Cool Walls, Less Air Conditioning

Keeping your exterior walls cooler during hot weather can really help decrease the amount of energy your air conditioner consumes and decrease the amount of very hot air your AC’s condenser expels outdoors. Since many regions are warming up and heat waves are more common, I felt moved to explore this topic a bit and share some of what I’ve learned from living in the desert.

Cool walls have the side effect of helping to decrease your home’s contribution to the urban heat island effect, which has major climate change and human health implications. You’ll also save a buck on energy costs.

My “slump block” concrete 50 year old walls in the desert. I’m pretty sure they are hollow inside (not filled with insulation). They are painted white which is good, but the east and west walls get a lot of direct sunlight and release it into the home at night. So I wanted to do something about this.

Podcast

“Tucson is 11 degrees warmer on average than it was 100 years ago. Half of that increase (5.5 degrees) is attributed to the urban heat island.”
WMG’s Fall 2023 Newsletter

And climate change solutions at home are a major win and something we can do now.

“Roughly 1/3 of the solutions to combat climate change are best implemented at the household or family level.”
Project Drawdown post

The non-profit Watershed Management Group (WMG) based here in Tucson, AZ has a neat new “Cool Tucson 5 Degrees” initiative that has tips that can probably apply to most warming cities. I’ve been gradually implementing many of WMG’s suggestions on my property, focusing mostly on low cost, easy to implement DIY options to start.

rainwater harvesting site plan — Rainwater harvesting site plan from Watershed Management Group, a local non-profit. They suggested rainwater basins (blue areas) be dug and gave guidance for placement of native trees, shrubs, and a possible large water cistern for the future. Water from the roof feeds the basins and in time this may be the only water many of our native plants need, drastically reducing our home’s water usage. I did a LOT of digging to create the basins, and now we have an excellent “pollinator garden” that attracts bees, hummingbirds, butterflies, and even our dog! Rock was replaced with mulch and the yard seems much cooler.

Last summer Tucson, AZ actually broke heat records with 51 days (consecutive I think!?) of at least 105 degree temperatures, compared to 26 days normally over the same period. Toasty.

Why Focus on Cool Walls?

First, I am focusing on cool walls just to keep it simple for this article, and because this concept has so much bang for the buck, but I have looked into other components as well.

Here’s a general starting point you may want to consider: A full “home energy audit” can be helpful as it looks at many different pieces (air leaks, insulation, HVAC system, windows…). Review professional and DIY options on the energy.gov site and their infographic. If going with a professional audit or some home upgrades make sure to research the company/individual and check on certifications, training and reviews, etc, and consider any federal tax credits.

Back to the walls! They often generally have less insulation than roofs, which I’ve read can mean they sometimes have about half the amount of resistance to heat flow that a roof has. So, cool exterior walls are a very important energy efficiency and climate consideration and can be achieved in several ways:

Decreasing direct sunlight on walls and nearby ground.
Using wall materials that strongly reflect sunlight (solar energy) and cool themselves by efficiently emitting any heat that was absorbed. The exterior wall surface stays cooler and reduces the amount of heat conducted into the building.
It isn’t too surprising that white paint is great! I still need to research “special” wall paints or coatings to see if they may outperform standard, cheaper white paint significantly.

For more details on cool walls also see the CRRC’s information about What is a Cool Exterior Wall? and review this graphic:

How to Cool Walls

Sure, shading an external wall can help cool it down. But don’t forget that if the ground next to and even under your home is excessively hot it will contribute to higher wall temperatures and higher AC usage, as will a hot roof.

wall insulation — Insulation was added to this west wall in our utility room where the water heater and HVAC system is. The other side of the wall has a shade sail cooling the wall because we don’t have enough room to grow trees there.

I’ve been optimizing our external wall temperatures in several ways:

Shade sails have been installed for east/west walls, while I wait for newly planted native shade trees to take hold. The west wall is particularly hot, so it is “shade sail city” for now.

Shade sails are relatively inexpensive and easy to take down in the winter when we actually want the walls to be warmed by the sun. But they are often made of HDPE (plastic) which isn’t the greatest material in terms of fossil fuel usage and pollution during the manufacturing and delivery steps.
So far I’ve removed small rocks (hundreds of thousands?) previously covering much of the yard, since they retained a lot of heat and my dog disliked walking on them. I have replaced the flat rocky/barren/hot/ugly vibe with undulating “passive” rainwater harvesting basins that use water that hits our roof to grow beautiful native plants and shade trees. I’ve added light colored mulch and need to do add much more. Also, our grey water system feeds a basin, and we collect the AC condensate for use on native plants. All of this will help cool things down in the yard.
Insulated the utility room’s inner wall on west side of home. The AC unit and water heater are in this room, and keeping this small space cooler should make those systems run more efficiently and last longer.
Insulated the northwest side’s laundry room door’s “window”—this majorly cooled down that area. It was shocking how much cooler the room became, but there’s still more work to do there.
Installed a 20″ box fan in garage that expels hot air through a whirly bird—total DIY hack. Parking a hot car in your garage releases heat for hours! The hot attached garage warms up my kitchen walls.

shade sails cooling a wall of home — My friend added the flag as a joke… East facing wall has plenty of shade sails while we wait for native Ironwood and Palo Verde trees to mature and shade the wall and mulched ground. The trees are getting water from a rainwater basin just to the right of the photo, which is fed by a downspout and line from NE roof section. Shading with native trees has a long list of benefits beyond just cooling your home, such as providing perfect growing conditions beneath them for other native shrubs and cacti 🌵 and food for animals and insects (🌺 🫛)

Measuring Wall Temperatures

I happened to have an older FLIR ONE unit that plugs into an iPad/iPhone and it produces infrared images which show the striking differences between areas of a wall that have full sunlight for hours versus shade.

When I first moved into the home I walked the interior and exterior and took infrared pictures. I found that the ceiling and windows get quite warm during extended heat waves. To my surprise there was a section of roof on the west side that was completely missing insulation, so I added some!

There are other options I’ve seen that are much cheaper, such as “infrared thermometers” like these on Amazon.

A picture tells a thousand words. I took this one below on a 90 degree afternoon in May. The top of our AC condenser (red dot) was 135 degrees and the side (2) was 95. This was while the AC was running. The bricks on the shaded ground (blue dot) were just 91 degrees. The shaded area under the roofline (1 dot) was 96 degrees and the exposed wall (3 dot) was 105 degrees.

cool walls - hot west facing wall with AC exhaust

All that hot air coming out of the top of the AC condenser is blowing right next to our west wall and hitting the side of the roof, which warms the rest of the house.

Wouldn’t it make more sense if a) the condenser was maybe further from the hottest wall in the house and b) the hot air coming out of the top of it was directed away from the wall and roof of the home? Perhaps I have another project on my hands…

Measuring Ground Temperatures

The image below shows the west side of my home in the afternoon of a different 95 degree day (if I remember correctly), with the brick on the ground (red dot) at 138 degrees in full sunlight on a warm day in May, and the brick under a shade sail at just 90 degrees. That’s almost 50 degrees cooler.

temperature of ground shaded versus in sun

Moral of the story: Bricks get hot. The bricks in this particular area really hold onto heat, and they don’t let rainwater infiltrate below them, so we get standing water here during big storms. This needs to be rethought eventually and I know there are other brick designs that allow water to pass through. But removing the bricks and mulching this area would likely help a lot.

Cool Ceilings

Keeping the roof and ceiling cool may involve various methods such as nearby shade trees, appropriate light color/coating, etc. Keeping the ground cool may involve proper roof overhang, limiting hardscape and using mulch instead, with native trees and shrubs nearby (trellises with vines are a nice option too), and/or use of shade sails.

roof overhang on home provides shade for walls and surrounding areas, keeping home cooler — The roof overhang helps keep our south facing wall cooler as the shade is there all day. In the winter the sun is lower and the wall is warmed. Good design! Some locals even put temporary extendable awnings (various designs) up in the summer, to further increase the amount of shade around the home. 🏠

The design of my home’s ceiling is not perfect. There is essentially no “attic” and the low slung roof doesn’t have thick enough insulation in my opinion. There is probably much more I can do, but so far, optimizing ceiling/roof temperatures has involved:

Using a high quality white elastnomeric roof coating and making sure it is re-coated on a regular basis. See link further below for ratings of roof coating products.
Installing rooftop solar, which partially shades some sections of the roof. Technicians For Sustainability did a great job on our design and installation and Clean Energy Credit Union handles the loan.
Making sure insulation is in place for the entire ceiling. As I mentioned, some of mine was missing.

General Comfort During Heat Waves

Obviously, setting your AC to a higher temperature will have it run much less. If you do this, here are some simple tips to stay comfortable:

In AZ we are big fans of… ceiling fans, which won’t reduce the actual temperature of the air but really help you feel better. They use very little energy and help mix the cool air near the floor with the warmer air near the ceiling. They are great in your bedroom at night.
Don’t forget about your AC’s built in fan. This processes a lot of air volume and can keep air circulating through the entire home. In addition to the on and auto settings mine has a circulate mode where it can turn on regularly for a short period.
Hydrate and eat cool foods. Ice cubes galore!
Keep windows/blinds closed.
Consider changing the AC settings on a schedule so that you use it less during high demand “peaks” like afternoon/early evening. Many thermostats are programmable.
Water! Wiping down with a wet towel or using a fan/spritzer, or taking a very short shower can really help. People survived in Tucson for thousands of years and often had nearby rivers flowing year round that they could cool off in (and back then air temps were generally cooler than they are today).
Sleeping in a cool room can be very helpful health-wise. Consider linen sheets and other lightweight bedding options like cool pillows. There is much more to be said about this topic, perhaps in another article.

More thoughts: My dog loves of laying on cool floor tiles ;) We notice that as heatwaves go on, the entire home, even the tile floors and shower water, gradually warm up. So the less total heat the home absorbs over the summer, the less the AC will need to run in order to keep fido happy on those chilly tiles.

Home Design for Hot (And Cold) Weather

One of the most important considerations when it comes to energy efficiency, especially if you are moving to a new-to-you home, is the general layout and design. For example, when I see dark colored roofs in Tucson, AZ I think “oops.”

When it comes to my home, it has some plusses in terms of the general design:

The east/west walls are short, while the north/south are long. This means the sun hits smaller wall surfaces in general. The roof has an overhang built in so that in the summer the sun doesn’t directly hit our south facing walls/windows and warm them up but in the winter it does.
The home is not large and it is single-story with a low roofline (read: total air volume indoors is not significant so there’s less air to cool). In Tucson a second story can get very hot.
The home has a white elastnomeric coated roof which is re-coated regularly. Tucson Rubberized Coatings is one common company. See the link to cool roofs below.
The home is about 50 years old and isn’t air tight. This can actually be seen as a plus and a minus. Perhaps better indoor air quality, but at the expense of some energy efficiency.

But there are so many ways the design and implementation of my home could be leveraged to improve energy efficiency, with varying associated costs:

More insulation of various types in various locations.
Thicker walls. Ours are concrete slump block, one layer, likely without insulation inside, and only some have a sheetrock layer over the top with an insulation sheet behind – OmniBlock looks like an interesting option w/insulation inside.
Better windows, and/or coatings.
Better shades (like cellular/honeycomb versions) and drapes.
More shade trees especially on the east/west and north sides to create an “arc of shade.”
Less exposed concrete and hard surfaces near walls.
Air conditioner condenser exhaust moved away from wall/roof.

Another consideration is that we are in a single family home, detached from any other structures. While this is great for privacy, a multi-unit building is often much more energy efficient in general.

Thoughts on Modern Tight Buildings

Finally, “tight” buildings that leak very little air to the outdoors may be more energy efficient, but if they don’t have proper ventilation in place they may also have poorer indoor air quality. Lots of factors go into this of course. So this topic may bring up the concept of balancing planetary health and your personal health. This could be especially important to consider if you are buying or moving into a new home that has building materials that are still off-gassing (think of how some people react to new car smells, for example—enclosed in a small space). A new home that “breathes” more or has proper ventilation may actually have better indoor air quality, but also higher utility costs. So, it is something to consider.

Either way, regular maintenance and cleaning (my article here) and perhaps some additional portable air filters (my article here) will help with both energy efficiency and indoor air quality.

Helpful Links

Cool Roof Rating Council – coolroofs.org
The CRRC is a 501(c)(3) nonprofit organization that develops fair, accurate, and credible methods for evaluating and labeling the radiative properties of
roofing and exterior wall products
CRRC’s directory of roof and wall products, with detailed initial and 3 year solar reflective and thermal emittance figures for many manufacturer’s products
CRRC’s resources for home and building owners

Brand Lancaster’s site and books offer multi-pronged approaches to cooling homes and yards
Also see his articles on sun and shade harvesting

Contact your electric company about programs and rebates. Ours has an “Efficient Home Program” that partners with AC companies, for new AC/heat pump installations, which take advantage of local and federal rebates.

Home Energy Rebates on energy.gov

Questions?

@diyecoguy on YouTube | BlueSky

Or Email Me (chris@diyecoguy.com)

If you like my content you can tip me on Ko-Fi so that I can buy a new keyboard and some more books!