Standard R-30 batts from Owens Corning are about 9½ inches thick. The denser cathedral version, R-30C, is about 8¼ inches. Other brands may vary slightly, so check the product label.

Does blown-in fiberglass settle?

Only slightly. DOE data puts loose fiberglass settling at 2 to 4 percent. Manufacturers' coverage charts show the depth needed to keep the labeled R-value.

Is batting or blown-in better for a ceiling with no attic?

Neither form can be blown loose over a closed ceiling. Closed rafter bays are usually handled with dense-packed blown insulation or, when the ceiling is open, with cathedral-style batts sized to the available depth.

The Takeaway

Fiberglass batts and blown-in fiberglass are the same material in different forms:

• Batts and rolls are best where framing is open and regular: walls, garages, crawl spaces, and simple attic layers. They only deliver their rated R-value when they aren't compressed and have no gaps.
• Blown-in fiberglass is usually the better choice for existing attics with obstructions or uneven insulation. Its R-value depends on reaching the depth on the coverage chart.

For a typical San Antonio attic top-off, blown-in is usually the practical route. Batts shine wherever the framing is exposed. If you're not sure which form fits your attic or walls, Get an Estimate and we'll check the space first.

References

• Owens Corning: EcoTouch PINK Fiberglas Insulation Submittal Sheet and Product Data Sheet. owenscorning.com
• Owens Corning: PROPINK L77 Loosefill Insulation Submittal Sheet and Manufacturer's Fact Sheet. owenscorning.com
• U.S. Department of Energy, Building America Solution Center (PNNL): Loose-Fill Insulation. basc.pnnl.gov
• ENERGY STAR: A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR. energystar.gov
• ENERGY STAR: Well-Insulated and Sealed Attic. energystar.gov