Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Moisture, Psychrometrics and Relative Humidity - Their Effect on Structure and Air Quality

Jeff Aderholdt - Wednesday, August 03, 2011

Earlier, I briefly addressed the importance of moisture and humidity issues in a home on the Why Moisture Since this is such an important issue for both structure health and for the health of the those living in the home, it seemed appropriate to expand on the subject. There is a lot of information available from a variety of sources, a lot of it is very technical and difficult to understand. What I will attempt to do is to filter this data into something a little easier to understand and hopefully more useful.

Relative Humidity

Humidity is usually designated in percentage of relative humidity. The percentage is in relation to, or relative to, the total amount of moisture air can hold. 100% relative humidity, or 100% RH, means that the air is holding absolutely as much moisture as it can hold, it is completely saturated.Humidity is also relative to the temperature of the air. Hot air can hold a lot of moisture, where as cold air can hold very little. The measuring of these differences is called “psychrometrics”. I have included a simplified psychrometrics chart that shows the relationship of moisture content and temperature, and the resulting percentage of relative humidity. (This chart is simplified because other factors such as air pressure and elevation have a limited effect on the calculations).

Insulation Psychrometric Chart Moisture and Humidity

The vertical lines designate the temperature. This chart shows temperatures from 20° F to 100° F. The horizontal lines designate the actual moisture content of the air. The curved lines show percentage of relative humidity, % RH. With this chart, we can predict how air will respond to changes.

For example, let's say that the indoor air temperature of our home is 70° F with 40% RH. We find the 70° F vertical line and follow it up until it intersects with the 40% RH curved line. The second chart shows this with the red line.

Insulation Psychrometric Chart  Moisture and Humidity

From the point of intersection, follow the horizontal line until it intersects with the 100% RH curved line. Going vertical back down to the temperature scale, we find that the temperature is 44° F. This is a significant temperature. If we cool the 70° F air with 40% RH to 44° F, the relative humidity will reach 100%. This is call the “Dew Point”. Dew point is the temperature where air can hold no more moisture. It is also the temperature where condensation happens. We can now predict what out 70° F/40 RH air will do if it comes in contact with a 44° F, or colder surface. Condense! To prevent condensation we need to either prevent the air from coming in contact with a surface at or below the dew point, or lower the relative humidity. First we need to determine what relative humidity we want our indoor air to be.

Indoor Humidity and Air Quality

There are a number of different factors that can have an adverse effect on indoor air quality. Carbon monoxide, high carbon dioxide levels, low oxygen levels, chemical fumes, VOCs, etc. all can have a negative effect on indoor air quality. Not to be ignored is humidity. Air that is too dry creates some problems, and so does air that is too humid. A number of studies have identified that the optimal humidity range for indoor air is between 40% RH and 60% RH.

Optimal Relative Humidity for Health

Depending on climate and other considerations, we may not be able to stay in that range all the time, but the more time we can keep inside this target zone, the better indoor air quality we will have.

Indoor Humidity and The Structure

Flash and Batt Insulation Moisture RiskAs has already been mentioned, to prevent condensation we need to either lower the relative humidity of the air, or prevent the air from coming into contact with a surface at or below the dew point. If we are trying to maintain our indoor relative humidity at a minimum 40% RH for as much of the time as we can in a cold climate, we need to be extra vigilant in plugging and sealing holes from the inside of the house into the structure. Earlier I wrote about the risks of “Flash and Batt”  in the article Flash and Batt - Risky in Cold Climates. This is the reason why! The air seal is in the wrong place. The air seal needs to be at the point of air access. The air needs to be stopped before it can get into the walls.

Moisture in the exterior walls and other places in the structure can have a lot of negative effects. First, if we have enough leaks, it may be difficult to maintain our indoor humidity without adding a humidifier. This adds to the operation cost. Next, moisture in a cool dark place makes it an ideal breeding ground for mold and mildew. Remember, if air can leak from the inside into the structure, it can leak back inside too, carrying with it the smells and spores of what is growing in the walls. Third, wood that is damp is more given to deterioration and decomposition. Wet wood is also a favorite of termites and carpenter ants. Fourth, wood that is damp has a higher relative humidity. Wood with a higher relative humidity expands and swells. This can create a lot of possibilities. Let's say that we have foam sprayed between the studs of a wall cavity. As the seasons change, the studs repeatedly expand and contract, pushing against the foam and then retreating. What will happen over time? There is a definite possibility that the foam will start to separate from the studs creating cracks that allow more moisture in to the cracks, moisture that can do more damage. How much better to prevent the moisture from getting into the structure in the first place!Maintaining our indoor relative humidity at a constant level also has benefits to the structure. Materials like wood and drywall expand and contract less. The result? Wooden doors that work consistently without binding because they are not damp and drywall that is less likely to crack.

Best Practice – Controlling Humidity

So, how does one build a house to properly manage indoor air quality and moisture? It comes down to two basic principles. First, make the structure as air tight as possible, putting the air tight barrier at the right place. In cold climates, this means sealing as close to the inside of the walls and structure as possible. Second, mechanically control your indoor humidity. In the summer, air conditioning usually will take care of this. In the winter, a ventilation system, such as a heat recovery ventilation system, or even a central exhaust system, controlled by a humidity sensor with make-up air will do just fine.

At NTC, we specialize in helping you have a home that is not only energy efficient and cost effective, but also in making sure that you end up with a healthy home. We can help your home to be healthy in structure, and we can help you to keep the people living in the structure healthy too. Call us and let us put our expertise and experience to work for you.

Anonymous commented on 28-Nov-2012 06:10 AM
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