Mold Chart for Temperature and Humidity Monitors

By adam
September 23, 2015   Comments Off on Mold Chart for Temperature and Humidity Monitors


All crawlspaces and basements should have a wireless temp/RH gauge and printouts of the above “MOLD CHART”.

(This chart should be accompanied by primer (see below) or previous knowledge of wood moisture and mold-contamination.)

It can be hard to understand moisture in air, so a few starting points:

1) Hot air can hold more water & cold air less. Temperature makes the humidity “relative”. At 75 degrees Fahrenheit & 65% RH there is “no risk for mold”.  At a cooler 70 degrees Farenheit, a more humid 66% RH is still “no risk for mold”. In this regard, if your objective is to “curtail the formation of biological contaminants”, you could either drop the temperature, or the humidity, or a combination of both and be successful.

We are most healthy right at 50% RH and our goal for sealed crawlspaces in humid climates is 65% RH or less and with stability of temperature & humidity fluctuating just a few % points daily. The absolute best scenario is that on average there is no biological contaminants growing on the wood and very few critters. Often with our sealed crawlspaces we see humidity rising to “problem levels” briefly in the spring and fall (when HVAC run-times are down, nearly zero). A key take-away from our experience, is that wood is awesome, because it takes a very loooong time to destroy it. Eventually with a reasonable crawlspace project under your belt you should be able to drive the %RH and %WMC down to good standing in 6 to 8 weeks without a dehumidifier but by driving it down by retaining energy within the envelope of the crawlspace, thus protecting the floor system, the wires, pipes and ducts, and the soils from moisture issues. We want our soil saturation to be stable as well as below the vapor barrier. Do not allow soil moisture to rise into your crawlspace or the ambient humidity in through your vents.

2) Mold likes WET, WARMER, porous, organic, objects so it can multiply but it needs time to develop.

3) Wood takes a long time to absorb and release water vapors so the average conditions in a crawlspace are much more relevant that the highs and lows.

4) Mold and Mildew and Fungus are the 3 topical biologicals and all of them should be treated first, like a moisture problem. Just measure the Temp and RH and solve the moisture problem based on the chart. Finding out what type of mold or contamination you have is not as useful as how much. By all means clean up the place after 8 weeks of osmotic stress has been placed on all the surfaces, as a courtesy to the next service professional so they don’t think there is still a moisture problem and have to breath it all in.

5) If you must “test the crawlspace air for mold” understand that the amount of mold in the air may be inversely linked to the humidity in the air. At any given time that there is “drying period”, like in a crawlspace that just got sealed, like every November when everyone gets sick, like when the dandelions blow… the conditions are getting “healthier” but the topical biologicals are trying to get air-born to spawn elsewhere. Most “mold tests” pump 75 liters of air through a canister with a “sticky spot” that goes under a microscope. Your analysis may say something like 7,000 mold spores per cubic meter of air on a humid vented crawlspace during the middle of the summer, and then you may seal it all up, install a dehumidification system, or do waterproofing… and 8 weeks later after your mold tester has crawled through the dusty crawlspace knocking everything down and picking everything up to start your follow up test after the work done, you may have solved the humidity problem, made the space much more inviting and clean, but get an analysis back that there are 77,000 mold spores to the cubic meter. You will know the crawlspace is ripe for vacuuming or “dry-scathing” alone (no chemicals) and you will remove a large majority maybe 98% of the growth. You could be mistaken to think the mold situation is worse just because the air-born mold was higher. Isn’t that weird? Even this simple inverse connection, air mold v surface mold is unknown to many.

5) A properly sealed crawlspace will dry it out in about 6-8 weeks in the summer starting from a pretty wet recent history of conditions.  Ideally an old nasty crawlspace would be sealed in February when it is bone dry to start with. Code requires a drying mechanism in sealed crawlspaces and there are several options. The best drying mechanism to have is a small adjustable supply air device, such as a flapper, from an existing air-handler and duct system and for the occupants to understand these lessons, and how to operate their HVAC in a way that promotes drying and and a dry crawlspace. If you let the temperature rise in your home and then you crank down the A/C 10 degrees, it will run for hours, processing lots of air, and get really cold, providing lots of de-humidification for several hours in a row. Some new Thermostats can drive down humidity simply by “increasing the float” from 3 degrees to 4 degrees because the coil gets colder and colder the longer it runs to an extent to which it is pulling gallons of water per hour rather than a few drops at a time.

Almost 100% of our clients are steadily in the NO RISK ZONE within 8 weeks of having a Bare-Minimum Sealed Crawlspace Installed.

When you convert from a vented crawlspace with HVAC systems “outside” the heated and cooled envelope to a sealed crawlspace with HVAC systems and everything else in the crawlspace “inside a favorable environment” there is likely a net energy savings for sealing the crawlspace and a direct gain on drying the space out. Often, because the energy lost by the existing duct work or air-handler is simply retained at a savings to keep the crawlspace dry and comfy. That is right; you might not need a drying mechanism at all. Yet, in this scenario, it could make sense to proactively replace old leaky duct work or equipment with tight radial flex ducting and have an “ADJUSTABLE DRYING MECHANISM” so you can fine-tune the crawlspace conditions and take the dirty old leaky ducts out.

SHS has installed over 500 Temperature and Relative Humidity Monitors in Crawlspaces and Attics across the state each also revealing the temperature and relative humidity of the location of the indoor base-station. It gives you 2 data points in 2 locations visible from one location. This Temp/RH information is critical for understanding the actual moisture situations we face. Our chart, which was adapted from helps immensely to analyze the moisture. The chart has abbreviations that are explained at

We recommend installation of a wireless monitor so that we can be sure the conditions in the crawlspace are favorable, and not worse, than the ambient conditions.

— We prefer a unit like this one shown below for many resons:

– The bezel can be labelled with a Sharpie. CT, CH, iT,  iH can be written on the light grey part to make it easy to see Crawlspace Humidity quickly. You can actually mark-out the word Outdoor on the back-lit display to make it perfectly not confusing unless it is actually outdoors. Sharpie is erasable with Sharpie.

– There are no screws for either battery compartment.

– They work great with basic alkaline batteries for up to 2 years. (5) AA batteries.

– Set-Up is easy and relatively automatic as the time and time-zone and DST is automatic.

– The display is back-lit when needed.

– Wall or Table or Shelf mount.

– Affordable.



In this crawlspace just one third of the drying potential is needed to keep the crawlspace well within the target zone.
In this crawlspace just one third of the drying potential is needed to keep the crawlspace well within the target zone.