# Humidity

Originally Posted By: tgettier
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What if any is the maximum amount of allowable relative humidity in a crawl space or basement?

Originally Posted By: Caoimh?n P. Connell
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Hello Mr. Gettier ?

Regarding your humidity question. You first have to ask ?Why do I want to control relative humidity?? If the answer is to meet local building code, then that becomes the de facto maximum allowable %RH (all reason notwithstanding).

However, if you want to control %RH for, say mould growth, then that is a very complex issue. There is no correlation between relative humidity (%RH) and mold growth outside the context of dew point temperatures and especially dew point temperatures on surfaces

In mycological terms, moisture is described by ?water activity,? (aW). Water activity is expressed as the ratio of the vapour pressure of the water in the air in equilibrium with a substance divided by the vapour pressure at the same temperature of pure water. The aW values range from 1.0 (for free standing water) to 0.7 for hard candy and dried cereals and fruits.

The percent relative humidity is the ratio of the partial pressure of water present in an air mass to the partial pressure of water at saturation for the same temperature and pressure. Therefore, although a physical relationship exists between the relative humidity and the aW, it is very complex and not particularly easy to use in everyday life. This is because the aw will be different for different substrates even if the %RH and the temperatures are exactly the same.

Let?s look at a simple example of just the temperature and moisture complexities - Posit a summer day in Colorado with a relative humidity of only 35% and an outdoor temperature of 98?F, the outdoor air will contain approximately 16 grams of water per cubic meter of air (16 g/m3). Crawlspace vents are closed and %RH in the crawlspace is also 35% relative humidity. However, the temperature in the crawler is 65?F and, therefore, the air in the crawler only contains about 5.6 g/m3.

The minimum temperature needed to precipitate the water from the outdoor air is only 66?F (the dew-point). That means that if the air temperature or surface temperatures in the crawlspace are less than or equal to 66?F, and outdoor air enters, the %RH in the crawlspace will skyrocket to 100% RH and the moisture from the outdoor air drawn into the crawlspace will precipitate onto those surfaces (even though the actual moisture content in the air has not changed). The condensed moisture may provide a water source for mold to grow. If the effect is prolonged, and wood may remain damp, and the probability is that molds will proliferate. In this way, fans meant to ?dehumidify? crawlspaces actually defeating their own purposes by increasing the potential humidity in the crawlspace.

OK .. now same house? Outdoor temperature is 10?F and 35% relative humidity, therefore the air outside contains about 1.9 g/m3. The crawler is 37? and also 35% relative humidity (the air contains 2.1 g/m3). What happens when outside air is drawn into the crawler? %RH drops, because the not only does the incoming air contain less moisture, as the air mass is warmed the %RH does down.

The point of all this is that one CANNOT discuss %RH in the absence of 1) Temperature 2) Ventilation considerations 3) Surrounding structures 4) context (comfort, bugs, regulations, etc).

Now, weather is weird - at the moment, on this fine Colorado July afternoon, in Denver it is 104?F, but where I am in Colorado, I am sitting at my desk looking at ? SNOW! That?s right, snow. Furthermore, the relative humidity will change with altitude, so even if you specify a particular %RH, at what elevation is that value good for?

More simplictically, overall, what we see is that very high relative humidities (in excess of 90% sustained) are required to achieve a sufficient aW to support microbial growth of xerophilic/xerotolerant fungi at room temperatures at 2,000 meters elevation. Some of my work indicates that in the absence of dew point considerations on surfaces, mould growth will not occur on inoculated wooden surfaces even at sustained relative humidities of 90%

I hope that clears things up a bit. I know it was good for me.

Cheers,
Caoimh?n P. Connell
Forensic Industrial Hygienist
www.forensic-applications.com

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG

Originally Posted By: tgettier
This post was automatically imported from our archived forum.

I now see why it took so long to get a response. Since it seems that either you know what you are talking about or you know enough to make it look like you know what you are talking about. Hear is my reason for asking I recently inspected a crawl that “you could feel the moister in the air” and there was a fair amount of dry rot on the joist and girder. I was hoping that there was a device that I could use to check the amount of moisture in the crawl. Now you have opened a whole new set of questions for me. From what you said (and what I understood) it is possible to raise the amount of humidity (moisture) in the crawl using one of the humidity controlled fans. "Or any fan increasing the air flow in the crawl? I do not do MOLD as of now I am trying to learn what is involved in mold testing. With the said I did not see any mold in this crawl but there was a big moisture issue that the new owner needs to address to prevent further damage. There is a moisture barrier on the dirt floor. What would you unofficially suggest to do to lower the moisture in the air if indeed installing a fan could raise the level?

Originally Posted By: Caoimh?n P. Connell
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Hi Patrick-

A couple of things ? first it is impossible to speak in anything but generalities not having see the house.

Next, from a fungal perspective, there are a couple of notable ?rots.? One of the more common organisms associated with ?dry-rot? is the Serpula lacrymans. This organism is so names since it can flourish on otherwise dry wood, transporting the necessary moisture dozens of feet to where it needs it. Therefore, often, it will be growing in otherwise dry areas (thus the name), and the relative humidity in the crawler won?t really have much effect. Two other potential dry-rotters (or associates) are the genera Inonotus, and Phellinus.

Brown rots, and white rots are other types of rot one encounters, and the organisms for these maladies can vary, but both types can compromise the integrity of the wood. In each of these cases, the organism usually does require immediate moisture.

You certainly can check the %RH in the crawlspace, but without appropriate experience, you may not have much luck in interpreting the result ? for example in one case, 80% RH may not be particularly high and in another case 50% may indicate a serious moisture problem. Conditions, conditions, conditions.

Next: Each structure has the ability to squirrel away moisture to some degree. Once that capacity is exceeded the water becomes available. The ability to hide that water is known as the structure?s ?hygric buffer.? When houses were built of stone and masonry, the water loading within an house (the hygric buffer) was very large (on the order of say, 500 gallons for a 1,500 square foot house). Then, as houses became built of dimensional lumber, the water capacity of the assembly decreased by perhaps an order of magnitude. That is, the moisture was still in the environment of the structure, but now 450 gallons of that moisture had no place to go. Then, as houses were built out of drywall, steel studs, steel joists, OSB, particle board and hard board, the water capacity of the assembly fell by another order of magnitude (by some estimates). Where is that 495 gallons of excess moisture going to go? Answer: It is still there, but now because of the concept known as ?available water,? I mentioned earlier, the structure is at saturation and mould and other macrofungi grows on any available food source (such as your joists).

So? ironically, it is entirely possible that the poly vapour barrier (moisture barrier) on the earthen floor that you mentioned is in fact creating the problem? moisture barrier indeed! Moisture barriers can stop moisture from leaving a structure as well as entering a structure. Indiscriminate use of a moisture barrier will consistently degrease the hygric buffer of the structure; judicial use, however, such as when the moisture is entering via the earthen floor, may make its application prudent.

Unless I saw the house and understood the dynamics, I really couldn?t make much of a suggestion. Now, if you REALLY want to find out? My billing rate is a mere \$95/hour portal to portal (time and materials plus 15%), and I would love to see beautiful Bedford, VA. Just send me the plane ticket.

Having said that, here is the easy answer: 1) determine where the moisture is coming from, 2) give it someplace to go, or stop it from coming in.

Cheers!
Caoimh?n P. Connell

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG

Originally Posted By: Caoimh?n P. Connell
This post was automatically imported from our archived forum.

Oops… sorry… you’re not Patrick. Patrick was the other guy with the mould question at:

http://www.nachi.org/bbsystem/viewtopic.php?t=14142

Sorry!
Caoimh?n

Originally Posted By: tgettier
This post was automatically imported from our archived forum.

WOW thanks I definitely learned something today. I wish I could bring you out but that?s not feasible I do thank you for your detailed post.