Ideal Indoor Humidity

The ideal indoor humidity for a southern New Hampshire home is 30 to 45 percent in winter and 45 to 55 percent in summer. These ranges balance occupant comfort and health against the risk of moisture damage to the home itself. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends a broader 30 to 60 percent range for human comfort. The narrower NH-specific bands account for cold winter walls and humid summer dew points.

Here is why the numbers are what they are and how NH homeowners can control humidity year-round.

Why these specific ranges

The lower bound (around 30 percent)

Below 30 percent relative humidity, the dry-air symptoms most NH homeowners associate with winter start to appear:

Dry, itchy skin and chapped lips
Sore throats and nosebleeds
Dry, itchy eyes
Increased susceptibility to colds and flu (viruses survive longer in dry air)
Static electricity discharges
Cracking hardwood floors, shrinking woodwork, separated trim
Out-of-tune pianos and cracked acoustic instruments

The upper bound (45 percent winter, 55 percent summer)

Above the upper bound, condensation on cold surfaces becomes a risk:

Condensation on window glass (especially older single-pane or aging double-pane windows)
Moisture buildup in wall cavities where warm interior air meets cold exterior sheathing
Mold growth on cold surfaces in basements, bathrooms, and exterior walls
Bacterial growth in damp areas
Dust mite growth above 50 percent humidity

The winter upper bound is lower because outdoor temperatures are colder and indoor surfaces (especially windows) are correspondingly colder. The summer upper bound can be a few percentage points higher because indoor surfaces are warmer.

How to measure indoor humidity

A digital hygrometer (also called a humidity meter) costs under $20 and is the most reliable way to know what your indoor humidity actually is. Many smart thermostats (Ecobee, Nest, Honeywell Home) also display indoor humidity from a built-in sensor. Place at least one measurement device in a central living area, not in a bathroom or kitchen where readings are unrepresentative.

Controlling winter humidity in NH

NH homes are routinely too dry in winter. The cold outdoor air entering the home (through controlled ventilation and incidental infiltration) holds very little moisture. Once that air is heated to room temperature, its relative humidity drops to 5 to 15 percent without active humidification.

Options to raise winter humidity:

Whole-home humidifier installed on forced-air ductwork. Connects to the home's water supply, controlled by a humidistat. Capacity sized to the home (typically 15 to 30+ gallons per day). The best solution for homes with central forced-air heat.
Portable humidifiers for individual rooms. Useful for bedrooms but cannot keep up with whole-house demand. Require frequent refilling and cleaning.
Lifestyle sources add some humidity: showers (close the bathroom door after), cooking (use the range hood appropriately), houseplants. Together these add a few percentage points but not enough to reach the target range alone.

Controlling summer humidity in NH

NH summers are humid. July and August dew points routinely climb into the high 60s and low 70s, and indoor humidity can rise into the 60+ percent range without active dehumidification.

Options to lower summer humidity:

Central AC removes humidity as a byproduct of cooling. A properly sized and well-running AC keeps indoor humidity in the 45 to 55 percent range during cooling season.
Oversized AC is a humidity problem. Short-cycling AC drops temperature without removing enough moisture. The home feels cold and clammy. This is one reason correct sizing matters.
Whole-home dehumidifier installed on the duct system. Operates independently of the AC and is the right solution for finished basements and homes that need humidity control on cool, humid days when the AC is not running.
Standalone basement dehumidifier for homes with chronically damp basements. Empty the reservoir or connect to a drain.

Humidity issues we see in NH homes

Window condensation in winter

Condensation on windows in cold weather usually means indoor humidity is too high for the window surface temperature. Solutions:

Lower the humidifier setpoint by 5 percent and see if condensation stops
Add storm windows or upgrade aging double-pane windows
Check that the humidifier humidistat is responding to outdoor temperature (some models reduce target humidity as outdoor temperature drops)

Musty basement in summer

Damp basement air feels musty and supports mold growth. Solutions:

Run a basement dehumidifier targeting 50 percent relative humidity
Address obvious moisture sources: leaks, condensation on cold pipes, foundation infiltration
Ensure the basement is not directly venting outdoor humid air through open windows

Cold, clammy upstairs in summer

Often a sign of oversized AC. The system drops temperature quickly without removing enough humidity. Diagnostic visit recommended.

Schedule a humidity consultation

If your NH home is consistently too dry in winter, too humid in summer, or you are dealing with window condensation or basement moisture, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.

Frequently Asked Questions

What is the ideal indoor humidity in New Hampshire winter?

30 to 45 percent. Below 30 percent feels dry and triggers respiratory symptoms. Above 45 percent risks condensation on cold windows and exterior walls.

What is the ideal indoor humidity in summer?

45 to 55 percent. Properly sized AC typically maintains this range during cooling season. Oversized AC often produces too-low temperatures with too-high humidity.

How do I measure indoor humidity?

A digital hygrometer for under $20, or check your smart thermostat (Ecobee, Nest, Honeywell Home all display indoor humidity).

Why is my house so humid in summer even with AC running?

Most commonly an oversized AC short-cycling. Other causes include leaky ducts pulling in attic moisture, a clogged condensate drain, or a system low on refrigerant. A diagnostic visit identifies the cause.

Can I use a portable humidifier instead of a whole-home unit?

For a single room (bedroom, nursery), yes. For a whole home, portables cannot keep up with the typical 5 to 15 gallon per day moisture demand. A whole-home humidifier connected to the home's water supply is the only practical answer for whole-house humidity control.