Vapor Barriers
Posted: February 15th, 2012 | Author: Toni | Filed under: Architecture, Building Materials | No Comments »
Vapor Barriers are a big topic these days in the construction industry. And the issue, whether to include a vapor barrier or not, came up during a roof assembly discussion on one of my latest projects. The roof supplier and installing contractor recommended that the vapor barrier be omitted. But they were not able to provide anything more than field experience as for reasons why. It’s just what they had successfully installed over the past 10 years with no major maintenance issues. All of my reference information, created to provide architects with good rule of-thumb-guidance, said the roof should have a vapor barrier on the warm side, but not a lot else. Again, I could find no real reasons why. So, I decided that I needed to have a better understanding on the subject and started researching the need for vapor barriers.
For some basic background information vapor barriers are included in the building envelop, exterior walls, roof, and floor, as a way to prevent the migration of moisture through the process of vapor diffusion. Vapor diffusion is the migration of water vapor from higher concentrations to lower concentrations. Water vapors naturally move from more to less. The traditional response to fight this process within a wall system in a cold climate is to place a vapor barrier on the warm side of a wall assembly forcing the vapor to move from inside the wall system to the outside of the building. However, out is not always where the vapor wants to go.
Water in the building envelop is a major concern. Architects strive to design buildings which do two things. First keep water out, and second, let water out when it gets in. This is easy enough with liquid water or even solid water, that’s snow and ice of course, but it gets more complicated when we think about vapors. One way of dealing with these tiny droplets of water in the air is to allow air movement within the assembly to dry out any moisture that does get inside the walls. However, this air drying method has gone by the wayside as building shells get more energy efficient leaving us to re-open the question of vapor barriers.
The industry has found that by adding air barriers to building exteriors, the energy consumption of the building can be greatly reduced. We’ve also discovered that moving insulation farther outside the wall assembly can reduce energy usage even more. Both of these changes reduce air movement within the wall assembly so using air to dry out the building envelop is no longer a smart option. This is a significant change to envelop design and the architectural rule-of-thumb guidance on vapor barriers does not seem to have caught up yet.
I am over simplifying here. Engineers have argued about the placement of and need for vapor barriers within building envelopes in different climates for years, but I am an architect in search of a broad understanding. So, armed with this knowledge of why traditional vapor barrier placement is now in question, I decided to figure out how to determine whether or not they are needed at all. What I have discovered is that I have to let go of the overarching rule-of-thumb vapor barrier placement that I love so much.
To determine vapor barrier placement each building assembly must be considered independently. To do this, I’ve used the Dew Point Method for calculating the anticipated point of condensation within a wall system. Again, engineers have modeling programs that take into account a multitude of variables with incredibly accurate results. As an architect, I’m okay with the less precise but conservative Dew-Point Method. This method is explained in Chapter 22 of the 1997 ASHRAE Fundamentals Handbook. After performing a series of these calculations, I have a much better understanding of how vapor moves through a wall. Here is some of what I’ve learned:
- Figuring out the anticipated migration of moisture in a wall or roof assembly is dependent on three things: the assembly, the indoor conditions, and the exterior conditions.
- It’s okay to have some moisture in the building envelope, as long as it doesn’t exceed the maximum safe moisture content of the material where condensation occurs. Construction materials in building assemblies inherently store and release water vapor.
- All materials have a water vapor permeance (perm). If this permeance is low enough, generally understood to be less than 1.0 perm, the material is considered a vapor barrier. Some only consider a vapor barrier any material with a permeance under 0.1 perm such as polyethylene sheeting.
- A material is a vapor retarder if it has a permeance greater than 0.1 and less than 1.0 like kraft facing on batt insulation. Materials in this category are sometimes referred to as “smart vapor barriers.”
- Any material with a permeance under 10 perm but higher than 1.0 perm is considered a vapor retarder. Lots of materials fall into this category including different types of foam insulation and paint.
- Vinyl wall coverings, which tend to act as vapor barriers, on the interior side of exterior walls can be very problematic.
Now that I’ve run these calculations, I understand why a vapor barrier on the warm side of the wall can do more harm than good. As we incorporate new energy saving techniques in wall construction, there is more of a chance that water vapor will get trapped within the wall assembly leading to the degradation of materials. If vapor does get into an assembly, it must be allowed to dry either to the interior or the exterior of the wall system. Often times the construction materials themselves act as sufficient vapor retarders and additional barriers only cause problems. Where to place the vapor barrier, and whether or not one is needed at all, is something to be determined by the design team for each individual building project. Consult an architect familiar with these issues before jumping to a decision on the location of a vapor barrier.
The question of whether or not a vapor barrier is needed on the warm side of a flat roof, the recent issue which provoked my interest in vapor barriers, is actually a little simpler. Oh sure, calculations can be performed to determine if a vapor barrier is needed and where condensation is likely to occur, but I think this can also be addressed through a bit of common sense. Since most low slope roof membranes have very low water permeance, the majority of drying must occur to the interior of the building. When a vapor barrier is placed on the warm side of the roof, there is a good chance that water can be trapped within the assembly. Moisture actually degrades elements of a roofing system, reducing insulation value. For these reasons, I tend to agree with the installers about omitting the vapor barrier. I’m thinking the better question might be, “How much water can be safely stored in the roof assembly materials to prevent condensation?” but that’s another topic.
Sources:
1997 ASHRAE Handbook Fundamentals Inch-Pound Edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. copyright 1997.
Building Science Digest 106, Understanding Vapor Barriers, 2006-10-24, Joseph Latiburek, Building Science Press copyright 2006.
The Influence of Low Permeance Vapor Barriers on Roof and Wall Performance, Research Report-1101, John Straube, Building Science Press copyright 2011
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