Joe Watson lives in a three-story 1993 house in Richmond, Virgina, with a walkup attic, part of which he’d like to turn into living space. The question is how.
“I am looking to insulate all the rafters and bring the HVAC into semi-conditioned space,” he writes in a Q&A post at GreenBuildingAdvisor. “I would get a minisplit system for the 20×20 room that would be drywalled in on the third floor. (The rest of the attic space would be left unfinished with a fire barrier of the insulation).”
He’s been told that R-30 will be adequate for his Climate Zone 4 home, and Watson so far is looking at two insulation options: fiberglass batts and spray foam.
“If I go fiberglass, I would attach 2x4s to the 2×8 rafters to give me a space of 10.75 inches to add insulation and leave a gap for a vent from soffit to ridge,” Watson says. “If I go spray foam (professionally done only) I am unsure on open-cell vs. closed-cell in my climate zone. Two of the three quotes I got said open-cell, but could do closed-cell for more $$$ (shocker). I get my third quote tomorrow.
“After reading so many posts/blogs/articles, I am lost on the open-cell vs. closed-cell question for my area,” he continues. “We get really hot in the summer, and, last winter hit the low teens at night (unusual, though). Any thoughts?”
That’s the topic for this Q&A Spotlight.
R-30 worth of insulation just isn’t enough
No matter what his local building office says, Watson’s climate zone calls for more insulation than R-30, says GBA senior editor Martin Holladay. “In your climate zone,” he writes, “the 2012 International Residential Code calls for a minimum of R-49 roof insulation — so even if your local code official is fine with R-30, it would be much better if you could aim higher.”
Watson could use either open-cell or closed-cell spray foam insulation, Holladay adds, but he should be aware of “deceptive” sales pitches from spray-foam contractors.
“It’s common for these contractors to try to convince homeowners that ‘R-20 spray foam performs as well as R-38’ and other similar nonsense,” Holladay says, providing a link to a relevant article: It’s OK to Skimp On Insulation, Icynene Says.
Dana Dorsett thinks that closed-cell foam would be a waste of money in this situation because thermal bridging through the rafters will erode so much of its insulation potential.
“Using R-6 per inch closed-cell foam where it is thermally bridged by framing is a waste of good foam, since its performance is so severely undercut by the framing fraction,” Dorsett says. “With 7 inches (R-42) of closed-cell foam in a 2×8 rafter, you’d almost make code minimum on an R-value basis, but would still be a long way away from making it on a performance basis (a U-factor of U-0.026, which is a whole-assembly R of about R-38) due to that thermal bridging.”
Instead, Dorsett recommends open-cell foam trimmed to the rafter edges along with an air barrier in the form of painted wallboard, OSB or a smart vapor retarder such as MemBrain or Intello. After that, he suggests adding 2x6s perpendicular to the rafters and adding a layer of rock wool or R-21 fiberglass insulation. A big advantage of open-cell foam, he says, is that it’s blown with water rather than a hydrofluorocarbon, a powerful greenhouse gas.
Choosing the right batts
Should Watson choose batt insulation, he’ll have several choices, including fiberglass in several densities or rock wool.
“If you’re going with soffit-to-ridge venting everywhere, you can go all-fiberglass, but not with low-density R-19s or R-22s,” Dorsett writes. “R-19s are crap — they only perform at R-18 when compressed to 5 1/2 inches in a framing bay (!), and perform at R-13 compressed into a 2×4 bay. They are a “fluffed” R-13 (same weight per square foot), which makes them more of an air-filter than an air-retarder. They are way too air-permeable to use without an exterior air barrier. R-22s are about as bad, performing at only R-19 when compressed to 5 1/2 inches.”
The only batt that will meet its nominal R-value rating is a high-density R-21 version made for cathedral ceilings, he adds. They’re twice as dense as either the R-19 or R-22 fiberglass batts and provide enough resistance to air flow not to lose much insulation value to wind-washing even without an exterior air barrier. Better yet are R-23 rock wool batts, Dorsett says.
Installing 2×6 framing perpendicular to the rafters for a second layer of batt insulation will dramatically reduce the thermal bridging when compare to sistering the 2x6s to the rafters in the same direction, Dorsett says, and Watson also should consider adding a layer of housewrap as an air barrier between the rafters and the 2x6s.
“With R-23s in both the vented rafters and the 2×6 cross members you are at R-46 center-cavity, but it will outperform R49 between joists due to the much lower thermal bridging,” he says.
With a vented roof assembly, Holladay adds, Watson should install a ventilation baffle in each rafter bay so that the ventilation gap extends uninterrupted from the soffit to the ridge vent. “The only situation in which the ventilation gap can be omitted is if you have an adequate layer of foam insulation above the mineral wool, in which case you no longer have a vented assembly — you have an unvented assembly,” he says.
The unvented roof option
Installing one or more layers of rigid foam insulation on top of the roof deck will address the thermal bridging problem. Because Watson says that his roofing is about due for replacement, this option might be attractive.
If so, Dorsett says, the exterior foam should represent at least 30% of the total R-value of the roof assembly.
“So, if you install 4 inches of EPS (R-16 to R-17-ish), or 3 inches of polyiso (R-16 to R-17, de-rated for climate and location within the stack-up), or 3 1/2 inches of nailbase polyiso, or 4 1/2 inches of nailbase EPS on the exterior you could install up to about R-35 to R-38 of fibrous insulation tight to the underside of the roof deck without much moisture risk,” he says.
“Roxul sells some high density rock wool R-30s that fit 2×8 framing. If you install 3 inches of polyiso or 4 inches of EPS above the roof deck, you’d only be at about R-46 to R-47 in the center of each cavity, but the insulated assembly would still meet code minimum requirements on a U-factor basis due to the thermal break that the continuous foam provides over the rafters.”
Filling the rafter bays completely with spray foam insulation also produces an unvented roof assembly, but this option would bring Watson back to one of his original questions: open-cell (vapor-permeable) foam or closed-cell (vapor-impermeable) foam?
A layer of open-cell foam 5 inches to 6 inches thick applied directly to the bottom of the roof deck, and no interior vapor retarder, will mean moisture cycling on the roof deck, Dorsett tells him — “count on it.” But, he adds, moisture that accumulates during the winter should dry out in the spring.
“You really need 10 to 11 inches of open-cell foam (installed in two passes) to hit R-38, but with a smart vapor retarder or half-perm vapor-barrier latex paint on the ceiling gypsum, the moisture risk would be pretty low,” Dorsett says. “At 10-inch thickness most half-pound foam is already under 5 perms, and much less risky to the roof deck already. But it can still accumulate more moisture than is prudent if you don’t bring the vapor permeance a bit lower.”
A roof assembly consisting of 2×8 rafters and 2x4s perpendicular to them, all of it filled with open-cell foam, would give Watson 10 3/4 inches of insulation and hit the R-38 target perfectly. Later, he could think about adding a layer of rigid foam on top of the roof deck.
Dealing with the sales pitch
Watson has been in touch with a local rep for Icynene, a brand of open-cell spray foam, and he’s been given some different advice.
“Open-cell spray foam is an excellent choice in our climate zone 4,” says the message from Icynene. “The climatic conditions associated with Richmond are mild enough that the chances of interior condensation occurring on your roof deck are rare. This is confirmed by the 2015 IBC which does not require air impermeable insulation (i.e. spray foam) to have a vapor retarder if installed on the underside of structural roof sheathing in climate zone 4. Even in extreme cold weather if open-cell spray foam is used, moisture that may be present on the underside of the roof deck would be able to dry to the inside through the water-vapor-permeable open-cell foam. Our dealers have successfully conducted this construction approach many times.”
Further, the Icynene rep says, an alternative performance path to meet code would allow lower R-values in the attic insulation when balanced by improvements elsewhere — in high-efficiency heating and air conditioning equipment, for example.
“The typical R-value found in this performance method for an attic assembly in climate zone 4 is R-20 or about 5 1/2 inches of open-cell foam,” the rep says.
To Holladay, the message has a familiar ring. “My thought is that Icynene has gotten a little bit more sophisticated in the way it recommends thin insulation, but that it is still peddling the same weak tea that it was peddling in 2010,” Holladay says. “If a builder hires an energy consultant to do the performance path calculations, then the approach suggested by Icynene might be justified.
“But there is no justification whatsoever for these sentences: ‘The typical R-value found in this performance method for an attic assembly in climate zone 4 is R-20 or about 5 1/2 inches of open-cell foam. This is the reason why in a retrofit situation many localities will allow 5-6 inches of open-cell foam to pass even without a specific model being done for the building.’
“If a local code official allows the use of R-20 roof insulation in Climate Zone 4 without insisting that the builder show evidence of performance path calculations, one might conclude that the official is either lazy or ignorant. Perhaps a more charitable explanation for such a ruling is possible, but I can’t think of one right now.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
In any climate, getting that attic-roof air barrier continuous is job number one. And that is, of course, much, much easier if continuous rigid insulation topside is possible. The topside rigid insulation addresses the thermal bridging at roof framing and also warms everything to its interior (in the winter).
Following the 30% rule (30% of the total roof assembly R-value is in the exterior rigid insulation) means avoiding any serious risk of condensation within the roof assembly, regardless of the vapor and air permeability of the cavity insulation and the vapor permeability of layers to the interior of the roof framing cavity. In fact, selection of interior layers with relatively high vapor permeability means good drying potential of the roof assembly to the interior.
But this configuration does eliminate soffit-to-ridge ventilation at the vulnerable existing roof sheathing, so getting that roof-attic air barrier continuous becomes that much more important. It is possible to add soffit-to-ridge ventilation topside of the new nailbase sheathing for the new cladding, but this also adds to the depth of the new roof assembly, challenging eave and fascia aesthetics.
So where does the bulk water barrier (the roofing underlayment) go when you add topside exterior rigid insulation? It always makes sense to “warm” that membrane by locating it under the new rigid insulation — but it also makes sense to locate the water barrier layer where it is easiest to connect it to the flashing protecting any roof penetrations.
I must admit that I have never used rigid mineral wool or fiberglass insulation board on the exterior side of roof sheathing, but I am attracted to the hygrothermal performance and environmental profile of these types of rigid insulation. In my neck of the woods, these materials are pretty challenging to use, because they are hard to source and expensive. And of course, the air permeability of these rigid insulation boards means that you can’t tape them as your continuous air barrier system.
In summary, select the materials for your new roof assembly that yield the most continuous air and thermal and bulk water barriers while also providing the best overall drying potential and lowest environmental impact.
Does this sound like I am avoiding actual full specs for this roof assembly? You bet! There are enough possible combinations that prescribing performance is not really possible.