Why are my pipes sweating in summer?

Frost King's manufacturer guide gives the short version: "Sweating pipes happen when moist, humid air in your home is warmer than cold pipes." Air holds water vapor. When that air touches something colder than its dew point, the vapor turns to liquid on the surface. A glass of ice water does the same thing on a patio table.

Phoenix is dry most of the year, so summer sweating surprises people. The monsoon is the reason. The National Weather Service explains that strong moisture surges "can cause surface dewpoints in the Arizona deserts to soar above 65°F, and sometimes as high as 75°F." A cold water line running through a garage, a wall cavity, a crawl space or a closet on a humid July night is right where that moisture lands.

The usual spots are cold supply lines in garages and utility rooms, pipes near the water heater's cold inlet, lines running through a cool, closed space, and the supply line to a refrigerator or ice maker. Toilet tanks sweat for the same reason; see why a toilet tank sweats.

The humidity doesn't have to be extreme. Condensation forms whenever a surface is colder than the dew point of the air touching it. In Phoenix that means a cold water line in a closed garage or a wall cavity can sweat on a monsoon afternoon even though the house feels dry. The pipe gets colder than the air's dew point, and the water on it is the proof.

Do this quick check:

1. 1Wipe the pipe completely dry, then hold a dry paper towel against it.
2. 2Check again after 15 to 30 minutes.
3. 3Condensation shows up as a light film or scattered beads along the whole length of the pipe, and it's worst on the coldest, most humid days.
4. 4A leak shows up as a steady drip from one spot, usually a fitting, valve or joint, and it doesn't care about the weather.

A leak is present at all hours. Condensation comes and goes with humidity. If you find a drip at a single point, or the water keeps coming when the house is dry and cool, treat it as a leak and see how to detect leaks in your home. Also rule out an air conditioner drain: see the air conditioner condensate drain rules if the drip is near a furnace or air handler.

Constant dripping soaks whatever's beneath the pipe. The EPA's mold and moisture guide treats condensation on pipes as a moisture source and notes that hidden mold can grow in "areas inside walls around pipes (with leaking or condensing pipes)." Its photo of a rusty drainpipe carries this caption: "Rust is an indicator that condensation occurs on this drainpipe. The pipe should be insulated to prevent condensation." Over time, sweating can rot a cabinet floor, stain a ceiling, and corrode the pipe itself. If the water has reached the ceiling, see whether the stain is a roof leak or a plumbing leak.

1. Insulate the pipe. The EPA's advice is direct: "Cover cold surfaces, such as cold water pipes, with insulation." Insulation works because it keeps humid air from touching the cold metal. Foam tube insulation slips over the pipe and is sold in sizes to fit. Frost King describes its version this way: you "cut them to the right length and slide them into place, then press the adhesive-coated ends together for a tight seal." A few practical tips:

• Dry the pipe first. Insulation put over a wet pipe traps the moisture against it.
• Cover the whole run, including valves and elbows. One bare gap lets moisture form there and drip.
• Seal every seam. A gap defeats the purpose.
• Use pre-formed fittings for tees and elbows so there are no bare spots.

The steps, in order. You don't need to shut off the water for this job, because the insulation goes on the outside of the pipe.

1. 1Wipe the pipe dry and let it sit until it's dry to the touch. Run a fan if it keeps sweating.
2. 2Measure the pipe's outside diameter and buy foam tube insulation sized for it. The label lists the pipe sizes it fits.
3. 3Slip the tube over the pipe. Pre-slit tubing opens along its length so you don't have to take anything apart.
4. 4Press the seam closed and seal it. Use the adhesive strip on self-sealing tube, or tape the seam for the type that doesn't have one.
5. 5Cover valves, elbows and tees with pre-formed fittings, and butt each new piece tightly against the last.
6. 6Check it the next humid day. If the outside of the insulation is dry, you're done. If water still shows up at one spot, it's probably a gap, or it's a leak.

2. Lower the humidity around the pipe. The EPA advises you to "keep indoor humidity below 60 percent (ideally between 30 and 50 percent)" and to "use air conditioners and/or de-humidifiers when needed." In a closed garage or utility room, a small dehumidifier, better airflow, or sealing a gap that lets in humid air can stop sweating even before you insulate.

3. Fix the source of extra moisture. A running evaporative cooler with closed windows, a wet crawl space, or an unvented laundry area all add humidity. Cutting that down helps every cold surface in the house.

If insulation isn't enough. Insulation stops condensation on the pipe but doesn't change the air. A pipe in a space that's always humid, such as a garage with a wet floor or a closet next to a leaking drain, needs the moisture fixed too. Refrigerant lines on an air conditioner are insulated for the same reason. Their insulation is part of the HVAC system, not the plumbing, so ask your A/C technician if that line is bare or the foam is falling apart.

Quick fixes that don't work. Towels and buckets under the pipe manage the drip but don't stop it, and they hide the moisture that feeds mold. Wrapping a wet pipe in tape or cloth holds water against it. Duct tape peels off in the heat. Stick with proper foam insulation, and clean up any water that's already collected. The EPA's advice for water problems is short: "Fix plumbing leaks and other water problems as soon as possible. Dry all items completely."

Call a licensed plumber if the drip is at a fitting or joint, if a pipe is corroded or crusty, if water is stained through a ceiling or wall below, or if you can't tell whether it's a leak. HQ Plumbing & Air can inspect the line and find out. We're licensed and insured, we give free estimates, and you get the price up front before work starts. If water is actively coming through a ceiling, that's an emergency and we're available 24/7.

Sweating itself is harmless in the short term and easy to stop with the right insulation. Just confirm it's condensation before you assume it is. For the hot side, see whether you should insulate hot water pipes.