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Q&A Spotlight

Determining the Best Attic Option

Choosing between a conditioned or unconditioned attic hinges on the merits of spray foam insulation

Spray foam in the attic? Spraying the bottom of the roof deck with polyurethane insulation turns the attic into a conditioned space, a hospitable environment for air ducts. But is foam the best choice?

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.


  1. User avater
    Armando Cobo | | #1

    Not so fast, my friends... (as Lee Corso says)
    Issues I didn’t hear in these discussions are house designs (style and size) and labor. I try to work with my clients on which attic system to use, as long as the HVAC and the duct system is in conditioned space or encapsulated. Sometimes the design of the house, especially if it’s a large house, with a high-pitched roof creating a huge unconditioned attic, dictates how to proceed. Also, a house designed with a basement has easier ways to deal with HVAC systems than houses that are built on a slab-on-grade. Smaller house designs have different ways to deal with HVAC and duct systems in much simpler ways than large homes. The bottom line is that not one system and/or application work for all houses.
    At the present moment we are working on a 5K sq. ft., 2-story, slab-on-grade, 36’ wide open house plan. The homeowner didn’t want to see any exposed drop beams or drop ceilings on the second floor, so the most cost effective way to create a space for the ducts was to chase the roof trusses, with a 25’x54’x2.5’ built-in chase box. That became a labor intensive issue with the builder, his superintendent, the framers and insulators, since they never done that before. For the unexperienced framers having to cut OSB and nailers to build the box was a huge deal. Then it also became an issue with the insulators about sealing all box ceiling joints with EcoSeal or Energy Complete, 2” CC on the box walls and full blown R38 above the box ceiling.
    On paper, and having designed this system several times in the past with experienced builders, framers and insulators, is not a problem. However, trying a new system in an area not know for high-performing houses is a very difficult challenge and it made me wonder if I’ll do it again, not just the cost issue, but all the headaches and drama involved with this process.

  2. Richard Beyer | | #2

    Response to.... Not so fast, my friends
    Armondo your plan is not that complicated when you use great craftsman who know what they are doing. There are many articles addressing your concerns as illustarted below. The science is published.

    Allison Bailes, Phd., tackled your scenario in a previous blog article shown here....

    GBA touched on it here....

    US Department of Energy dug into it here...


    Martin Holladay dealt with it in 2013

    California Energy Commission 2003

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Should have ordered room in the attic trusses Amando
    Should have ordered a truss like a room in the attic truss. Nice square space for the crews, same as any other part of the home to work on. Done.

  4. User avater
    Armando Cobo | | #4

    Responce to Richard and AJ
    This attached drawing may help understand better. Lets try this time...
    - Truss chases are 22' wide, not 25'.
    - The air handlers ARE inside closets in the conditioned space.
    - BUILT CHASED ROOF TRUSSES, with a 22’x54’x2.5’ built-in chase box. The trusses are 36' wide, 12/12 pitch, at 24" o.c., 22' wide chase had to have vertical webs in between to support 31 PV panels. Also, if the built-in duct chase is 54' long, it means that there are 27- 24"x30"x22' individual cavities that needed to have end-walls and ceilings.
    As I said, we done this before many times with experience folks, and no problems. I did not think this was too complicated... apparently I was wrong (first time this year) Ha!

  5. Matthew Nolette - So Maine CZ 6A | | #5

    Truss shapes
    Armando, maybe I'm not following the detail but seems to me you're saying those vertical webs in the chase were factory built and the bottom chord was continuous which from my point of view would make it a great deal of work. The sheathing would all need to be notched around the webs,

    For what it's worth, I happen to be sitting in front of Mitek truss software. There's very little limit on truss shape. Next time around, something like a tray built in would make it easy for any crew to sheath and air seal 90* corners. The flat ceiling can be framed in afterward with conventional framing. There's a few details like this kicking around this site and others.

  6. User avater
    Wojtek Karp | | #6

    Yes – there are alternatives to spray foam insulation

    For contractors who offer attic insulation services, there is always an alternative to spray foam. We recommend two types of blown insulation for the attic: blown-in cellulose and blown-in fibreglass. Both are cost effective options, with benefits that maintain long term, however our preference is to install cellulose insulation due to its long lasting and effective performance.

    Blown-In Cellulose Insulation

    Blown-in cellulose is an excellent insulation for spaces like the attic or for cavities with limited access. This is an eco-friendly product because it’s made primarily from recycled paper. Blown-in cellulose is a quick and easy installation, making it quite cost effective for most homeowners.

    When professionally installed, the thermal performance of blown cellulose compares favourably with other insulation products. The application can also be used to effectively boost the R-Values of existing insulation. In other words, this is the ideal “top up” when older insulation has settled.

    Cellulose insulation has a number of benefits, particularly when installed in the attic and roof cavity. The finished application resists mold; it’s treated to be fire retardant; and the blanket of insulation reduces noise penetration. Properly installed, cellulose provides years of performance.

    Blown-In Fibreglass Insulation

    As an insulation product, blown-in fibreglass is similar to cellulose, but made of glass fibres. The fibres are spun together to create a fluffy material that creates an insulation blanket throughout the installation area. Fibreglass is quite well known as a batt, but works well as a blown product.

    Blown-in fibreglass insulation is ideal for topping up existing insulation in the attic. Providing the existing insulation is clean and unsoiled, blown fibreglass provides a comparable R-Value to the blown cellulose, but performs better in a fire test (glass does not burn at high temperatures).

    Fibreglass insulation is impervious to mold growth and will not attract insects or rodents. This is especially relevant in the attic space. When professionally installed, blown fibreglass insulation will improve a home’s overall energy efficiency and save on monthly heating and cooling bills.

    EcoComfort can help you to make an informed decision

    At EcoComfort, our in-house insulation professionals ( can recommend the best product option for the job. Both types of blown insulation are cost-effective with long-term performance. With a professional install, energy efficiency is enhanced and utility bills are measurably reduced.

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