Fiberglass batts give about R-3.1 to R-4.3 per inch, depending on how dense they are. Blown-in fiberglass gives less, usually about R-2.2 to R-2.9 per inch. So to hit the same R-value, blown fiberglass needs a deeper layer. That’s fine in an attic, where you have room, and it’s why batts usually win in walls.
People see two very different depths for the same R-value and assume one bag is lying. Neither is. Our Grizzly Insulation Co crews install both kinds in Denver homes every week, so here’s the plain math behind the numbers.
What Does R-Value per Inch Mean for Fiberglass Insulation?
It’s how much heat resistance you get from each inch of material. Multiply it by the thickness and you get the total R-value. If the term is new, our page on what R-value means keeps it simple.
Fiberglass works by trapping still air between glass fibers. Packing the fibers tighter traps the air in smaller pockets, which slows heat a bit more. That’s the whole reason density changes the per-inch number.
How Do Fiberglass Batts and Blown-In Compare per Inch?
Here’s the side-by-side, using the batt sizes the Department of Energy lists in its guide to types of insulation:
Fiberglass type
Example
R-value per inch
Standard batt
R-11 in a 3.5 in wall
About R-3.1
Medium-density batt
R-13 in a 3.5 in wall
About R-3.7
High-density batt
R-15 in a 3.5 in wall
About R-4.3
High-density batt
R-21 in a 5.5 in wall
About R-3.8
Blown-in fiberglass
Attic loose fill
About R-2.2 to R-2.9
Exact blown-in numbers vary by brand, so always check the chart printed on the bag.
Why Does Blown-In Fiberglass Need More Depth Than Batts?
Because it’s fluffier. Loose fill is blown in at a low density so it can spread across an attic floor and around every wire and joist. Lower density means bigger air pockets, which means a lower R-value per inch.
That trade is usually worth it in an attic. Batts leave seams and gaps at every joist and wire. Blown-in fills those gaps, so the real-world performance often beats a batt layer with the same label. Our guide to blown fiberglass insulation covers it in more detail.
Does Installation Quality Change Fiberglass R-Value?
Yes, a lot. Home energy raters grade insulation installs from Grade I, which is near perfect, down to Grade III, which has gaps and squashed spots. A batt with gaps around the edges loses a real share of its label R-value.
That’s why we fuss over fit. Every batt should touch all four sides of the bay, with no voids at the top. Our page on batt insulation shows what a clean install looks like.
How Deep Does Fiberglass Need to Be for Common R-Values?
Rough depths, using mid-range numbers:
R-13 batt: 3.5 inches, made for 2×4 walls. See R-13 insulation.
R-21 batt: 5.5 inches, made for 2×6 walls.
R-38 blown-in: about 13 to 16 inches.
R-49 blown-in: about 17 to 20 inches. See R49 insulation.
For an attic in a Denver home, plan on the deeper end.
Is a Higher R-Value per Inch Always Better?
Not on its own. You’ll hear that the densest batt is always the best buy. That idea skips the most important part, which is fit. A high-density batt with gaps around it can lose more heat than a lower-density layer that fills every corner.
If your attic were ours, we’d blow in fiberglass to the right depth and air seal first. In walls, we’d pick the batt that fills the cavity exactly, without squeezing. A squeezed batt loses R-value, so a thick batt crammed into a thin wall is money wasted.
Does Blown-In Fiberglass Lose R-Value Over Time?
A little, if it was installed too thin. Loose fill settles slightly, which is why bag charts list both an installed depth and a settled depth. Installers should blow to the installed depth so the settled layer still meets the target.
Denver’s dry air works in fiberglass’s favor here. It doesn’t soak up moisture the way cellulose can, so it holds its loft well in our attics. Our attic depth guide shows how to check yours.
How Much Fiberglass Does a Denver Attic Need?
Most Denver attics should land between R-49 and R-60. That works out to roughly 17 to 22 inches of blown fiberglass, depending on the brand. Many older homes we visit have only 4 to 8 inches, which is closer to R-11 to R-19.
You don’t have to start over. Dry, clean fiberglass can stay, and new blown-in goes right on top until you hit the target depth. For the whole-house picture, see how much insulation you need. A quick look with a tape measure in a few spots across the attic tells you where you stand.
When Should You Choose Batts vs Blown-In Fiberglass?
Use the space to decide. A quick guide:
Open walls and floors: batts. They fit stud bays neatly and give more R per inch. Our batt insulation crews do this daily.
Attic floors: blown-in. It fills gaps and is easy to top up. Our blown-in insulation service handles it.
Closed walls: blown-in through small holes, done by a pro.
Frequently Asked Questions About Fiberglass R-Value per Inch
Yes. Squeezed fiberglass loses some of its rated R-value.
Pick the Right Fiberglass Insulation R-Value per Inch for Each Space
Batts pack more R into each inch and suit walls. Blown-in needs more depth but fills attics better. Choose by the space, install to the full depth, and seal the leaks first.
Want the right depth in your Denver home? Grizzly Insulation Co can help. We’re BPI Certified and an Xcel Energy rebate partner. Call (303) 854-6221.
How Fiberglass Insulation R-Value per Inch Compares Between Batts and Blown-In
Fiberglass batts give about R-3.1 to R-4.3 per inch, depending on how dense they are. Blown-in fiberglass gives less, usually about R-2.2 to R-2.9 per inch. So to hit the same R-value, blown fiberglass needs a deeper layer. That’s fine in an attic, where you have room, and it’s why batts usually win in walls.
People see two very different depths for the same R-value and assume one bag is lying. Neither is. Our Grizzly Insulation Co crews install both kinds in Denver homes every week, so here’s the plain math behind the numbers.
What Does R-Value per Inch Mean for Fiberglass Insulation?
It’s how much heat resistance you get from each inch of material. Multiply it by the thickness and you get the total R-value. If the term is new, our page on what R-value means keeps it simple.
Fiberglass works by trapping still air between glass fibers. Packing the fibers tighter traps the air in smaller pockets, which slows heat a bit more. That’s the whole reason density changes the per-inch number.
How Do Fiberglass Batts and Blown-In Compare per Inch?
Here’s the side-by-side, using the batt sizes the Department of Energy lists in its guide to types of insulation:
Exact blown-in numbers vary by brand, so always check the chart printed on the bag.
Why Does Blown-In Fiberglass Need More Depth Than Batts?
Because it’s fluffier. Loose fill is blown in at a low density so it can spread across an attic floor and around every wire and joist. Lower density means bigger air pockets, which means a lower R-value per inch.
That trade is usually worth it in an attic. Batts leave seams and gaps at every joist and wire. Blown-in fills those gaps, so the real-world performance often beats a batt layer with the same label. Our guide to blown fiberglass insulation covers it in more detail.
Does Installation Quality Change Fiberglass R-Value?
Yes, a lot. Home energy raters grade insulation installs from Grade I, which is near perfect, down to Grade III, which has gaps and squashed spots. A batt with gaps around the edges loses a real share of its label R-value.
That’s why we fuss over fit. Every batt should touch all four sides of the bay, with no voids at the top. Our page on batt insulation shows what a clean install looks like.
How Deep Does Fiberglass Need to Be for Common R-Values?
Rough depths, using mid-range numbers:
For an attic in a Denver home, plan on the deeper end.
Is a Higher R-Value per Inch Always Better?
Not on its own. You’ll hear that the densest batt is always the best buy. That idea skips the most important part, which is fit. A high-density batt with gaps around it can lose more heat than a lower-density layer that fills every corner.
If your attic were ours, we’d blow in fiberglass to the right depth and air seal first. In walls, we’d pick the batt that fills the cavity exactly, without squeezing. A squeezed batt loses R-value, so a thick batt crammed into a thin wall is money wasted.
Does Blown-In Fiberglass Lose R-Value Over Time?
A little, if it was installed too thin. Loose fill settles slightly, which is why bag charts list both an installed depth and a settled depth. Installers should blow to the installed depth so the settled layer still meets the target.
Denver’s dry air works in fiberglass’s favor here. It doesn’t soak up moisture the way cellulose can, so it holds its loft well in our attics. Our attic depth guide shows how to check yours.
How Much Fiberglass Does a Denver Attic Need?
Most Denver attics should land between R-49 and R-60. That works out to roughly 17 to 22 inches of blown fiberglass, depending on the brand. Many older homes we visit have only 4 to 8 inches, which is closer to R-11 to R-19.
You don’t have to start over. Dry, clean fiberglass can stay, and new blown-in goes right on top until you hit the target depth. For the whole-house picture, see how much insulation you need. A quick look with a tape measure in a few spots across the attic tells you where you stand.
When Should You Choose Batts vs Blown-In Fiberglass?
Use the space to decide. A quick guide:
Frequently Asked Questions About Fiberglass R-Value per Inch
Batts run about R-3.1 to R-4.3. Blown-in runs about R-2.2 to R-2.9.
The color is just the brand’s dye. Density sets the R-value. See why insulation is pink.
It has less R per inch but fills gaps better. In attics it often performs better overall.
About 13 to 16 inches, depending on the brand. Check the bag chart.
Yes. Squeezed fiberglass loses some of its rated R-value.
Pick the Right Fiberglass Insulation R-Value per Inch for Each Space
Batts pack more R into each inch and suit walls. Blown-in needs more depth but fills attics better. Choose by the space, install to the full depth, and seal the leaks first.
Want the right depth in your Denver home? Grizzly Insulation Co can help. We’re BPI Certified and an Xcel Energy rebate partner. Call (303) 854-6221.