Humid, Dry and Marine Regimes

Why this matters

The same air conditioner, duct layout and wall assembly behave differently depending on how much water is in the outdoor air. In a humid climate a correctly sized system that runs short cycles leaves the house at 72 F and 65 percent humidity, which the customer reads as broken. In a dry climate an evaporative cooler does most of the work of an air conditioner for a fraction of the power. On the marine coast a house that almost never needs cooling grows mold on the north closet wall because nothing ever dries the air. The moisture regime, the letter after the climate zone number, is the quickest way to know which of those problems the job is walking into.

What the letter tells you

The IECC labels most zones A (moist), B (dry) or C (marine). The definitions and how to look them up live in a sibling card; see related: Climate Zones and Design Temperatures in Plain Terms. This card is about what each regime changes once you know it.

Two terms carry most of it:

• Latent load is the moisture a cooling system has to remove, as opposed to the sensible load, the temperature it has to remove. Both are in Btu/h.
• Sensible heat ratio (SHR) is sensible capacity divided by total capacity. A coil running at an SHR of 0.75 spends three quarters of its work cooling and one quarter condensing water out of the air. Lower SHR means more moisture removal.

Humid regimes: the moisture is the job

In moist climates, especially the hot-humid Southeast and Gulf, the latent load is a large share of the cooling load and it does not drop off when the temperature does. A mild, muggy 78 F day can have almost no sensible load and plenty of latent.

What changes:

• Equipment selection checks both capacities. A Manual S selection compares the equipment's sensible capacity against the sensible load and its latent capacity against the latent load, at the design condition and the airflow you will actually set. A unit that covers the total but not the latent leaves the house damp. Read it as two lines, not one. Say the load is 22,000 Btu/h sensible and 8,000 latent, 30,000 total, so the house needs an SHR of 22,000 over 30,000, about 0.73. A candidate unit's data shows 30,000 total at an SHR of 0.78 at your airflow: that is 23,400 sensible and 6,600 latent (30,000 minus 23,400). The total matches exactly and the latent comes up 1,400 Btu/h short, about 18 percent of the 8,000 latent load. That house will be cool and sticky on design days, and the fix is in the selection (a lower-SHR setting or a different unit), not in the thermostat. These figures are illustrative; the real ones come from the manufacturer's expanded performance tables at your design condition.
• Airflow per ton goes down. Running the indoor blower slower, a common starting point is around 350 cfm per ton rather than 400 (set within the equipment's allowed range), keeps the coil colder and raises moisture removal. Variable-speed equipment that runs long at low speed does this naturally.
• Oversizing is a moisture failure. An oversized system reaches the thermostat setpoint before the coil has run long enough to drain, and the water on the coil re-evaporates into the airstream when the compressor stops.
• A dedicated dehumidifier becomes a real option for tight houses where the cooling load is small but the moisture load is not.
• Ducts in a vented attic sweat if the insulation jacket is torn or the duct is uninsulated, because attic air at a high dew point meets a 55 F duct surface.

The target the customer can check: EPA's mold guidance is to keep indoor relative humidity below 60 percent, ideally between 30 and 50.

Dry regimes: evaporation becomes a tool and a hazard

In dry climates evaporation is fast and the outdoor wet-bulb temperature sits far below the dry-bulb. That opens one option the other regimes do not have and creates a few problems they do not have.

• Evaporative cooling works. A direct evaporative cooler pulls outdoor air through wet media; the air cools toward its wet-bulb temperature as water evaporates. How close it gets is the media's effectiveness, often in the 80 to 90 percent range for good media; read the cooler's data for the number. The worked example below runs the arithmetic.
• Evaporative coolers need a way out for the air. They push 100 percent outdoor air into the house, so the house needs relief: open windows, or ceiling relief dampers into a vented attic. A sealed house with a cooler running just pressurizes.
• Hard water scales the media. Evaporation leaves the minerals behind. A bleed-off line that dumps some water continuously is what keeps the media from crusting, and in a hard-water area the bleed rate goes up. See related: How to Check Water Hardness on a Service Call.
• Dust loads filters and coils faster, so maintenance intervals shorten.
• Low winter humidity shrinks wood, raises static, and dries out occupants; whole-house humidifiers belong here and in any cold climate, not just dry ones.

Marine regimes: mild, damp, and nothing running to dry it

The marine coast has mild winters, cool summers, and a long damp shoulder season. Cooling loads are small, so the air conditioner, if there is one, rarely runs long enough to dehumidify anything. Heating runs at mild temperatures for many months, which suits heat pumps well.

What bites:

• Cold surfaces stay damp. Exterior walls behind furniture, closets on the north side, and window frames sit near the indoor dew point for months. Mold grows where a surface stays above roughly 80 percent relative humidity for long periods.
• Ventilation does the drying. Bath and kitchen exhaust that actually runs, and balanced ventilation (an HRV or ERV) in tight houses, matter more here than cooling capacity.
• Crawlspaces stay wet through the long rainy season.

When you find mold, do not disturb it as part of an unrelated job. Cutting, scraping or blowing air across it releases spores into the air you and the occupants breathe, and cleanup is a separate scope with its own containment. EPA's homeowner guidance treats an area over about 10 square feet as a job for its cleanup guidance or a professional.

What the regime does to the wall

Vapor retarders are the place the regime becomes a code requirement. In the 2018 IRC, R702.7 requires a Class I or II vapor retarder (polyethylene sheet, kraft facing) on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4. The 2021 edition reorganized that section into tables that allow Class III (latex paint) in more assemblies under stated conditions, so read the edition your jurisdiction adopted.

The opposite failure is regional too. In a hot-humid climate, a polyethylene sheet on the inside of an air-conditioned wall is where outdoor moisture driving inward meets a cold surface, and the wall gets wet behind the drywall. A remodeler carrying a northern habit south builds that.

The evaporative cooler question in two cities

A customer asks for an evaporative cooler instead of an air conditioner. A reasonable shop gate, a starting point to tune: compute the cooler's supply temperature at the cooling design condition; under about 70 F it is a candidate for primary cooling, 70 to 75 F supplemental at best, above 75 F do not quote it as cooling.

The formula: supply temperature equals dry-bulb minus effectiveness times the wet-bulb depression (dry-bulb minus wet-bulb). Effectiveness is taken as 0.85 for this media in both cases. The design conditions below are illustrative.

Case A, a dry interior city. Design dry-bulb 100 F, coincident wet-bulb 64 F.

• Wet-bulb depression: 100 minus 64 is 36 F.
• Cooling achieved: 0.85 times 36 is 30.6 F.
• Supply: 100 minus 30.6 is 69.4 F.
• 69.4 F is under the 70 F gate. Quote it as primary cooling, with the relief air path and the bleed-off sized, and tell the customer that on the rare humid monsoon afternoon it will do less.

Case B, a humid Gulf city. Design dry-bulb 92 F, coincident wet-bulb 78 F.

• Wet-bulb depression: 92 minus 78 is 14 F.
• Cooling achieved: 0.85 times 14 is 11.9 F.
• Supply: 92 minus 11.9 is 80.1 F.
• 80.1 F fails the gate by a wide margin, and that is only half the problem: the cooler gets its 11.9 degrees by adding water to air that is already humid, so indoor humidity climbs past the 60 percent line in the EPA guidance. Do not quote it. Quote a properly selected air conditioner or heat pump, set up for latent removal.

Same customer request, same formula, opposite answer, and the difference is entirely the wet-bulb temperature the place supplies.

What would change Case A. A house with no practical relief path, or an owner unwilling to open windows, turns a technically good cooler into a pressurized, clammy house. A water-restricted area may limit the water use. A two-stage (indirect-direct) cooler would push Case B closer, but still not past the gate at a 14 F depression.

How to check the regime on site

• Measure, do not assume. A digital psychrometer gives dry-bulb, wet-bulb and relative humidity in a minute. Take outdoor and indoor readings on the visit and write them on the ticket with the time.
• Read the coil, not just the thermostat. In a humid-climate complaint, a supply temperature only 12 F below return, with no condensate running after 15 minutes of operation, is a coil that is not dehumidifying.
• Look where moisture collects. Window frames, closet corners and the crawlspace tell you more about a marine-climate house than any load number.

References

• EPA, A Brief Guide to Mold, Moisture and Your Home (indoor humidity and cleanup-size guidance)
• International Residential Code, R702.7 vapor retarders, in the edition your jurisdiction has adopted
• ACCA Manual S, equipment selection against sensible and latent loads
• See related: Climate Zones and Design Temperatures in Plain Terms; How to Check Water Hardness on a Service Call; Reading the Place Before the Quote