Fiberglass Kraft Faced Insulation

Kraft-faced fiberglass is thermal and acoustical insulation with a paper vapor retarder bonded to one side. Kraft facing is the brown paper layer stapled or friction-fit toward the conditioned side of a wall, floor, or ceiling assembly. It is not the insulation itself; it is the facing attached to the insulation to help slow moisture movement and make installation easier.

For Montana builders, one decision matters more than every other: faced or unfaced, and which side the paper points. Get it wrong and you trap moisture in a wall through eight months of heating season. Get it right and the wall stays dry.

Here’s the short version. Use faced kraft-paper insulation for new exterior walls in cold climates (zones 4 Marine through 8) with the facing toward the warm interior side. Use unfaced when adding insulation over existing, in hot-humid climates (zones 1–3), for interior walls, or when another vapor retarder is already present. The cardinal rule: never trap a vapor retarder between two insulation layers — it creates a moisture sandwich that can’t dry in either direction. Montana sits squarely in cold climate territory, so the warm-side-facing-in rule applies to nearly every exterior wall built here.

What the Facing Actually Does

The paper is a vapor retarder. Nothing more, nothing less. Faced insulation is a standard fiberglass or mineral wool batt with kraft paper laminated to one side. That paper layer is a Class II vapor retarder with a permeance of approximately 1 perm — it slows moisture vapor from passing through the insulation from the warm, humid side of the assembly to the cold side. Under ASTM E 96, kraft facing tests at 1.0 perm permeability.

Builders confuse this constantly. Three things the facing is not:

• Not an air barrier. Kraft paper has staple holes, seams, and isn’t sealed at edges. It does almost nothing for air leakage. For air sealing, you need caulk, foam, or tape at every penetration and gap — the insulation facing doesn’t substitute.
• Not a moisture barrier. At ~1 perm, kraft facing slows moisture movement — it doesn’t stop it. Bulk water (leaks, flooding) passes right through.
• Not optional in cold climates. The 2021 IRC (Section R702.7) requires a vapor retarder on the warm side of exterior wall insulation in climate zones 5 and above. Kraft facing satisfies this requirement.

The fiberglass itself is the part that does the insulating, and it holds up well in unheated crawlspaces, garages, and basements. It won’t rot, mildew or deteriorate, and is also noncorrosive to pipes, wiring and metal studs. Fiberglass won’t retain moisture, which can cause permanent loss of R-value in other products.

The physics behind the warm-side rule: moisture vapor moves from warm/humid air toward cold/dry air. In winter in a cold climate, that means from inside to outside. The vapor retarder goes on the warm side to slow this movement before moisture reaches a cold surface and condenses. In Montana, the warm side is the interior. Always.

Faced vs. Unfaced — Where Each One Belongs

Faced batts earn their keep in a handful of specific spots. New exterior wall construction in cold climates, kraft facing toward the interior. This is the most common application and the scenario the product was designed for. The facing provides the Class II vapor retarder required by code in zones 5+.

Cathedral ceilings: faced batts between rafters with the kraft paper toward the interior heated space. In vented cathedral ceiling assemblies, you still need a ventilation channel between the insulation and the roof deck. For floors over unconditioned spaces, when insulating a floor above an unheated crawl space or garage, faced batts go between the joists with the facing UP — toward the heated room above.

Unfaced is the right call in several cases, and the most important one shows up constantly in older Montana homes with attic insulation already in place. Adding insulation over existing insulation is the most critical unfaced application. If you’re blowing cellulose or fiberglass on top of existing attic insulation, or adding a second layer of batts, the new insulation must be unfaced. A vapor retarder sandwiched between insulation layers traps moisture with no drying path.

Same goes for interior partitions and any wall that already has a retarder. Walls between two conditioned rooms have no temperature differential and no meaningful moisture drive. Vapor retarders are unnecessary. Use unfaced batts — typically for soundproofing purposes only. When another vapor retarder already exists — polyethylene sheet, vapor-retarder paint, or foil-faced sheathing — adding kraft-faced batts creates a double vapor retarder. Use unfaced.

The reason layering two retarders fails: when installing layers of insulation, there should only be one vapor retarder installed in the appropriate location. It’s not good practice to incorporate two vapor retarders in an insulation system. Moisture could become trapped between the two layers and not be able to dry.

R-Values, Sizes, and Forms

Kraft-faced comes two ways. Pre-cut batts fit standard wall cavities and are faster to install than roll products. Rolls can be cut to fit any size cavity and installed in any part of a building. Johns Manville Formaldehyde-Free Kraft-faced thermal and acoustical insulation offers R-Values up to R-49.

Batt width should match the framing — assemblies framed with 12” to 19.2” on-center cavities, wide-spaced wood trusses or I-joists all work, along with standard 16” and 24” o.c. wood and metal framing. Standard product lengths include 48”, 93” and 96” batts.

The R-value table below covers the full range. Thicker batt, higher R, deeper cavity required.

R-Value	RSI	Thickness	Common Framing
R-11	1.9	3-1/2”	16, 24 o.c. wood/metal
R-13	2.3	3-1/2”	16, 24 o.c. wood/metal
R-15	2.6	3-1/2”	15”, 23” wood
R-19	3.3	6-1/4”	16, 24 o.c.
R-21	3.7	5-1/2”	15”, 23” wood
R-22	3.9	7-1/2”	15, 19, 23 wood
R-25	4.4	8-1/2”	15”, 23” wood
R-30	5.3	8-1/4”–10-1/4”	12, 16, 19, 24 o.c.
R-38	6.7	10-1/4”–13”	16, 24 o.c.

Western Building Center’s lineup covers the common wall and floor values. Stock includes R-13 kraft batts and rolls in both 16” o.c. and 24” o.c. widths (3-1/2” x 15” x 93” and 3-1/2” x 23” x 93”), R-19 kraft rolls in 15” widths, and R-21 kraft batts at 5-1/2” thick in both 15” and 23” widths for 2x6 wall framing. The R-21 in a 2x6 cavity is the workhorse for Montana exterior walls where the deeper stud gives room for more R.

Installation Basics and the One Safety Rule

Facing orientation comes first and never bends in a heating climate: paper toward the warm interior. When using faced insulation, the facing or paper will typically face the warm-in-winter side of the assembly. Consult your local codes for the use of a vapor retarder and the proper installed orientation. For floors over unheated space, that means facing up toward the heated room.

Cutting and fitting is straightforward. Kraft-faced insulation cuts easily with an ordinary utility knife. Stapling tabs are provided for attachment to wood framing. The insulation can also be installed with fasteners or simply pressed in place between studs or joists. On walls, place the batts into the wall cavity and staple the flanges of the batts to the inside or face of the studs about every 12 inches. Overhead in ceilings, tighten the schedule — staple the flanges to the joists on both sides every 6” - 8”.

Fit it snug, but never crush it. It’s very important that it fits snugly on all sides. If the insulation is too long for a space, cut it to the correct size. If it is too short, cut a piece to fill the void. Expand the insulation to its full thickness in the cavity to insure complete coverage. Avoid compressing the insulation material, because compression will reduce its R-value. Tight gaps get filled with scrap unfaced pieces, not jammed batts.

Now the rule that gets people in trouble. The kraft paper burns. The kraft facing on this product will burn and must not be left exposed. It must be covered with gypsum board or another approved interior finish. Where an exposed application is required, use FSK-25 flame-resistant faced insulation. An unfinished basement or garage wall with bare kraft showing is a code problem and a fire problem both.

Keep clearances around heat. Leave a 3” space between the insulation and electric motors or lighting fixtures. Only IC (Insulation Contact) rated lighting fixtures may be covered or be in direct contact with insulation. Use only unfaced fiberglass insulation between wood framing and masonry chimneys. Do not place insulation in air spaces surrounding metal chimneys/flues or fireplaces.

One more for the jobsite: store it dry. Store insulation indoors. Keep insulation clean and dry at all times. Wet batts before they go in the wall are wasted batts.

Frequently Asked Questions

Which way does the kraft paper face in a Montana wall? Toward the warm interior. In winter in a cold climate, moisture moves from inside to outside. The vapor retarder goes on the warm side to slow this movement before moisture reaches a cold surface and condenses. In a heating-dominated climate, the warm side is always the inside of the house.

Can new faced batts go over the old attic insulation? No. The new insulation must be unfaced. A vapor retarder sandwiched between insulation layers traps moisture with no drying path — the moisture sandwich. Use unfaced for any second layer.

Is the kraft facing an air barrier? No. Kraft paper has staple holes, seams, and isn’t sealed at edges. It does almost nothing for air leakage. For air sealing, you need caulk, foam, or tape at every penetration and gap.

Can kraft-faced insulation be left exposed in a garage or unfinished basement? No. The kraft facing will burn and must not be left exposed. It must be covered with gypsum board or another approved interior finish. Where an exposed application is required, use FSK-25 flame-resistant faced insulation.

Will fiberglass rot or lose R-value if it gets damp in a crawlspace? The glass itself holds up. Fiberglass will not retain moisture, which can cause permanent loss of R-value in other products. It won’t rot, mildew or deteriorate, and is noncorrosive to pipes, wiring and metal studs. Bulk water and leaks are still a problem to fix at the source.

What R-value goes in a 2x6 exterior wall? R-21 is the standard fit. The 5-1/2” thick kraft batt matches a 2x6 cavity and comes in both 15” and 23” widths for 16” and 24” on-center framing.

Does compressing a batt to fit hurt it? Yes. Avoid compressing the insulation material, because compression will reduce its R-value. Cut it to fit instead of stuffing an oversized batt into a tight bay.