Which is better for an attic?

The Knauf R-30. Its 10 in. thickness gives R-30 in a single layer, which is what most attics need. The Owens Corning R-21 would need a second layer to reach similar R-values.

Which fits 24 in. on-center walls?

The Owens Corning R-21. Its 23 in. width is made for 24 in. framing, while the Knauf's 16 in. batts suit 16 in. on-center cavities.

Is the higher R-value worth the extra cost?

Compare price per square foot at the R-value you need. R-30 does the work of roughly 1.4 layers of R-21, so the Knauf wins if its pack price is under about 1.4 times the Owens Corning's.

Do both have a vapor barrier?

Yes. Both are kraft-faced with a vapor retarder. Face the kraft side toward the heated space and follow local code for your climate.

Which is better for sound?

Both absorb sound. The Owens Corning lists acoustic insulation as a use, and its thicker 5.5 in. batts in walls help with airborne noise.

Both are glass batts that last the life of the building if kept dry. Neither degrades with age; moisture and compression are what shorten performance.

Knauf R-30 Kraft Faced Fiberglass Insulation vs Owens Corning R-21 Faced Fiberglass Insulation

Buy the Knauf R-30 for attic and ceiling jobs where you want the most heat resistance per square foot; buy the Owens Corning R-21 for standard 24-inch-on-center wall cavities. The Knauf covers 69.33 sq ft per pack at R-30, while the Owens Corning covers 89.13 sq ft at R-21 in wider 23 in. batts. Compare live pack prices across stores before you buy, since both are sold in multi-batt bundles.

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