Humidity and Indoor Air Quality in Maryland

Dew point is the temperature to which air has to be cooled before the water vapor in it starts condensing. It is an absolute measure: it tells you how much moisture is actually in the air, regardless of how warm that air is. Relative humidity is a ratio instead, comparing the moisture present to the maximum that air can hold at its current temperature. That is why the same relative humidity feels completely different in different seasons. Seventy percent at 60 degrees on an October morning is pleasant. Seventy percent at 88 degrees in July is oppressive, because warm air holds far more water.

The rough scale most people can feel: a dew point around 55 is comfortable, 60 to 65 starts feeling sticky, and anything at 70 or above is the heavy, soupy air the Mid-Atlantic delivers for weeks at a stretch. Through July and August this region regularly sits in the upper 60s and low 70s. All that moisture pushes into the house every time a door opens, and steadily through every gap in the shell. Your air conditioner has to condense that water back out, and the amount of work involved is set by the outdoor dew point, not by the percentage on a weather app.

Aim for 40 to 50 percent relative humidity indoors, and treat 55 as the upper edge of acceptable in the worst of summer. Above 60 percent, problems start compounding. Mold does not need standing water, only sustained moisture at a surface, and in a room sitting at 60 percent the cool surfaces are wetter than the room average: uninsulated basement walls, the inside of a closet on an exterior wall, the wall behind a dresser that blocks air movement. Dust mites are the other reason. They cannot drink, so they absorb water from the air, and their populations fall off sharply when indoor humidity stays below about 50 percent.

Too dry is its own problem. Below 30 percent you get static shocks, dry sinuses and irritated airways, gaps opening in hardwood floors and trim, and instruments and furniture that move with the season. The practical target is a band rather than a number: roughly 45 to 50 percent in summer, drifting down toward 35 percent in the coldest part of winter, because trying to hold 50 percent in January will condense water on the windows of most older houses around here. A cheap hygrometer, which is just a humidity gauge, is worth buying. Put one upstairs and one in the basement, because they will not agree.

Cooling and drying happen in the same place. Indoor air is pulled across the evaporator coil, a surface kept well below the dew point of that air, and moisture condenses on it and runs off to a drain. That is the whole mechanism, and it is a side effect of making the air cold rather than a separate function. It also means dehumidification depends entirely on run time. The coil takes several minutes to get properly cold and wet, so the first stretch of every cycle removes almost nothing. The real water removal happens later in a long cycle.

This is why an oversized system dehumidifies badly. It satisfies the thermostat before the coil has done any real work, and then the blower running on after the compressor stops evaporates the condensate sitting on the coil straight back into the house. Two settings make it worse. Running the fan continuously instead of on AUTO does the same re-evaporation every cycle, so use AUTO in humid weather. And a blower set to move a lot of air keeps the coil warmer, which cools well but dries poorly, which is why better systems have a dehumidification mode that deliberately slows the fan down.

A whole home dehumidifier is a ducted appliance with its own humidity control that drains to a floor drain or a condensate pump. Its real advantage is that it works when the air conditioner is not running. In late May and again in September, a 68 degree day at 80 percent humidity never calls for cooling, so the air conditioner stays off and the house slowly loads up with moisture. A portable unit is a reasonable tool for one room, but you have to empty the bucket or run a hose, it is noisy, and the heat from its motor warms the space it is drying.

Basements are their own case, and inside the Beltway they are usually the wettest part of the house. Below grade walls stay close to ground temperature all summer, so when warm humid air reaches them, moisture condenses on the concrete and on anything stored against it. A basement can read 66 degrees and 75 percent while the upstairs is perfectly comfortable. Fix the water first: gutters, downspout extensions, and grading that slopes away from the foundation solve more basement humidity than any appliance. Then dehumidify what is left. Do not finish or seal a basement wall over an active water problem, because you will only hide it.

Cold air holds very little water. Take outdoor air on a 25 degree January morning, bring it inside, heat it to 70, and its relative humidity falls to somewhere in the teens. Every house exchanges air with outside all day, so in winter your indoor air is mostly former outdoor air that has been warmed and thereby dried. Forced air heat gets blamed for this, but the ducts are not removing moisture. The furnace simply makes cold dry air noticeable, and a leaky duct system running through an attic pulls more outside air in, which makes it worse.

A whole home humidifier addresses this by adding water vapor into the supply duct, either by evaporating it off a wetted pad in the airstream or by boiling it in a steam unit. It needs a control that backs off as the outdoor temperature drops. Holding 45 percent indoors during a single digit cold snap will put condensation on the coldest windows in the house, and in an older bungalow with original wood sash that water runs into the joinery and rots it. Set the humidifier conservatively, watch the windows on the coldest morning of the year, and turn it down if they are wet.

MERV stands for Minimum Efficiency Reporting Value, a scale from 1 to 16 that describes how well a filter captures particles between 0.3 and 10 microns. MERV 8 handles ordinary household dust, pollen and mold spores. MERV 11 to 13 also catches finer material, including a good share of smoke particles and the fine dust that carries many allergens. Higher is not automatically better, because a denser filter resists airflow more. The fix is surface area rather than density: a four or five inch media cabinet holds far more filter material in deep pleats, so it can reach MERV 13 without choking the system.

That distinction matters more than most homeowners are told. Residential equipment is designed to work against a modest amount of resistance, and a one inch MERV 13 filter dropped into a slot meant for a cheap fiberglass panel can cut airflow enough to freeze the evaporator coil in summer or trip the furnace high limit switch in winter. It also drives up the electricity the blower uses. If you want better filtration, have the system evaluated for it and change the filter housing rather than just buying a denser filter. And change filters on schedule, since a loaded filter of any rating becomes the restriction.

Fresh air is the last piece. Older leaky houses ventilated themselves by accident through gaps that also wasted energy. Newer construction and heavily air sealed older homes do not, and without deliberate ventilation, moisture from showers and cooking, carbon dioxide, and off gassing from finishes and furniture accumulate. The basic steps are unglamorous: bath fans that actually terminate outside rather than in the attic, a range hood ducted outdoors, and a carbon monoxide detector on every level, since a tight house can affect how naturally vented gas appliances draft. Where more is needed, a balanced ventilator brings in outdoor air and recovers heat from the air it exhausts.