IRC R318 Subterranean Termite Protection: What an Arizona Home Needs

Termites are the number one structural pest in the Phoenix and Tucson metros. The desert subterranean termite works year round in warm soil. Section R318 of the International Residential Code (IRC) makes a builder design that pressure out of a new home. It applies to any home in an area the code flags for termite risk, and all of the Arizona low desert qualifies.

What R318 requires

R318 does not force one product on you. A home in a termite area has to use at least one approved method of protection:

In areas subject to damage from termites ... methods of protection shall be one of the following methods or a combination of these methods.

The code lists six methods. The most common is a soil termiticide, a chemical put in the ground under and around the house. The others are a termite bait system, pressure-treated wood, naturally durable termite-resistant wood, physical barriers such as steel mesh or a treated sand layer, and steel framing. Most Arizona homes use the soil termiticide. A licensed applicator sprays it on the graded pad and the footing trenches, right before the slab is poured. It is the low-cost method, and it covers the whole footprint at once.

Why Arizona sits in the very heavy zone

The code ties the rule to a map. The 2021 IRC uses the Termite Infestation Probability map, Figure R301.2(7). It sorts the country into four bands, from "none to slight" up to "very heavy." Arizona's low desert, including the Valley, falls in the "very heavy" band, the top one. That rating does two things. It makes termite protection mandatory on a new home. It also triggers a second rule for foam plastic.

Under R318.4, you cannot put foam plastic insulation on a foundation below grade in a "very heavy" area. That means no rigid foam (polystyrene or polyiso) on the outside face of a foundation wall, or under a slab, below the soil line. The one exception is a design that adds an approved termite barrier for the foam and the framing. Foam gives termites a hidden path up into the wood, and an inspector cannot see them tunnel through it. Where foam is used above grade, the code wants at least a 6-inch gap between the foam and bare soil. That leaves a strip you can inspect.

What it means for an Arizona custom home

For example, take a custom home in Cave Creek. The termite treatment is a set step in the foundation work. After the pad is graded and the plumbing is set, the applicator sprays the soil termiticide across the pad and into the footing trenches. Then the crew lays the vapor barrier and pours the slab. The treatment is cheap next to the build, often a few hundred dollars, and it carries a renewable warranty. If your energy plan calls for rigid foam at the slab edge, your builder has to detail it so it does not break the foam rule. Usually that means keeping foam off the below-grade face, or adding a termite barrier and a visible inspection gap. That is one more reason the footing and slab-edge details get drawn with care on a desert home.

Edition note and related rules

R318 is stable across the 2018, 2021, and 2024 IRC, though the map's figure number has shifted between editions. Termite protection also works with R317, which calls for treated or naturally durable wood where framing sits close to concrete or soil. Which edition your city enforces is on the Arizona building code editions by city page. For the soil work at the same stage, see do I need a soil test to build a house in Arizona. Read the full section on the ICC site: 2021 IRC R318 on codes.iccsafe.org.