Ted has more than a few cobwebs in his attic. The unconditioned space also houses his HVAC system.
The 1,800-square-foot brick ranch in Climate Zone 4 dates from the 1960s, but the previous owner installed both a furnace and ductwork in the attic just four years ago. Ted also has inherited a powered attic ventilator. Although both the attic floor and the ductwork are insulated, Ted recognizes the situation isn’t ideal.
Summer hasn’t even arrived, he notes in a post at GBA’s Q&A forum, and when the temperature outside climbs to more than 95°F, the temperature in the attic tops 120°F. That has kept the ventilator working, but Ted surmises the high temperatures are reducing the efficiency of his air conditioning equipment.
“Should I turn off the ventilator so it won’t suck cool air from the conditioned area?” Ted asks. “Should I put more insulation material on top of the attic floor and the ductwork? Will a radiant barrier under the rafters help?”
Those are the questions to get this Q&A Spotlight rolling.
Add more insulation
Dana Dorsett would have Ted get the level of insulation much closer to the R-49 recommended in the International Residential Code.
“In a DOE Climate Zone 7 (not to be confused with a USDA plant hardiness zone 7) a mere 8 inches of fluff is woefully inadequate,” Dorsett says. “IRC code-minimum for zone 7 is R-49, and you have barely half that. Air sealing all the ducts and air handler connections and seams, and air sealing all the duct boots to the ceiling gypsum (and any other penetrations) is a critical first step prior to adding any insulation.”
(As it turns out, Ted initially confused the two climate zone designations. His house is located in DOE Climate Zone 4, not Zone 7 as he had originally indicated. But the code recommendation for attic insulation is the same — it’s R-49 in both in Zone 4 and Zone 7.)
Dorsett suggests that a low-e silver-colored paint on the underside of the roof deck, or a perforated radiant barrier under the rafters, would make a difference during the cooling season (with only modest increases in heating costs during the winter). The first step, though, is more insulation.
“But you might start with over-blowing the floor-fluff with 6-8 inches of cellulose, to bring the combined depth to about 15-16 inches,” Dorsett says. “(As a DIY, do what the pros do — install multiple depth gauge strips to know when you’re there.)”
Turn off the fan
GBA senior editor Martin Holladay notes that the fan is problematic: “The best approach is to immediately disconnect the powered attic ventilator, and then to hire a contractor to transform your vented unconditioned attic into an unvented conditioned attic,” Holladay says.
No matter what the climate zone, a powered attic ventilator is not a good idea, he adds. (For more information on this issue, see Fans in the Attic: Do They Help or Do They Hurt?)
“Creating a conditioned attic to bring HVAC equipment inside the home’s thermal envelope is always good advice — it’s never a good idea to locate HVAC equipment or ducts outside of your home,” Holladay writes. “That said, the required work is expensive, so the investment often makes little sense from an economic payback perspective.”
Holladay continues, “Installing a radiant barrier under your rafters may make sense in your case. Moreover, it’s always a good idea to seal leaks at ductwork seams, to seal leaks in your home’s ceiling (the attic floor), and to improve the R-value of your attic insulation and duct insulation.”
The cooling effects of an attic ventilation fan are in most cases “negative,” Dorsett says. That is, Ted will end up using more energy, not less.
“Attic ventilation is really never about cooling,” he says. “It’s primarily about moisture control, to keep the wood in the attic from getting moldy or rotting. In more humid climates such as on the Gulf Coast, in air conditioned homes even passive attic ventilation puts more moisture into the attic than it removes.”
Attic conversions can be expensive
As if to underscore Holladay’s point, Brian Gray, writing from Chicago, says that he faces a nearly identical situation as Ted does, and the fix is looking to be anything but cheap.
“I’ve been debating my situation for what seems like forever,” he says. It comes down to three options, the first of which is to convert his attic into a conditioned space by installing 4 1/2 inches of polyiso insulation above the roof deck and then adding a new standing-seam metal roof. He’d add open-cell foam below the roof deck to give the assembly the required R-value, but the bids range from $40,000 to $60,000.
His other options are cheaper, but they have some disadvantages.
“$40K+ is a lot of money to solve a problem, but I get a new roof, better curb appeal, and surely a higher resale,” he says. “That said, it’s a lot of money.”
Encapsulating ducts in spray foam
One option Gray is considering is to encase the ducts in his attic with a 3-inch layer of closed-cell foam, a job he believes would be possible with one or two DIY spray kits and a total cost of between $600 and $1,200. Would this idea have any value in Ted’s situation?
Because closed-cell foam can only be safely applied in layers, or “lifts,” of 2 inches at a time, Dorsett replies, that’s probably the limit of what should be considered for ductwork.
Using different versions of closed-cell foam, applied with different blowing agents, would be slightly more expensive but also allow thicker layers to be applied in a single lift. “At 2 inches, most [hydrofluoroolefin] blown 2-pound foam would be north of R-14, and north of R-20 at 3 inches,” Dorsett says. “It still may not be financially rational to go more than 2 inches.”
Open-cell foam, he adds, is another option. Cheaper and less of a threat to the environment, open-cell foam is relatively vapor-permeable, meaning some moisture will collect during long periods of heavy air conditioning use, he says, “but unlike fiber insulation, open-cell foam isn’t air-permeable, and the rate of accumulation is still quite slow, probably slow enough to be OK in a Zone 5A climate.”
Dorsett says that most spray foam would have to be coated with an intumescent paint to be fully code-compliant.
How much insulation do ducts really need?
Ted notes that flexible ducts sold in retail stores seems to be insulated to R-4 or R-8, but by his own measurements conditioned air still shows a 10° to 15° drop from one end of the supply trunk to the other. Should there be such a steep drop in temperature in a run of only about 10 feet?
“You are correct that the drop in temperature indicates that the duct is poorly insulated,” Holladay replies. “R-4 or R-8 duct insulation is certainly better than nothing, but for those of us who care about insulation, even R-8 doesn’t make much sense for ducts that are outside of the home’s thermal envelope.
“After all, most new homes have R-49 insulation on the attic floor,” Holladay continues. “When the attic is 20°F, that insulation is addressing a delta-T of 50 F°. But the air in the ducts is facing a delta-T of maybe 100 F° or 120 F° — a greater delta-T. So (logically) the ducts deserve more, not less, insulation. Obviously, it’s hard to install R-60 duct insulation. But I raise the delta-T discussion to demonstrate why it’s so important to bring your ducts inside your home’s thermal envelope.”
Dorsett adds this thought to the discussion about duct work insulation: “The reason ducts don’t need R-60 insulation despite the higher temperature difference is that total square footage of the duct surface area is much less than the attic floor, and the duty cycle on most systems isn’t anywhere near 100%. R-8 is usually fine for supply ducts, and less is OK for return ducts (due to the smaller temperature difference).”
This may be true, Holladay says, but even R-8 insulation doesn’t solve the problem when ducts are located in the wrong place. “Ducts need to be indoors,” he says.
Our expert’s opinion
Peter Yost, GBA’s technical director, adds this:
No organism we know of puts its lungs, unsheltered, outside its body; so putting HVAC systems outside a conditioned space is hardly biomimicry. But there we are, with ducts and HVAC units in the hostile environment of an unconditioned attic.
First thing: Kill the attic ventilator. Unless you have a perfect air barrier at the ceiling line — and why would you if you are already in a building where someone chose to put the space conditioning outside? — you will be pulling makeup air along the paths of least resistance, including from the conditioned space below. You can’t vent your way out of this predicament.
The best option is to move the conditioned boundary to the roofline. Insulate and air seal to pull the attic inside the building. This could and should have been done in the first place, but now you have another chance to make it right.
The next best option is to encapsulate the ducts. See recommendations for accomplishing this at the Building America Solution Center. Note all the tabs for various items of key information.
For more information, see two other suggested solutions: Ducts Buried in Attic Insulation and Ducts Buried in Attic Insulation & Encapsulated. Note that the former approach is not recommended in moist and marine climates while the latter approach is OK for all climates.