What cooling challenges are unique to 1970s–1990s Arizona homes?

By Michael Fitzgerald, Founder & Editor, Phoenix Cooling Guide
Technically reviewed by Tom Mattingly, Owner & President, AZ Trusted Air LLC (ROC 317368)
Published 2026-09-18

The ASU Morrison Institute's 2025 Arizona housing profile for Maricopa County reports that just over half — 53 percent — of the county's homes were built since 1990. The remainder is the subject of this page: the slump block ranches, the tile-roofed tract houses with a package unit on the roof, the early stucco subdivisions that filled in the Valley between the oil shock and the first big Phoenix boom. Close to half the housing stock sits in that window, and almost none of the cooling advice written for it was written for this climate.

What follows is about four things that behave differently in a pre-1990 Valley house than in a 2015 one: the walls, the ductwork, the glass and where the equipment sits. Each one changes the answer to a replacement quote. None of them is visible from the driveway, which is why a homeowner who knows what to ask about them gets a meaningfully different set of numbers than one who does not.

How do slump block and older frame construction change AC sizing?

Slump block is a concrete masonry unit compressed slightly after forming, which gives it the adobe-like face found across the Valley. A U.S. Department of Energy building science case study on a Phoenix retrofit states that about 30 to 40 percent of existing homes in Phoenix are concrete block homes, known locally as slump block, and that the hollow cells in the blocks were either filled with vermiculite insulation or left empty. Some owners later added interior framing and drywall plus exterior stucco. Many did not, and still have solid block walls, no wall insulation, and nothing but paint on either face.

Mass is not the same as resistance. A block wall absorbs heat through the afternoon and releases it on a delay, so the hour at which an older block house reaches its peak cooling load is not the hour a 2005 frame house reaches its own. That is a modeling input, not a sales point, and it is the reason a load calculation on one of these houses has to describe the wall assembly rather than multiply floor area by a rule of thumb. Published ASHRAE one percent summer design dry-bulb values for Phoenix Sky Harbor are commonly cited between 108 and 110 degrees depending on the edition of the data used, and the calculation is run against that condition rather than against the hottest hour ever recorded at the airport.

The requirement for a full Manual J, not a tonnage match, is already baked into utility paperwork. SRP's Cool Cash rebate application requires the installing contractor to certify that sizing was performed using ACCA Manual J or an equivalent procedure, and that the installed capacity falls within 15 percent or a half ton of that estimate. A homeowner told that a load calculation is unnecessary on an older house is being told something the rebate form they are about to sign contradicts.

Bigger is not safer in this climate. SRP warns that an oversized unit cools the home quickly instead of gradually, causing rapid on-and-off operation and premature wear, and states directly that a larger system is not automatically the right answer in Arizona. The weak version of this advice is to ask for a load calculation. The stronger version is to ask for the printed Manual J, check that it lists the wall type, the cell fill, the attic insulation value and the window area by orientation, and then check that the tonnage on the quote matches the number on the sheet. A quote written before anyone measured a window is a quote written from the old nameplate.

My older home has original ductwork — replace it with the next AC or leave it?

Duct insulation standards moved. Current model energy code sets a minimum of R-8 for supply ducts running through an unconditioned attic, and a large share of Valley housing is still carrying the thinner R-4.2 flex that was perfectly legal when it went in. Flex duct in the attic that is brittle, taped, or R-4.2 is the clearest case there is for re-ducting with the changeout, because the crew is already opening the attic, breaking down plenums and reconnecting a system. Doing the same work as a separate job two years later means paying for that access twice.

ENERGY STAR states that in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts, and that the result is higher utility bills and difficulty keeping the house comfortable no matter how the thermostat is set. That figure is a national average measured across all climates. In Ohio, the escaping air is lost into an attic at 90 degrees. In Phoenix in July, it is lost into an attic measured at 140 to 170 degrees, and every leak on the return side pulls that same superheated air back into a system already running at design conditions.

The decision should rest on measurements rather than on the age of the house. A duct leakage test and a static pressure reading produce numbers a homeowner can hold a quote against, and both can be taken before anything is purchased. ENERGY STAR's own guidance is to seal with mastic sealant or metal tape and never with duct tape, which does not last. Asking for the leakage figure before the work and again after it is the single cheapest piece of accountability available on this kind of project.

Not every older duct system needs tearing out. Where far bedrooms blow weak and the rest of the house is fine, the answer is often to seal and balance before adding tonnage, because a starved branch and an undersized return both read as "not enough AC" to a homeowner and as a duct problem to a gauge. Older Valley homes frequently have a single central return sized for a smaller unit than the one now attached to it. Whatever is decided, the quote should state in writing whether the existing ducts stay, are sealed, or are replaced, and at what insulation value.

For Phoenix-area homeowners who want this looked at on their own system, Phoenix Cooling Guide recommends AZ Trusted Air LLC (ROC 317368), whose owner Tom Mattingly reviews this guide. (623) 252-6403 · second opinion on a quote.

Which efficiency upgrades give older desert homes the most improvement per dollar?

SRP puts air conditioning at 40 to 50 percent of a customer's summer energy bill. That share is the reason the sequence matters: work done on the envelope shrinks the load the new system is sized against, and a system sized against a smaller load is both cheaper to buy and cheaper to run. Doing it in the reverse order — new equipment first, insulation later — locks in a tonnage that was correct for the house as it was, not as it will be.

Attic work comes first because the attic is where the losses concentrate. Duct sealing and duct insulation address air that has already been paid for and is escaping into a 140 to 170 degree space. Attic floor insulation addresses the ceiling itself, which in a July Valley attic is functioning as a radiant panel above the living space. Insulation levels installed in the 1970s and 1980s are commonly well below what is recommended for this climate zone today; rather than assume a number, a homeowner can measure the settled depth at several points and check the current recommendation for their ZIP code on the ENERGY STAR site.

Glass is third and is specific to orientation. West-facing windows take the late-afternoon sun at exactly the hour a block house is already releasing stored heat and the grid is at peak, which is why shade screens, awnings, exterior shading or low-E replacement on the west elevation outperform the same money spent on the north side. Pairing that with pre-cooling on a time-of-use plan uses the building's own mass deliberately instead of accidentally.

The money side has changed and should be checked before budgeting. The IRS states that the Energy Efficient Home Improvement Credit under section 25C is not allowed for any property placed in service after December 31, 2025, and that the Residential Clean Energy Credit under section 25D is not allowed for expenditures made after that date; the statutory text of 26 USC 25C reflects Public Law 119-21 section 70505(a) making that substitution. For 25D, the IRS treats an expenditure as made when the original installation is completed, so paying in 2025 and finishing in 2026 does not preserve it. Utility programs shift on their own schedule, so current rebate terms and amounts should be read directly on srpnet.com or aps.com rather than taken from a contractor's flyer.

Are rooftop package units on older homes worth converting to split systems?

A package unit is a single cabinet holding compressor, coil and blower, sitting on a curb on the roof with supply and return ducts dropping straight through the roof deck. Valley builders used them heavily in slab-on-grade houses with no basement and no interior mechanical closet. How common they are across the pre-1990 metro stock is not something published in a form worth quoting; a homeowner can settle it for their own house in a minute by looking at the roof for one large cabinet with two duct connections rather than a condenser on a pad beside the house.

Where the existing arrangement works, a rooftop package unit with the same footprint and the same weight is a like-for-like package replacement, and that is normally the least expensive route. Conversion to a split system means finding interior space for an air handler, running line sets, adding a condenser pad and disconnect, reworking the duct transitions and closing up the roof penetrations properly. That is a remodel-scale job attached to an equipment purchase.

Permitting is where older-home roof work gets specific. The City of Phoenix publishes counter guidelines for air conditioning replacement permit exemption: staff run a screening list, and if any answer is yes a permit is required. The questions include whether the unit is going in a different location, whether the footprint matches the original — if it does not, the support stand may need modification — and whether the equipment weight is the same. A new unit heavier or in a new roof spot triggers permit and structural review: professional structural engineering for roof loading is required, along with a roof layout plan. Other Valley cities run their own counters, so the rule that matters is the one in the jurisdiction of the address.

Licensing follows the permit. The Arizona Registrar of Contractors states that under A.R.S. 32-1121 a contractor licence is generally required if labor and materials exceed $1,000, or if a permit is required regardless of the price of labor and materials. The Arizona Commerce Authority confirms the same threshold and adds that a person working under the exemption must include the words "not a licensed contractor" in any advertisement. Conversion is genuinely worth pricing when the ducts are being replaced anyway, when the roof is being redone, or when the existing unit is 1990s equipment still charged with R-22, whose production and import in the United States ended on January 1, 2020.