Preventing Condensation by Calculating Temperature Gradients

By GAF Roof Views 04-10-2018
Tag Icon

A vapor retarder's primary function is to minimize or reduce water vapor diffusion into a low-slope roof assembly and help prevent the formation of condensation. Once a designer determines a vapor retarder is to be used, the objective is to make sure the temperature at the vapor retarder membrane is always warmer than dew-point temperature.

To ensure the temperature at the vapor retarder remains higher than the dew-point temperature, a sufficient amount of roof insulation must be installed above the vapor retarder to maintain the vapor retarder at a warm enough temperature. So what can a designer do to determine this?

Vapor drive occurs in two directions. When it is colder outside than inside (e.g., winter), the warm interior air moves towards the exterior. When it is warmer outside (e.g., summer), the warm exterior air moves towards the interior. When the outside temperature is warmer than the interior temperature, the roof membrane acts as a vapor retarder. Where there are more heating degree days than cooling degree days, vapor retarders are installed on the interior side of the thermal insulation layer.

Calculating the temperature gradient through a roof assembly is not as difficult as it would seem.

During winter, warm interior air wants to move to the exterior of the building. In doing so, the warm interior air will cool as it passes through a roof assembly. The rate at which the air cools depends on the building material because each material has its own distinctive thermal-resistance property. This heat loss, or temperature gradient, can be illustrated on a roof assembly diagram. A designer can use this method to corroborate whether a vapor retarder will be effective in a roof assembly and where it should be located. Calculating the temperature gradient through a roof assembly is not as difficult as it would seem. The following step-by-step procedure will show you how to do this graphical analysis.

For example

Let's consider a natatorium (aka a building with a swimming pool inside) located in Minneapolis, MN. The proposed roof assembly is a TPO roof membrane, ½" thick highdensity polyisocyanurate insulation cover board, two layers of 2½" thick polyisocyanurate insulation, two-ply felt and asphalt membrane vapor retarder, ½" thick gypsum board and a metal deck.

Step 1: Determine the winter interior design dry bulb temperature. This design value is essentially the temperature that the building's interior will be set at during the winter months. This information typically can be obtained from the HVAC system designer. However, if it's an existing building, the building maintenance engineer is a possible resource. For this example, let's use 80°F for the winter interior design dry bulb temperature.

Step 2: Obtain the winter exterior design dry bulb temperature. Design values can be found in Chapter 14—Climatic Design Information of the 2017 ASHRAE Handbook—Fundamentals; or in the Appendix of The NRCA Roofing Manual: Architectural Metal Flashing and Condensation and Air Leakage Control—2018. For Minneapolis, the winter exterior design dry bulb temperature is -16°F.

Step 3: Draw the roof assembly to a practical scale. A scale of 3 inches=1 foot works well and typically fits on a standard 8.5×11 size sheet. Underneath the roof assembly, draw a temperature scale below the roof assembly. The starting temperature, i.e., the left side of the scale, will be the winter interior dry bulb design temperature. The scale will end with the winter exterior design dry bulb temperature. Then, place evenly spaced vertical lines between these two lines to represent 5- and/or 10-degree increments. See below.

draw roof assembly to scale

Step 4: Determine the roof assembly's overall R-value. Thermal properties of roofing materials can be obtained from manufacturers' product literature. Other possible sources include: Chapter 26—Thermal Transmission Data of the 2017 ASHRAE Handbook—Fundamentals; or the Appendix of The NRCA Roofing Manual: Architectural Metal Flashing and Condensation and Air Leakage Control—2018.

Below are the R-values for the proposed roof assembly.

materials and r value

*Note that an "air film" exists on both the inside and outside of a roof assembly. Air films actually have an R-value and they contribute to a roof assembly's overall R-value. Also, there are different R-values for interior and exterior air films for winter and summer.

Step 5: Calculate the amount of heat loss occurring at the top surface of each material/layer. The temperature at the top of each material is lower as we move from interior to exterior. The heat loss formula for temperature drop (Td) is:Td= Ti– [(R/Rt) x ΔT]


  • Td = temperature drop (temperature at top surface of material), degree Fahrenheit
  • Ti = design inside (interior side) temperature, degrees Fahrenheit
  • ΔT = (winter interior design dry bulb temperature) – (exterior design dry bulb
  • temperature)
  • ΔT = 80 degrees – (-16 degrees) = 96 degrees
  • R = cumulative R-values of materials starting from the interior
  • Rt = R-value of the total assembly

The table below are the calculations for each roofing material in the roof assembly example.

materials and temperature at top face

Step 6: Plot the calculated temperature gradient values for top surface of each material on the roof assembly drawing. Draw a line from the winter interior dry bulb design temperature to the next value and continue to "connect the dots" until you reach the winter exterior design dry bulb temperature.

plotted calculated temperature graident values

Step 7: Determine the dew-point temperature. Dew-point temperature can be determined by using a simplified version of the ASHRAE psychrometric chart, shown below.

simplified version of ASHRAE psychrometric chart

For this example, look at the top row of the chart and locate the design dry bulb (interior) temperature column, which, in this case, is 80°F. Next, locate 60 percent relative humidity on the left side of the chart. The dew-point temperature is at the intersection of the design dry bulb temperature column and relative humidity row. For this example, the dew point temperature is 65°F.

Note: If the design values don't exactly match the values in the chart, you can use linear interpolation to determine the dew point temperature. Another option is to use a dewpoint calculator app.

Step 8: Locate the dew-point temperature value on the temperature gradient line on the roof assembly drawing. If the dew-point temperature falls within the insulation that is above the vapor retarder, there is sufficient roof insulation above the vapor retarder. So the vapor retarder should be effective in preventing or minimizing condensation from occurring within the roof assembly.

dew-point temperature value

If the dew point falls below the vapor retarder, additional insulation is needed. After selecting a new amount of roof insulation, confirm the new amount is sufficient by doing the graphic analysis again. Conversely, if the dew-point temperature falls within the upper layer of insulation—say in the upper one-third of the total insulation layer—the amount of roof insulation might be able to be reduced to prevent condensation, as long as the dew point stays within the insulation and, importantly, the revised insulation amount still meets energy code requirements. Of course, perform another graphic analysis to verify the reduced insulation amount is adequate.

In closing

It is very important to note that this graphical method is a simplified procedure using theoretical constant values. In the real world, actual relative humidity and dew-point temperature values constantly change. This procedure is just one way to determine whether a vapor retarder will perform its intended function. Designers should further substantiate their analyses by using other methods.

NOTE: This blog is for information purposes only. GAF does not provide professional design services. You should always consult with a design professional to determine whether the roofing system to be installed is suitable for the particular needs of your building.

More homes and businesses in the U.S. are protected by a GAF roof than by any other product. We are the leading roofing manufacturer in North America, with plants strategically located across the U.S. As a Standard Industries company, GAF is part of the largest roofing and waterproofing business in the world.
Don't miss another GAF RoofViews post!
Thinking of upgrading your roof and shingle colors? You're not alone. In the last year, 76% of US homeowners made at least one home improvement project—and for good reason. Beyond adding visual appeal, home improvements make good financial sense: for example, upgrading a roof with Timberline shingles can increase the value of a home by an average of 10%*.
Workplace conflict can arise across many working environments and can lead to unmotivated staff and poor working relationships, both on and off the roof. The right de-escalation techniques can help defuse situations as well as foster more cohesive working relationships going forward.
Commercial property managers and owners should consider how to make the most of their partnerships in order to improve the effectiveness of their building management decisions. Partnering with manufacturers allows managers to plan for improvement projects and monitor their buildings' health even from a distance. Manufacturers can provide effective property management solutions that give owners vital information and save them money.
Cool roofs can help save you money on cooling costs in the North as well as the South. Here are some cool truths to ponder when planning a new roof.* TRUTH 1: It's All About Reflectivity A cool roof reflects the sun's rays away from the roof, helping to reduce rooftop temperatures. While a sunlight-absorbing black roof can reach up to 190°F in the summer, a reflective roof's temperature can be as much as 55°F lower. Dark surfaces also contribute to the urban heat island effect. (SOURCE:
Low slope roof penetrations can be a source of problems if not done correctly. Pipe, vent, and conduit penetrations through low slope roof assemblies can cause problems for an otherwise tight membrane, insulation, and deck design. With many intermediate layers in roof assemblies, such as a vapor retarder and cover board, there are opportunities for things not to be done correctly somewhere in the assembly. It gets even more complex when we consider that in order to use a vapor retarder, there might be an additional cementitious board above a steel deck.
One year after the pandemic began, Trent Cotney of Cotney Attorneys & Consultants has seen firsthand how COVID changed roofing. Here, he shares insights into what contractors are concerned about as well as tips on how to successfully navigate the roofing industry in this new environment.
This blog contains information created by a variety of sources, including internal and third party writers. The opinions and views expressed do not necessarily represent those of GAF. The content is for informational purposes only. It is not intended to constitute financial, accounting, tax or legal advice. GAF does not guarantee the accuracy, reliability, and completeness of the information. In no event shall GAF be held responsible or liable for errors or omissions in the content or for the results, damages or losses caused by or in connection with the use of or reliance on the content.

Interested in sharing or republishing our content? We kindly ask you to adhere to our guidelines.