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Q&A Spotlight

Why Is My House So Hot?

A GBA reader believes that after he added more attic insulation, his air conditioning equipment runs longer than it used to

Can more insulation increase your cooling bills? After blowing more insulation onto his attic floor, a homeowner has the impression that he has to run his air conditioner more often and longer than he used to.
Image Credit: Dennis Schroeder/National Renewable Energy Laboratory

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.


  1. Brent Eubanks | | #1

    why no night flushing
    it makes no sense to try to combine night flushing with daytime air conditioning

    I do not understand this conclusion, at least in the context of a location with a decent diurnal swing. If you can reduce the temperature of the mass of the house by 5-10 degrees overnight, why is this not helpful? Obviously you do not want to run both systems at the same time, but I can't see the problem with running them in series.

    I agree that if the climate is humid there is the question of removing latent heat. Without doing a calculation, I cannot estimate whether the latent outweighs the benefit of cooling the house's thermal mass. But that seems like it would only be a concern in a fairly humid climate, yet your advice suggests that you think it's a bad idea across the board. Why?

  2. Hobbit _ | | #2

    holding heat
    Okay, I want to see the NREL study on the specific heat capacity
    of girlfriends. Since it would invariably be tied to mass, the
    followup study on eating disorders would probably be interesting...

    Anyway, Dana set me straight a while back on the "capacity" of
    most insulating materials, be they fluffy or polyiso or whatever.
    It's not much. The structure and materials probably account for
    a lot more "holding" than the blown-in. I thought I was seeing
    temporal-lag effects through polyiso on unusually warm winter
    days, but it turned out to be an illusion. A foot of brick,
    maybe, but not layers of 'iso or 'glass. [How would something
    like dense-pack cellulose weigh in here?]

    I've found that it takes a *significant* delta between accumulated
    interior temp and nighttime outdoor temp to be useful at all, and
    only when windows are appropriately opened on different levels to
    allow the little bit of available stack effect to actually move
    air up through the house. But how much flow is that really from
    the gentle drift, maybe 50 CFM all told [SWAG] if you've thrown
    everything open? Say it went down ten degrees outside, you've got
    50 * 1.08 * 10 sensible, all of 540 BTU/h you're expecting to actually
    cool anything. Conductive loss that might have helped in the past is
    down in the noise with the improved envelope! Now add the downside
    that the night air probably got quite a bit more humid as the temp
    dropped ... did you notice that mist on the car roof outside as
    you were opening windows? Now you've let all that into your nice
    dehumidified air-conditioned space, and have to get rid of all
    that latent whenever the A/C next fires up.

    So I only try any of this on cooler *dry* nights.


  3. Richard James | | #3

    Cooling at Night
    90's built CA ranch style house, R30 batts in ceiling, R19 in walls. Low solar gain due to good orientation of double pane windows and generous roof overhangs (porches). This article hits home and this is what I have done for 15 years to good effect. Thermostat is set @ 80 degrees all day. It is set @ 74 degrees at night. We like it cool at night for sleeping. I'd guess the KWH to be a wash, could be less, but it is no more for sure. If I choose a blocked time tier rate plan I could save some $$, but not enough to have to think about it. With the large drywall thermal mass being cooled overnite these are real world observations: 85 degree day the AC does not come on until 6 pm, 90 degree day 4 pm, 100 degree day 2 pm. It gets down to 60 degree a at night in CA but the humidity goes night so window venting makes no sense. Have sealed the usual offenders in floor and ceiling planes and there is still enough natural infiltration to keep air quality good. In CA the daytime infiltration air is dry so it dehumidifies the house. Air in the house stays clean thanks to a high MERV in duct return air filter. My only complaint is the AC load calculation done on this house was wrong. The AC is about 25% oversized, all the more so since plugging attic leaks If we lived in FL would need to downsize it to address humidity. One day I'll upgrade the attic to cellulose (for winter heating) and see what happens.

  4. User avater
    Dana Dorsett | | #4

    CA air is generally pretty dry
    When it's 60F overnight the outdoor dew points are probably in the mid-50s F or lower, which is just fine for using a night-ventilation strategy That air might be at a high relative humidity at 60F, but when you bring it indoors and raise it's temp into the 70s the RH will be under 60%, maybe even under 50%. The RH of fully saturated air on a 60F foggy night is only 59% when raised to 75F, which is below the mold-explosion threshold.

    In much of the eastern half of the US the night-time outdoor dew points average in the mid-60s to low 70s F, which isn't that healthy at 75F indoor temps. When the pollen counts are low I'll open up windows to cool the house overnight when the predicted dew points are expected to be in the mid-50s or lower, but at 60F+ dew points I use mechanical dehumidification & air conditioning to manage the loads. (When pollen counts are high I keep the windows closed even at low outdoor dew points, due to allergy & asthma conditions among family members.)

  5. Terry Lee | | #5

    Here is the explanation...
    BAD DESIGN - A misunderstanding of the heat capacity and moisture storage content(an electrically charged material that attracts moisture which causes it to expand at the surface, store and release moisture based on RH) and their ability to breathe. Has less to do with dew point, temp, attic insulation, etc...more to do with dynamic material properties (specific heat, density, thickness) design that most do not begin to understand that you have to understand the valuation and difference between dynamic mass and steady state r-value.

    Synthetic manufactured gypsum is a bad choice, most concretes with high levels of portland cement as the binder is another. Neither have much useful mass, high heat-cool storage capacity, or the ability to expand-store when wet, shrink when can not force these materials to perform in ways they are not design for by changing the environment they are placed in.....wrong approach!

    Hempcrete, limecrete, and especially earthcrete with the right types of clay are much better than OPC_concrete. Better IAQ and for the environment too. You won't find much complaining like this from these homeowners, house stays cool in summer, warm in winter, little heat-cool loads, no humidity control required.....allergy problems solved, no mold and mildew.

    Knowing where to place mass is the other challenge, as are the building permits, getting labor cost down, the chemistry is a big challenge.

  6. User avater
    Nate Adams | | #6

    Reduced load and latent heat
    It seems a discussion of sizing is largely missing here. My humid summer bias will show here, though.

    Adding attic insulation and air sealing can reduce cooling loads by 1/2-1 ton fairly often, making what is likely already an oversized AC even bigger. If it short cycles, which it should with little load, any vents at the end of the runs, often on a second floor (if the house has one) don't get as much latent (water heat) or sensible (air heat) removal. This leaves them uncomfortable.

    Ironically, after spending money on insulation to save energy and be more comfortable, it may be a good idea to buy a smaller AC. If the homeowner prefers higher temperatures and it is not the primary heat source, undersize it a bit, too. This will do a really good job at latent removal because it runs a lot. Considering a heat pump upgrade could be smart, too, since it will heat well at higher temperatures that have low loads.

    I also wonder if the building materials are getting warm (the thermal mass) and the night time open windows technique is not removing the heat well from them. Setting it and forgetting it with a smaller AC could help leach heat and moisture out of the materials day and night and lead to better mean radiant temperatures.

    This is one of the dangers of prescriptive upgrades, they often cause other unforeseen problems. A thoughtful upgrade design could have avoided this.

    Just some musings...

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