Interior and Exterior Films
We are all familiar with window films (sometimes referred to as tint with automobiles). Modern films are designed to block UV light that damages plastic, cloth, and furniture; optical light (what we can see) to reduce glare; and infrared (what we can feel) to reduce total thermal transmission from the sun into our homes or cars. It’s all light, just shorter and longer wavelengths (yes, you learned this in high school, but probably forgot :).

Most films are applied to the interior of glass windows, or at the factory to the interior of the first pane of glass in a double-pane insulated window. The films are rated for transmission as a percentage, i.e. “75% optical transmission” means the film blocks 25% of the in-bound light we can see, and allows 75% to pass through. Most block 99% of UV and upwards of 90% IR.

In a casual study I conducted at my place of work, I noticed that non-treated glass remains relatively cool to the touch (ambient room temperature) as full spectrum sunlight enters the room and heat the objects it strikes. When an interior window film is applied, yes, the objects in the room remain cooler, but the glass itself heats up. If the glass is heating, that means it is generating heat energy, or infrared light, and is in fact heating the room, even if to a lesser degree (50% in / 50% out). This deserves further study, to learn how much the glass is heated through the process of reflecting IR with an interior film, and how much heat energy ends up in the room anyway.

That said, if you apply a film on the outside of the glass window the heat energy is reflected without heating the glass, and as such is more effectively keeping the interior space cool. But most translucent films are not intended to be applied on the exterior as they will dry-out, get scratched, and fall off prematurely.

There is an alternative …

Save a bird. Reduce your electric bill too.
We live in the San Pedro River valley where more than 300 species of migratory birds travel twice each year. And year-round we enjoy a local population of doves, finches, woodpeckers, humming birds, ravens, hawks, and many more. Sadly, they too often see the reflection of the blue sky in the early or evening hours and fly into our windows, either stunned for a few minutes or breaking their necks.

We tried curtains on the inside, and hand-cut paper snowflakes taped to the windows too. We successfully hung shade cloth in front of two windows, but this solution is not feasible all around for a variety of reasons, including the amount of structural modification and resulting loss of visibility.

Then we discovered the not-for-profit company CollideEscape whose various exterior window films provide up to 100% guaranteed elimination of bird collisions. This vinyl film is the same as that applied to city buses, painted-on advertisements making the windows appear opaque on the outside when in fact the passengers can see just fine.

We applied these to all upper windows. It worked perfectly. Not a single bird collision on those windows with the film. In the process we also discovered that the film drastically reduces the interior temperature where sunlight falls, by as much as 30F. This was initially noticed by sitting in our loft to determine how our view was changed, inside-to-out.

With two windows side by side, in the same morning light, one had the vinyl film, the other did not. The temperature difference was immediately noticeable on our faces and forearms, and confirmed with a mercury thermometer placed in the light of each window. We have since covered all our basement windows too and noticed a significant change in the temperature of the carpet where the sunlight falls. An infrared thermometer noted the temperature of the carpet (yes, carpet does get hot) dropping from 107F in direct sunlight to 85F behind a glass pane covered with the CollidEscape film, the lower temperature just seven degrees over the 78F room temperature (as measured on the carpet beyond the sunlight of any windows) at that time. Our basement is noticeable cooler than the last few years, which aids in cooling the entire house.

This essay is part of a series about Keeping Cool in your Home …

History
It doesn’t take a physics lab full of PhDs to find simple solutions to complex problems. Sometimes we need only look to other parts of the world and what they’ve been doing for centuries. All along the Mediterranean coast homes are painted white with limewash or whitewash to reflect the intense sunlight, keeping the interior of the homes cool. In Iceland, they do the opposite, building with black roof tiles to absorb solar heat.

The modern (with lab and PhDs) version of whitewash is Purdue University’s world’s whitest paint. Developed by Purdue professor of mechanical engineering Xiulin Ruan, this new paint is fighting global warming by keeping surfaces cool to reduce the need for internal air conditioning. According to Ruan and his team’s models, covering 1% of the Earth’s surface in their technology could mitigate the total effects of global warming, a fact encouraging them to continue pursuing formulas suitable for surfaces like asphalt and roadways.

Our Home
Colleen and I have spent the past year mitigating the increasingly warm summers by reducing the amount of thermal energy our house gains during the day, and increasing the amount of thermal energy released at night.

It is important to note that our home is a rather unusual construction, not at all ideal for anywhere but the temperature climate of San Diego or coastal Hawaii. That said, it came with the property, provides exceptional views of the surrounding wildlife year-round, and is enjoying a successful remodel toward thermal mitigation.

It is important to note that we live at 3000 feet elevation with winter, night lows dipping into the mid-20s with days at 50-70F. Summer will see three months in the mid-90s by mid-afternoon with a few weeks over 100F, sometimes pressing 110F. With each summer night, even on the hottest day in the year, the air cools to the low 70s or high 60s. This is the way the desert is suppose to be, and was until the introduction of concrete, asphalt, and air conditioning (which we will address later).

Too hot to touch?
Our first major effort in thermal mitigation was painting the roof white. As with most of the homes in this southeast Arizona region, galvanized sheet metal is a preferred material as it lasts, with minimal care, thirty to fifty years.

However, as anyone who has touched sheet metal in the Arizona sun knows—it gets really hot—dangerously hot. When you touch but cannot hold your hand to the surface for the intensity of the heat, you have reached your ‘threshold of pain‘. This is the minimum temperature at which your body feels pain and you have a natural reaction to remove yourself from that situation. This varies from person to person, and from object to object. 110F air is tolerable while a 110F Jacuzzi will require some getting used to. We can generally hold our hand to or walk barefoot on 110F concrete. But if that temperature climbs to 120F or 130F, it becomes unlikely you will stand there for long. I use 132F as my own threshold of pain for what I can tolerate with bare feet or my hands.

In the course of our work on our home, we have used an infrared thermometer which has been compared to both a mercury and bi-metalic coil thermometer and validated to within 2 degrees Fahrenheit. This gives us a high degree of accuracy up to twenty, even thirty feet away.

Our house is built such that our roof extends over the outer walls by 4 feet. This casts needed shade in the summer, and with the low sun in the winter allows direct sunlight to enter our home and heat the concrete floor through the large, double-pain windows.

With the infrared thermometer we are able to measure the temperature of the metal roof from the underside of the overhang such that as we painted each section, we could readily determine the effect of the new paint application with the same ambient air temperature and immediate solar gain.

Choosing the right paint
There are many brands of paint on the market today. Most of the products are now water-based (acrylic), moving away from oil-based to reduce toxic chemicals consumed (and wasted) in manufacturing. While acrylics have come a long way, and make sense for bedroom walls and refinished desks, nothing beats the durability and weather resistant nature of a good oil-based stain or paint.

At my work at Biosphere 2 I became familiar with the oil-based Rust-Oleum brand Rusty Metal Primer. My team found it to be an incredibly durable product, readily applied with brush, roller, and sprayer. The Gloss White top coat is far more reflective of solar radiation than an elastomeric, and without the need for pressure washing every six months to keep it from collecting dust and losing its reflectivity.

Rust-Oleum will tell you that you need to use a special, water-based primer to adhere to galvanized metal. However, my test proved otherwise—a screwdriver only marginally able to scratch the primer after 24 hours drying. This is likely due to the fact that the metal roof on our house is nearly thirty years of age, with the galvanized metal losing its sheen.

In July 2023 we worked from 4:30 am ’till 7:30 am three mornings in a row to apply the primer. Due to our work schedules we returned to the project a week later and applied Rust-Oleum High Gloss White, again with an airless sprayer. With just one coat we achieved a quality finish (a second coat will even the highs and lows). We painted the two main sections (north and south) that together encompass more than three quarters of the total surface area. This [2024] summer we completed the east section of the roof with one day of prep and two days painting (primer and white respectively). The west end remains.

When complete, the total number of gallons of paint for our 1500 sq-ft roof will be 7 gallons primer and 7 gallons white. At $37 per gallon that is roughly $500 in paint. A new roof of the same size would be between $10-30,000 for materials and at least double for labor, if contracted.

From 153F to 115F
Using or infrared thermometer we were thrilled to discover that we reduced the surface temperature of the galvanized steel from ~150F to ~110F (actual high temperature ranges between 135F and 153F; with the underside low ranging from ambient air to 115F for the painted surface, corresponding to humidity, cloud cover, smoke particles, and time of day).

While we have 4″ foam insulation beneath the corrugated steel over 2″ tongue-n-groove pine ceiling, over the course of a day the heat eventually gets through. We used to feel the radiation (infrared) on the backs of our necks and bare arms despite the air temperature maintained at 80F with mini-splits, much in the way that a desert canyon wall will radiate heat after sunset.

Now, that radiant heat penetrating our home is reduced, the thermal gradient from ground level to the loft (20 feet) has been reduced to just ~5-8F degrees, which is 10F less than before the paint. Furthermore, in a comparison of May 2023 to May 2024, despite the 3F increase in average temperature, our electric bill went down $22. There are other factors, perhaps, but the point is—we are both feeling and seeing a difference.

What we experienced first hand is confirmed in this and many other similar articles:
The surprisingly simple way cities could save people from extreme heat.

“New research suggests cities are ignoring the power of cool roofs at their own peril. A study in the journal Geophysical Research Letters earlier this month modeled how much cooler London would have been on the two hottest days in the extra-hot summer of 2018 if the city widely adopted cool roofs compared to other interventions, like green roofs, rooftop solar panels, and groundlevel vegetation. Though simple from an engineering standpoint, cool roofs turned out to be the most effective at bringing down temperatures.”

So much of what we hear in the news and read on-line about climate change is the rising cost of electricity to cool our homes, and the increasing burden on already oversubscribed electric power grids. Emphasis is placed on the consumption of electricity, not the reduction of electric consumption. In a warming climate, reduction of thermal gain equates to a lower electric bill.

Yes, local solar photovoltaic systems augment (grid-tied) or remove (off-grid) reliance on the electric grid, but not everyone can afford PV arrays, or do not have permission from the apartment manager or rented home owner to modify the roof or electric panel.

No matter if you are moving toward renewable energy or simply wanting to reduce your electric consumption and associated electric bill, and improve the quality of your interior comfort in the midst of increasingly uncomfortable summers, there are a number of things you can do to reduce thermal gain—the amount of heat trapped inside your home during the day, and then remove that heat by night.

Above graph: “Ext. Ambient” is the air temperature at the time of recording. “Conc. Full Sun” is concrete in full sun. “C. Shade Cloth” is an adjacent slab of concrete positioned below a shade cloth. “C. Shade Struct.” is a slab of concrete that remains in shadow all day, each day, i.e. beneath a porch roof. “Eave – Bare” is the temperature of the underside of the steel roof overhang, original galvanized coating. “Eave – White” is the temperature of the underside of the steel roof overhang, the top side painted gloss white. “Int. Ambient” is the interior air temperature. “Ceiling – E. Bare” is the temperature of the interior wood ceiling below original galvanized steel roofing, with 4″ foam insulation between. “Ceiling – E. White” is the temperature of the interior wood ceiling below the painted gloss white steel roofing, with 4″ foam insulation between. “Window – Bare” is the temperature of the glass itself without any film. “Window – White” is the temperature of the glass with an exterior application of the while vinyl film.

In this series of essays we will introduce, explain, and demonstrate various means to reduce the interior temperature of your home. Many of the concepts are explored are the direct result of work we have done to our own home, here in Cascabel, Arizona. The results are immediate and noticeable.

The concepts explored will include:

• Reflecting sunlight via white paint and white vinyl film; and
• Reducing sunlight transmission via window films
• Reducing sunlight transmission via shade structures
• Thermal mass
• Passive versus active cooling
• Swamp cooler versus air conditioning; and
• Types of air conditioning (cooling)
• Cooling by plants, and green roofs

This landing page will be updated, with links to each new essay as added.

The first two essays are posted:

• Keeping Cool – paint your roof white
• Keeping Cool – window films

Stay tuned!