Not even the most powerful residential air conditioning unit on the market is enough to keep Ryan Welch’s Alabama home cool and comfortable.
With the HVAC equipment located in what sounds like an underinsulated attic, temperatures in the second-floor living area may not reach 72°F until 10 in the evening—only after the attic has cooled.
“Our attic has varying levels of insulation,” Welch writes in a Q&A post, “and the weak spots are under the attic floorboard, which has been nailed down, making it difficult to remove.” Parts of the attic floor are insulated to R-19, other areas to R-30 (Alabama is in Climate Zone 3, where the International Residential Code, or IRC, requires attic insulation rated at R-38).
Welch runs two powered attic ventilators constantly, with the AC set at 75° during the day. But the system simply can’t keep up.
Moving the HVAC equipment is not an option, Welch says. So, he is considering several other options: disconnecting the attic ventilators, adding more insulation to the attic floor, and spraying a layer of foam on the underside of the roof deck, which would create a conditioned attic.
What’s the best plan? Welch’s dilemma is where we begin this Q&A Spotlight.
Make it conditioned space
GBA Editor Brian Pontolilo suggests that if moving the HVAC equipment is truly impossible, Welch should make the attic part of the conditioned space. But he adds a caveat.
“Keep in mind that your insulation R-values are only part of the equation,” he writes, “and that air-sealing is likely a bigger issue, including around those [non-IC-rated] can lights that need to be isolated from your insulation asap.”
Pontolilo points Welch toward a GBA article on the issue (see the “Related Articles” sidebar for a link). The article says that in some situations, it makes more financial sense to abandon the old HVAC system and install a new one—such as a few minisplit heat pumps—than it does to convert the attic into conditioned space.
“If your current ceiling is not well air-sealed and insulated, however, that work would still need to be done,” he says, adding that making sure ducts are sealed also is very important for energy efficiency.
Powered attic ventilators make things worse
To Dana Dorsett, powered attic ventilators are part of the problem, not a way to make the house cooler. The ventilators, he says, usually make the problem worse by sucking conditioned air up through all the leaks in the ceiling. That may be why the HVAC system can’t keep up.
“With leaky can lights and other large leaks in the attic floor, even stack effect can create infiltration drive, but powered attic vents can amplify that by orders of magnitude,” Dorsett continues. “Turn them off now, and never turn them on again. The attic will get hotter, but since you don’t live up there, who cares?”
With the ventilators turned off, the system might be able to keep up with the second-floor cooling load, Dorsett says. If not, if might be worth Welch’s time to determine whether supply and return ducts are properly balanced, and that neither the ducts nor the air handler is leaking.
He adds: “If the duct design checks out and the upper floor ceiling is as tight as you can make it, and it’s been brought up to R-38, it should be fine. Failing that, for relatively low money as a DIY, painting the roof deck with a silvery ‘radiant-barrier paint’ and installing perforated aluminized fabric-type radiant barrier on the underside of the rafters will lower the peak temperature of the attic by at least 10°F and the direct parasitic load on the ducts and air handler would drop.”
The air conditioner is probably too big
Welch says he is cooling roughly 2200 sq. ft. with a 5-ton, two-stage air conditioner (a ton of air-conditioning capacity equals 12,000 Btu per hour). Parasitic loads probably account for a ton of capacity, Dorsett says, leaving 4 tons for cooling the house. That’s 550 sq. ft. per ton when most houses have a floor-area-to-ton ratio of about 1 to 1300.
“A 5-ton AC unit for a moderate-size home is extreme to the point that it can not just burn extra energy due to short-cycling, but also make cooling the house less effective,” says GBA reader Burninate.
Duct design probably isn’t helping, either
Welch writes that there are two return vents for the HVAC system in the hall, but no supply vents. This, Dorsett replies, may also be part of the problem.
Pumping air into rooms that can be closed off from the hall creates pressure imbalances that make the cooling problem worse. “Even without the attic fans running, the pressure is driving air from the rooms through all the leaks in the ceiling into the attic and out, and pulling air into the hallway space by whatever paths it can find,” Dorsett says. With the attic fans running, even more conditioned air is drawn into the attic.
This amounts to using “the great outdoors” as part of the return path, he adds, increasing the total load substantially.
It would help to leave all doors open during the day, but it would make more sense to create correctly sized return paths from each air-conditioned room to the central hallway. Rooms that share a partition wall with the hallway could make use of stud bays for an air return. For more on jumper ducts, he adds, see this suggestion from GBA blogger Allison A. Bailes.
Tackling the insulation challenge
Welch has a couple of options for adding to the insulation already in the attic. They include tearing up the floorboards, then sealing and insulating the attic floor; spraying the bottom of the roof deck with foam; and air-sealing all accessible areas, then adding blown insulation or more fiberglass batts on the attic floor.
Don’t do any additional insulating, Dorsett replies, until all the air-sealing has been done. This would include sealing air-duct boots to the ceiling, sealing any seams in the duct boots and any accessible ducts with duct mastic, and sealing all seams in the air handler.
Can lights are another issue. They can be sealed from above with a cardboard box that’s large enough to provide 3 in. of clearance on the sides and top. Existing fixtures could be replaced with airtight versions rated for contact with insulation. All penetrations into the attic—such as plumbing and electrical runs—should be sealed.
Once those steps have been taken, adding 6 in. or more of blown-in cellulose over the insulation that’s already there should bring the attic floor up to code requirements. A radiant barrier also would help; silvery paint ($75 per gal.) isn’t as effective on its own as an aluminized fabric radiant barrier ($75 for a 500-sq.-ft. roll), he adds.
If Welch decides to insulate the roof plane, a 2-in. layer of closed-cell polyurethane foam followed by Rockwool would get the R-value to between R-35 and R-37, or R-42 to R-44, depending the depth of the rafters.
Our expert adds some thoughts
Here’s how GBA technical director Peter Yost sees it:
I thought that the answers or preferred approach(es) depended quite a bit on the actual attic configuration, so we decided to see if Welch would send more details about his attic, and, voila, great pictures! In addition to the photo at the top of this post, Welch also sent us the photo below showing his HVAC system.
We all have seen complex attics with steps up, steps down, adjacent cathedral ceiling spaces, dormers, you name it. But Welch’s attic is pretty straightforward: a simple square footprint and hipped roof. To me, this attic is ready-made for conversion to a conditioned attic or “cathedralized” attic (as a general reference on cathedralized attics, see this article.)
We did not ask Welch the age or remaining service life of the roof cladding, but we should have. If the service life of Welch’s roof is nearing its end, he could consider topside rigid insulation as part of his conversion to a conditioned attic, as opposed to all of the insulation being interior at the roofline.
Welch should know from the Q&A that any insulation solution to his attic starts with air-sealing and then moves to insulation.
There appear to be supply ducts that are mighty close to the roof eave, so getting full R-value around those perimeter supply ducts may be challenging. The other advantage to moving the air-seal and insulation to the roofline is that Welch maintains full access to just about every element of his HVAC system as he works to improve its performance.
And Welch still needs to answer Martin’s five questions about his new conditioned attic, posed here.
In terms of moisture management in a conditioned attic in Welch’s climate, the recommended ventilation rate is 50 cu. ft. per min. of supply air per 1000 sq. ft. of floor area (for more, see this reference).