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.
Japanese equipment giant Kubota brought 22 new or updated machines to the 2025 bauma expo earlier this year, but tucked away in the corners was a new retrofit kit that can help existing customers decarbonize more quickly, and more affordably.
No matter how badly a fleet may want to electrify, harsh economic realities and the greater up-front costs typically associated with battery electric remain high hurdles to overcome, but new retrofit options from major manufacturers are popping up to help lower those obstacles.
The latest equipment maker to put its name on the retrofit list is Kubota, who says its kit can be installed by a trained dealer in a single day.
That’s right! By this time tomorrow, your diesel-powered Kubota KX019 or U27-4 excavator (shown) could be fitted with an 18 or 20 kWh li-ion battery pack and electric drive motors and ready to get to work in a low-noise or low-vibration work environment where emissions are a strict no-no. Think indoor precision demolition or historic archeological excavation.
Then, if necessary, it can go right back to diesel power.
From diesel to electric and back again
If that sounds familiar, that’s because we’ve talked about a similarly flexible power solution from ZQUIP. The battery packs and diesel engines are much larger in that application, but the basic sales pitch remains the same: electric when it benefits your operation, diesel it doesn’t.
Kubota says its modular retrofit kits is a response to the increasing global demand for sustainable alternatives by focusing on making machinery that’s flexible and repairable enough to be “reusable,” and offer construction fleet managers a longer operational lifespan, superior ROI (return on investment), and lower TCO (total cost of ownership) than the competition.
Kubota’s solution also notably reduces maintenance costs and operational overheads. With no engine and associated components, servicing time and expenses are considerably reduced, saving customers both time and money. Additionally, with electricity costing far less than fossil fuels, it offers a highly economical advantage.
International Rental News reports that other changes to the excavators include a more modern cab controls with a digital instrument cluster, a 60 mm wider undercarriage for more stability, and an independent travel circuit allows operators to use the boom, dipper, bucket, and auxiliary functions without an impact on tracking performance.
Kubota’s new kit, first shown at last year’s Hillhead exhibition in the UK, will officially be on sale this summer – any day now, in fact – though pricing has yet to be announced.
Electrek’s Take
If you’re wondering how it is that we’re still talking about bauma 2025 a full quarter after the show wrapped up, then I haven’t done a good enough job of explaining how positively massive the show was. Check out this Quick Charge episode (above) then let us know what you think of Kubota’s modular power kits in the comments.
SOURCE | IMAGES: Kubota, via International Rental News.
If you’re considering going solar, it’s always a good idea to get quotes from a few installers. 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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Elon Musk isn’t happy about Trump passing the Big Beautiful Bill and killing off the $7,500 EV tax credit – but there’s a lot more bad news for Tesla baked into the BBB. We’ve got all that and more on today’s budget-busting episode of Quick Charge!
We also present ongoing coverage of the 2025 Electrek Formula Sun Grand Prix and dive into some two wheeled reports on the new electric Honda Ruckus e:Zoomer, the latest BMW electric two-wheeler, and more!
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Environment
FERC: Solar + wind made up 96% of new US power generating capacity in first third of 2025
Solar and wind accounted for almost 96% of new US electrical generating capacity added in the first third of 2025. In April, solar provided 87% of new capacity, making it the 20th consecutive month solar has taken the lead, according to data belatedly posted on July 1 by the Federal Energy Regulatory Commission (FERC) and reviewed by the SUN DAY Campaign.
Solar’s new generating capacity in April 2025 and YTD
In its latest monthly “Energy Infrastructure Update” report (with data through April 30, 2025), FERC says 50 “units” of solar totaling 2,284 megawatts (MW) were placed into service in April, accounting for 86.7% of all new generating capacity added during the month.
In addition, the 9,451 MW of solar added during the first four months of 2025 was 77.7% of the new generation placed into service.
Solar has now been the largest source of new generating capacity added each month for 20 consecutive months, from September 2023 to April 2025.
Solar + wind were >95% of new capacity in 1st third of 2025
Between January and April 2025, new wind provided 2,183 MW of capacity additions, accounting for 18.0% of new additions in the first third.
In the same period, the combination of solar and wind was 95.7% of new capacity while natural gas (511 MW) provided just 4.2%; the remaining 0.1% came from oil (11 MW).
Solar + wind are >22% of US utility-scale generating capacity
The installed capacities of solar (11.0%) and wind (11.8%) are now each more than a tenth of the US total. Together, they make up almost one-fourth (22.8%) of the US’s total available installed utility-scale generating capacity.
Moreover, at least 25-30% of US solar capacity is in small-scale (e.g., rooftop) systems that are not reflected in FERC’s data. Including that additional solar capacity would bring the share provided by solar + wind to more than a quarter of the US total.
With the inclusion of hydropower (7.7%), biomass (1.1%), and geothermal (0.3%), renewables currently claim a 31.8% share of total US utility-scale generating capacity. If small-scale solar capacity is included, renewables are now about one-third of total US generating capacity.
Solar is on track to become No. 2 source of US generating capacity
FERC reports that net “high probability” additions of solar between May 2025 and April 2028 total 90,158 MW – an amount almost four times the forecast net “high probability” additions for wind (22,793 MW), the second-fastest growing resource. Notably, both three-year projections are higher than those provided just a month earlier.
FERC also foresees net growth for hydropower (596 MW) and geothermal (92 MW) but a decrease of 123 MW in biomass capacity.
Taken together, the net new “high probability” capacity additions by all renewable energy sources over the next three years – i.e., the bulk of the Trump administration’s remaining time in office – would total 113,516 MW.
FERC doesn’t include any nuclear capacity in its three-year forecast, while coal and oil are projected to contract by 24,373 MW and 1,915 MW, respectively. Natural gas capacity would expand by 5,730 MW.
Thus, adjusting for the different capacity factors of gas (59.7%), wind (34.3%), and utility-scale solar (23.4%), electricity generated by the projected new solar capacity to be added in the coming three years should be at least six times greater than that produced by the new natural gas capacity, while the electrical output by new wind capacity would be more than double that by gas.
If FERC’s current “high probability” additions materialize, by May 1, 2028, solar will account for one-sixth (16.6%) of US installed utility-scale generating capacity. Wind would provide an additional one-eighth (12.6%) of the total. That would make each greater than coal (12.2%) and substantially more than nuclear power or hydropower (7.3% and 7.2%, respectively).
In fact, assuming current growth rates continue, the installed capacity of utility-scale solar is likely to surpass that of either coal or wind within two years, placing solar in second place for installed generating capacity, behind only natural gas.
Renewables + small-scale solar may overtake natural gas within 3 years
The mix of all utility-scale (ie, >1 MW) renewables is now adding about two percentage points each year to its share of generating capacity. At that pace, by May 1, 2028, renewables would account for 37.7% of total available installed utility-scale generating capacity – rapidly approaching that of natural gas (40.1%). Solar and wind would constitute more than three-quarters of installed renewable energy capacity. If those trend lines continue, utility-scale renewable energy capacity should surpass that of natural gas in 2029 or sooner.
However, as noted, FERC’s data do not account for the capacity of small-scale solar systems. If that’s factored in, within three years, total US solar capacity could exceed 300 GW. In turn, the mix of all renewables would then be about 40% of total installed capacity while the share of natural gas would drop to about 38%.
Moreover, FERC reports that there may actually be as much as 224,426 MW of net new solar additions in the current three-year pipeline in addition to 69,530 MW of new wind, 9,072 MW of new hydropower, 202 MW of new geothermal, and 39 MW of new biomass. By contrast, net new natural gas capacity potentially in the three-year pipeline totals just 26,818 MW. Consequently, renewables’ share could be even greater by mid-spring 2028.
“The Trump Administration’s ‘Big, Beautiful Bill’ … poses a clear threat to solar and wind in the years to come,” noted the SUN DAY Campaign’s executive director, Ken Bossong. “Nonetheless, FERC’s latest data and forecasts suggest cleaner and lower-cost renewable energy sources may still dominate and surpass nuclear power, coal, and natural gas.”
To limit power outages and make your home more resilient, consider going solar with a battery storage system. In order to 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. They have 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 you share your phone number with them.
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