Passive Cooling in a Tiny House
Originally published on RMI.org.
By John Matson
As the world warms and the demand for cooling increases, many homes will require an “all of the above” approach to keep cool without further contributing to global warming. That can include high-performance cooling systems that use climate-friendly coolants and consume relatively little energy, as well as building design approaches that offset the need for mechanical cooling in the first place.
This tiny, ultra-efficient Passive House was built in just 10 days in #Australia http://t.co/glUiZVSN1o pic.twitter.com/pjYxo496ti
— The Architect’s Newspaper (@archpaper) February 10, 2015
In this post, we look at some passive cooling strategies that help keep an innovative tiny house comfortable during California summers, without the use of a mechanical cooling system.
Brett Webster, a manager in RMI’s Carbon-Free Buildings program, lives in a 170-square-foot home in Sonoma County, California. Brett and his partner helped design and build the solar-powered tiny house as part of a graduate project, and they have lived in the demonstration home for about five years. The home itself was built on a 24-foot-long trailer and can be hitched up to a truck for relocation. So even though Brett and his partner have lived in their tiny home for years, they have moved twice in that time between Northern California locations (and their respective microclimates).
Strategic Shading
The walls of the tiny house are clad in reclaimed cedar slats over one-inch-thick panels of cork, which provides a layer of continuous insulation, reducing the thermal bridging of the wooden wall framing. Because the carbon sequestered in cork trees can exceed the carbon emissions of producing cork products, cork is often considered a carbon-negative material. The cedar siding is separated from the cork by an air gap, which allows the wooden slats to shade the cork and absorb solar radiation, while slowing the rate of heat transfer directly to the house. The walls of the structure are insulated with recycled denim to further limit heat gain in warm weather and heat loss in cool weather.
Pulley-mounted shade awnings, made from cedar slats to match the siding, cover the largest expanse of glass on the tiny house: a sliding-glass door at the entry to the home. Webster says that the shade structure extends far enough to block solar radiation from pouring through the glass entryway in summer, but it can let in sunlight and heat in winter, when the sun is lower in the sky.
The ability to shade the windows in summer and admit sunlight during the winter is critical to maintaining passive comfort in the house. The windows that the design team chose for the tiny house are well-insulated (low U-value) but are also designed to let the sun’s heat in (high solar heat gain coefficient), because the Bay Area is mostly a heating-dominant climate zone. During the summer, when that heat gain is not desirable, shading the windows is a necessity.
Ceiling and Roof
A layer of BioPCM phase change material in the ceiling acts like thermal mass to absorb and store heat that would otherwise warm the interior space. Adobe buildings and concrete-walled structures similarly benefit from thermal mass that prevents the interior from becoming overheated during the day. But phase change material is lightweight, making it more appropriate for applications like the ceiling of a tiny house, and it doesn’t have the carbon footprint of concrete. (Cement production alone accounts for about 8 percent of global carbon emissions.)
The phase change material, which comes embedded in sheets that can be rolled out between ceiling joists like high-tech bubble wrap, melts from solid to liquid at 77 degrees F (25°C). As it changes phases, the material absorbs a lot of thermal energy, preventing the temperature from exceeding 77 degrees until its heat-absorbing capacity has been reached, like a sponge that can’t soak up any more water.
The tiny house’s roof is designed to harness much of the sun’s energy and reject the rest. A 2.3-kilowatt solar array shades much of the tiny house’s roof and feeds into a Tesla Powerwall to store electricity for nighttime use. The “cool roof” is also covered with a light-colored acrylic roofing membrane to minimize heat gain from solar radiation.
Some Energy Required (But Not Much)
In addition to the passive cooling approaches described above, the tiny house relies on a few efficient electric devices to provide airflow and ventilation. Even though they don’t qualify as strictly “passive” technologies, ceiling fans and other efficient electric devices have long gone hand-in-hand with passive cooling approaches. The ventilation and airflow systems in the tiny house consume very little energy and allow the building to remain comfortable without a dedicated mechanical cooling system.
A high-efficiency overhead ceiling fan consumes 4–18 watts of electricity and ensures occupant comfort in warmer temperatures. “Airflow creates a cooling sensation that’s extremely effective,” Webster says. According to the US Department of Energy, using a ceiling fan can significantly offset the need for air conditioning, allowing occupants to raise the thermostat by about 4 degrees F without sacrificing comfort.
The well-insulated structure is designed to be closed off to the outside during hot days in the summer, so the windows do not provide any natural ventilation during the daytime. The tiny house therefore relies on an energy recovery ventilator to bring fresh air into the house. An energy recovery ventilator uses a heat exchanger to reduce the thermal energy of the outside air before it enters the house, thereby providing ventilation without flushing warm air into the building. In the winter, it does the reverse, using the heat of the outgoing stale air to warm the incoming fresh air.
Unplugging
The tiny house’s passive design and minimal energy requirements for ventilation make it fully capable of going off-grid, especially in the summer months when solar energy is abundant. And even if most of us aren’t ready to commit to living in a 170-square-foot house on wheels, the lessons from Webster’s tiny house and other passive homes provide a powerful reminder: Even for energy-intensive applications like cooling, with thoughtful design, you can do a lot with a little.
Image gallery courtesy of RMI.
GM sold over 21,000 electric vehicles in the US last month, its best yet. Despite the surge in August sales, GM warned that with the “irrational discounts” on EVs set to end soon, the market is due for a shake-up.
GM sells record EVs in August as irrational discounts end
August was GM’s best month ever for EV sales. The company sold over 21,000 electric models under the Chevy, GMC, and Cadillac brands last month.
The higher demand comes as buyers rush to secure the $7,500 federal tax credit, which is set to expire at the end of September.
Driven by the hot-selling Chevy Equinox EV, Cadillac Lyriq, and GMC Sierra EV, GM remains the second-best seller of EVs behind Tesla.
GM expects to see strong demand again this month, but without the credit, it expects changes next quarter. GM said, “There’s no doubt we’ll see lower EV sales next quarter.” The company anticipates it will take several months for the market to correct, adding that “We will almost certainly see a smaller EV market for a while.”
Like several automakers in the US, GM will adjust production accordingly, promising not to overproduce. Despite slower sales, it remains confident that its EV market share will continue to grow.
Since affordable EVs and luxury models have been the strongest segments, GM believes it’s in a better position than most. It already has “America’s most affordable 315+ range EV,” the Chevy Equinox EV. The electric Equinox is one of the few EVs with a starting price under $35,000 in the US.
Soon, the new Chevy Bolt EV will debut, which is expected to be even more affordable, starting at around $30,000.
With a full line-up of electric SUVs, Cadillac is the leading luxury EV brand, but that doesn’t include Tesla. And then there’s the Chevy and GMC electric pickup with segment-leading range, features, and more.
GM said as it adjusts to the “new EV market realities,” its ICE vehicles will provide flexibility while driving profits. We will learn more on October 1 when GM reports full third-quarter sales results.
Although I wouldn’t call it “irrational,” GM is offering generous discounts on EVs with the deadline approaching. The Chevy Equinox EV is listed for lease starting at just $249 per month with a new $1,250 conquest bonus. Chevy is also offering the $7,500 credit on top of 0% APR financing until the end of September.
Thinking about trying one of GM’s EVs for yourself? You can use the links below to find Chevy, Cadillac, and GMC models in your area.
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Global solar installations are breaking records again in 2025. In H1 2025, the world added 380 gigawatts (GW) of new solar capacity – a staggering 64% jump compared to the same period in 2024, when 232 GW came online. China was responsible for installing a massive 256 GW of that solar capacity.
For context, it took until September last year to pass the 350 GW mark. This year, the milestone was achieved in June. That pace cements solar as the fastest-growing source of new electricity generation worldwide. In 2024, global solar output rose by 28% (+469 terawatt-hours) from 2023, more growth than any other energy source.
Nicolas Fulghum, senior energy analyst at independent energy think tank Ember, said, “These latest numbers on solar deployment in 2025 defy gravity, with annual solar installations continuing their sharp rise. In a world of volatile energy markets, solar offers domestically produced power that can be rolled out at record speed to meet growing demand, independent of global fossil fuel supply chains.”
China’s solar dominance
China is leading this surge by a wide margin. In the first half of 2025, the country installed more than twice as much solar capacity as the rest of the world combined, accounting for 67% of global additions. That’s up from 54% in the same period last year. Developers rushed to complete projects before new wind and solar compensation rules took effect in June, fueling the spike. While that may lead to a slowdown in the second half of the year, new clean power procurement requirements for industry and bullish forecasts from China’s solar PV association (CPIA) suggest that 2025 will still surpass 2024’s record high.
The rest of the world
Other countries are adding solar at a healthy clip, too. Together, they installed an estimated 124 GW in the first half of 2025, a 15% year-over-year increase. India came in second with 24 GW, up 49% from last year’s 16 GW. The US ranked third with 21 GW, a 4% gain year-over-year despite recent moves by the Trump administration to suppress clean power deployment. Germany and Brazil saw slight dips, while the rest of the world added 65 GW, a 22% rise over 2024.
Africa’s solar market is also stirring. The continent imported 60% more solar panels from China over the past year, though a lack of reliable installation data makes it a challenge to track the true pace of deployment.
With installations surging across major markets and China driving the charge, 2025 is on track to be another record-breaking year for solar power.
Read more: China-made panels drive Africa’s 15 GW solar import milestone
The 30% federal solar tax credit is ending this year. If you’ve ever considered going solar, now’s the time to act. To make sure you find a trusted, reliable solar installer near you that offers competitive pricing, check out EnergySage, a free service that makes it easy for you to go solar. It has hundreds of pre-vetted solar installers competing for your business, ensuring you get high-quality solutions and save 20-30% compared to going it alone. Plus, it’s free to use, and you won’t get sales calls until you select an installer and share your phone number with them.
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Porsche just axed two of its most iconic models. The gas-powered 718 Cayman and Boxster sports cars have been discontinued, with their new EV successors set to debut next year. However, Porsche isn’t the only brand killing off a popular nameplate.
Sports cars are due for EV successors in 2026
As it prepares for the all-electric replacements, Porsche has stopped taking new orders for the 718 Cayman and Boxster. For now, you can still order the vehicles from stock.
We’ve known for years that an electric replacement was on the way for the 718 lineup. Porsche CEO Oliver Blume confirmed in 2022 that the electric 718 successor would follow the Taycan and Macan EVs.
Although the new Cayman and Boxster EVs were expected to launch by the end of this year, it was pushed back due to software and battery sourcing delays.
Porsche initially planned to build the EV versions alongside the current ICE models at its Zuffenhausen plant, but that will no longer be the case. Despite rumors that Porsche was planning to extend 718 production, “high-ranking Porsche sources” told Autocar that’s not the plan.
The luxury sports car maker has dialed back its EV plans recently, with ICE Macan and Cayenne models now due to be sold alongside the electric versions.
Meanwhile, Porsche isn’t the only sports car maker killing off models with new EV successors on the way. Audi confirmed with Autoblog that the A7 and S7 will be discontinued after the 2025 model year.
In a statement, Audi said, “There are no 2026 Model Year A7 or S7 being offered as production shifts to the new A6 TFSI coming later this year.” However, the RS7 will live on as a 2026MY. The ICE A7 will be rebranded as the A6 TFSI, while the EV version will retain the A6 E-tron name, featuring a similar sportback design to the outgoing model.
Porsche and Audi have leaned into a more flexible “multi-energy” strategy, blaming slowing EV sales and a changing market.
Just last week, Porsche announced it no longer plans to build EV batteries in-house. Instead, it will focus on research and development.
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