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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.
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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
Our citations refer to research articles found at the end of this article and also in this article's separate REFERENCES section.
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.
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.
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
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:
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.
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.
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. Click to enlarge.
Interesting question; I think we need more information to make an answer beyond mere arm-waving;
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.
Also take a look at the collection of suggestions under CRAWL SPACE DRYOUT - home
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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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 t'stat. 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
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 dehumidifcation 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?
(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 .
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.
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.
(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
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.
(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
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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