Insulation Compression Calculator
The Insulation Compression Calculator tells you the real, effective R-value of a fiberglass batt once it's squeezed into a framing cavity shallower than the batt's labeled thickness. Pick your batt's rated R-value and the cavity depth (or a standard framing size like 2x4 or 2x6), and it instantly shows the compressed R-value, how much you lose, and the share of rated performance you keep — using the standard NAIMA / Insulation Institute compression chart, so you don't have to hunt down a manufacturer PDF. It runs entirely in your browser — no signup, no app to install.
Tap your batt's rated R-value and the cavity depth (a framing preset like 2x4/2x6, or an exact number of inches). The effective R-value, the R-value lost, and the percentage of rated performance kept update instantly above — based on the standard NAIMA compression chart.
Know your batt and your wall depth? Get the effective R-value in two taps.
Use the free Insulation Compression Calculator →What Is Insulation Compression?
Fiberglass batts are manufactured to a specific loft — an R-19 batt is about 6.25 inches thick, an R-21 batt is 5.5 inches, and so on. Insulation compression happens when you stuff a batt into a cavity that's shallower than that labeled thickness: around wiring, behind a shallow soffit, or when a thicker batt is used in a smaller stud bay. The batt still fills the space, but it no longer delivers its full rated R-value — and the amount of the shortfall is exactly what this calculator reads off the standard fiberglass insulation compression chart.
How to Use the Calculator
There are just two inputs, and the result updates the moment you tap — no submit button:
- Open capsuletools.app/compressed-insulation-calculator/
- Batt's rated R-value — tap the R-value printed on your fiberglass batt (R-11 through R-49)
- Cavity depth — tap a standard framing size (2x4, 2x6, 2x8, 2x10, 2x12) or type the exact depth in inches
The result shows the effective (compressed) R-value, the R-value lost to compression, and the percentage of the batt's rated value you keep. If the cavity is actually deeper than the batt, the tool tells you the batt isn't compressed at all and keeps its full R-value.
Cavity: 2x6 wall (5.5″ deep)
Effective R-value = R-18
R-value lost = 1 · kept = 95%
Why Compressed Insulation Loses R-Value
When you compress fiberglass, the glass fibers pack closer together. That actually raises the R-value per inch a little — but because you now have fewer inches of insulation in the wall, the total R-value drops. This is why compressing insulation reduces R-value overall, even though the material itself hasn't lost any quality. A uniformly compressed batt can still be a Grade I installation; you just have to rate the wall at the lower compressed number, which is the figure this tool returns. It's an estimate from aggregated manufacturer data — for a spec-critical job, your specific product's compression sheet always takes precedence.
R-Value by Framing Size (2x4, 2x6, 2x8, 2x10, 2x12)
A few of the most common batt-in-cavity combinations, straight from the standard chart. Cavity depths are the actual (dressed) lumber depths, not the nominal name:
| Batt | Cavity | Effective R-value |
|---|---|---|
| R-19 (6¼″) | 2x6 (5½″) | R-18 |
| R-21 (5½″) | 2x4 (3½″) | R-15 |
| R-25 (8″) | 2x6 (5½″) | R-21 |
| R-30 (9½″) | 2x8 (7¼″) | R-25 |
| R-38 (12″) | 2x10 (9¼″) | R-32 |
The big takeaway: matching the batt to the cavity beats compressing a thicker one. R-13 or R-15 batts are made to fill a 2x4 wall at full value, and R-21 fills a 2x6 wall — forcing an R-19 into a 2x4, by contrast, is heavy compression that drops it below where the standard chart even lists it.
Frequently asked questions
Does compressing insulation reduce R-value?
Yes. When a fiberglass batt is compressed below its labeled thickness, its total R-value drops. The R-value per inch actually rises a little as the fibers pack tighter, but because you now have fewer inches of insulation, the overall R-value comes out lower than the number printed on the batt. For example, an R-19 batt squeezed into a 2x6 (5.5-inch) cavity performs at about R-18.
How much R-value do you lose when compressing insulation?
It depends on the batt and how far it's compressed. A small squeeze loses little: an R-19 batt in a 2x6 wall drops only to about R-18. A large squeeze loses a lot: an R-21 batt forced into a 2x4 (3.5-inch) cavity falls to about R-15, losing roughly a quarter of its rated value. This calculator gives the exact effective R-value and the amount lost for your batt and cavity, based on the standard NAIMA compression chart.
Can you put R19 in a 2x4 wall?
You can physically fit it, but it isn't a good idea. An R-19 batt is about 6.25 inches thick and a 2x4 stud cavity is only 3.5 inches deep, so the batt is heavily compressed — dropping to roughly R-14, below where the standard chart even lists it — and it tends to bulge past the studs and interfere with the drywall. R-13 or R-15 batts are made to fill a 2x4 wall at full performance.
What R-value can you get in a 2x6 wall?
A 2x6 stud cavity is 5.5 inches deep. An R-21 batt is sized exactly for it and delivers its full R-21. An R-19 batt (6.25 inches) fits with slight compression and performs at about R-18. So for a standard wood-framed 2x6 wall filled with a single fiberglass batt, R-21 is the practical maximum.
What happens when you compress fiberglass insulation?
The glass fibers pack closer together, so R-value per inch goes up slightly while total R-value goes down, because there is less thickness overall. Compression doesn't ruin the insulation — a uniformly compressed batt can still be a quality (Grade I) installation — but you get less R-value than the label promises, so you should use the compressed figure when checking whether a wall meets its target.
Use the free Insulation Compression Calculator →
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