Condensation Ket Art

Condensation, Dew Point, and Roofing

Print Friendly, PDF & Email

Prepared with Co-authors Thomas J. Taylor, PhD, and James Willits

Except in extremely arid climates, there is always some amount of water vapor in the air around us. When that air comes into contact with a cold surface, that water vapor condenses as a liquid onto the surface. A good example of this is the water droplets on the side of the glass of ice water. Those droplets are commonly known as “condensation” and are what results when the air gets too cold to hold the water vapor that is in it. Even when a cold surface is not available, if the air temperature suddenly drops, water vapor condenses out as mist or fog. Air can only hold so much water – more at higher temperatures and less at colder temperatures.

Let’s examine this in a little more detail, taking a closer look at…

Relative Humidity

We know that air contains water vapor but we need to define how much it contains. At any temperature, there is a maximum amount of water that air can hold. When we measure how much water is actually in the air, we express the number as a percentage of that maximum amount. For most people, 50 to 60% relative humidity is very comfortable, but most of us can easily tolerate anywhere from 30 to 70%. Relative humidity below 30% is noticeably dry and above 70% is when people start commenting about how humid it feels.

Let’s compare Miami and Phoenix to see how relative humidity comes into play. In Miami, a cold beverage may be served with a napkin wrapped around it to absorb the condensation that forms on the glass. But in Phoenix, there may be so little condensation on the cold glass that a napkin might not be needed. Why is that? Relative humidity is the major contributing factor. The reason is that the relative humidity in Miami is likely to be above 65%, i.e., the air is holding 65% of the moisture it is capable of holding. In contrast, the air in Phoenix would likely be dry with a relative humidity around 35%, causing very little condensation to form. So, to recap, relative humidity is a ratio of how much water vapor is in the air in relation to how much the air can contain at a given temperature. The “relative” part refers to the fact that air’s capacity to hold moisture changes with temperature. The warmer the air is, the greater amount of moisture it can hold. The more moisture it holds, the greater the volume of condensation forms on a cold surface. Now, let’s discuss dew point.

…air’s capacity to hold moisture changes with temperature.

Dew Point

The dew point is a specific temperature at a given humidity at which water vapor condenses. Let’s consider Miami and Phoenix again as two extremes. In the summer, Miami’s relative humidity can reach 85% at a temperature of 80°F. Obviously, a lot of condensation will form on a chilled beverage glass. But it actually does not take much of a drop in temperature to reach 100% relative humidity and have condensation form. So, a lot of cool surfaces will have condensation on them. At the same temperature in Phoenix (80°F), the relative humidity could be 35%. The temperature would have to be much lower before condensation could form. Cool surfaces would not have condensation on them.

The Dew Point is the temperature at which condensation forms. It is a function of the relative humidity and the ambient temperature. In other words, the amount of water vapor that is in the air and the temperature of the air. Take a look at the chart below (which is a very simplified form of what is actually used by HVAC engineers). Let’s pick the 40% relative humidity line in the first column, and follow that line across to the 70°F column. The 40% line and 70°F column intersect at 45°F, meaning that in an environment that is 70°F and 40% relative humidity (RH), water in the air will condense on a surface that is 45°F.

Dew Point Temperatures for Selected Air Temperature and Relative Humidity

Dew Point Temperature (°F)
Relative HumidityDesign Dry Bulb (Interior) Temperature (°F)
Chart adapted from ASHRAE Psychometric Chart, 1993 ASHRAE Handbook—Fundamentals.

So, what does that have to do with roofing? Well, consider your building envelope: It separates the interior conditioned environment from the outside. The foundation, walls, and roof are all systems that intersect to make this happen. Although this pertains in some respect to all of the systems, we will focus on roofing. The insulation layer in the roofing system resists heat loss or gain from the outside, depending on the season. Within the insulation layer, the temperature slowly changes until it reaches outside. Let’s talk about a building in the winter to illustrate the point. The interior is 70°F with 40% RH, like our example on the chart above. As you move through the insulation layer from the inside toward the outside, the temperature gradually drops until it reaches the colder temperature outside. The plotting of those temperatures is referred to as the temperature gradient of that system.

Now, if the temperature gets to 45°F at any point in that system (the dew point temperature on the chart), then water would be expected to condense on the nearest surface. This is shown in the following diagram:

To recap, the interior air has 40% of the total water vapor that it can support. But as the air migrates up through the roof system, it gets cooler until the point where it can no longer hold on to the water vapor and condensation occurs. In the example shown above, this would happen at 45°F and just inside the insulation layer.

Lessons for the Roof Designer

Condensation — which is liquid water — can negatively affect the building in many ways. It can lead to R-value loss of the insulation layer by displacing the air within the insulation with water, as well as premature degradation of any of the roofing system components, such as rotting wood or rusting metal (including structural components). It can also contribute to unwanted biological growth, such as mold.

However, prevention of these negative effects is possible. Remember that water vapor needs to get to a surface or location that is at or below the dew point temperature.

In the schematic of the roof assembly shown above, it is clear that interior air must be prevented from moving up into the roof as much as possible. This was discussed in detail in an earlier GAF blog. One method to limit air movement into a roof includes using two layers of overlapping foam insulation. Another method is to place a vapor retarder or air barrier on the warm side of the insulation. The vapor retarder/air barrier can prevent the water vapor from reaching the location where it can condense.

Also, penetrations for vents and other details that involve cutting holes through the insulation must be looked at closely. If the gaps around penetrations are not sealed adequately, then the interior air is able to rapidly move up through the roof system. In cold climates that may lead to significant amounts of condensation in and around those penetrations.

Additionally, the billowing effect of a mechanically attached roof can exacerbate the potential for condensation because more air is drawn into the roof system. An adhered roof membrane may help limit the potential for air movement and subsequent condensation.

One important thing to remember is that relative humidity, and interior and exterior temperatures in summer and winter, should be considered when designing the envelope.

Typically, commercial buildings have an environment designed by an HVAC engineer that will determine interior temperature and relative humidity taking occupant comfort into account, as well as a design exterior temperature based on the weather in the building’s location. These and other factors help engineers determine what type and size of equipment the building requires. The building envelope designer will use those values, plus the designed use of the building, and local codes to determine the construction of the building envelope. One important thing to remember is that relative humidity, and interior and exterior temperatures in summer and winter, should be considered when designing the envelope. An envelope design that works in one area of the country may not work in another part of the country which could lead to adverse conditions and the types of degradation mentioned earlier. Consider how your wardrobe would change if you moved from Minneapolis to Phoenix (here, we are relating your clothes to the building envelope).

In a perfect world, the location of the building would be the entire story. Unfortunately, building use can (and often does) change. Factors that can adversely affect temperature and humidity, and therefore hygrothermal performance of the envelope, can include: a dramatic change in the number of occupants, the addition of a kitchen or cooking equipment, the addition of a locker room workout area or shower, and sometimes even something that seems insignificant like an aquarium or stored wood for a fireplace. This is not meant to be an exhaustive list, but a few illustrative examples to communicate a general understanding. Believe it or not, even changing the color of the exterior components could contribute to greater or less solar gain and effectively change the dew point location within the building envelope. Changing the dew point and/or dew point location can lead to unwelcome condensation, and potentially result in damage.

Changing the dew point and/or dew point location can lead to unwelcome condensation, and potentially result in damage.

Consider a situation where an owner decides to invest in energy efficiency upgrades on their property while replacing the roof. The owner upgrades windows, doors, and weather-stripping at the same time. The building could have had latent moisture problems that were previously hidden by air leaks across the building envelope. After the retrofits, those issues may surface, for example, in the form of stained ceilings. Was the water damage caused by the retrofit? Most likely the answer would be no. The previous inefficient design disguised the problem.

Keep in mind that a holistic approach should be taken with building envelope design. If you change one part, it could negatively affect something else. This blog is for general information purposes only. It is always a good idea to consult a building envelope consultant to help prevent condensation issues and ensure that small changes do not become large problems.

There are 15 comments

Add yours
  1. Karim Forsat

    Hello James,

    This is a great article. I believe I have a small air leak in my vaulted ceiling that is causing condensation and water droplets. The amount is small and only in one location. How can I confirm this (thermal imaging)? What type of specialist can assist me? My roofing installation company has no clue. Hoe do I solve this issue?
    The house is 1 year old. Tile roof, 20x30x20 living room. Tongue and Groove ceiling. Condensation happen after 5 PM on hot and humid days in Southern California. Portable dehumidification has not helped as the humidity is up in the cavity of the room.


    • James R. Kirby, AIA

      Hi Karim,

      It seems that warm, moist air is coming in contact with a surface that is below the dew point. Perhaps there is pipe/conduit/etc that is cold. Stopping the air is one avenue to prevent the condensation, but given this is one location, it seems that warming up the cold surface has merit. Wrapping a pipe with insulation may be a solution. The use of a thermal imaging camera should help locate the cold location(s) and may help lead you to the cold surface. Does the condensation occur downslope of or line up with a rooftop penetration? Smoke guns can also be used to track the path of airflow. These types of scenarios can be tricky, so it’s important to find a roofing consultant who is well versed in this type of problem who can also provide a proper solution. The current president of the SoCal IIBEC Chapter, Kyle Eazor, should be able to assist with a local roofing consultant. Kyle can be reached at 949-315-0211. I’m sorry you have an issue, but there are solutions! Best of luck. Jim

  2. Bill Thomas

    Yes, there is insulation. We are an architectural and engineering firm. We designed the space and the roof system is one we use often. The interior is open to roof deck. There are a few office areas with drop ceilings but for the most part it is all an open concept space. Roughly 9,000 sq ft. Thanks for your reply. We are looking for a local roofing consultant.

    • Thomas J Taylor, PhD

      Bill – thanks. It is possible that condensation occurs even in well designed roof assemblies with dew points well below the membrane. Some situations that come to mind include using single layers of insulation, where the gaps between boards allow air flow up and against the membrane. Similarly, penetrations that haven’t been sealed between the pipe (for example) and the insulation allow air flow upwards. We recommend always insisting that two layers of polyiso be used, with staggered joints and that gaps around penetrations be filled with expanding foam. Of course, as I’m sure you know, further downstream in the process value engineering gets in the way all too often. Fixing such issues after the event can be very difficult. We’ll reach out to you with some specifics.

  3. Bill Thomas

    Is it possible for a roof to develop condensation between the membrane and top of metal deck? We have a new roof system that is dripping water all over. Mostly at the center as the roof slopes to the center of the building. There is no condensation forming on the “underside” of deck. We have had two independent contractors assure us there are no leaks from the outside due to bad seams or penetrations. The humidity in the building space is very high. many times above 70%. The outside humidity is also high and hot. We moved into the building in February. (Indiana) we did not start seeing leaks until sometime around May. Could this be moisture that was trapped during construction working it’s way out? How likely is it that this is condensation? Any help would be greatly appreciated.

    • Thomas J Taylor, PhD

      Bill – great question that’s not as unusual as we would like! Yes, you could be getting condensation under the membrane. Can you confirm that there’s insulation between the deck and the membrane? It is difficult to diagnose roofing issues via correspondence and we strongly advise getting a roof consultant involved in problematic cases. Our advice is generic and for specific situations eyes are needed to really investigate. But, having said that, let us know about the insulation and also what kind of use the building sees.

  4. David Epps

    Would a skating rink have the inversed effect…warm air above the roof system trying to drive to the cold inside the building? And does adhered TPO roof serve as the vapor barrier above the insulation?

    • James R. Kirby, AIA

      Hi David. That’s a great question, and you are thinking about this correctly. A building with a cold interior–a cold storage building, for example–means the moisture drive is from the warmer exterior to the colder interior the majority of the time. So, yes, when the moisture drive is from exterior to interior, the roof membrane serves as the vapor retarder above the insulation. We wrote a blog about this: And the very last sentence in the blog links you the GAF’s “Guide to Cold Storage Roof System Design.”
      Thanks for reading!

  5. Hannan Ahmad

    Weather has a great effect on buildings especially the roof area and that’s why it is important to build roofs while keeping these things in mind. A roof build with weather-resistant material can save you from repair costs in coming years.

    • James R. Kirby, AIA

      Thank you, Dwayne! I’m glad you’re reading our blogs and find them informative. Feel free to share them with your colleagues! Jim

  6. Ron Pickle

    Hi James,

    Never before any body explained the atmospheric condensation so well and the effect it has on different surfaces specially roof and the damage this moisture can lead to. Just brilliant! Many thanks for this eye opener!

    • James R. Kirby, AIA

      Thank you, Ron! It truly was a collaborative article with our Building and Roofing Science Team. It’s great to know we’re getting the message out. Look for more blogs about roofing science in the near future. Thank you! Jim

Post a new comment