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Every summer, Phoenix homeowners brace themselves for the inevitable: electricity bills that seem to climb as fast as the mercury. When the thermometer hits 115 degrees and your AC runs nonstop, it’s natural to wonder if your walls are doing their job. You may have heard neighbors claim that older homes with thicker walls stay cooler, or that upgrading to thicker drywall will slash your cooling costs. But here’s the thing, wall thickness alone isn’t the hero in this story.

Thicker walls should provide better insulation, much like adding an extra blanket in winter. But when it comes to keeping your home cool in the Arizona desert, physics tells a different story. Understanding what truly affects your cooling costs can save you from wasting money on upgrades that barely make a difference.

Wall Thickness VS R-Value

The R-Value Truth Every Homeowner Should Know

R-value measures how well a material resists heat flow. The higher the number, the better the insulation. But here’s where things get interesting: adding thickness doesn’t automatically mean adding meaningful insulation, especially when you’re talking about materials like drywall that weren’t designed to be thermal barriers in the first place.

Your walls are a system. The drywall is just the interior finish. What happens between that drywall and the outside world matters far more than whether you’ve got half an inch or five-eighths of an inch of gypsum board on your studs.

Half-Inch vs. Five-Eighths Drywall

Standard half-inch drywall has an R-value of about 0.45. Five-eighths-inch drywall? Roughly 0.56. That extra eighth of an inch gives you an additional R-value of about 0.11. While this doesn’t significantly impact cooling costs, the type and thickness of your wall material can affect how well the paint adheres and lasts. To put the R-value in perspective, a single inch of fiberglass insulation provides an R-value between 3.0 and 4.0.

When your wall cavity could hold R-13 or R-15 batt insulation, or when building codes recommend R-30 in your attic, that 0.11 difference from thicker drywall becomes almost invisible. Your AC won’t notice. Your electricity bill definitely won’t change. The professionals at Jr’s Painting have worked on countless Phoenix homes, and they’ll tell you the same thing: don’t count on drywall thickness to keep your house cool.

Plaster, Stucco, and the Thermal Mass Factor

Now, older homes with thick plaster walls or substantial stucco exteriors do sometimes feel different than modern homes with thin drywall. But that’s not really about insulation value. It’s about thermal mass, which is a concept altogether different.

Thermal mass describes a material’s ability to absorb and store heat. Dense materials, such as concrete, plaster, or thick stucco, can absorb heat during the day and release it slowly over time. In climates with significant temperature swings between day and night, this can help moderate indoor temperatures. Phoenix does cool down at night, especially in spring and fall. During those months, thermal mass can reduce cooling needs by storing daytime heat and releasing it after sunset when outdoor temperatures drop.

However, during June, July, and August, when overnight lows still hover around 90 degrees, thermal mass offers little assistance. The heat absorbed during the day never entirely dissipates because the nights stay too warm. That stored heat continues to radiate into your living space, causing your AC to work harder.

What Actually Saves Cooling Costs in the Desert

Insulation Upgrades That Make a Measurable Difference

To significantly reduce your cooling costs, start where it counts: your attic and exterior walls. Most Phoenix homes were built with minimal insulation by today’s standards. Adding blown-in insulation to your attic to reach R-38 or R-49 can cut your cooling costs by 15 to 25 percent. That’s real money back in your pocket every summer.

For walls, especially those with concrete masonry unit (CMU), the situation becomes more complex. Those hollow block cores can be filled with foam insulation, dramatically improving the wall’s thermal performance. Exterior walls that once conducted heat like a highway suddenly become barriers.

Continuous Exterior Insulation

Here’s something many Phoenix homeowners don’t realize: adding rigid foam insulation to the outside of your block walls, underneath new stucco or siding, creates a continuous thermal barrier. Unlike cavity insulation that gets interrupted by studs or mortar joints, continuous exterior insulation wraps your entire home in protection.

This approach also moves the dew point outside your wall assembly, reducing the risk of moisture problems. In our dry climate, that’s less of an issue than up north, but it’s still good building science. More importantly, it stops thermal bridging—those pathways where heat sneaks through structural elements that connect your interior to the scorching exterior.

Building codes in Phoenix now recognize this. New construction often includes exterior foam sheathing, and retrofit applications are becoming more common. The upfront cost runs higher than simply blowing insulation into wall cavities, but the performance improvement is substantial.

Air Sealing and Solar Heat Gain: The Forgotten Factors

Your home loses conditioned air through countless small gaps: around windows and doors, where pipes penetrate walls, at electrical outlets, where the walls meet the foundation. Sealing these leaks can reduce your cooling load by 10 to 20 percent. It costs relatively little and delivers fast payback.

Then there’s solar heat gain through windows. Those beautiful Arizona sunsets come with intense solar radiation. Low-E window coatings, solar screens, and strategic exterior shading can block tremendous amounts of heat before it enters your home. South and west-facing windows are the biggest culprits. Even interior treatments like cellular shades help, though exterior shading works better by stopping heat outside your thermal envelope.

Phoenix-Specific Considerations for Maximum Efficiency

CMU Construction and How It Performs Here

Concrete block homes dominate Phoenix construction, and for good reason. CMU walls resist fire, insects, and high winds. They’re durable in ways that wood-frame construction isn’t. But uninsulated CMU walls have a terrible R-value—around R-2 to R-3 for a standard 8-inch block wall. That’s worse than a wood-framed wall with no insulation.

The thermal mass of concrete block can moderate temperature swings during mild weather, but it’s not enough during peak summer. That’s why retrofitting CMU homes with insulation makes such a dramatic difference. Whether you fill the block cores, add interior furring and batt insulation, or install exterior continuous insulation, the improvement is noticeable immediately.

Taking Advantage of Utility Rebates

Both APS and SRP offer rebates for energy efficiency upgrades. Attic insulation, air sealing, and HVAC improvements all qualify for tax credits. Some rebates cover a significant portion of your costs, making upgrades more affordable than you might expect. Before investing in efficiency improvements, check to see what incentives are available. The utility companies want you to use less energy during peak hours, and they’re willing to help pay for upgrades that achieve that goal.

These programs change periodically, so verify current offerings before you start work. But historically, Phoenix-area homeowners have been able to offset hundreds of dollars in upgrade costs through utility rebates alone.

Why Exterior Foam Beats Thicker Drywall Every Time

Let’s return to the original question. If you’re renovating and trying to decide between upgrading to thicker drywall or investing in proper insulation, the choice is clear. Thicker drywall might improve sound transmission slightly and offer more impact resistance, but it won’t meaningfully reduce your cooling costs.

The same money spent on blown-in attic insulation, exterior rigid foam, or quality air sealing will deliver returns that you can measure on your utility bills. The physics doesn’t lie. Focus your budget where thermal performance actually improves.

Practical Advice for Making Smart Upgrades

Simple Upgrades With Big Returns

Not every efficiency improvement requires tearing your house apart. Start with the low-hanging fruit:

Air seal around windows, doors, and penetrations using caulk and weatherstripping. Upgrade attic insulation if you’re below R-30. Install solar screens on west and south-facing windows. Add a radiant barrier in your attic to reflect heat away from your insulation. Replace old, inefficient AC units with modern, high-SEER models. Each of these improvements pays for itself relatively quickly through reduced energy bills.

If you’re planning interior painting work, that’s a perfect time to add outlet and switch plate gaskets for air sealing. If you’re updating your home’s exterior, consider adding rigid foam insulation before applying new stucco. Coordinate upgrades to minimize disruption and maximize value.

When to Invest in Insulation vs. Fresh Paint

Paint refreshes your home and protects surfaces, but it doesn’t reduce cooling costs. Insulation is invisible but delivers measurable savings every month. If your budget forces a choice, prioritize performance over aesthetics. That said, you don’t always have to choose. A professional painting contractor can work around insulation contractors, and they can assess whether you need stucco or drywall repairbefore painting, ensuring your walls are ready for a flawless finish. You can also phase projects across different seasons.

What matters is understanding what you’re buying. Paint color can affect how much solar heat your exterior absorbs (lighter colors reflect more heat), but that’s a minor factor compared to proper insulation. Don’t expect paint alone to solve cooling problems.

Setting Realistic Expectations for Your Home

Older homes will never perform as well as new construction built to current energy codes. That’s okay. You don’t need perfection. You need improvement. Cutting your cooling costs by 20% or 30% through strategic upgrades is achievable and worthwhile. Expecting to cut costs by 50 percent without major renovations isn’t realistic.

Focus on the worst thermal weak points first. An uninsulated attic is usually the biggest offender. Single-pane windows on west-facing walls come next. Prioritize based on where you’re losing the most conditioned air, not on what’s most visible or easiest to address.

Frequently Asked Questions

Quality closed-cell spray foam insulation lasts 80 to 100 years or more, even in extreme Arizona heat. Open-cell foam has a similar lifespan. The material doesn’t degrade from temperature exposure, though UV light can damage exposed foam (which is why it’s always covered by drywall or other finishes). Spray foam also doesn’t settle or compress over time, unlike some batt insulation. The high upfront cost gets balanced by decades of performance without needing replacement or topping off, making it a genuine one-time investment.

 

If your garage shares a wall with your living space or sits beneath a room, insulating the shared walls and ceiling makes sense. Heat from an uninsulated garage can radiate into adjacent living areas, forcing your AC to work harder. However, insulating garage exterior walls and doors provides less return on investment unless you’re actively cooling the garage itself. Focus first on the thermal barrier between your garage and conditioned space. Many Phoenix homeowners see noticeable comfort improvements after insulating the wall and ceiling between an attached garage and the main house.

 

Thicker walls can keep a house cooler if the thickness is achieved through insulation or thermal mass, combined with cool nights. But simply installing thicker drywall won’t make a noticeable difference. The minimal increase in R-value gets overwhelmed by other factors. To achieve cooler indoor temperatures, consider investing in proper wall cavity insulation, exterior continuous insulation, or both. The wall thickness matters less than what’s inside those walls.

Building codes recommend R-30 to R-38 for attic insulation in Phoenix. For walls, R-13 to R-15 is standard for wood-framed construction. Block walls should be insulated to at least R-10, though higher values deliver better performance. These are minimums. Going beyond code requirements makes economic sense given our climate. The additional insulation cost is modest compared to the decades of energy savings it will yield.

Absolutely. Exterior insulation creates a continuous thermal barrier that does not interrupt the interior living space. It eliminates thermal bridging through framing members and dramatically improves wall performance. The investment is higher than cavity insulation, but the performance gain is substantial. For block homes, especially, exterior insulation can transform comfort and efficiency. If you’re residing or restuccoing anyway, adding exterior foam becomes even more cost-effective since you’re already paying for that work.

 

Stucco itself provides minimal insulation value. What helps is the thermal mass of thick stucco combined with proper insulation behind it. In older homes with thick, traditional three-coat stucco over block or concrete, the mass can moderate temperature swings during mild weather. But during peak summer, stucco alone won’t keep your house cool. You need insulation working in combination with that thermal mass. Modern synthetic stucco systems (EIFS) can include integral insulation, which performs significantly better than traditional stucco alone.


Conclusion

Phoenix heat is relentless, but your cooling costs don’t have to be. The myth that thicker walls automatically mean lower bills has persisted because it sounds logical. The reality is more complex. Thermal performance depends on R-value, air sealing, and how you manage solar heat gain, rather than simply the thickness of your drywall.

When you’re ready to make upgrades that actually matter, reach out to yourprofessional residential painterto start with proper insulation and air sealing. Those improvements deliver measurable results. And when you’re prepared to give your home a fresh look after all that efficiency work is done, reach out for a free estimate. Your home deserves to look as good as it performs.

Adrian Perez painter JR's painting

Author: Adrian Perez

Co-Owner | Project Manager at Jr’s Painting

Adrian Perez, co-owner and project manager at Jr’s Painting, brings years of experience and dedication to the painting industry.

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