Mold in New Hampshire: one half of the job is regulated

Concord is New Hampshire's only long-record NOAA humidity station, reporting a 66 percent annual mean relative humidity, an 80 percent morning mean against a 53 percent afternoon mean, a calculated mean dew point of 36 F, and 42.0 inches of precipitation. That 27-point daily swing is one of the widest in New England, and it is the mechanism: air that reaches near-saturation every night and dries out every afternoon puts water on cold surfaces on a daily cycle without ever producing a visible leak.

The seasonal peak is sharp and late. The morning mean climbs to 88 percent in September and 86 percent in October, the highest pair in New England, landing in the window when air conditioning has stopped and heating has not started. Then the daily mean holds near 70 percent through the winter while the dew point sits at 36 F, which flips the problem indoors: in a sealed, heated New Hampshire house the moisture that matters comes from showers, cooking and laundry, and it condenses on cold glass and inside cold wall cavities.

The state has also gotten measurably wetter. NOAA reports that precipitation since 2005 has averaged 6.8 inches more than the 1895 to 2004 average, that the highest number of extreme precipitation events on record occurred during 2005 to 2014, and that annual precipitation plus the frequency and intensity of extreme events are projected to keep rising with associated increases in flooding. Statewide annual precipitation now normals at 48.8 inches for 1991 to 2020, heavier in the south and along the eastern border. Temperatures have risen more than 3 degrees F since the start of the twentieth century with winter warming outpacing every other season, which shifts snow toward rain and prolongs wet-envelope conditions. NOAA counted 21 billion-dollar disasters affecting New Hampshire from 1980 to 2024: 11 winter storms, 4 tropical cyclones, 3 severe storms, 2 floods and 1 drought.

New Hampshire against the national picture

Relative humidity figures are the mean of the morning and afternoon observations NOAA publishes for each first-order station, rounded to whole percent. Dew point is calculated from the NOAA normal mean temperature and that mean relative humidity using the Magnus formula, so treat it as a derived figure rather than a published one.

One station, and the number to read is the swing rather than the mean. Concord's 80 percent morning against a 53 percent afternoon is a 27-point daily range, one of the widest in New England. A range that size means the air reaches near-saturation overnight, every night, and then dries hard by mid-afternoon. Surfaces that stay cool through the morning, a basement wall, a bathroom tile, a north-facing window frame, collect that water daily and never get the benefit of the afternoon.

What Concord will not tell you is the mountains or the seacoast. NOAA puts statewide precipitation at 48.8 inches, heavier in the south and along the eastern border, which is six inches more than Concord's own 42.0. The seacoast runs damper with a narrower swing; the White Mountains carry far more snow and a much longer heating season. Read Concord as the central-valley baseline. Neighboring reports for Vermont, Maine and Massachusetts cover the states next door, and all three regulate this work differently.

EPA and CDC name the same core genera for cold humid climates. In New Hampshire they concentrate wherever a surface stays cool through an 88 percent September morning, and in bathrooms where a 27-point daily swing meets weak ventilation.

Species information is general and informational. Health responses vary. See a licensed physician for symptom evaluation. Species sources: EPA Mold Remediation in Schools and Commercial Buildings, EPA A Brief Guide to Mold Moisture and Your Home, CDC National Center for Environmental Health guidance.

No New Hampshire agency publishes a mold remediation price schedule, and the state has no mold web page at all. That absence was measured rather than assumed: the DHHS sitemap of 1,874 URLs and the Department of Environmental Services sitemap of 1,801 URLs were both swept and neither contains a mold page.

The New Hampshire cost driver that no other state in this report has is the assessment and remediation split. Because the assessor must hold a national certification and the remediator need not, you will often be buying two things from two firms with very different credential requirements. That is not a bad thing. It happens to enforce the independence that experts recommend everywhere: the party sizing the problem is not the party billing to solve it. Budget for both, and treat the certified assessment as the document that defines the scope.

The second driver is cold-climate access. Ice dam damage sits at the wall-ceiling junction on the top floor, behind blown-in insulation, in an attic with limited working height, so the labor is the cost rather than the material. A finished basement that takes water is a demolition and rebuild.

The third is that the remediation half has no credential to check, so bids will differ in scope. Get an itemized written scope from each. Our national mold remediation cost guide covers what moves a quote in any market, and the cost estimator lets you scope your own job first.

On insurance: a sudden and accidental discharge such as a burst pipe is usually covered, ice dam damage often is, gradual seepage usually is not, and flooding is excluded from a standard homeowners policy entirely. Take coverage questions to the New Hampshire Insurance Department, and read our guide to homeowners insurance and mold.

New Hampshire regulates mold work, and it regulates exactly half of it. Since January 1, 2016, RSA 310-A:189-b has made it unlawful to perform residential mold assessment services for remuneration without a valid national third-party certification for mold assessment.

The statute defines mold assessment broadly: inspecting, investigating or surveying a dwelling or other structure to inform the owner about the presence, identification or evaluation of mold; writing a mold remediation specification or protocol; or collecting a mold sample for analysis. If someone is charging you to look at your mold, write your protocol or take your sample, they are covered.

Note what the state does not do. New Hampshire issues no license of its own. It requires a credential from a national nonprofit whose program is accredited by ANSI, CESB, NCCA, or an equivalent ISO-compliant body, and the Office of Professional Licensure and Certification's Board of Home Inspectors points the public to the American Council for Accredited Certification. So the thing to ask for is the national certificate, not a New Hampshire number.

And mold remediation is not covered at all. New Hampshire also has no state general contractor license, so the person who tears out the drywall needs no state credential while the person who wrote the protocol does. That is an unusual shape and it has a practical upside: the certified assessment is your leverage. Get the assessment from a certified assessor, use its protocol as the scope of work, and then require the remediator to bid against that protocol and to document daily moisture readings by material. Ask for IICRC certification and ANSI/IICRC S520 compliance on the remediation half, since nothing else requires it.

New Hampshire has no mold standard by name. RSA 48-A:14 sets the statewide minimum standards of fitness for rented residential premises in any municipality that has not adopted its own housing code, barring a landlord from maintaining rented premises in eleven listed conditions. The one a mold claim runs through is subsection IV, roof or walls that leak consistently, backed by subsection II on defective internal plumbing or sewage backup and subsection VI on substantial holes in floors, walls or ceilings.

The teeth are in RSA 540:13-d. A landlord cannot win a possession action for nonpayment of rent if the premises substantially violate the RSA 48-A standards or a local code and the violation materially affects habitability, provided the tenant proves by clear and convincing evidence that, while not in arrears, the tenant gave notice to the person who collects rent, and the landlord failed to correct within 14 days of written notice or as promptly as an emergency requires.

Three conditions attach. The violation must not have been caused by the tenant, repairs must not have been blocked by extreme weather or the tenant's refusal of access, and the tenant must not have been in arrears. If the defense is raised, the court can continue the case up to one month for the landlord to fix the problem, requires the withheld rent to be paid into court, and on a failure to remedy enters judgment for the tenant and refunds the deposited money.

The other New Hampshire specific: enforcement runs through a local health officer, not a state mold program. DHHS's Health Officer Liaison Unit processes health officer appointments on behalf of the Commissioner, trains them on topics that expressly include rental housing concerns and mold and moisture issues, provides case-specific technical assistance, and publishes a Health Officer Manual and a find-your-local-health-officer tool. A New Hampshire renter with mold is directed to that local health officer. Send dated written notice first using our landlord letter generator, then contact the health officer. Our guide to mold and apartment rights covers the general playbook.

General information, not legal advice. For your situation, talk to a New Hampshire attorney, a legal aid office, or your local tenant council.

A 27-point daily humidity swing, an 88 percent September, and a long sealed winter. Three moves cover most of it.

A. Fix the bathroom fan before you clean the grout

New Hampshire's daily swing puts the air near saturation every morning, so a bathroom that cannot clear its own moisture within an hour of a shower never dries. Confirm the fan actually vents outdoors rather than into the attic, which is a common and damaging shortcut in older New Hampshire houses. Check that it moves enough air by holding a tissue to the grille. Run it on a timer for 20 minutes after the shower rather than switching it off with the light. Cleaning caulk and grout without fixing the fan is a maintenance schedule, not a repair.

B. Own the cellar from August through October

The morning mean reaches 88 percent in September while cellar surfaces are still cool and neither heating nor cooling is running. Opening a cellar window on a humid September morning makes the walls wetter, not drier. Run a dehumidifier with a condensate pump and hold the space below 50 percent. Outside, direct downspouts at least six feet from the foundation and grade soil to slope away. Our indoor humidity calculator reports measured relative humidity plus calculated dew point.

C. Use the certification split to your advantage

New Hampshire is one of the few states where the law itself separates assessment from remediation. Hire a nationally certified mold assessor first, as RSA 310-A:189-b requires anyone charging for that work to be, and use the protocol they write as the scope of work. Then put that protocol out to remediation bidders. You get an independent scope, comparable bids and a document to hold the work against, which is exactly the structure experts recommend and which most states leave you to arrange yourself.

Monthly mean relative humidity at Concord. The daily mean is unremarkable and the morning mean is not: it reaches 88 percent in September, the highest reading in New England.

September and October are the cellar window. The morning mean runs 86 to 88 percent while below-grade surfaces are still cool and nothing mechanical is dehumidifying. This is the Penicillium window on stored cardboard, and it is when most New Hampshire cellars first smell musty.

December through March is the indoor-moisture window. The daily mean holds near 70 percent while the air outside carries almost nothing at a 36 F dew point. In a sealed, heated house the water comes from showers, cooking and laundry and condenses on cold glass and inside cold wall cavities. The fix is ventilation and a lower indoor target of roughly 30 to 35 percent, not a dehumidifier.

April is the driest month at 59 percent and the ice dam reckoning. Staining that appears at the top of an exterior wall in spring is usually February's ice dam showing itself. Check the ceiling under every roof plane after the thaw. The mold risk by ZIP tool gives you the local read.

New Hampshire has no state mold page, and its mold enforcement runs through locally appointed health officers. The liaison unit below is how you find yours.

Every source chip on this page points here. Agency and statute pages first, then the federal technical references.