It is now a given that high-performance houses have high levels of insulation. It is not uncommon for a new cold-climate home to have R-40 walls and an R-60 roof, as builders do their best to lower a home’s energy requirements.
But is this premise in favor of thick insulation weighted toward houses in cold climates, where heating is a higher priority than cooling? Does it make just as much sense to insulate houses as heavily in hot, humid climates? Or does a lot of insulation actually make it more difficult to keep the house cool?
Those are the intriguing questions raised in a recent Q&A post at Green Building Advisor, and the topic for this week’s Q&A Spotlight.
Reading a long thread on a website called Design By Many about a Passivhaus project in New Orleans, John Brooks had come across this comment: “In a cooling climate, the delta T is much smaller than in a heating climate, and due to the internal heat gains insulation actually starts to work against you at a certain point. I think anything up to R-30ish is doable/defendable for all components, including roof and suspended floor. Beyond that it will only add to your cooling problem.”
Brooks gets the conversation rolling with this: “I understand how insulation can ‘work against you’ in a building without air conditioning….
“Or when the conditions outside are ‘better’ than conditions inside. The problem is that most Southerners DO air condition their buildings in order to be comfortable …. and for many of the hours during the cooling season the conditions outside are ‘WORSE’ than conditions inside.”
The issues is humidity
High humidity makes air feel uncomfortably sticky. Even when outside temperatures are lower than inside temperatures, opening a window to cool down the house may be counterproductive because it raises indoor humidity levels, as J Chesnut suggests.
“I can’t see how insulation can work against you in a thermal sense if you can open the windows to equalize interior and exterior temps,” he writes, “unless you are trying not to introduce more humidity into the home.”
Chesnut wonders whether an energy-recovery ventilator, or ERV, would help. Like a heat-recovery ventilator, an ERV incorporates a heat exchanger that moderates the temperature of incoming ventilation air. Unlike an HRV, it also captures some of the moisture in the air.
But in this scenario, GBA senior editor Martin Holladay adds, an ERV isn’t going to help. “If you are ventilating, running the ERV can only lower the indoor humidity when it is dryer outdoors than indoors — in which case an HRV or an ordinary fan would do the same thing,” Holladay writes. “In most summer situations, the ERV raises the indoor humidity — but not quite as much as the humidity would be raised if you were ventilating with an HRV or an ordinary fan.”
Less insulation makes no sense
James Morgan accepts the idea that “delta T,” the difference between inside and outside temperature, is smaller in New Orleans than in a heating climate. That, he writes, will “certainly affect the economics of extra insulation.”
But he’s not onboard with the original assertion that because of internal heat gains, insulation “starts to work against you at a certain point.”
“Emphatically no,” Morgan writes. “This is flawed conceptual thinking, like calling for help but not too loud so as not to bother anybody.
“The extra insulation may not be particularly cost-effective but it will never increase your energy use for cooling as long as outside temperatures are higher than the interior. The AC pumps the heat outside: the insulation stops it getting back in. If it’s cool enough outside that you’re not needing to use the AC then standard natural ventilation options apply.”
Brooks still wonders whether a house might perform better over the entire cooling season with less insulation. “I also think that during the shoulder seasons in the South, there will be ‘times’ when LESS insulation equals better performance,” he writes.
He adds that in using modeling software, he found that superinsulating a house in New Orleans would actually push cooling costs down, while in Denver and Chicago, superinsulation causes cooling costs to rise.
“I think the reason is related to internal gains and the fact that the enclosure can not cool as easily at night,” he writes.
Controlling internal heat gains
If superinsulated buildings in the South trap heat produced inside the house, maybe the problem is the source of internal heat rather than the insulation, Keith Gustafson says.
“Perhaps I am being obviousman, but if your cooling loads go up due to internal heat loads, it is time to look a those internal heat loads, not the level of insulation,” he writes. “In heating climates all of these things are your friend. With so many living in cooling zones, maybe some enterprising company will develop appliances with external vents (ovens, dishwashers) and fridges with separate compressors…
“Perhaps this is a spot where the human brain takes over for the spreadsheet, a signal, if you will, that you are approaching the point of zero gain when you are pondering putting the fridge in the driveway to lower your AC bill…”
Morgan would agree that controlling internal sources of heat is the key. That would mean using high-efficiency lighting, insulated hot-water storage or an on-demand water heater, and by cooking outside whenever possible.
“And if you pack the house with bodies for a dance party and the AC just can’t cope,” he adds, “that’s when you open the darn windows and sweat it out.”
Cost is the other issue
No one, it seems, is arguing in favor of less insulation. But weighing the cost/benefit ratio of increasing amounts of insulation is another question.
“The payback on the added insulation lengthens because a portion of the time it is providing no benefit or a much lesser benefit,” writes Bob Coleman, “and in some cases not allowing helpful passive gains like reflectivity on the roof during the winter.”
Morgan adds, “On the subject of cost/benefit of Passivhaus in the south: count me interested but not yet convinced. I’m hoping some time in the not distant future to see logged performance data from the small but growing number of Passivhaus buildings in mixed/cooling climates so that those of us who prefer to stay away from the bleeding edge can make a realistic assessment on behalf of our clients.”
Our expert’s opinion
We asked GBA advisor Michael Chandler what he thought of the discussion. Here are his comments:
It seems obvious to me that the year-round advantages of more insulation far outweigh the occasional times when internal heat gains would make insulation less beneficial. My clients generally will just open windows for cross ventilation on those times when the air outside is cooler than inside while not also being oppressively humid.
We do need to educate our homeowners to consider humidity and latent heat as much as sensible heat. We don’t install whole-house fans but do put roof monitors with operable awning windows for edge season cooling.
My own house has no AC in the master bedroom, just passive cooling. It gets a bit oppressive in late afternoon but is fine by 9:00 or 10:00 p.m.