Sears Dehumidifiers are available in a range of capacities and with automatic humidistat controls to set the desired end indoor humidity.Indoor Humidity Standards / Settings

  • HUMIDIFIERS & HUMIDITY TARGET - CONTENTS: how & why to set & control indoor relative humidity levels. Variations in indoor humidity in the same space depending on exactly where and how humidity is measure
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How to control indoor humidity to avoid indoor air quality complaints, mold and dust mites.

This article answers the question "What indoor humidity level should I maintain to avoid mold and indoor air quality issues?" We explain the need for maintaining an anti-mold low humidity level indoors to avoid mold and other indoor pathogen growth in buildings.

We also discuss where and how to measure indoor humidity, what indoor humidity targets to set, and we explain relative humidity, dew point, and moisture condensation in and on building materials.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Problems Caused by Excessive Indoor Humidity include Mold Growth & Dust Mites

Photograph of chopped fiberglass insulation[Click to enlarge any image]

Article Series Contents

Target Indoor Humidity Level - Relative Indoor Humidity to avoid Mold & Dust Mite Problems

The recommended indoor relative humidity level for human comfort is in the 45% to 55% range.

Both lower and higher indoor relative humidity (RH) are uncomfortable, and as we will explain below higher indoor relative humidity levels are not just uncomfortable, they contribute to health risks and indoor air quality problems. Indoor RH also affects the level of other indoor air contaminants such as Formaldehyde & Nitrogen Dioxide (Gilbert 2006), and VOCs (Won 2001).

At the same time, too-low indoor RH also is uncomfortable.

There are also ideal indoor RH levels for the preservation of building materials and their contents (Bratasz 2007).

Humidity, Condensation, and the Dew Point

To avoid moisture condensation on cool basement or other building surfaces, we need to keep the RH down below the dew point at those surfaces. The "dew point" is the temperature at which moisture will condense out of the air.

The dew point is determined by the combination of the current temperature of the surface, the air temperature, and the humidity level.

If we were being scientifically precise we'd monitor all of the pertinent data - surface temperature, air temperature, relative humidity, and indoor air movement across surfaces. For our purposes, setting a reasonably low room-center target RH will usually be enough.

But remember, even if you don't see water condensing on and running down your basement walls, it doesn't mean that the walls won't be at a notably higher moisture level than the air in the center of the room.

See DEW POINT TABLE - CONDENSATION POINT GUIDE for details about the dew point and how to measure or calculate it for a building area or surface.

How low should we keep the interior moisture level to avoid a mold problem?

Suppose a building does not currently have a mold problem, or a mold cleanup project has been completed. How can we avoid a future mold problem in the building?

1. Be sure there are no ongoing building leaks, water entry, or venting problems.

2. Keep the indoor humidity level in the mid-comfort range.

A maximum indoor relative humidity of 55% RH may be acceptable,

50% RH better,

45% RH is a good target for an attic knee wall provided there are no ongoing leaks and the attic space is not one which is being vented to outside (in that case you're not in control of the humidity.

Indoor Relative Humidity Safety Margin

A client told us that he could keep the basement at 55% Relative Humidity (RH) but he didn't want to push it below that. Is this enough safety margin?

At 60% indoor RH we're entering the indoor problem mold-formation risk zone of high interior moisture in building wall or ceiling cavities or on wall and floor surfaces, possibly conducive to mold growth.

If you set the RH target at 55%, you're operating with not much safety margin of dryness. A small change in outdoor conditions (spilling water by the foundation) or indoor conditions (a nearby roof, wall, window, plumbing leak) can increase the moisture and RH into the problem zone.

If for reasons of dehumidification cost you have to operate close to the edge, extra attention to leaks, moisture proofing, roof and surface drainage are even more important.

The text below offers more technical background on indoor relative humidity (RH) control. This is getting slightly more technical about measuring the relative humidity - knowing a little more about how indoor air moves, how moisture levels vary in air and in building materials, and how to set the best humidity targets will improve the management of indoor moisture levels.

How To Measure Indoor Humidity

Tecpel hand held hygrometerRelative Humidity vs. Absolute Humidity in buildings

A variety of instruments can measure the amount of moisture in air, which we call "humidity." For example an inexpensive indoor "weather station" often includes a "humidity" gauge along with a barometer and thermometer.

But just knowing the level of moisture in air (absolute humidity) is not enough. Usually, the humidity targets we use in these articles, and in academic or scientific texts are numbers expressed as relative humidity which takes into account not only the absolute water level in the air, but also the air temperature.

Relative humidity, by taking into account both the absolute humidity in the air and the temperature of the air, is telling you the humidity level as a function of the maximum amount of water that the air is capable of containing at a given temperature.

If we're trying to control mold and other indoor pathogens for which water is a gating factor, it's relative humidity that is important.

Why? Because water condenses out of air onto a building surface (and thus supports mold or other indoor pathogens) only when the air at that surface contains more water than it can hold at that temperature.

When warm, moist air contacts a cool surface, your basement drywall near the floor, for example, the air touching that surface may cool and give up some of its moisture to condense on the surface.

See TOOLS for MEASURING HUMIDITY for accuracy and options for indoor humidity measurement equipment.

Variations in Indoor Relative Humidity by Building Area and Surface Type

The relative humidity, or "RH" will vary significantly in a building at a given moment, depending on where you make your observations.

Here are some example RH measurements from a recent investigation at a 1970's wood frame two story home in generally good condition, after an extensive mold remediation and dryout project, where the owner had been running two dehumidifiers in the basement, and where there were no building leaks:

Notice, with no surprise, that the RH is higher close to the (cool) masonry surface? This explains our reasoning in suggesting a fairly low basement RH target for buildings if we're going to measure the RH in the center of the room.

Some dehumidifiers have an RH meter built right into the machine, so it will tell you what RH level it's seeing in its incoming air. But for operating efficiency you'll often run the machine in the center of the room.

The target humidity for a building, if measured at room center, needs to be low enough to avoid condensation out on cool surfaces at the room perimeter or floor.

How Long to Reach the Indoor Humidity Target?

How long should it take to reach your indoor relative humidity RH target?

Using central air conditioning or even portable indoor dehumidifiers in a building that has not been flooded or wet by leaks, even in humid weather conditions we ought to be able to bring indoor relative humidity (RH) down to the target 50% range in twenty-four hours.

But that will not be the case if the building has been flooded, wet or unusually damp.

When a building has been damp for some time, moisture has been absorbed into various materials such as wood framing and masonry surfaces.

It may take weeks or even longer to drop the humidity in such an area, as the moist materials also have to dry out, not just the air. Using a fan to increase air movement in the area being dehumidified can speed this process.

Watch out: if you cannot get the indoor RH down to a low level in a below-grade area such as a basement or crawl space, I'd suspect that too much moisture is continuing to enter through the slab or masonry walls. Attention to outside drainage may not be enough. In such cases, coating the walls with a masonry sealer (Thoro-Seal™ or Dry-Lok™ are example products) might help.

If you want to get past this practical discussion of indoor humidity and mold, check out "Understanding Ventilation," by John Bower. The Healthy House Institute, 1995.

More than a normal person can stand to read about what to do about mold in buildings is at this website. You might start at MOLD CONTAMINATION IN BUILDINGS - home

Moisture Movement in Building Materials

Water molecules are very smart. They will naturally move from a moist area or surface to a more dry one, tending to seek equilibrium moisture across all surfaces and materials in a building, always considering the factors We have discussed above: temperature, relative humidity, and dew point.

So if humidity increases in a basement from warm moist air entering that space, moisture will begin to enter the more dry drywall and insulation materials.

Conversely, as you run a dehumidifier in the basement, moisture will be removed first from the basement air, and then as that dry air contacts more-moist basement surfaces (drywall and insulation, for example), moisture will move from those materials back into the air.

Moisture moves in either direction, into the air from materials, or into materials from air, always moving from the more-moist to the less moist substance, seeking equilibrium. This is why there will be a lot of water output from a basement dehumidifier when it is first run in an area, and then later water output will slow.

Using a Central Humidifier Attached to a Heating Furnace

Copper tubing supplies water to a central humidifier that leaked into the supply plenum and ductwork (C) Daniel FriedmanQuestion: how to connect hot water to a central humidifier

2018/01/11 Eric said:

I've added a humidifier to my furnace and would like to use hot water (recommended by manufacturer) from my hot water heater to supply water to the humidifier.

Can I screw a T-Valve into my water shutoff valve, add a new shutoff on the other end of the T-Vavle, and use this to supply my humidifier without violating codes?

Correction: I meant to ask if I can screw a T-Valve onto my water heater drain valve on the bottom, not onto the water shutoff valve.

This question and answer were posted originally at HEATING SYSTEMS

Reply: OPINION: this connection is not-recommended


T-valve from Mondeo at InspectApedia.comI do not think a T-valve would be the proper application for providing hot water to your system humidifier.

Typical T-valves such as the Mondeo T-valve I illustrate below, are ball-valves used to divert flow from a primary input source to one of two different destinations. The ball of the valve is drilled straight through and then an intersecting drill opening is made at a 90 degree angle to the first passage.

Ball valves are intended to be used in the fully-open or fully closed position, though some suppliers describe using a T-valve to permit sending output to two destinations simultaneously - you'd see reduced flow rate to both of them compared with a straight ball valve or an L-valve.

Also there could be a risk of pressure-loss and backflow of unsanitary water from the humidifier to the building water heater or plumbing system (though double check valves could mitigate that risk).

The simplest installation uses a saddle valve that punctures a hot water line (or in my OPINION better, a cold water line) and uses a small diameter flexible copper tube to conduct water to the humidifier. However many plumbing codes no longer permit these valves.

Instead, in my OPINON, it would make more sense to simply install a tee in the hot water supply piping (or in my OPINION better to cold water piping) and then connect a simple ball-valve or gate valve (to permit flow adjustment) in line with the outlet of the tee that will feed the humidifier.

Unless you are going to keep filling your humidifier manually, and unless your humidifier includes a water feed control in its design, you'd want a sanitary float control valve to admit water into the humidifier.

Central humidifier leaks and corrosion in ductwork (C) Daniel Friedman

BOTTOM LINE: drop the whole idea of connecting hot water to the humidifier and I would reconsider using a central humidifier at all

Why do we express this opinion?

There might be a small improvement in the production of moisture being sent into your furnace supply plenum or supply air duct by using hot water, but that benefit is likely to be offset by the formation of mineral scale and crud that HVAC technicians find troublesome as scale fouls controls and moving parts and increases service cost.

OPINION: frankly I am not a fan of central humidifiers that feed water into a supply plenum over a furnace heat exchanger. In over 40 years of building inspections I have seen that most of the central humidifiers I've seen installed were not maintained, were not working, and many of them had leaked into and caused costly, even dangerous damage to the furnace heat exchanger.

Central humidifers also can serve as mold amplifiers and mold distributors in buildings. (Solomon 1976).

For those reasons I prefer point-of-use portable humidifiers such as in sleeping areas.


Watch out: do not yry feeding hot water from your water heater's drain valve. You will have chosen the worst place to feed hot water into a humidifier as you'll be draining sludge and scale from the bottom of the water heater.

Research on Using a Central Humidifier in Homes

Excessive Humidity Encourages Indoor Air Quality & Health Hazards

Wet basement inspection points (C) Carson Dunlop Associates InspectApediaDiscussion moved - see HUMIDITY CONTROL INDOORS

Common problematic indoor molds

Discussion moved - see COMMON INDOOR MOLDS

Control Attic Humidity to Reduce Mold Risk

Discussion moved - see ATTIC HUMIDITY CONTROL

Upper Floor Humidity Control


Control Basement Humidity to Reduce Mold and Dust Mite Allergen Risk

Discussion moved - see BASEMENT HUMIDITY CONTROL

Do Water Pipes or Stone or Brick Surfaces "Sweat" in buildings?


Supply-only Vent System Operation: PIV System Humidity Level Variations

This discussion has moved to HUMIDITY CONTROL with SUPPLY-ONLY or PIV VENTILATION SYSTEMS - separate article

Bottom Line on Excessive Indoor Condensation: What to Do About It

  1. Identify and correct sources of un-wanted indoor moisture
  2. If necessary use a dehumidifier in damp areas like crawl spaces or basements
  3. Review building ventilation details in general, such as attic venting (avoiding attic condensation)
  4. Investigate the details of construction of building exterior walls and top floor ceilings to see what vapor barrier is present or absent.
  5. Review indoor temperatures, relative humidity, and air movement.

InspectAPedia is an independent publisher of building, environmental, and forensic inspection, diagnosis, and repair information provided free to the public - we have no business nor financial connection with any manufacturer or service provider discussed at our website.

Research on Indoor Relative Humidity Level Targets


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