How to control indoor humidity:

Advice on controlling indoor relative humidity in attics, upper floors, basements, crawl spaces, or on sweating walls & pipes.

This article series 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.

Indoor Humidity Control by Area

Article Contents

• HUMIDITY CONTROL INDOORS
• COMMON INDOOR MOLDS
• ATTIC HUMIDITY CONTROL
• UPPER BUILDING FLOOR HUMIDITY CONTROL
• BASEMENT HUMIDITY CONTROL
• SWEATING PIPES & WALLS INDOORS

Also see these articles:

• CENTRAL HUMIDIFIERS in BUILDINGS
• DEHUMIDIFICATION BASICS
• DEHUMIDIFICATION PROBLEMS
• HUMIDIFIERS & HUMIDITY TARGET

Excessive Humidity Encourages Indoor Air Quality & Health Hazards

[Click to enlarge any image]

Once indoor relative humidity is in the 60% range we are inviting indoor mold growth.

High indoor humidity can encourage more problems than indoor mold.

The same moisture conditions that support growth of problematic indoor molds also cause or contribute to these hazards

• Bacterial hazards,
• Asthma (Strachan 1989) possibly related to dust mite populations
• Dust mite fecal allergen problem, The same measures to control humidity to prevent mold growth are what's needed to discourage the dust mite population that exists in all living areas.
• Viral hazards, (Lowen 2007),
• Possible insect problems in buildings

Our citations refer to research articles found at the end of this article and also in this article's separateReferences or Citations section.

Common problematic indoor molds

We recommend use of dehumidifiers and humidity instruments or humidity transmitters to monitor your building. But no dehumidification system will be up to the task of preventing mold if a building has serious leaks, flooding, or water entry.

No dehumidifier, no "air cleaner," no "ozone generator," nor other magic machine, spray, or air treatment will correct a mold problem in a building if there is a significant problem reservoir.

For that case, what's needed is to find the mold problem, remove it, and correct its cause. And as a last warning, there are about 1.5 million mold species - some of them may be able to grow in very dry or very wet or other inhospitable conditions.

Mold spores are everywhere all the time, entering from outdoor air as well as on pets and clothing.

A mold spore landing on an indoor surface is likely to be insignificant and amount to little more than a common component of indoor dust, until such a mold spore lands on an organic surface (such as drywall) and the indoor humidity level and thus the humidity or moisture level of the surface on which the mold spore rests, is sufficiently high.

Since a mold spore requires moisture to propagate and grow, the indoor humidity level is a key gating factor in the control of indoor mold (and dust mites) in buildings.

Certain common mold genera and species, such as some members of the Aspergillus sp. and others grow readily on common building materials if they also have enough moisture. While there are fungal species that are able to grow under a remarkably wide range of environmental conditions, keeping indoor humidity at the appropriate level will reduce the chances of growth of the most common indoor problem molds.

We refer to common problem indoor toxic or allergenic molds such as Aspergillus sp., Penicillium sp., Stachybotrys sp. /S. chartarum /Memnoniella echinata, Trichoderma sp. /T. viride, Ulocladium sp. /U. chartarum, and at a less significant level of concern, Cladosporium sp. and its common indoor species such as C. sphaerospermum and C. cladosporioides.

A number of Basidiomycetes and Ascomycetes also appear indoors as wood rotters and on other wet or damp building materials, though they may as a group be less often toxic or pathogenic to humans and more often an indicator of wet or damp mold-conducive indoor conditions.

Our table of the most commonly found indoor molds growing in buildings has been moved to a separate online document.

See TABLE OF MOST COMMON INDOOR MOLDS FOUND IN BUILDINGS. Use the back button on your browser to return to this page.

Control Attic Humidity to Reduce Mold Risk

In the case of an attic crawl space, perhaps a knee-wall area abutting an upper floor bedroom, the risk of excessive inside humidity at a wall is much less than in a basement. In the attic we don't face a cool concrete-block wall surface in the attic.

But what about an un-vented attic in a cold weather climate? Heat loss into such a space and warm moist air leaking into such a space can indeed create high levels of problem moisture - enough to wet surfaces or even form frost and later drip onto the attic floor.

On the other hand, if the attic is vented to outside (ridge vents and soffit vents as I recommend) you'll never control the attic RH. You'll be trying to control the whole outdoors.

On the third hand (if that's possible), if an attic is not vented to outside, the RH there is most-likely a function of and approach the levels of the humidity levels in the air in the rooms abutting and below the attic area.

Articles on Attic Ventilation

• ATTIC CONDENSATION CAUSE & CURE
• ROOF VENTILATION SPECIFICATIONS
• ROOF VENTIILATION INTAKE-OUTLET RATIO

Upper Floor Humidity Control

Reader Question: how can I decrease the indoor humidity level upstairs

I have 3 dehumidifiers going plus the central air and the basement and first floor are at 50 - 55% humidity but I can not get the top floor where the bedrooms and bathroom is below 60% in the spring/summer/fall. Some days it even goes to 65%.

We had a mold problem 7 years ago and had professional remediation and have not seen any evidence of mold since but I have developed chronic sinus and bronchial problems that I wonder if it is being caused by mold spores.

I thought maybe humidity coming from the attic but then the floor downstairs would be affected also as it is a multilevel house and the kitchen/dining/living room area is directly under the attic also and it is 50% down there so I do not know how to fix the humidity problem.

Any help would be appreciated as I am tired of being sick. Thank you. (this is the first summer I have been sick like this and can not get over it) - Anonymous 9/12/11

Reply: a strategy for correcting high indoor moisture levels and possible mold or dust mite reservoirs

Although sometimes we find surprising down-currents of air and moisture from a building attic, that's not the most common indoor moisture problem source.

Since we haven't inspected your building and know next to nothing about it, we have to outline a more general strategy for reducing high indoor moisture:

1. Find and fix sources of high indoor moisture;
   
   If your building uses central air conditioning also
   
   see WET CORRODED DUCT WORK
2. Then run dehumidifiers seasonally as needed,
   
   if necessary using fans to improve indoor air circulation - a step that will in turn significantly increase the effectiveness of your indoor dehumidifiers.
3. Make a more expert and thorough inspection of your building
   
   for a remaining mold reservoir; the fact that you previously had enough of an indoor mold problem to justify paying a professional mold remediator suggests that the problem was significant.
   
   Too often a "remediator" focuses on spraying, or better, removing moldy materials and cleaning the building.
   
   But still, if s/he forgot to find and fix the sources of high indoor moisture, a problem with mold, dust mites, or other indoor allergens or air quality problems is likely to return.

At CONDENSATION on WINDOWS, SKYLIGHTS we discuss the causes & cures for excessive condensation on windows

At DEW POINT CALCULATION for WALLS we discuss how and why moisture can accumulate within building walls

At INDOOR AIR QUALITY & HOUSE TIGHTNESS we discuss indoor air quality problems in homes with very little ventilation or fresh-air makeup

At MOISTURE CONTROL in BUILDINGS and also with more focus on sources of indoor moisture

At MOISTURE METER STUDY we include examples of the difficulty of measuring moisture in building walls and ceilings and we show points of hidden leaks that may affect indoor humidity levels.

At WATER ENTRY in BUILDINGS we discuss the importance of finding the source of excessive building moisture and doing what we can to correct that problem first.

Control Basement Humidity to Reduce Mold and Dust Mite Allergen Risk

If the RH in the center of a basement is 55% it is likely that at the walls or corners, where there is less air circulation, the RH may be different, often higher.

The local temperature difference close to a cool masonry wall surface means that both temperature and measured relative humidity close to the wall will be different than in the center of a room.

But it's at the cooler wall surface that condensation may be expected to occur. If you measure the RH at the worst-case location such as the most-suspect-of-dampness corner of a basement and you're 55% close to the wall you're likely to be OK.

Reader Question: what is the best way to reduce humidity level in a basement? Vent the drop ceiling?

I have a 1,400 square-foot basement with a drop ceiling below the main level subfloor. The house is on a slope, so one wall is not underground. In some areas, the RH is 62 percent.

I was thinking of installing an attic-style vent fan horizontally through that wall in the area between the drop ceiling and the subfloor to create air movement and to vent the air to the outside. Will that help reduce the RH? Thanks. - D.M. 6/4/2013

Our illustration at left shows where to look for moisture in and leak problems in basements, courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ]. Click to enlarge.

Reply: here are the basic steps in reducing & controlling basement humidity level

Interesting question; I think we need more information to make an answer beyond mere arm-waving;

• If you just install exit venting, where will the make-up air come from?
• Will that makeup air be higher in humidity than the air we are exhausting? Sometimes? If so we're making matters worse not better.

While years ago we thought that we should vent crawlspaces and perhaps basements with outdoor air - still reflected in crawl space or foundation vent opening size ratios found in some building codes.

Current best construction practices, now better informed, have shifted to making the indoors a closed, conditioned space. That's because the moisture level and temperature of outdoor air vary so widely in many areas that under many conditions mere venting makes indoor moisture levels worse not better.

Your best approach to reducing the high humidity in a basement or crawl space combines these methods

• Identifying and fixing sources of water entry
  
  See BASEMENT LEAKS, INSPECT FOR
  
  Usually wet or damp basements are due to outside leaks from roof runoff.
  
  See BASEMENT WATER ENTRY PREVENTION
• Sometimes high indoor humidity combined with cool basement surfaces can also show up as condensation problems that might be mistaken for leaks.
  
  See CONDENSATION or SWEATING PIPES, TANKS
• Using fans and dehumidifiers and other methods to dry out a damp or wet crawl area.
  
  See CRAWL SPACE DEHUMIDIFICATION
  
  See CRAWL SPACE DRYOUT for advice
• Using an indoor dehumidifier and setting an appropriate indoor humidity target (discussed in this article).
  
  Also see MOISTURE CONTROL in BUILDINGS
  
  and MOISTURE PROBLEMS: CAUSE & CURE
• Adding heat when seasonally appropriate, or air conditioning (which dehumidifies) when appropriate.
• Where fresh air is needed indoors, consider an air-to-air heat exchanger type of ventilation system
  
  See VENTILATION, BALANCED
  
  and VENTILATION, BALANCED HEAT COST SAVINGS)

Also take a look at the collection of suggestions under CRAWL SPACE DRYOUT - home

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

No. Why does water condense on your cold water pipes overhead in the basement before it condenses on the steel Lally columns supporting your main girder?

Perhaps because cold water (at 40 deg. F.) is running through the water pipe, cooling its surface to a lower temperature (40 deg. f.) than that of the Lally column (perhaps 55 deg. f.). Water pipes do not "sweat" as people say - water is not exuding out of pores in the pipe.

Water is condensing from moist air onto the surface of the cold water pipe. Insulate your cold water pipes to avoid condensation and drips onto the floor. It looks like sweat, but it's not.

For a different reason, that of energy efficiency, you might want to insulate your hot water and heating pipes in a basement as well, though in some conditions we are so desperate to warm and dry a problem area that we deliberately leave the hot water and heating pipe insulation off so that we can steal some of their heat to warm and dry an area.

Similarly, moisture will condense out of moisture-containing air on cool building surfaces like stone, brick, metal, concrete floors or walls or ceilings, and on tile floors or walls set over cool or cold surfaces.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question/Comment: excessive indoor humidity traced to high velocity air conditioning system coil condensation blow-off

For years I've been trying to figure out the excessive humidity problem in my home too. I finally found it after working with foundation people, plumbers, a/c techs -- nobody could figure it out, but I finally did.

When the cooling kicks on, the moisture level skyrockets. It has affected the inside of my home tremendously. We thought it was the a/c drain. They re-routed it, made sure it was draining well and clear. It is. Leak near or under the foundation. We checked everything-that wasn't it.

What's happening is the fan is actually sucking the moisture out of the evaporator coils before the condensation off the coils can drain away.

The design of this Lennox horizontal system in the attic is such that the small space right above and right below the squirrel type fan, creates a venturi effect, increasing the velocity of the air being sucked into the fan.

The velocity of the air is so strong that the coils (about 12 in. away) have the moisture sucked right off of them and into the fan, which, of course is then blown into the ducting.

I'm not sure if this is an engineering design problem, if the a/c co. wired the fan to a speed that's too high, or if perhaps a part is missing that is supposed to prevent this.

Regardless, the inside of the unit is now so covered with mildew and mold and the electrical connections on the heating elements as well as all the electrical connections on the inside of the unit, are so corroded and rusted, it's a wonder that love thing works at all.

(This also probably explains why sometimes the heat works and sometimes it doesn't. The a/c repair guys have never been able to figure out why.

They always seem to think it's the thermostat It's not. It's new and has recently been completely re-wired when I moved it from an outside wall to an inside wall.)

Anyway, that's where MY moisture problem is coming from. Good luck! - Mike / DFW 12/13/2012

Reply:

Mike, gold star to you for good detective work. We will keep your note in this article, as it will surely help other readers.

More about dehumidification problems traced to central air conditioning systems can be read

at DEHUMIDIFICATION PROBLEMS.

Do you think that the root problem, then, is an improper duct or plenum size or design?

Question: effect of spray foam insulation on relative humidity with different vent systems

(Apr 21, 2014) if you have sprayed foam insulation in an attic in Louisiana and fiberglass insulation in rafters at correct r factor will the relative humidity be better with no ridge vents or whirlybirds.i think the attic is air tight with foam sprayed on the underside of the roof.

The attic does stay cooler but i was just wondering about air flow or ventilation .

said:
If a home in Louisiana with 100 degree summers has the attic sprayed with foam insulation on underside of roof,and fiberglass insulation in rafters at correct r factor with no ridge vents or whirlybirds for air flow will the relative humidity be better in the rooms below.the home has central air with unit in the attic.

Reply:

Adding insulation and cutting ventilation do not themselves reduce indoor humidity in a warm humid climate, but operating air conditioning, provided it is not over-sized, will reduce the indoor humidity level.

The contribution of the insulation in this case is the reduction of heat gain by the building, reducing the cooling costs.

Beware that if the building design drives moisture into any building cavities (roof, walls for example) trapped moisture there invites mold, rot, and insect trouble.

So the building design needs to include attention to ceiling and wall vapor barriers (in LA typically on the warm outside) and penetrations or air leaks.

Question: will adequate roof venting reduce indoor relative humidityu

(May 22, 2014) Chi said:
It's 68 degree outside and humidity at 60%…my inside RH is 52% at 66 degree without AC…is that normal during Spring/Summer?

Should a sufficient roof/attic ventilation reduce the RH??
During Winter, my indoor RH was 35% at 68 degree while outside was below 20 degree with humidity at 100% due to snow

Reply:

Chi

Without trying to re-calculate your numbers against a standard, I'd say of course in humid weather your indoor RH won't be down at 35%, but probably more useful is this comment:

IF you run your A/C and the interior cools off enough to satisfy the thermostat but the humidity is still uncomfortably high then your A/C system may be oversized - and so not dehumidifying.

Roof ventilation, or attic ventilation, properly balanced between intake and exhaust may cool an attic but it would not directly affect the humidity of the occupied spaces below - with an exception that in circumstances of excessive condensation or even frost formation in an attic, that space can become a moisture source against the ceiling or occupied spaces below.

Question: where to start with excessive condensation on windows

(Oct 6, 2014) TonyC said:

Living in CT, experiencing condensation on the inside glass of my windows on the main living area, and second floor, none in the basement. The indoor temperature read 66 degrees and humidity indicated at 46%, not sure if this is RH or absolute.

The outside temperature was 39 degrees with the weather station indicating 94% humidity.

We seem to experience these problems during the Fall seasons as temperatures gradually change to Winter where we require the heat to be on.

We can find no visible drivers for excessive moisture in the home. We use bathroom fans that ventilate to the outside when showering and for 20 minutes afterward.

We have an exhaust fan that ducts to the outside over our cooktop and that is on when we cook. Some windows are worse than others particularly those that face the north and east and this is always noticeable in the morning. Is it possible all my windows are failing.

They are 10 years old, but we have had this problem for several years, I am only just finding this site to ask a question. We are lost as to where to begin to try to solve this. Any ideas would be appreciated

Reply:

Tony

IF windows are "failing" and we are referring to insulated glass, you'd more likely see moisture collecting between the panes.

If windows are failing by becoming leaky or drafty you can detect that using a smoke test or perhaps thermography.

I'd start by asking why moisture might be higher in the problem room than in others.

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Continue reading at DEHUMIDIFICATION BASICS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CLOTHES DRYER VENTING
• CONDENSATION on WINDOWS, SKYLIGHTS
• CRAWL SPACE DRYOUT - home
• DEW POINT TABLE - CONDENSATION POINT GUIDE
• DEW POINT CALCULATION for WALLS
• FOUNDATION WATERPROOFING
• GUTTERS & DOWNSPOUTS - home
• HUMIDITY CONTROL & TARGETS INDOORS
  • CENTRAL HUMIDIFIERS in BUILDINGS - add needed moisture to building air
  • DEHUMIDIFICATION BASICS - remove un-wanted moisture indoors
  • DEHUMIDIFICATION PROBLEMS
  • DEW POINT CALCULATION for WALLS
  • HUMIDITY MEASUREMENT INSTRUMENTS
  • HUMIDITY RESPONSE, NEST THERMOSTATS
  • MOISTURE CONTROL in BUILDINGS - home
• INDOOR AIR QUALITY & HOUSE TIGHTNESS
• HUMIDITY CONTROL with SUPPLY-ONLY or PIV VENTILATION SYSTEMS
• HUMIDITY LEVEL TARGET
• HUMIDITY MEASUREMENT INSTRUMENTS
• INDOOR AIR QUALITY & HOUSE TIGHTNESS
• MOISTURE in BUILDING WALLS, EFFECTS
• MOISTURE CALCULATIONS
• MOISTURE CONTROL in BUILDINGS - home
• MOISTURE PROBLEMS: CAUSE & CURE
• PERM RATINGS of BUILDING MATERIALS
• SWEATING (CONDENSATION) on PIPES, TANKS
• VENTILATION in BUILDINGS - home

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Citations & References

In addition to any citations in the article above, a full list is available on request.

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