Open-Cell Spray Foam Under a Roof in Western PA: The Catch

Air barrier and vapor retarder are two different jobs

Both foams stop air once they are thick enough: closed-cell at about 1 inch, open-cell at about 3½ to 5½ inches depending on the product. That fixes the biggest moisture problem in a cold attic, warm indoor air leaking up through the ceiling.

Water vapor also moves through solid materials, by diffusion, from the warm humid side to the cold dry side. Materials are graded in perms (ASTM E96):

Open-cell foam holds the air back and lets the vapor walk through. Our article on whether spray foam traps moisture has the manufacturer figures behind the foam rows.

Where the dew point lands in an Erie roof

Erie's January normal is about 28°F (average high 35°F, average low 21°F). Under shingles, with about R-21 of foam below it, the sheathing runs only a couple of degrees warmer than the outdoor air, about 30°F on a January average. On clear nights it gets colder than the air, because the roof radiates heat to the sky.

Inside, 70°F at 35% relative humidity has a dew point of about 41°F. At 40% it is about 45°F; at 50%, about 50°F.

The foam's room-side face stays near 69°F, and the temperature falls steadily through the foam toward the deck. With either foam, the 41°F plane sits inside the foam: about 1.6 inches below the deck in full-depth open-cell, about 0.8 inch in 3 inches of closed-cell. Everything above that plane is colder than the dew point of the air diffusing through it. The difference is how much vapor gets there.

A steady-state estimate for a normal Erie winter puts the vapor reaching the sheathing through full-depth open-cell at roughly 14 to 34 times what gets through 3 inches of closed-cell. The moisture math case study shows the month-by-month numbers and their limits.

Building Science Corporation puts it in one line for closed-cell roofs: "In cold climates the ccSPF provides the first condensing plane." Closed-cell is built to take that job. Open-cell is not, so the code asks for a vapor retarder over it.

What the code requires

Pennsylvania's Uniform Construction Code is based on the International Residential Code. Section R806.5 allows an unvented attic or rafter assembly (a sealed attic) when it meets a list of conditions. Two matter here:

• Item 4: "In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation."
• Item 5.1.3 and Table R806.5: when foam goes against the deck with batts or blown insulation under it, the foam must provide at least R-20 in zone 5 for condensation control. At about R-7 per inch, a typical aged value for HFO closed-cell, that is just under 3 inches. Flash and batt in zone 5 works through that hybrid.

An ignition-barrier coating is a fire product. Unless its data sheet gives a tested permeance of 1.0 perm or less, it is not also a vapor retarder. See thermal barriers and ignition barriers.

Why the job gets sold without it

None of this is new in the trade. The step gets left out for ordinary reasons:

• Open-cell costs less per inch and goes on faster, so the quote wins.
• One pass fills a 2×10 bay. Closed-cell at R-20 takes two or three passes with cure time between them.
• "It breathes" is an easy line. It is true for vapor, which is the problem.
• The coating is a second product, often a second visit, and easy to leave off a low quote.
• Many attic jobs are done without a permit, so nobody checks R806.5.

The homeowner sees a bay full of foam, the house feels tighter, the bill is lower. If the sheathing later fails, the story becomes "spray foam rotted my roof." The foam did what open-cell foam does. The job left out the part that protects the wood.

What the record shows

In a 2012 GreenBuildingAdvisor thread, a Pittsburgh-area homeowner described open-cell foam sprayed on the roof deck in 2007. In 2011 a water spot led to the foam being cut back, and they found "a very large air gap in which the foam was not adhered to the decking. Mold and wetness was present." An engineer traced the mold to "condensation and voids," and all of the foam was removed in 2012. The site's editor answered that open-cell "should not be used in Climate Zones 5 or colder to create an unvented conditioned attic," citing Building Science Corporation's warnings, including that "vapor-retarder paint does NOT work on cured foam."

The other side is fair too. Allison Bailes, writing on the same site in 2014, argued that "open-cell spray foam is not the enemy" and that the moisture in failed roofs comes from inside the house, not through the shingles. His advice for zone 5 and colder: don't use open-cell under the roof "unless you also install a vapor retarder over it."

So the disagreement is about whether a coating on open-cell is reliable enough. Neither source recommends open-cell alone under a cold roof deck.

How wet is too wet? Home Innovation Research Labs treats OSB and plywood as dry below 16% moisture content and calls 20% the long-recognized threshold, with a margin of safety, for keeping decay from starting. Mold risk on OSB starts near 13%. The ridge, and the north slope, which sees the least sun, are the places to check.

If you already have open-cell under your roof

Some open-cell roofs in zone 5 do fine, often because the house runs dry in winter. Measure instead of guessing:

1. In February or March, take pin-type moisture meter readings on the sheathing at the ridge, on the north slope and near the eaves (pull back a small patch of foam where needed and seal it afterward).
2. Under 16% is good news. Over 20% means the assembly is not keeping up.
3. Keep winter indoor humidity at 30 to 35%: run bath and kitchen fans, vent the dryer outside, turn down any humidifier.
4. If the foam face is clean and reachable, a rated Class II vapor-retarder coating is the fix the code describes. Ask for the product's tested permeance and coverage rate in writing.
5. If readings are high and the sheathing is soft, the repair starts on the roof: replace the damaged panels, then change the insulation plan below.

What to ask before you sign

A quote for foam under a roof deck in Western PA should say, in writing:

• Open-cell or closed-cell, and how thick.
• For closed-cell: whether the thickness reaches R-20, and the aged R-value per inch on the data sheet.
• For open-cell: which vapor-retarder coating, its tested permeance (1.0 perm or less), and the coverage rate.
• Whether the attic also needs an ignition or thermal barrier, and what covers that.
• Whether a permit will be pulled and who inspects it.

A contractor who sprays both foams and quotes closed-cell for your roof deck is not upselling you. In this climate it is the assembly that passes without a second product on top. Our guide to choosing a spray foam contractor covers the rest of the questions worth asking.

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