Heat Waves and How to Plan for Them

Posted by on Jun 20, 2017

Temperature variation (in degrees Celsius) from long-term averages in the southwestern U.S. and California on June 16, 2017. Image: National Centers for Environmental Prediction

The heat wave that has blanketed much of the Southwest and California for much of the past week is likely to set new temperature records throughout the region. It raises interesting questions about how to plan for heat waves in our building design and with community planning.

Phoenix reached over 120°F yesterday for the first time since 1990—so hot that commercial airlines have had to cancel dozens of flights. Sacramento hit 106°F on June 18thr—and the city is expected to reach over 100°F for nine consecutive days during the current heat wave. The San Francisco Airport reached 97°F on June 18th, breaking the previous record for that date (1981) by 9°F, and even the notoriously cool City of San Francisco hit 88°F that day, also a daily record (by 2°F).

What is different about the California heat wave this June is that the air is more humid than usual. Even though the air temperature in Sacramento was 104°F on the 19th, it felt like 109°F. A moist air mass is producing that humidity—and increasing the danger to residents. Of particular concern is that nighttime temperatures in some areas haven’t been falling, as they normally do.

According to climate scientists, as global warming progresses, such heat waves will become more common and the magnitude will be greater.

How do we plan for heat waves?

A number of us on the U.S. Green Building Council Resilience Working Group are currently trying to figure out what building design measures could help to mitigate heat waves, and I’d love to hear your suggestions. If heat waves are considered a significant hazard for a location, what should we be doing about that as we plan and design buildings?

Below are some ideas to get your creative juices flowing. Please provide your suggestions in the comments field, below—or you can e-mail me directly (alex@resilientdesign.org).

Provide cooling-load-avoidance measures

Heat waves lead to warmer indoor temperatures in three primary ways: conduction of heat from outdoors through the building envelope; transmission of radiant solar heat through windows; and the convection of warm air through open windows and doors and air leakage. For each of these heat-gain pathways there are solutions to minimize that heat gain: added insulation in the building envelope to slow down conduction; overhangs, vegetative shading, movable window treatments, and other measures to reduce unwanted solar gain through windows; and tightening up the building envelope (relying on mechanical ventilation for fresh air) and carefully managing when windows are opened and closed to control convective heat gain.

Oversize air conditioning equipment

We may need to rethink our sizing guidelines for mechanical cooling equipment. Cooling loads are increasing in general, and during heat waves, those cooling loads can far exceed typical conditions. While I have long been loathe to oversize heating and cooling equipment, it may make sense to plan for higher cooling loads that are resulting from climate change.

Incorporate an emergency cooling station with backup power

In larger building complexes and campuses, it may be advisable to create indoor or outdoor cooling stations that can be powered with backup generators or (preferably) islandable solar-electricity systems with battery storage. In arid areas, such cooling stations can rely on mist-generating evaporative coolers—of the sort one may see on the sidelines at football games on hot days. These are much more energy-efficient than refrigerant-cycle air conditioning equipment, which can be critically important if a generator or stored electricity is being used as the power source.

Provide air movement

Low-tech fans and natural ventilation can make a huge difference with emergency cooling, especially for people without mechanical air conditioning. Moving air evaporates moisture from our skin, cooling the skin surface. If emergency power is being rationed during a power outage, fans use far less electricity than refrigerant-cycle air conditioners.

Educate building occupants

How a building is operated can make a huge difference in indoor temperatures during heat waves. A lot of this is common sense: closing up a house or apartment during the day when the outdoor air temperature is higher than the indoor temperature, and opening up the house in the evening when it’s cooler outdoors. In an apartment building, it may make sense to create an emergency plan for what should be done in the event of a power outage during a heat wave.

What am I missing? Above are some quick ideas. What other concrete measures can be taken? Provide your thoughts below—and keep cool!

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Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. To keep up with his latest articles and musings, you can sign up for his Twitter feed. To receive e-mail notices of new blogs, sign up at the top of the page.


  1. Alex, one note of caution regarding equipment size: oversizing cooling equipment can actually decrease its ability to reduce humidity. The air cools too fast and the system kicks off before it’s had a chance to remove enough moisture. The result can be both extra humidity, and also moisture-related health & durability issues. It may be preferable to size systems normally and have them a little over-worked during heat waves.

    On the non-technical end of the spectrum, I think a key strategy is to provide common facilities where people can go to cool down — a library, community center, or other public venue — ideally, as you said, powered by islandable PV.

    • Great input, Carl. Thanks much.

  2. thx for your great musings. Higways are buckling around Sacramento due to expansion — guess they were not designed with much safety margin or temps of 108 (or even more in the concrete). T

    re: your suggestions: OK, I’ll bite on the AC oversizing bait. That strategy should be avoided or be the last resort. Here is the rationale:

    1) It does not address passive survivablity/livability (even if you have renewable E you would need huge batteries for storage, which bumps up your environmental footprint quite a bit).
    2) It increases GHG emissions and environmental pollution from power generation, manufacturing, transport, and repair of AC equipment and the electrical power infrastructure.
    3) Even if you are using “clean” energy, running AC equipment adds waste heat to the environment, esp. in dense urban areas where UHI is already a problem. Also, PV panels themselves heat up and may add the warming the atmosphere, UHI, etc.
    4) Passive strategies require much less maintenance, so they tend to be more reliable and durable.

  3. What are you missing? Here are some quick thoughts (priorities to be determined).

    1) Phase in at every decade or two the more aggressive passive and, if necessary, active measures, if first cost is a major problem. For example, plan for adding shading, thermal storage, green roofs, and better technology (Phillips, 2017, p. 57-61, http://www.bigconference.green/pdfs/The-Heat-is-On-Future-Proof-Buildings-for-Climate-Change_Cyndy-Comerford-Tom-Phillips-and-Marco-Arnesano.pdf).
    Planning for future neighborhood, EV, & microgrid connections and future AC oversizing might also make sense here.

    2) Optimization of thermal mass. Phase change materials to flatten peak loads should be considered. More energy-dense PCMs are being developed (EU CORDIS project, Research EU 2017), so we should also consider modular systems where the material can be swapped out easily for future upgrades. Also, I like the method of controlled thermal mass such as rock bed storage with a controllable ventilation damper. This helps avoid the problem of having too much heat stored over a long lag period. Ground coupled systems kind of fall under the thermal topic too.

    3) Sensor technology should be planned for, if not installed now. Here I am mainly thinking of affordable thermal comfort sensors such as the Comfort Eye (Arnesano et al., p. 1 25+, http://www.bigconference.green/pdfs/The-Heat-is-On-Future-Proof-Buildings-for-Climate-Change_Cyndy-Comerford-Tom-Phillips-and-Marco-Arnesano.pdf)
    and even DIY sensor packages for WBGT measurement (Salamone et al., 2017, http://www.buildup.eu/en/practices/publications/low-cost-environmental-monitoring-system-how-prevent-systematic-errors-desi-0).

    4) I hate to beat a dead carriage horse, but we need to address extreme humidity trends EXPLICTLY in building designs and energy efficiency. Most climate models that are used for vulnerability assessments rely on temperature predictions but lack humidity predictions. Humidity increases will amplify the heat stress on humans, buildings, plants, the grid, etc.
    …BTW, the current heat wave in CA is not very humid compared to the killer heat waves auch as CA 2006, Chicago 1995 etc.
    Caif. is currently in a heat wave from a Dry Tropical system, while the Moist Tropical system is currently hitting parts of the SW and eastward.
    See p.32-34, link to archived synoptic weather map for the 2006 and others, and link to Sheridan et al predictions of synoptic systems, at http://www.bigconference.green/pdfs/The-Heat-is-On-Future-Proof-Buildings-for-Climate-Change_Cyndy-Comerford-Tom-Phillips-and-Marco-Arnesano.pdf

    5) The weak link is still our own behavior. So not only is educating building occupants critical, but so is the training of designers, installers, and maintenance pro’s. Conversely, any technology or strategy needs to be very user friendly, thoroughly pilot tested with all the user types, and very rigorous. Fault detection & diagnostic systems are already being required in commercial buildings (at least in CA).

    • Really helpful input, Tom. Thanks so much.

  4. You mentioned natural ventilation. Do you know of info that explores how we might use natural ventilation in our existing housing,(Toronto for us). Beyond trying to catch prevailing breezes such as stack effect,attempts to draw cooler air from bsmnt and moving it up where it is usually much hotter on floors above.

    • Richard, I think there is potential to re-examine natural ventilation in a comprehensive way. I’ll be at the PLEA Conference in Scotland the first week of July and look forward to discussing such opportunities with others at the conference. In the 1970s and ’80s there was some great work done on solar chimneys and other induced ventilation strategies; I’d like to see a renewed focus on such measures. Doing so will become more and more important.

  5. Good topic Alex,

    First, I would look closely at revising building design on a community level. The current builder model of SFH design would be better served by eliminating all those exposed walls (think Taos pueblo). Reducing surfaces exposed to the heat makes sense.

    Building on that, thermal mass with interior insulation (thermos model) has been shown to be effective, but I’ve heard arguments for insulation outside, thermal mass inside too. The mass counts, and shading is critical.

    I have to agree with others on oversizing AC…the unit will repeatedly cycle, and proper mixing will not occur.

    Finally, lived in AZ for a decade, and could never understand why 65 degrees was considered a comfortable indoor temperature in July. Expectations and behavior need to be controlled in a critical event. If the interior temperature can be held at 80 degrees, that is not a life threatening event. Building and system design should reflect that goal.

  6. The passive route can be married with the ‘active’ by minimizing materials, using optimal materials (e.g., white long sleeves in heat … not poisons on skin). To feel cool, you need to get rid of humidity, and that is best done by ‘shade’ (cool air can’t hold the moisture that warm air can hold), ‘moving air’ (evaporates the moisture on your skin, and that ‘evaporative change of phase from liquid to gas produces -80 cal./gram, and thus air conditioners work.’). The active air conditioner moves cooled, drier air, and both evaporates ‘sweat’ and so you feel cooler very quickly.

    That’s all good, but what to do? My go to plan is a bit ‘Spartan’, but it’s worked for me in the Winter with Mtn. night temps at -10F, and I’m cosy without any central heat. The solution: Sleeping Tents. Purchasing the quietest window AC (not reverse cycle) and cheap. Modify it so it rolls around and has a duct for hot air, and modify tent with that ‘chimney’, leading it to a window. You will also have to collect the condensate… and if you’re handy develop a switch to shut off when the tank is full. (now there are roll arounds already there, but they evaporate the condensate back into the exhaust, which makes them less efficient (but hey, it’ll work). If you can develop a solar system to just run this system at night, along with enough for a refrigerator, and induction hot plate, you’ll have it made. You might go to propane for cooking.

    Details: I just tossed an old quilt to cover the top of the tent screening. Make sure you have a portal for electric (cut a small hole the diameter of the cord and cut the cord, then reattach it using wire nuts and tape, all outside the tent…. or go wild and put a new male plug on the household current line.

    Now my good buddy instructed me on how to attach a generator on a 220 line and plug that male into the dryer socket. This is dangerous and you also must be sure to turn off the main incoming, so you don’t sent power back to the line. Also NEVER start the engine without first plugging it into the dryer (a hot male plug…no, just don’t go there, no jokes please). With a solar system, the connection into the main circuit breakers, be it thru the dryer or elsewhere (a permanent connection), a real shut off, like on marine boats which divert to different batteries would be great, but make sure appropriate for the voltage and wattage.

    Of course, I use an old eider down quilt, not an electric blanket. I have an 10 x10 tent which would be comfortable for 2 and a young child; adding tents and connecting to the AC using a duct with a built in fan, might also be a good idea.

    I spent a great deal of time when at FEMA as a Hazard Mitigation Specialist abhorring DRYWALL. It’s made with gypsum, a Calcium Sulfate material which acidifies when damp and the sulfate is the host to the living space of anaerobic bacteria, molds and other toxins. Because of the closed in of the paper sides, when the vapor pressure of the HOT, OUTSIDE air pushes into the interior, or is actually drawn in by the drier and cooler air, a ‘dew point’ can be reached inside the drywall, and start the process of growing mold. Therefore, I will never purchase a building made after 1974, when Gerald Ford, pressured by the NAHB to allow drywall as a building material. I NEVER should have been allowed for exterior walls.

    So if you live in a climate of high heat and humidity and have drywall on the interior surface of your exterior walls, make sure you NEVER WATER ANY PLANTS NEAR YOUR BUILDING. A dear friend of mine died because this black mold was always appearing on the West wall of her bedroom, and the neighbor and or the condo building sprinklers wetted down the wall each day… to make the grass and pretty plants grow…

    Vapor barriers are very important in the North, cold climates on the interior walls, to keep the inside moisture from getting into the wall fiberglass insulation and degrading it’s R-value. Likewise in hot climates, paint with polyurethane on the exterior limits the vapor getting into the wall. Either way, if you have a tent inside in the emergency, you more closely match the exterior temperature, and mitigate or maximize any insulation you do have. So still keeping your home closed in the heat is good, except opening and putting an intake fan into a window to capture that early morning coolness might be good, and to get fresh air. A whole house fan system might work, but as long as it seals well in the daytime to limit the exterior heat AND HUMIDITY from getting inside.

    Radient barriers, and old 40 gallon or so water heaters as ‘preheaters’ in the sun for a hot water line and an outside shower is a good idea. Valves and protecting the storage from freezing might work in the NORTH with a great Southern exposure. (make sure you don’t do a ‘Jimmy Carter’, where his solar systems didn’t require a shut off valve… and at night the water heater would syphon the hot water and warm the cooler night air… but hey… the government helped pay for the big Nothing Burger.

    Oh, yeah, and last point. Size matters in AC systems .. Keep it Small… forget Macho. In South Florida, I’ve been comfortable with 800 sq. ft. per ton (12k BTU/hr). And the systems are 25 yrs. old. I have a huge shade tree on the South side of house. The upstairs has a system at 500 sf/ton, and it’s made me think that 600 sf. /ton is sufficient with shade and a moderate temp. attic (I have clay tile). Hotter roofs could use a radiant barrier on the roof rafters, vented at the soffits and peak or gable ends. I’ve been in new buildings with great radiant passive insulation and on a 90 degree day it was low 80’s inside with lower humidity and moving air, before the windows and doors were installed… it was like under a shade tree.

  7. I once lived in an apartment back in the early 80’s in Houston, Texas that had NO air conditioning! However, it did have an attic fan that exhausted air out at the high point of the apartment. My bed was next to a large second story window, so during summer I slept with that window open and the fan on. Not only did I sleep very comfortably even though Houston, with all it’s concrete mass, doesn’t cool down much at night, I actually found that I would wake up chilled in the night sometime and have to pull the sheet over me! To avoid additional penetrations in the roof in a place notorious for torrential downpours, it was vented out the side of the building just under the peak of the gable roof. While it was a bit noisy (I’m sure fan motors can be found that are whisper quiet now days), it actually was perfect because the white noise covered up the sounds of the city outside my open window, keeping it from disrupting my sleep. This is a low tech, low energy, relatively inexpensive solution that would be easy to install even as a retrofit in existing homes.

    Another thing I’m considering for the house we’re building on a couple of acres in the countryside in the next few years is a “cooling tube,” something commonly used in Earthships. While I’ve been unimpressed with the performance of Earthships and wouldn’t consider building my home out of toxic tires, the cooling tube concept seems like it could be a very useful one to us out there as we’d have room to make one that is long enough to draw nice cool air up into the house passively just by opening it up and opening the windows in the house upstairs.


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