Which is better for an exterior 2x4 wall?

The Knauf faced batt. Its kraft facing is the vapor retarder, so you skip a separate poly sheet. Put the facing on the warm side, which is the interior in cold climates.

Which is better for an attic?

The unfaced R13. Facing in an attic can trap moisture against the roof deck. Use unfaced batts and keep a separate air barrier.

Can I stack two unfaced batts for more R-value?

Yes. Two unfaced R13 batts give about R-26 in a 2x4 cavity only if the cavity is deep enough; in a true 2x4 wall you are limited to R13 to R15.

Is the Knauf worth the extra cost?

Usually yes on exterior walls, because it replaces a separate vapor barrier and staples in place. On interior walls or attics, the unfaced batt does the same job for less.

Does the 23 in. width fit my studs?

23 in. fits 24 in. on-center framing with a friction fit. For 16 in. on-center studs, cut it down or buy a 15 in. batt.

Are both GREENGUARD certified?

Yes. Both list GREENGUARD certification, so indoor air quality is not a deciding factor here.

Knauf R-13 Kraft Faced Insulation vs R13 Unfaced Fiberglass Batt Insulation

Buy the Knauf faced batt for standard 2x4 walls where you want a built-in vapor retarder and a clean fit in 16 in. or 24 in. on-center framing. Buy the unfaced batt if you already have a separate vapor barrier, are insulating an open attic or interior wall, or need to stack two layers. The Knauf is a known 23 in. by 93 in. kraft-faced batt rated 4.7 from 101 ratings; the unfaced R13 is a plain 2x4 wall batt with no facing and no listed size or rating.

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