Mold in a semi-arid climate
The National Weather Service describes Boise as semi-arid, with just over eleven inches of precipitation a year at the airport, most of it falling from November through May. Dry outdoor air doesn't prevent indoor mold, which grows wherever water collects: basements that take on water during snowmelt, pipes that freeze and split, condensation on cold surfaces in winter, and bathrooms without good ventilation.
Basements
Many Boise homes have basements, and finished basements are especially vulnerable. Water entering through foundation cracks or window wells during snowmelt can soak carpet, pad, drywall and insulation. Because finished surfaces hide moisture, mold can grow in wall cavities behind baseboards before anyone notices. If you smell mustiness in a basement, check the bottom of drywall and carpet edges along exterior walls, and look for water stains. Fix the water entry before remediation.
Frozen pipes
NOAA's engineering weather data for Boise records about 78 freeze-thaw cycles a year. Pipes that freeze and split in exterior walls and crawlspaces can leak into cavities. After a burst pipe, have materials dried professionally and checked with moisture meters, not just the visible surfaces.
Winter condensation
Warm, moist indoor air condenses on cold windows, exterior wall corners and attic sheathing in winter. Exhaust fans that vent outdoors, good insulation and air sealing, and keeping indoor humidity moderate help. If you run a humidifier, monitor humidity so it doesn't climb too high.
Attics and ice dams
Ice dams at the eaves can push meltwater into the attic and walls, and bathroom fans that vent into the attic add moisture to cold sheathing. Check the attic in late winter for frost, damp insulation and dark staining.
Crawlspaces
Crawlspaces with exposed soil, poor drainage or blocked vents can grow mold on floor framing. A vapor barrier over the soil and proper drainage are the foundations of prevention.
Before you hire
Ask Boise mold contractors what moisture source they've found, how they'll contain the work area, what they'll remove and clean, how they'll confirm the job is complete, and whether they recommend independent clearance testing. Look up the contractor's Idaho registration with DOPL.
Evaporative coolers and humidifiers
Some Boise homes use evaporative coolers in summer or humidifiers in winter, both of which add moisture to indoor air. They have their place in a dry climate, but a leaking cooler, a humidifier set too high, or either one running in a poorly ventilated room can raise humidity enough to feed mold. Service coolers every season, check for leaks around the unit and ducts, and use a hygrometer to keep indoor humidity moderate when a humidifier is running.
After the cleanup
Once remediation and rebuilding are finished, keep an eye on the area for the first few months, especially through the next snowmelt. A return of musty odors or new staining means moisture is still getting in.
Window wells
Window wells that fill with snow and meltwater can soak basement window frames and the drywall around them. Covers and drains on window wells prevent many basement mold problems.