We hear it often: “It is so dry here, why do we have so many mold problems?” This article will explain to you in detail all of the typical places and causes of fungal colonization in buildings. The following information is applicable to cold climates so let’s begin with some general knowledge of how moisture and mold problems happen. We are not going to touch on obvious things like leaks but we will focus on failures due to building construction and use.
You are right, it is really dry in Colorado. Some of the driest air you can find is high mountain winter air (which is a useful thing to know if you ever have a water damage). But, it is not outdoor humidity that is the problem, it is outdoor temperature combined with indoor humidity that is the cause of almost all environmental mold problems in buildings. To understand this we are going to delve briefly into the psychrometric chart. The psychrometric chart is a graphical representation of the relation between moist air and temperature. Let’s look at an arbitrary indoor air condition in a typical mountain town such as Telluride or Aspen; 70 degrees Fahrenheit and 30% relative humidity; comfy. The chart tells us this: there are 43 grains of moisture per pound of dry air and the dew point is 37 degrees (you can also use a psychrometric calculator). The dewpoint is the temperature at which water in the air condenses or “forms dew”.
So what have we learned? Well, that is why your glasses fog over when you come inside after shoveling snow because they are colder than the dewpoint temperature. So it gets you thinking: “Hmmm, what else inside my house is colder than the dewpoint temperature?” Good question; but before we answer that we need to know the actual dewpoint temperature based upon temperature and R.h. in your house. In our experience, indoor dewpoint conditions commonly vary from somewhere around 20 deg. to as high as 55 deg. Usually, the first indication of a dewpoint issue is when your windows “sweat” in the winter.
Here are few things in or on your house, during the winter, that is likely colder than the dewpoint temperature: every exterior component including the exterior walls, windows and roof structure–yes, pretty much the exterior of the whole building during most of the winter. And guess what, those places are the most common places we find cold weather induced fungal growth. Don’t worry, as we get into specific examples of fungal growth in Part 2, we’ll include a description of the cause and a prescription for the solution.
So we have deduced that it’s the cold climate combined with internally generated moisture vapor (humidity) that is the problem. However, a properly constructed building does not have issues with mold, in any climate. You can even build an indoor swimming pool in a ski town if you do it right (trust me, it doesn’t always work, but it is done). So if humidity combined with cold temperatures is a problem, what about the summer? Well, we are not going to have as many cold surface issues, that is for sure (although, it can be pretty cool at night at 10000 feet or during the spring and fall down low).
So we have to delve into the process that makes mold grow. We have isolated one factor; moisture. The other main factor is food and temperature. Buildings are good mold food. There seems to be a species for every material and substrate. There is not much elaboration needed here, other than to say if conditions are right, mold will grow just about everywhere and on everything in a building.
Temperature is also very important. To understand the relationship between humidity and temperature and their effect on mold growth we need to look at a diagram called a “mold isopleth”. The isopleth is a graph of mold growth rate relative to combinations of moisture content (actually water activity) vs. temperature. Incidentally, the plot of an isopleth for each species of mold looks remarkably like a topographic map of a mountain. What we see with these graphs is that ideal growth conditions vary from fungus to fungus but typically are from around 0.7 water activity to 1.0. Mind you, there are other variables such as the composition of the substrate.
So bear with me, we have one last little complexity we need to discuss before you become an expert: “water activity.” Water activity essentially describes the relative humidity of a material. Building materials are generally hygroscopic. They absorb and adsorb moisture (think about how the gaps in your wood floors change seasonally). At any given relative humidity, under stable conditions, each material has a certain unique water activity. When water activity is high enough, mold can grow. These factors can be indirectly inferred from material moisture content. Water activity is related to moisture content but each material has its own relation. For instance, at a water activity of 0.8, drywall with have a moisture content of 0.7% and pine will have a moisture content of 17%. In each case, that is just enough for mold to begin to grow. The important takeaway here is that a material does not need to be palpably wet to grow mold, just wet enough, and that varies from material to material and is related to atmospheric humidity (R.H.). That means that under conditions of high humidity, the water activity can rise enough for mold to grow. Interestingly, measuring the moisture content of a material is the best way to determine average atmospheric humidity if you understand the water activity relationships of the particular material.
Well enough of that– what are the practical implications? Essentially, whether it is summer or winter, we can find conditions in a building where materials adsorb moisture, it’s just that they are bit more pronounced in the winter. In the following part, we will discuss particular examples.
Part 2, Examples of Mold Growth
Our discussion of moisture and mold problems is going to begin at the bottom of the building. Why? Because as we progress through these examples you will see how “what happens in the crawlspace or basement” affects everything else in the building. So let’s start our consideration with a glass jar. Take a glass jar and put some water in it… I’ll wait. Now insert a hygrometer (relative humidity meter) and put a cap on the jar… wait. After a while read the hygrometer and it will show 100% R.h. or very near thereto. Now punch a few holes in the lid and watch the humidity drop as the moist air inside the jar exchanges with the drier air outside. Now take some hot liquid paraffin and pour it into the jar so that it completely covers the water, let it harden and re-install the cap. The humidity drops some more, very close to the humidity outside the jar. That is exactly how a building works.
Example 1: Our first example concerns fungal growth on the crawlspace rim board. In our experience, fungal growth on rim boards is the most common type of growth in cold climate buildings. The rim board is a board that sits vertically on the foundation stem wall and is oriented parallel thereto. Its purpose is to stabilize the ends of the floor joists, transfer the load to the foundation and to enclose the joist ends so you don’t have big openings around the base of your house. The reason why rim boards fail so commonly is that of high humidity in the crawlspace and cold temperatures on the rim board. Often they can get so bad that wood rot begins to decay the rim board (wood rot requires persistent high water activity).
The mold growth will usually be the worst on the north side of the building because it is usually colder there due to the lack of solar gain. It doesn’t take a lot of humidity to create this problem as the moist air interacts with the cold rim board to raise the moisture content of the wood. The solution to this problem is discussed here: https://plateauenv.com/crawlspaces-what-you-need-to-know/ .
Example 2, Crawlspace; As humidity levels in the crawlspace rise, say towards 75% or so, things change a little. We will see the worsening of the problem from example 1. But now we are approaching a relative humidity that is sufficient to raise the moisture content of wood, even in the absence of cold temperatures. Building products constructed of wood will see a moisture content exceeding 15%. This is the danger zone. Xerophilic (capable of growing at low water activity) molds begin to grow. They first colonize wood columns and the bottoms of floor joists. If conditions are bad enough, the sides of the joists eventually become colonized.
Example 3, Crawlspace; We occasionally see isolated mold growth on the crawlspace side of the subfloor. However, it is rarely due to the conditions described in examples 1 and 2 (though it can be). This is because the subfloor, being in association with the conditioned indoor air is usually very close to normal temperature and thus will not reach the moisture level found in other crawlspace components. Further, hygroscopic subfloor materials not only absorb moisture but they release it on the warm side as well, essentially drying out towards the interior of the building. But there is a unique situation that often causes widespread fungal growth and it happens only during the construction phase. During construction, the interior of the building is not heated. In the summer months this is may not be a problem, but as construction heads into the cold season, things change. And this happens a lot as the framing and finishing stage of building construction often extends into late fall and through the winter. The building may never be heated until nearly complete, so the subfloor stays cold. The elevated humidity in the crawlspace interacts with the cold subfloor; well by now you know the rest of the story. This condition is often exacerbated by uncontrolled precipitation, prior to installation of the roof and windows (dry-in). In that case, we may see heavier growth near window openings. The solution? Crawlspace moisture control installations must be implemented immediately upon construction of the floor system and if necessary, supplemental drying may be needed where precipitation is intense (Crested Butte, Telluride, etc.).
Example 4, Windows; Okay, we are out of the crawlspace, which is good because those places are full of mold, spiders, and rodents. We are now sitting safely within the conditioned confines of our house; what could go wrong? Answer: humidity. You knew I was going to say that. The humidity in the living area is a combination of that diffusing or leaking through the subfloor from the crawlspace or basement + moisture from breathing, cooking, bathing, growing marijuana (don’t laugh, we have inspected several grow operations) and whatever else you do to generate humidity like using a humidifier (careful with this one). The first sign of excessive humidity will be condensation on windows during the winter. If it is not due to bad windows, which it often is, this is a tell-tale that all is not right. If it is only in places like the bedroom, bathroom or kitchen, (humidity generating areas) then you may have caught in time. The key here is to control your humidity sources. First, make sure you have properly addressed your crawlspace problems. Moist air can travel from the crawlspace by air leakage, duct leakage, and diffusion. Next, use your bathroom and kitchen exhaust fans. Leave the bath fan on for about 15 minutes after showering. Make sure your clothes dryer exhausts outside. Turn down the humidifier and reduce occupancy loads. Incidentally, one of the leading causes of mold in mountain homes is occupant overload. It seems that the ski-bums like to pack as many as they can into space so they can save rent money for PBR and lift tickets.
We are still in the living area; So where else does mold grow if we don’t adequately control interior humidity?; Within the wall and ceiling cavities and along the upper story rim boards of multistory buildings. Well, how does the humidity get inside the walls? Two mechanisms: diffusion and exfiltration.
Diffusion is the penetration of your wall systems by moisture vapor, driven by a difference in water vapor pressure. Moisture moves from areas of higher vapor pressure to lower vapor pressure. Again, think about how wood responds to changing humidity by expanding or contracting. Exfiltration is the transport of moisture into your wall by a difference in air pressure. This happens as interior air is forced through cracks and penetrations in your wall system by positive air pressure in the building relative to the outdoors. Of the two, exfiltration is far worse.
Example 5 Diffusion; Diffusion failures are rare. They only happen under ideal conditions. Typical causes are usually a missing vapor retarder, low permeance of exterior building components or very elevated humidity. The truth is, even with a vapor retarder moisture vapor moves into your wall system in the winter, and then back towards the indoors at other times. Usually, it all balances out, but sometimes there is not enough time to dry out during the summer and the moisture keeps building, year by year. Keep in mind that water vapor acts like a gas and has a pressure of its own that is dependent upon temperature. The small water molecules will diffuse readily through most building materials and the rate of diffusion increases with temperature and the number of molecules of water in the air (aka, absolute humidity). And, like all gasses, there is movement from areas of high pressure to areas of low pressure. In a building, this means that warm, moist air will move towards cooler or drier areas. To compensate for this phenomenon, buildings often incorporate vapor retarders. Usually, this is a plastic film or paint film that drastically reduces the diffusion of water molecules through the building. In a building, the location for the highest diffusion rate is the north side.
The diffusion phenomenon can also be seen in your siding. As moist air diffuses from the indoors, it raises the moisture content on the back side of your siding. The result: cupping. Fibrous sidings will expand and contract, sometimes causing very visible bowing. In some cases, if you investigate the interior of the wall, you may see widespread fungal growth.
Example 6, Exfiltration; This phenomenon is much more common. Air movement around penetrations and cracks allows the moisture-laden air to impinge directly upon the insides of the exterior wall surfaces. The result will be fungal colonization around near switch plates or other penetrations.
Example 7, Second Story Rim Joist; This case is a rather unusual circumstance that may be a combination of exfiltration and diffusion. As we have alluded, it is important to have vapor retarding features in your wall system (among other things). The second story rim joist is often ignored as the first and second-floor wall systems receive whatever vapor retarding feature is required. The result is moisture accumulation and eventual fungal colonization. You may even notice that the rim joist develops a pronounced cup, which can displace the siding.
Let’s move to the roof /attic system, second only to the crawlspace as the location of most building moisture and mold problems. The roof system on your home is probably the most technologically complex aspect of your building. It must control the vagaries of weather while ensuring a stable indoor environment and still be designed to endure internally generated moisture and heat loss. What happens when roof systems fail? Aside from leakage, we may see mold growth anywhere within the roof but most commonly at the eaves and ridge. We may see the failure of the truss plates as seasonal moisture slowly dislodges them. Excessive variations in moisture content will cause the trusses to bow, dislodging roofing fasteners and cracking drywall.
There are two basic types of roofs installed today; ventilated and unventilated. The unventilated roof system is commonly used where ventilation is not possible. Ventilated roofs do not always work so well where heavy snow can block ridge vents, with cathedral type ceilings, on low slope roofs or for a number of other factors. Even with the best of modern technology, unventilated assemblies still have as many problems as ventilated ones. It is building specific. Oh yes, I forgot. Everyone thinks that spray-applied foam insulation is a cure-all for roofing problems…not. Sometimes it is the only solution but it has its own pitfalls.
Example 8, Ventilated Roofs; Ventilated roof assemblies need good cross flow ventilation, throughout the attic space. This will include ventilation openings at the eave (low) and openings up high at the roof ridge or gable end. Today’s more complex roof systems can sometimes make it difficult to achieve good cross flow ventilation. When the eave vents are properly installed but the ridge and gable end vents are not adequate we will see mold growth near the eave. We may see similar results if the eave area is packed tight with insulation. In each case, moist air that has entered that attic in this location ends up affecting the roof sheathing or trusses in a localized manner, since the humidity cannot be carried away by ventilation.
When the ventilation at the eave is blocked but is adequate at the ridge, we may see mold growth at the ridge as the humid air interacts with the cold outdoor air (interestingly, we see a similar occurrence in inadequately ventilated crawlspaces, where the first place mold grows is near the vents). When the ventilation is terrible all around and the humidity in the home is high, we will see mold pretty much everywhere.
Here are some good hints for building owners. To evaluate your own ventilation visually inspect your attic looking the following signs:
Mold: As described, at either the ridge or eave, move insulation at the eave as necessary to view.
Nails: Look closely at the tips of nails or screws that have penetrated the sheathing. Are they corroded or rusty? Moist air has been condensing on them if so, suggesting your ventilation is marginal. Look at the insulation directly under them, are there marks from water droplets?
Truss Plates: Do the truss plates look corroded or worse; have they begun separating from the wood?
Soffits: Look carefully at your soffits, especially on the north side. Are there signs of moisture staining or damage, warping, cracking, etc. that appear worse on the north side?
Roof Surface: Look for nails or screws that have backed out– usually a sign of perennial expansion and contraction from moisture.
Snow Cover: A great indication of a solid roof system is un-melted snow cover. If your snow is melting when no one else’s is, it may be a sign of poor ventilation or excessive heat loss with commensurate moisture impact.
Example 9, Unventilated Roofs: Here we are specifically referring to roofs that were intended to be unventilated. The most common way to properly accomplish this these days is with the installation of a high level of insulation that also incorporates a class II vapor retarder and is air impermeable (aka, spray foam). There are some interesting variations in roof design that include above-deck insulation but are more commonly found in commercial buildings. A discussion of attic insulation configurations is contained in paragraph R806.5 of the 2012 International Residential Code. The choice is yours but beware; the common approach to this type of roof system has a few pitfalls. First, roof leaks may go unnoticed until the roof has rotted out. Polyurethane spray foams are field mixed and applied, thus there are some variations in quality that can lead to “out of specification” insulation. This may cause a reduction in R-value, delamination, permeability, and off-gassing. So be careful. If the spray foam is out of spec. water vapor may diffuse through or around the failed foam.
That about covers it. Remember, that all buildings experience perennial moisture variations due to temperature and humidity. That is normal. But just about any building can experience humidity failures of some sort. However, there are always extenuating circumstances related to how the building is used and to how it is constructed. All of the failures described herein are related to humidity in a building and I would go so far as to say that most of the time there is an unusual or excessive source in the building. Even poorly constructed buildings may not have a problem with humidity failures if the humidity is controlled. But even the best building can be subject to moisture damage from the causes described herein if sufficient humidity is present in the building.