When Jeff Watson realized that the insulation on his attic floor was rated at R-11, he did what any energy professional would have suggested: he added more insulation. He air sealed the attic floor, added ventilation baffles where necessary, and blew in a thick layer of R-60 insulation. But he isn’t entirely satisfied at the results.
“As expected, the temperature in the house doesn’t fluctuate as much,” Watson writes in a Q&A post at GreenBuildingAdvisor. “However, I feel as if I’m using AC more.
“Before the R-60, the AC would be on when it’s 80+ degrees out,” Watson continues, “Now I’m using it even when it’s in the low 70s outside, because the house refuses to budge in temperature when I’m trying to air it out by cracking some windows open.”
For example, with the daytime outside temperature at 85°F, Watson’s air conditioner has cooled the interior to 78°F. At night, outside temperatures have dropped to 70°F, but after leaving windows open all night, the indoor temperature is still at about 79°F.
What’s up? Watson’s hunch is there isn’t enough attic ventilation. All the heat stored in the insulation filters into the house rather than venting to the outside. He has eight soffit vents and two “turtle vents” at the top of the roof, he reports, and he wonders whether he should maybe add some more.
Is he on the right track? That question is the topic for this Q&A Spotlight.
Attic ventilation is overrated
Judging by articles posted at GBA in the past, attic venting doesn’t accomplish much in the way of cooling, says Peter L. The main reason to install vents, he says, is to dry any moisture that accumulates on wood in the attic, not cool the space. “It was debunked as hype, especially those selling attic fans,” he says.
Possibly the air conditioning itself isn’t working properly, Peter L. adds, or the relative humidity is higher this year than in previous years. But in any case, Watson doesn’t have enough data to come to a conclusion.
“The problem with ‘feeling’ is that there is no way to base any scientific results on it,” he writes. “What were your AC bills prior to the R-60 insulation? Compare the R-11 AC bill with the R-60 AC bill and make sure the outside temperatures were similar, as were the inside AC control temps.”
Also, adds David Meiland, it would be easy enough to check the air conditioning equipment. Make sure it’s charged with refrigerant correctly and meets other specs.
Why won’t the house cool down overnight?
Watson’s interest, however, is less about the performance of the air conditioning equipment and more about why the house is resistant to cooling off “via natural means,” meaning when he opens the windows at night. Forget about the air conditioning equipment, he says, and focus on why the house refuses to cool down overnight.
“Since I’m talking about the temperature within the house rising overnight, this means all lights and heat-generating things are off, most windows are open, and the sun has already set,” he says. “If it’s 78° inside with windows open, how can the temperature rise overnight within the house if it’s 70° or lower outside?”
Could it be, as Bob Holodinsky suggests, that the added insulation is trapping heat that might otherwise escape?
“If your roof is cooling way down, maybe due to night sky radiational cooling, then indeed it would have more effect on your living space with less insulation above the ceiling,” writes AJ Builder.
The insulation is, in fact, storing heat, adds Kevin Dickson, and this heat radiates downward. That should explain why the house won’t cool off at night. “When you have a large, well insulated mass exposed to outside air, it will tend to hover within a few degrees of the average daily temperature, which Jeff says is 79°F,” Dickson says. “I’ve personally lived with this, and that’s why for summer cooling I’d much rather have an unvented R-60 foam roof than a vented attic full of fluff.
“Stop the heat at the roof plane,” he continues, “and then you don’t have to cool down the attic somehow before bedtime. A vented attic with fluff gets heated all day by the attic ventilation air.”
Now, Dickson says, Watson will have to add forced ventilation in the attic that would be used only at night. “None of the articles and studies that denigrate forced attic ventilation have addressed this phenomenon,” he says.
Insulation doesn’t have much thermal mass
With an attic insulated to R-11, Watson had been getting a “bit of ‘free’ overnight cooling” because of heat radiating to the outside at night, says Dana Dorsett. The effect would be less with an attic insulated to R-60.
At the same time, the R-11 roof lets in a lot more heat during the day that makes its way into the conditioned space, more than off-setting the cooling effects at night. With an R-60 roof, most of the daytime heat gain will be from windows, not the roof.
The thermal mass of R-60 fiberglass is “negligible,” Dorsett says, with the thermal mass of open-blown R-60 cellulose three times as much — but still small. In fact, standard density 1/2-inch gypsum drywall has more than half the thermal mass of the entire cellulose layer, Dorsett says, and more than the thermal mass of the R-60 fiberglass.
“The thermal mass of all the stuff in your house, combined with interior heat sources like dogs, DVRs, refrigerators, and girlfriends, generally keeps the interior temps about 5 F° warmer than the outdoor air in an R-11 house, but 10 F° or more warmer than the outdoor temperatures in very well-insulated houses,” Dorsett writes.
“When the outdoor temperature is lower than the indoor temperature, you can get some cooling via ventilation, but unless you have a tall house and big windows to take advantage of stack effects to drive the air exchange, simply opening the windows won’t necessarily get you there.”
It’s simple: Your house is different
To Keith Gustafson, Watson’s problem is easy to explain, and easy to solve. “You used to have a house that needed 100 kWh to cool during the day, and 0 kWh during the night,” he writes. “Now you have a house that needs 50 kWh to cool during the day and 10 kWh at night.
“You need to start running it like the house you have, not like the house you used to have.”
Yes, says James Morgan, Gustafson has it right. “If you have a well-sealed and well-insulated house with decent air quality,” he says, “night venting during the AC season makes absolutely no sense.”
Our expert’s opinion
You may have noticed the absence of comments in this thread by GBA senior editor Martin Holladay. With Peter Yost away on vacation, Martin has agreed to step in this week as the Q&A Spotlight expert. Here’s what he had to say:
I think that Dana Dorsett and Keith Gustafson have provided good advice.
It’s worth emphasizing four points:
1 — There is no evidence that passive attic ventilation lowers cooling costs. Moreover, there is ample evidence that a powered attic ventilation fan usually increases cooling costs. (For more information on this topic, see All About Attic Venting and Fans in the Attic: Do They Help or Do They Hurt?)
2 — Even at night, most homes have many sources of interior heat. These include people, pets, refrigerators, TV set-top boxes, internet routers, fans, clocks, and small transformers.
3 — It is extremely unlikely — almost impossible — for attic insulation to get hot enough to add heat to the interior of your house, as Kevin Dickson proposes. In the case of insulation installed on your attic floor, the bottom inch of insulation is at about the same temperature as the ceiling drywall, while the top inch of insulation is at about the same temperature as the attic air. Jeff Watson reports that when he runs his air conditioner, he maintains his indoor temperature at 78°F. So when he turns off his air conditioner and opens his windows to go to bed, the ceiling drywall and the bottom inch of attic insulation are at about 78°F.
If the weather is hot enough for Watson to want to run his air conditioner, his attic isn’t cold. Watson wrote that he turns on his air conditioner when the outdoor temperature hits the low 70s, and that a typical outdoor temperature on a day when he operates his air conditioner is 85°F. During the summer, attics are often hotter than the outdoors, so when Watson goes to bed, the air in his attic is probably between 76°F (about the same temperature as his ceiling drywall) and 95°F. Under these conditions, the thick blanket of insulation on his attic floor either has no effect on his indoor temperature (when his attic is at 76°F) or is helping to keep his house cool (when his attic is 79°F or warmer).
4 — Flushing the interior air of your house at night (either by operating a whole-house fan or by opening your windows) is a sensible cooling strategy, but only for homes that lack air conditioning or for homeowners who don’t want to operate their air conditioner. As James Morgan correctly notes, it makes no sense to try to combine night flushing with daytime air conditioning. In many areas of the country, opening your windows at night introduces humid outdoor air. When the windows are closed the next morning, the air conditioner will operate longer than usual as it tries to address the latent load introduced overnight by the open windows.