Attics come in many shapes and sizes, but they are either conditioned or unconditioned. That is, they are insulated and heated like the rest of the house and can be considered conditioned space, or they are designed to allow the free circulation of outdoor air and become unconditioned spaces.
Is one option better than the other? That’s what Rus Pearson would like to know.
“Some say spray the roof deck, allowing the attic air to become conditioned, which is great for the [heating and cooling] ducts in the attic,” Pearson writes in a Q&A post at Green Building Advisor. “Others say spray the attic side or the ceiling to allow the attic to vent. I don’t understand this one since the attic air is now unconditioned air and the ducts a have only a wrap of insulation… I’m confused.”
Pearson’s question looks straightforward. But the ensuing discussion, the topic of this Q&A Spotlight, quickly turns to the merits of spray foam insulation, which is often used as an air and vapor barrier, as well as insulation.
First, the basics
A key consideration is whether, as Pearson suggests, there will be heating and cooling ducts in the attic. That’s not uncommon, and in some homes, air handlers and other HVAC equipment also may be located there as well.
“If there are any ducts in your attic, it’s always better to have the second kind of attic — an unvented conditioned attic — so you aren’t wasting heat during the winter or losing cool, conditioned air from your ducts during the summer,” says GBA senior editor Martin Holladay.
The topic has been covered at GBA in the past (for a couple of articles on this topic, use the links listed in the “Related Articles” sidebar).
A conditioned attic isn’t vented, he adds. If Pearson decides to go that route, he will have options other than spray polyurethane foam.
Spray foam has too many disadvantages
Think twice about the foam, advises AJ Builder. Not only is it expensive, but it also carries risk. “Have you considered the cost versus the savings versus the risk?” he writes. “When adding insulation, there is much more — or less, or different — that can be done.”
Suppose, AJ Builder continues, that you spend $5,000 on spray foam insulation. The house may use $200 less fuel per year, or Pearson may end up turning up the heat or turning down the air conditioning to give up the savings in return for more comfort.
Before deciding on foam, he says, weigh budget, comfort, and “save the planet” considerations.
Richard Beyer suggests that Pearson do some reading on spray foam insulation, including a backgrounder from the Environmental Protection Agency, which warns that exposure to isocyanurates such as MDI and other chemicals in the foam can lead to health problems.
Another article that Beyer recommends comes from the Centers for Disease Control and Prevention, which says that as popular as spray foam has become, “much remains unknown… specifically the health implications of its amines, glycols, and phosphate upon workers.”
“Environmentally friendly doesn’t necessarily mean worker-friendly,” David A. Marlow writes.
Spray foam is not only expensive, adds Charlie Sullivan, but it’s not so great for the planet. From a greenhouse gas basis, the blowing agents used in foam can be more than 1,000 times as potent as carbon dioxide, Sullivan says, adding, “In some cases, it’s hard to find another good option, but in this case there are much cheaper options that work just fine.”
What other options does he have?
Writing from Climate Zone 5, C.B. would like to know exactly what these other options might be. “I’m having my roof deck spray foamed to a depth of 4 inches and then 8 inches of blown-in fiberglass,” C.B. says. “I don’t think there is another option to the spray foam (besides cobble-and-fit rigid foam) in order to meet IRC 2012 Section R806.5 (see Table 806.5). Since I’m in Zone 5, I need R-20 of air-impermeable insulation — hence the 4 inches.”
Pay attention to the details, suggests Dana Dorsett. He points out the table C.B. refers to is based on attic insulation rated at R-49. If the R-value is higher, there should be more foam insulation. For Climate Zone 5, he says, the foam should be at least 40% of the total R-value of the insulation.
Even assuming that C.B. sticks with a combination of spray foam and fiber insulation in the roof, there are several ways of meeting code minimums.
“The true depth of 2×12 milled timbers is only 11.25 inches,” Dorsett writes. “With 4 inches of foam you’d have only 7 1/4 inches of room for the fiber. At R-4/inch (1.8-lb density Spider or Optima) that’s R-29. Add that to the R-24 foam and you’re at R-53. If you did only 3 inches of R-6/inch foam (R-18) you’d have enough foam to deal with R-27 in the fiber layer, and have 8 1/4 inches of room to play with for the fiber, which would need to be about R-3.3/inch to not have dew-point issues.
“But if you installed 3 inches of an R-6.5/inch product (R19.5) you could install up to R-29 in there, and a damp-sprayed cellulose product could work,” Dorsett continues. “That would put you right at the IRC 2012 R-49 code minimum, but use 25% less of the climate damaging foam.”
Dorsett says several foam products on the market use water as a blowing agent instead of a hydrofluorocarbon. One of them is Icynene’s MD-R-200, with an R-value of 5.1 per inch and a vapor permeance of about 1 per 4 inch, making it suitable for C.B.’s application, he says.
“With 4 inches of MD-R-200 you’d have R-20.4 of foam, which could accommodate R-31 of fiber,” Dorsett says. “With 7 1/4 inches of space for fiber and 1.8 lb. [per cubic foot] fiberglass, that would give you R-29 of fiber and R-20.4 of foam, putting you a hair above code minimum, with less damage than using an HFC-blown foam. The trick is to find an installer near you that has experience with 2-lb. water-blown Icynene foams.”
Or, skip the spray foam altogether
Another approach, Dorsett adds, is to use a combination of rigid polyisocyanurate and Type-II expanded polystyrene insulation on top of the roof deck, plus R-30 worth of fiber in the rafter bays. One advantage here is that the foam would provide R-20 of insulation to reduce thermal bridging through the rafters, “and would thus outperform code-minimum from a thermal point of view,” he says.
The reason for using two types of rigid foam is that the polyiso “falls off a performance cliff” when temperatures dip below 30 to 35 degrees F., while the EPS gains in performance.
Pointing to other articles on this topic, Holladay says that if the roof assembly includes a ventilation channel above the cavity insulation, a wide variety of insulation materials could be used.
Strong feelings about using foam
Although Dorsett mentions two foam manufacturers by name — Icynene and Aloha Energy — Beyer’s experience with one of them, presumably Icynene, was very disappointing. The chemical smell of the foam four years after it had been installed was so powerful that at a hearing in Connecticut the Commissioner of Consumer Protection “pushed his chair back so fast you would have thought I was handing him a skunk,” Beyer said.
AJ Builder no longer promotes the use of spray foam insulation, but he’s had experience with low-density Icynene and never had a problem with it. “I have checked the foam for smell and water moisture over the years due to the bad outcomes posted by others here and I can say there are no problems,” AJ Builder writes. “The foam is fine as far as I can tell without hiring a team of scientists and lawyers.”
But, says Beyer, whether the foam uses water or something else as a blowing agent, they both require isocyanate. “Both are toxic regardless of brand…” he says. “When you think there’s no odor there are chemical compounds in your indoor air directly linked to the foam insulation which is unlike any other insulation. Sometimes it’s what you can not smell which should concern you.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
If you have any mechanical equipment in an attic, you definitely need to go “cathedralized” (a funny term, but I like how it distinguishes a true cathedral finished ceiling from an attic space that has the air and thermal barriers at the roof plane). And in my experience, folks always appreciate the dry, stable, and protected storage space that a cathedralized attic provides.
I am just now working on a project where the client has similar concerns about the potential health and environmental issues associated with spray foam. My advice there (and here) is this: Should you go with spray foam, remember the installer is also the manufacturer; spray foam is a site-generated final product. To make sure you get a safe and quality installation and final product, follow the seven tips that Henri Fennell and I worked out in this recent BuildingGreen blog.
Should you choose not to use spray foam, make sure that you get the same level of airtightness without it. In attics, from the interior, this can be quite challenging.
I really like the idea of continuous exterior rigid insulation topside of the roof deck, just as long as you can connect that system to the wall air and thermal barriers. And while I share the concerns raised about the temperature-dependency of the R-value of rigid polyisocyanurate insulation, there is quite a bit of variation from one manufacturer to the next. This issue is covered well (although without resolution) in this Building Science Corporation document.
But stay tuned. Chris Schumacher of Building Science Labs and I are working on an article to get to the bottom of this topic as we speak.