Using Temperature to Control Condensation in Cold Climates

by Jonathan McGaha | 30 November 2015 12:00 am

A basic understanding on the way we look at condensation and wall design

 

Designers have been concerned about condensation in wall assemblies for decades. Since the mid-1970s, greater amounts of insulation specified in the building envelope have increased the likelihood for condensation somewhere in the assembly. Initially, water vapor diffusion was seen as the likely culprit for condensation problems. Using the Profile Method (also known as the Dewpoint Method), designers believed that the wall system should be tuned for maximum condensation resistance by selecting the appropriate permeability for each wall component.

The rule of thumb was to place low permeability retarders on the warm, inner side of the wall and higher permeability air-and-water barriers on the cold side of the wall. In this fashion the designer strove to make it difficult for water vapor to enter the wall
(making it difficult for much water to condense in the wall) and easy for water vapor to leave the wall (drying out any water that managed to get into the assembly). Manufacturers introduced high-permeability air barriers, water barriers and sheathings along with “smart” vapor retarders for the warm side of the wall.

 

Photo courtesy of The Dow Chemical Co.

Preventing Condensation Problems

All of this sounds good, but it wasn’t necessarily preventing condensation problems. There are some basic facts about permeability that designers need to understand to get a better grasp on controlling condensation. These facts change the way we look at condensation and wall design.

Fact 1: Water vapor will condense on or in a material no matter how high its permeability may be, if the temperature of that material gets low enough. High permeability is useless at low temperatures.

Fact 2: Cold water dries more slowly than warm water no matter how permeable the shell surrounding it. Increasing the drying potential of a wall system is an important and valuable goal. But water at lower temperatures will take a long time to dry because the evaporation rate of water at low temperatures is very slow. Imagine a puddle on a sidewalk, how long does it take to dry? If it is hot out, perhaps only a few minutes. But if it is cold, a puddle might take hours or days to evaporate. This is an example of the profound effect temperature has on evaporation rate. Robust drying potential cannot be achieved in the layers of a wall assembly that are at low temperatures.

Fact 3: Air movement transports far more water vapor than diffusion. Any installation flaw or penetration in the air/vapor barrier on the higher temperature side will result in an amount of air leakage that will overwhelm any benefit from the vapor retarder. This will result in a much greater potential for condensation in the wall.

Fact 4: Water vapor moves from areas of high concentration to low concentration, regardless of the direction of heat flow. Drying can take place in any direction.

 

A Common Thread

There is a common thread running through the facts regarding water vapor condensation in wall assemblies: temperature. The temperature of the wall components plays a critical role in condensation and drying.

It is easy to design a better wall system by taking advantage of these facts about condensation. It all really boils down to this: Place as much of the wall insulation as is possible on the outbound
(cold) side of the assembly. This is easy to do whether the base wall is metal stud, block or poured concrete. In cold-weather conditions this will warm the entire wall on the interior which increases drying and prevents condensation. This method has impressive benefits:

Photo courtesy of The Dow Chemical Co.

1) The wall components will be in the higher temperature portion of the wall temperature, which reduces condensation potential. This makes sense. After all, condensation is a caused by low temperatures.

2) The wall will have vastly better drying ability. Remember our puddle example? Higher temperatures mean much higher drying rates. It also means that the warmer portions of the wall will spend far more time at those higher temperatures, meaning there will be more drying time. Even if the exterior sheathing/insulation is completely impermeable, the drying potential of this wall is still very high because it is so warm. Any water in the stud cavity will have a much higher evaporation rate, which means a much higher drying rate. Also, it will easily dry to the interior of the building.

3) The wall doesn’t need a vapor retarder. Why worry about water vapor getting into the wall when most of the wall is too warm for condensation to take place? Besides, the stud cavity needs to dry to the interior, and an interior vapor retarder will only get in the way.

To create a truly robust wall system with the greatest condensation resistance and drying potential, put the right insulation in the right place in the wall: on the outside in a continuous layer. Keeping the wall warm is by far the best way to make a robust wall.

Daniel Tempas is building science expert at The Dow Chemical Co., Midland, Mich. For more information visit www.dow.com[1] or call (866) 583-BLUE

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