Flash and Batt in Zone 5: How Much Foam Before the Batts

What flash and batt is

A "flash" coat is a thin layer of spray foam against the inside of the sheathing. The rest of the stud or rafter bay is filled with fiberglass or mineral wool batts. Done with closed-cell foam, it gets you most of what full-depth foam does for air sealing at a lower price, because foam is the expensive part and batts add R-value cheaply.

It only works if the foam is thick enough. Too thin, and the foam's inside face gets cold enough in winter for moisture to collect on it, right where you can't see it.

Where the dew point lands in a wall

Picture a 2×6 wall: siding, OSB sheathing, closed-cell foam against the OSB, fiberglass filling the rest of the 5½-inch bay, painted drywall. Indoor air is 70°F at 35% relative humidity, with a dew point of about 41°F.

Water vapor from the house diffuses out through the drywall and the batts until it reaches the foam. Closed-cell slows it to a crawl, so the foam's inside face is the first cold surface it meets. That face has to stay above 41°F.

These are steady-state numbers using about R-7 per inch for the foam (a typical aged value for HFO closed-cell), R-3.7 per inch for fiberglass and ordinary values for siding, sheathing, drywall and air films. Erie's January average is 28°F.

A cold night isn't the whole story. Moisture builds up over weeks, not hours, which is why the code's own condensation rules for roofs are written around monthly averages. The month-long average is the number to design to, and a cold snap shows how much margin you have.

What the numbers mean

1 inch is the minimum commonly cited for zone 5. Martin Holladay, answering the question in the Journal of Light Construction, puts the minimum at 1 inch of closed-cell in zone 5 and 2 inches in zone 7. On an average January day its face sits right at the dew point of 35% indoor air, and below it if the house runs at 40% (a 45°F dew point). It can work, but it leans on a dry house and a good room-side vapor retarder.

2 inches keeps the face about 9 degrees above the dew point on an average January day, and still about at it on a 10°F night. This is the thickness with room for a humid winter.

3 inches gives the most margin, but leaves only about R-9 of space for batts in a 2×6, so it costs close to full-depth foam for less total R-value.

What the code asks for walls

In zone 5, the IRC requires a Class I or II vapor retarder on the interior side of frame walls. Kraft facing on batts rates about 1.0 perm, which makes it Class II. The code allows a Class III retarder such as latex paint instead when the wall has vented cladding (vinyl, polypropylene or horizontal aluminum siding, brick veneer with an air space, or a rainscreen) or enough continuous insulation outside the sheathing: R-5 over a 2×4 wall, R-7.5 over a 2×6.

For flash and batt, the practical rule is the one Holladay gives: choose "an interior vapor retarder that allows drying to the interior," such as kraft facing or vapor-retarder paint. Not sheet polyethylene. With closed-cell on the sheathing side, the wall already dries slowly outward, so poly on the room side would close off the other direction.

Flash and batt under a roof

Roofs have a hard number. IRC R806.5 item 5.1.3 allows foam against the roof deck with batts or blown insulation under it only if the foam meets the R-value in Table R806.5 for condensation control: R-20 in zone 5. At about R-7 per inch, that is just under 3 inches of closed-cell. Closed-cell at that thickness is also a Class II vapor retarder, which item 4 of the same section requires in zones 5 to 8. The batts then go directly under the foam.

Less than R-20 of foam under a roof deck is not a judgment call; it doesn't meet the code here. Sealed attics covers the rest of R806.5, and open-cell under a roof in Western PA covers why open-cell can't be the flash layer.

Details that make or break it

1. Batts touch the foam. An air gap between them lets air move and puts the cold surface in reach. Thicker batts, gently compressed, stay in contact better.
2. The flash is even. A thin spot is a cold spot. Ask how thickness will be checked.
3. Plates and penetrations are foamed too. The top plate, the rim and every wire and pipe hole are where air leaks.
4. Closed-cell, not open-cell. Open-cell lets vapor through to the cold sheathing; it can't do the flash job in this climate.

When flash and batt is the wrong choice

• Metal buildings and pole barns. Closed-cell goes directly against the steel, with no batts; see why your pole barn roof drips.
• Shallow rafters. A 2×6 cathedral ceiling has room for 3 inches of foam and only about R-9 of batt, about R-30 in all, well short of the R-49 the 2018 code asked of ceilings; full-depth foam or insulation above the deck is usually the answer.
• Basements and rim joists, where foam alone is the simpler, better assembly: see rim joists and basement walls.

For how much each inch of foam does on its own, see how thick spray foam should be.

Want a number for your own walls or roof? Price My Project or call (814) 208-3478.