Saturday, January 14, 2006

How Low Should You Keep Indoor Humidity to Avoid a Mold Problem

This Blog is about "What to do about mold in buildings"
When is mold a problem in buildings? What should be done about it? In depth information is at and the links at that page. Website content suggestions are most welcome.

Case histories: Here I post summaries of field and lab toxic or allergenic mold and other indoor air quality investigations. I omit private information. I describe observations, procedures, and findings helpful to readers who are trying to remedy their own mold, allergenic, carbon monoxide, odor, or other indoor air and related health concerns in their indoor environment.

What indoor humidity should we maintain in order to avoid a mold problem?

We're going to discuss maintaining an anti-mold low humidity level in a building which does not currently have a mold problem. No dehumidification system will be up to the task of preventing mold if a building has serious leaks, flooding, or water entry. (© Daniel Friedman 2006 All Rights Reserved.)

And 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.

We're therefore focused this article on the common indoor problem molds in this article. The genus I refer to include some common problem indoor 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 concer, Cladosporium sp. - the king of molds and its competitor Alternaria sp.

Suppose a building does not currently have a mold problem, or a mold cleanup project has been completed.

How low should we maintain interior moisture to avoid a mold problem?

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 should be ok, 50 better, 45 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. If you run humidity too low or too high the building occupants will be uncomfortable.

Technical background on indoor relative humidity (RH) control:
this is getting slightly more technical about measuring the relative humidity.

Controlling Humidity in Basements

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. 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.

Controlling Humidity in Attics

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 problme 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.

Choosing the target humidity RH level

One client said he could keep the basement at 55 but 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.

When have you reached your RH target?

If 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.

Warning: 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(TM) or Dry-Lok(TM) 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 my website. You might start at the "Mold Information Center - What to Do About
Mold in Buildings"

Hope this helps.

Daniel Friedman