by Daniel Tempas
Designers have been concerned about condensation in walls for decades. Since the mid-1970s, the greater amounts of insulation specified in the building envelope has increased the likelihood for condensation somewhere in the assembly. Many articles have been written over the years describing the physics of the problem and, for the vast majority of the time, there has been a laser-like focus on one solution.
Initially, water vapor diffusion was seen as the likely culprit for condensation problems and designers and consultants spent hours running and analyzing wall assemblies using the ‘profile’ (or ‘dewpoint’) method (Figure 1). With such analyses came the concept the wall system should be tuned for maximum condensation resistance by altering or selecting the appropriate permeability of the wall components.
The rule of thumb became to place low-permeability materials/retarders on the wall’s warm side, and higher permeability materials on the cold side (Figure 2). In this fashion, the designer strove to make it difficult for water vapor to enter the wall (lessening water’s ability to condense in the wall) and easy for water vapor to leave the wall (drying out any water that still managed to get inside). Manufacturers began to introduce high-permeability air barriers, water barriers, and sheathings along with ‘smart’ vapor retarders for the warm side of the wall.
This low-perm/high-perm strategy reveals two goals in wall design: the efforts to decrease condensation potential and increase drying potential. Reducing condensation potential is fairly well-understood but increasing drying potential is a less commonly sought after goal. Both are important for robust wall design.
Problems with permeability
While all this sounds good, it was not necessarily preventing condensation problems. There are some basic facts about permeability designers need to understand to get a better grasp on not only controlling condensation, but general wall design.
Fact 1: If a material’s temperature gets low enough, water vapor will condense on or in it, regardless of how high its permeability.
This is something to keep in mind in cold climates. This author has seen both fiberglass batts and high-perm air barriers with ice encrusted on their surfaces. When a material gets cold, its effective permeability dramatically drops. High permeability is useless at low temperatures. In other words, condensation is a temperature-related phenomenon.
Fact 2: Cold water dries slower than warm water, no matter how permeable the shell surrounding it.
Increasing a wall assembly’s drying potential is an important and valuable goal. However, water at lower temperatures will take a long time to dry because the related evaporation rate is slow. Simply put, robust drying potential cannot be achieved in the layers of a wall assembly that are at low temperatures.
For example, one can consider a puddle on a sidewalk (Figure 3). How long does it take that puddle to dry? If the ambient temperature is 32 C (90 F), it will not take long at all, perhaps only several minutes. However, when the ambient temperature is only 4 C (40 F), the puddle might take hours or even days to evaporate. This is an example of the profound effect temperature has on evaporation rate.
Fact 3: Air movement transports far more water vapor than diffusion.
This is something that has been understood by building scientists for quite some time, and has been filtering into the design community for decades. However, the subtle ramifications of this knowledge are just now finding their way into the world at large. The fact air movement is so dominant in water vapor transport (and subsequent condensation) means any vapor retarder must work either as, or in conjunction with, a near perfect air barrier.
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 planned benefit from that barrier’s diffusion characteristics. This will result in a much greater potential for condensation in or on any layer that is at a low enough temperature for condensation to occur. Additionally, this means diffusion-based analyses of the wall system are rendered moot.
Fact 4: Water vapor does not move from areas of higher temperature to lower temperature.
Thinking this is the only direction water vapor flows is incorrect. Water vapor moves from areas of high concentration to low concentration, regardless of the direction of heat flow. This is an important concept when it comes to understanding drying verses condensation.
Temperature to the rescue
After considering these four facts regarding water physics, it would seem there is a great deal of confusion and trouble regarding wall design. The manipulation of material water vapor permeabilities in a wall design cannot achieve a truly robust assembly. What can be done?
‘Temperature’ is the common thread running through the facts regarding water vapor condensation in wall assemblies. A wall assembly’s temperature profile plays a critical role in the ability to resist condensation and promote drying. This is not an unknown concept, of course—a quick search of building science literature will yield the occasional article mentioning the importance of the temperature profile. The problem is temperature profile manipulation is far down the list of the wall designer’s methods for creating a more robust wall. It is seen as unimportant when in reality, it is the opposite.
As much of the wall insulation as possible should be placed on the outbound side of the assembly (Figure 3). This is easy to do whether the base wall is metal stud, concrete masonry unit (CMU), or poured concrete. In cold-weather conditions, this will warm the entire interior wall, changing the temperature profile with far-reaching consequences (Figure 4).
For example, designing a wall assembly so more of the components will be in the higher temperature portion of the wall profile significantly reduces the potential for condensation. Not every part of a wall is equally sensitive to exposure to moisture. A standard rainscreen veneer wall assembly (Figure 5) is not sensitive to water, as it must be exposed to the elements on a constant basis. The support elements for the veneer are also not sensitive to water—they are in the drainage space behind the veneer and quite a bit of water reaches that space. As for the insulation layer on which the supports rest, it too must be moisture-resistant for the same reason. If condensation can be forced to happen only around components immune to water, then the wall design is completely robust in its resistance.
Designing a wall assembly so more of the components will be in the higher temperature portion of the wall temperature profile also significantly increases the drying potential for any water that does find its way into the wall. Referring back to the puddle example, higher temperatures means much higher drying rates. Combine the greater drying temperature with the longer drying time and one has a wall with a drying potential increased by an order of magnitude or more.
The importance of temperature modification to improve walls systems can be better understood when considering that both condensation and drying are two-step processes (Figure 6):
- movement of water vapor to or from the point of condensation or drying; and
- actual phase change of water from the vapor phase to the liquid phase (condensation), or vice versa (drying).
No matter how rapidly water vapor is transported to a given location in a wall assembly, either by the slow process of diffusion or the rapid process of air infiltration, condensation will not take place if the temperature of that location is high enough. This is also true in the drying process. No matter how easy it is for water vapor to exit a given location in a wall assembly, either by the slow process of diffusion or the rapid process of air infiltration, drying will not take place when the temperature of that location is too low. Again, temperature plays a critical role in the condensation and drying processes in a wall assembly. Altering the temperature profile of a wall assembly through judicious placement of materials is an effective method to control these processes.
The aforementioned Fact 4 about the true nature of the movement of water vapor makes it clear even when the exterior sheathing/insulation is completely impermeable, the drying potential of this wall is much greater than the previous design and the condensation potential is much lower. Since it is at a temperature near to that of the interior, 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 building interior.
Proper placement of the right insulation negates the need for a vapor retarder. Why worry about water vapor getting into the wall when most of it is at a temperature far too high for condensation to take place? If the insulation has been well-chosen, any condensation taking place toward the exterior of the building will be minute and meaningless. Besides, the stud cavity needs to dry to the interior, and an interior vapor retarder will only get in the way.
The overall robustness one gains from placing most wall components in the highest temperature part of the temperature profile overwhelms almost every other condensation/drying consideration in the wall design.
Using the temperature profile of a wall as part of the design process leads to a wall that is easier to build. Relying on permeability (to alter water vapor diffusion rates) in the design process for a wall assembly results in a dependency not only on material properties, but also on the quality of installation.
A critical part of any vapor retarder (or air barrier) is its continuity. Any flaw in the installation process of that air/vapor retarder that results in breaches of its continuity heavily compromises its ability to reduce condensation potential. This would include unrepaired construction damage or poorly sealed seams. Even normal penetrations in the wall assembly, like outlets and switches, present opportunities for discontinuity in the air barrier/vapor retarder.
On the other hand, manipulation of the temperature profile of a wall assembly is only about positioning the right amount of insulation in the right location in the wall. A board of insulation is far more robust that film of plastic, making insulation continuity far easier to achieve. Also, the outside of the wall typically has far fewer penetrations, making them far easier to handle.
Designing wall assemblies by adding or altering the permeabilities of the wall components is an artifact of the limited analysis tools relying on investigation of water vapor movement via diffusion. Such walls gain only mild improvements in condensation resistance and, more importantly, drying potential. To create a truly robust wall system with the greatest condensation resistance and drying potential, designers must look at altering the temperature profile of the wall assembly by moving insulation as far as possible to the wall’s exterior.
This does not mean one should no longer think about, or design with, the permeability of materials in mind, of course. Rather, it means the water permeability analysis/profile part of design efforts should be relegated to the proper place in the design consideration hierarchy: behind the wall temperature profile design effort.
Daniel Tempas is a building envelope technical service representative for Dow Building Solutions; he has held technical and engineering positions at the Dow Chemical Company for almost 30 years. Tempas is a (HERS) rater, a Leadership in Energy and Environmental Design (LEED) Green Associate, and a member of the RESNET Training Committee. He has also been a member of ASTM, Exterior Insulation and Finishing Systems Industry Members Association (EIMA), and Building Thermal Envelope Coordinating Council (BTECC). Tempas can be reached firstname.lastname@example.org.