In an Age of Climate Change, Passive Cooling Won’t be Enough

Posted by on Jul 27, 2015

The air-source heat pump on the far wall heats (and cools) our super-efficient house. Photo: Alex Wilson

The single, 18,000 Btu-per-hour, air-source heat pump on the far wall heats (and, if necessary, cools) our highly insulated house. Photo: Alex Wilson

In a July that is breaking heat records in many parts of the U.S. and worldwide and a week in which even Vermont is supposed to get into the mid-90s, the implications of a changing climate on building design are brought into focus.

I have long been a fan of passive design—and I still am. After we do our best to minimize energy consumption through energy-conserving design (high levels of insulation, high-performance windows, and such), passive solar heating, passive cooling strategies, and daylighting should dominate our energy design efforts. There are decades of experience we can draw from in passive design, starting with the knowledge gleaned from the passive solar energy conferences of the late 1970s and 1980s—many of which I attended (even editing the proceedings of one or two).

But I’m increasingly feeling that passive cooling won’t be enough as climate change advances and cooling loads increase. I’m now recommending that in most locations, even if passive conditioning is to be relied on initially, buildings be designed so that they can accommodate mechanical cooling measures down-the-road.

It’s getting hotter

Vermont weather this summer has (at least until this week) been very comfortable, but that’s not the case in much of the U.S. and elsewhere. Heat advisories have been issued in parts of the Southeast. Early July saw record heat in the Pacific Northwest: Seattle recording four consecutive days reaching all-time record highs; Boise recording nine consecutive days reaching over 100°F; and Portland approaching a record number of days with temperatures exceeding 90°F.

Western Europe is also having a record-breaking summer. In Germany, a temperature of 104.5°F in early July set an all-time record. Britain recorded its hottest July day ever (at Heathrow Airport—98.1°F), while Paris saw its second-hottest day ever (103.4°F), and Madrid setting a new June record at 103.5°F. In western France, hot weather recently led to power outages, leaving more than 100,000 without power.

In India, a June heat wave resulted in 2,300 deaths—the fifth-deadliest in world history—while a heat wave in Pakistan killed more than 1,200, with temperatures climbing to 113°F. In South America, a June temperature record was set in the Columbian city of Urimitia (107.6°F)

None of these disasters has been as deadly as the August 2003 heat wave that killed as many as 70,000 in Western Europe, and one has to be careful in ascribing specific heat waves to climate change, but the temperatures we’re seeing, coupled with the projections from climate scientists, should be setting off alarm bells.

This map shows how Vermont's climate is expected to effectively move south by the end of 2100 under two different emissions scenarios. In the worst-case model, our climate will be much like that of northwestern Georgia. Graphic by Alan Betts.

This map shows how Vermont’s climate is expected to effectively move south by late in this century under two different emissions scenarios. In the worst-case model, our climate will be much like that of northwestern Georgia. Graphic by Alan Betts.

Implications for building design

Rising temperatures and increasing cooling loads argue, first of all, for redoubling our efforts to incorporate high levels of insulation, cooling-load-avoidance measures, and passive cooling strategies into our buildings. This is particularly important given the prospect of increased risk of power outages during heat waves—as we saw in western France a few weeks ago.

Designing highly energy-efficient buildings is the best way to provide passive survivability, which RDI defines as ensuring that buildings will maintain habitable conditions should they lose power. This is something I’ve written about extensively on the RDI website and through BuildingGreen, including in this Fundamentals article and this 2006 in-depth overview.

Yes, we need to keep doing all that—and doing it better—but we should also be thinking about incorporating efficient mechanical cooling systems. Even if we don’t have near-term plans to rely on mechanical cooling, providing for the eventual use of such systems is smart design.

Fortunately, in residential and light-commercial buildings, that’s become much easier with the advent of high-efficiency air-source heat pumps (mini-splits). In buildings with very small heating loads, such as our Vermont farmhouse, this is often the best option for heating—especially when there’s a solar array to supply the electricity needed on a net-zero-energy basis—and the cooling comes as a sort-of bonus.

In larger buildings, designing for the later installation of cooling systems may require providing ducting or space for a larger heat pump, chiller, evaporative cooler, or some other cooling system to be added later.

Final thoughts

We should continue to strive for buildings that don’t require mechanical cooling—which usually means starting with an exceptional building envelope—but we need to be realistic also. If climate change will effectively move our buildings hundreds or even a thousand miles south, in terms of cooling loads and humidity, we need to provide for mechanical cooling.

In my house, with today’s temperature expected to rise to the mid-80s, I’ll close up the house this morning to keep the place cool, and I may lower some window shades to block solar gain (though porches and shade trees do a pretty good job with that already). That should keep the place cool enough so that I don’t need to turn on our mini-split heat pump—but it’s nice to know that it’s there just-the-same.

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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. After a decade of writing about how air conditioning is evil and unnecessary, (when I personally have the option of moving to a cabin by a lake) I have changed my mind about this. People almost have no choice anymore with changing climate and the great migration south. I finally gave in and wrote:

    We need a balance between the old and the new, an understanding of how people lived before the thermostat age along with a real understanding of building science. To discover what we have to do to minimize our heating and air conditioning loads and maximize comfort, we have to design our homes right in the first place.

    And then we can decide what kind of technology and hardware we need.

  2. We need to address temperature and humidity separately. For instance condensation will occur inside buildings if exterior conditions are humid enough even with all windows open and when not using an mechanical cooling. While conversely a person may be comfortable in an 85 degree space if the humidity is low, air movement is ample, and their clothing is suited to conditions. One place to start is requiring that vapor impermeable waterproofing be used on foundations and basement slabs. Simply reducing the 24 hour vapor drive inward through most basement walls and slabs would reduce humidity dramatically in tens of millions of US structures.

    • In a recent technical paper that I, along with Seth Holmes and Tom Phillips published in Building Research and Information, we suggested that a better metric than temperature for measuring potential health impacts from heat would be “wet bulb globe temperature.” This metric includes humidity and mean radiant temperature, along with air temperature; unfortunately, it’s harder to measure than air temperature.

      An abstract to that paper can be found here: E-mail me if you would like access to the full text; I have a few links that I can give out that provide full access to the article (

  3. Alex & Lloyd,
    This support of active cooling worries me. Having just lived through the Pacific Northwest heat wave in an unfinished deep energy retrofit, I can say it’s not fun at all. However I’d really like to caution you guys.

    Passive cooling and airsealing can get the job done. My freinds in completed passive houses in Portland and Whidbey Island did not get internal temps above the mid 70’s when exterior temps were sustained in the 90’s. If you want first hand reports, let me know and I’ll introduce them.

    If passive cooling really works like my friends are experiencing, then I think your giving up the concept too early. As we know, the largest issue with successful adoption of passive stategdies is the lure of a quick technological fix.

    Even at a zero balence. These cooling loads can be replaced by insulation and air sealing that will shift your home farther twards a smaller grid draw and in the direction of plus.

    Leaders need to take a position that zero is only a stop on the way to plus. energy.

    • Albert, I think you’re missing my point. I am not at all suggesting giving up on passive cooling (and high levels of insulation). I’m suggesting doing that AND including provisions to incorporate mechanical cooling now or in the future.

      We should definitely continue to focus on energy efficiency and passive cooling and heating strategies—and Passive House is a great way to achieve that—but I also believe that we need to accept that a warming climate will likely push the cooling loads way up. I’m not convinced that your friends in their passive house would remain comfortable with another five or ten degrees (F) of outside temperature and higher relative humidity.

      • Greetings.

        Given the need to let the cats in and out, our house on Whidbey did get up to 78 F while the temps were 90-92 or so.

        We do not have any kind of cooling but I would guess that temps in the 100’s would warrent a heat pump.

        But I grew up before air conditioning — we finally got “swamp coolers” when I was in high school. Mid 70’s is quite fine for summer temps.

        -KenD [Ken dot Dickey at Whidbey dot com]

        • Ken, the challenge is when it doesn’t cool off at night. That’s what happened in France in 2003, when so many people died. Those were buildings that had always benefited from the diurnal temperature cycle—the high-mass homes cooled down at night. We can’t be sure we can rely on that in the future.

      • I spent the first 34 years of my life in the hot humid east coast. We would leave Philadelphia to come to Vermont and other New England states to cope. As Alex say, (east of the continental divide) it is the humidity and lack of cool nights that make AC a reality for most. The pacific northwest is a lot easier, and the Inland Northwest where I live is very easy since the humidity is low and the nights are cool. My office built with highly insulated strawbale walls, lots of thermal mass, and well designed ventilation was no higher then 78 degrees during this summers high 90s/100s period. So with the right design and the right climate AC is a waste. When humidity is high, try southern India in the summer, AC is real. But the building envelope is still the issue, and buildings designed around climate and ventilation is the key. In addition, we in the western world need to get our bodies and mind out of needing one indoor temperature to be comfortable. A 10 to 15 degree swing is very easy to deal with, but we demand a constant temperature. This is going to be a lot more difficult with extreme weather of the next century. The answers are based our how we will adjust, and not just to indoor temperatures.

      • Alex,
        You, Lloyd, and Albert all make great points. And are in fundamental agreement. I believe it is simply the title of your message that concerns us.

        I’m one of Albert’s friends that he mentioned, and our futurefitted passive house did not exceed 76F throughout our 20 days of 90F or greater ambient.

        However, as everyone knows, we have it easy here in Portland compared to many climates with our cool nights and without high humidity.

        That will unfortunately continue to change. We’ve all noticed it here in Portland. We are slowly losing our beloved Mediterranean summers. The humidity is debilitating. And the occasional thunderstorm is not worth the build up. It’s a tragedy.

        Albert’s points are well-taken, and I sense that you agree. It’s a matter of nuance and messaging.

        Our concern: If we broadcast the eventual need for (and then plan for and build based on) a mechanical fix, then people may forgot/skip the more careful planning needed for and more expensive (first cost) passive design and construction. “We can always fix it by adding AC.”

        I know you don’t want to encourage that.

        Since we futurefit our home, we’ve largely not needed mechanical cooling. Our HRV works superbly as a “heat rejecting ventilator”.

        Other than that, we have to “be active for passive.” We throw open windows and doors in the early morning hours, trade cool ambient air for warm house air, and then close up the house before the heat of the day. During the heat of the day we rely on 20 watts/70 cfm of ventilation air (with free cooling) courtesy of our Zehnder HRV.

        If this fails to maintain comfort, we use the ceiling fans (which we hardly ever use after we futurefitted our house but routinely used in summers before).

        I hope that we’ll never need more than ceiling fans, but it will depend on humidity.

        We’ve gone to 100F ambient many times simply with passive measures. We could probably up to 105F. On those days, we rely on all of the built-in measures as well as putting up temporary, external reflective shades on any unshaded East/West-facing windows and doors.

        We also avoid cooking, take care to minimized opening doors near hot areas, and use the ceiling fans. Hotter than 105? I don’t know. We’ll probably plant more fruit trees close to our house for shade and install a reflective roof (when our comp roof needs replacing).

        I hope if we ever need dehumidification, I hope we’ll at least be able to use existing mechanical systems. For example, perhaps split heat pump water heater systems can be small enough and portable so that on the hottest/humid days, we can move them inside our homes and run them (probably at a lower capacity and for a longer period) for “free” cooling and dehumidification.

        If AC/dehumidifier becomes necessary, I expect a small portable or window AC/dehumidifier unit will be all we need given the many passive measures that lower the cooing load and the existing whole-house ventilation system to distribute the air.

        Thanks for sounding the alarm but promoting passive.

        Tad Everhart
        Portland, Oregon

        • Tad,

          Don’t forget that your Zehnder HRV almost certainly has a dehumidifier plug-in if you need one..


  4. I used to think AC in many parts of the country was for “wimps” but I too haved changed my mind. In Vermont, I don’t have A/C and can cope just fine, but the reality that heat pumps will be a necessity in most places AND used in cooling mode is just fact. I do worry that many people will just move to the mechanical cooling “ON” switch when it gets warm, swhen shading, cross ventilation, and common sense shoudl be used first, but the reality of climate change and unusually high temperatures is here. At the least we need to be sure all buildings are efficient, air sealed and insulated, then equipped with renewable energy to minimize the burning of fossil fuels to make them comfortable.

    I am scared about the added load of A/C as a norm in buildings. Good post Alex.

  5. Albert, I appreciate your concern, and your listing of successful examples. At the same time, success in Portland, OR, and Whidbey Island, WA, is success in two beautiful locations with pretty benign climates. There are quite a few locations in this country where thirty days over 100 degrees during the summer doesn’t make the news, unless they come all in a row. There are a few population centers where getting BELOW a hundred at night isn’t guaranteed.

    All building is local, and must address the local conditions. If everyone in this country looked at the summer weather for Whidbey and Portland each day of the summer, I bet 90% of them would say that the weather in one of those two places was better that day than where they live.

    Some people have chosen to live in extreme locations, without thought or concern about climate questions and energy concerns. However, the vast majority of us have chosen locations that used to be considered normal, and with climate change, those places are becoming more extreme. If we encourage all the people in this country to move to places like Portland and Whidbey, I imagine that the folks currently living there won’t be too happy.

  6. Having lived on both coast and the southeast, I would caution making judgements about who needs what sort of mechanical systems until you have lived in that climate. The USA has a tremendously diverse climate and many strategies that work best for one location will not work well for others.

    If anyone doubts the need for active cooling and humidity control, you can come stay with me in New Orleans next month and enjoy our August weather, where nighttime temps routinely stay above the mid 80s with 90%+ humidity. With these latent loads a well designed HVAC is more efficient at removing the humidity than a stand alone dehumidifier.

    Alex is just echoing what he has written many times about, building with resilient design in mind.

  7. Alex: living in hot and humid St. Louis your message hits home. Although our past two summers have been relatively mild, we certainly experience weeks of temperatures in the 90s with little temperature variation at night and continued high humidity. Based on the National Climate Assessment report there will be many more weeks of this over time. On the residential front, I agree with the split system solution when possible to allow for tuning of cooling based on specific time of day and needs versus the rampant over cooling that is the norm. We also need to find better ways to customize cooling in commercial settings to reduce consumption and allow for our bodies to appropriately adjust to summer temperatures – there’s nothing worse on my system than a sudden 20 degree temperature change when leaving a building.
    Thanks for raising this important point.

  8. This is a very important question and one I ve been researching for a while. I believe air conditioning can be a viable tool in order to adapt to a changing climate, but I would argue that only dwellings indicated as vulnerable should have the possibility of installing such devices. My rationale is driven by the fact that research indicates that the simple availability of mechanical cooling is a enhancing factor for its use.
    Thank you for the very fruitful discussion here.


    • Ricardo, thanks for these comments. What you suggest is a very important part of the discussion. How do you provide a mechanical cooling option yet discourage its use? I’d be interested in seeing some of your (and other) research on this. Is there a link you can provide?

    • Great post by Lloyd. Where he comes up with those old graphics is beyond me. He’s like a library.

      Alex & all, I do get it that there are varying needs in the country. I was writing from a Pacific Northwest point of view since Alex specifically mentioned our PNW heat wave. I have no experience in long term living in other climates.

      I still strongly resist the idea of advising cooling at current conditions. I know that you are not straight out advising cooling. Tad is correct that my concern is two-fold: Messaging and adjusting current practice.

      One of my concerns has a a good amount of self interest in it: For my retrofit I deliberately choose not to include a mini split. I did this because I knew I would use the cooling if it was available. The path I choose was to provide heat with a Sanden co2 refrigerant heat pump hot water heater. It does both DHW and space conditioning with a COP of 4.2 at 45 deg.

      My concern is that technology like this is really a breakthrough. However, if we start adding further expectations of cooling, in climates and conditions where we don’t really need them, them I think we are making a serious mistake. We might derail technology like this before it’s fully developed and implimented.

      For much of the US & Canada, I think this discussion is pre-mature. There are certainly areas where it is not premature and I think you’re position is valid. However, I think the distinction could be made a little stronger.

      Alex, I miss your smile out here in the West. Come on over to the Timber Framer Guilds 30th anniversary in Idaho this fall. We can continue the discussion in a panel format.

      Open to the idea?

    • Hi Albert, Ricardo Barbosa raised the key issue with this question and has collected evidence to support your fears: that if people have access to A/C they will use it. I haven’t looked at that data yet, but it is indeed something that needs to be considered.

      As for myself, I have a mini-split, so have access to A/C (and I even have excess electricity I’m producing beyond net-zero), but I still bend over backwards to avoid mechanical cooling, though on warm nights we do use a ceiling fan. Yesterday, as is my normal practice in hot weather, I closed up the house during the day, lowered blinds to exclude (some of) the sun striking those windows, and then opened up the house at night. With outdoor temperatures rising into the upper-80s, that has kept our house in the 72-73°F range. (But I realize I’m not a typical homeowner!)

      Thanks for the invite to the Timber Framer’s Guild meeting. I’d consider it if it doesn’t conflict with other fall conferences I’m committed to and if I could combine it with some other activities. I’ll be in Portland for an event in October.

  9. I have to ask about this sort of “planning ahead,” how do we have any clue what provisions we should put into our houses to help provide opportunities for future cooling installations? 25 years ago, who would have thought mini-splits were what one should provide for? The idea sounds good, but isn’t it impossible to actually implement?

    Unfortunately, I feel even introducing air conditioning into the discussion will push things in a bad direction.

    As others have pointed out, there are climates and there are climates, and all aren’t like Vermont or St. Louis (my sympathies there) or New Orleans (even more sympathies). Here on the Pacific Coast, we have sea breezes that cool during the day, fog that cools even more at night, a large swing between day and night temperatures, and nobody can convince me that passive isn’t enough. Yet builders are installing air conditioning in new houses that ignore every single passive design principle (along with solar panels pointing west to add meaningless green bling). That’s not green, it’s not sustainable. If anything, we need to re-up the emphasis on passive, and I fear introducing mechanical air conditioning into the discussion will simply provide a rationale for mainstream’s continuing in a non-sustainable direction.

  10. Alex, I’ve been grappling with this topic recently, and I have mixed feelings. I’m designing a Net Zero targeted house in Iowa (which has very hot & humid summers). We initially set out to avoid traditional AC through good passive design, plus a whole-house dehumidifier linked to the ERV for the peak summer weeks.

    But future climate analysis (based on IPCC projections) showed that, even with dehumidification, the occupants risked several weeks of uncomfortable conditions each year. Now we’re re-considering AC.

    My concern is similar to Tad’s: Once AC is installed, it’s too easy to rely on it — to use it all the time, not just during those peak weeks. When we don’t have AC we find other ways to adapt: light clothing, modifying our schedules, taking a cold shower before bed, drinking cold drinks. These cultural practices are hard-won and easily lost … and the changes to our expectations / behavior are critical: AC is a huge energy consumer, and we need to reduce our energy footprint, FAST.

    So, back to your question: “How do we provide a mechanical cooling option, yet discourage its use?” Better occupant education? Building monitoring that reminds occupants about the passive alternatives?


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