Air Barriers vs. Vapor Retarders — do you know the differences?
The use of air barriers and vapor retarders in construction is increasing rapidly, but many people — from building designers through specifiers to installers — remain confused as to the differences between them. There are many questions that frequently arise, such as:
- Is a vapor retarder the same thing as an air barrier?
- What is the difference between an air barrier and a vapor retarder?
- What have these materials got to do with roofing?
In this post, part 1 of a series, we'll cover the basics of these two materials. Subsequent sections will cover topics such as specification and installation.
One problem with the entire topic is that there have been other terms used, such as vapor barrier, air resistance layer, or moisture control layer. Are these related or different materials? Also, there is confusion about what the roof designer, or even the code requiring these materials, is attempting to achieve or remedy.
Let's begin with terminology. Leading experts and trade associations such as the Air Barrier Association of America (ABAA) suggest using just these two terms: Air Barriers and Vapor Retarders. Each might have some variations that we'll discuss later, but for the purposes of this article, let's forget all the other terms and just use these two.
It makes sense to start with air barriers and review why they are needed and where they are placed.
Why are they needed?
It's all about improving a building's energy efficiency. Adding more insulation to a building envelope has reached the point of rapidly diminishing returns. Also, insulation thicknesses are such that further increases would be difficult to accommodate without large increases in cost related to wall thickness, for example. Air leakage has been identified as a significant contributor to poor building energy efficiency. A building owner is paying a lot to heat or cool the interior air — why allow significant loss of that conditioned air to the outside?
What impact do tighter buildings have on indoor air quality?
As buildings get tighter, there can be concerns about indoor air quality. A detailed discussion on this topic is beyond the scope of this article, however, there are at least three general approaches that are used.
The first is to ensure that a commercial building's HVAC system allows for sufficient fresh air to be drawn into the building. This task is usually assigned to the HVAC design engineer, who follows standards such as ANSI/ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, to determine how much make-up air to deliberately draw into the system.
Secondly, tighter buildings can have indoor air quality issues if the materials used to finish and furnish the interior off-gas odorous and maybe noxious chemicals. For that reason, many interior designers are selecting materials that have Environmental Product Declarations (EPD), Health Product Declarations (HPD) and similar.
Third, we want to be in control of our fresh air intake and exhaust. In the past, leaky buildings allowed the unregulated transfer of air. With tight buildings, we are in control of the intake and exhaust, which means we are in control of our energy costs.
Where are they installed?
The goal of using an air barrier is to reduce the leakage of conditioned air out of a building. Therefore, air barriers must be part of the building envelope. This should be a hint that air barriers are systems — a combination of materials — and the individual components of the building envelope have to all link together in such a way as to prevent air leakage.
It should be obvious that roofing materials block air and might therefore be considered "air barriers." But to fully function as air barriers, they need to be installed correctly and tied into the wall elements and penetrations in such a way as to ensure there are no air leaks at those junctions and penetrations. Consider how a roof membrane often runs up and over a parapet wall or is flashed at an exhaust pipe. The building designer needs to specify that the membrane not just overlap the wall air barrier or surround the pipe, but be flashed or sealed to both in such a way that air cannot escape.
When are they needed?
A building envelope is required to function as an air barrier for new construction except in climate zone 2B, according to the 2015 IECC. Obviously, each component must block air and be tied to adjacent materials such that leaks do not happen. For existing buildings, air barrier installation is not required for replacement or recover roofs. Earlier versions of the IECC differed in terms of replacement and recover requirements as well as the climate zones impacted. For these reasons, along with differing adoption rates by state and region, always check with your local codes officials as to the local situation for any building or roofing project.
Roof membranes block air; when and why should I be concerned?
If you're working under the 2012 IECC, for new construction or in a tear off situation, everywhere except in climate zones 1 through 3, the roof membrane (which acts as an air barrier) must be tied into the wall component that acts as an air barrier. This is a code requirement that has been adopted by most states that are affected. Most roof membranes are already defined as suitable for an air barrier system and others have been tested to comply. If you're working under the 2015 IECC, the requirements are the same for new construction, but are relaxed for reroofing; there is no requirement to install an air barrier when reroofing. A future article will examine this topic in more detail.
What can go wrong?
When a building is closed up tightly, such as with an air barrier system, any small leak in that system can have catastrophic results. Tight buildings are typically pressurized, and as a consequence, for example, a small leak in the building envelope can result in a very large volume of air exiting through that leak. If that air encounters a cold temperature, then large amounts of condensation could occur within the building envelope. This typically occurs around features such as windows, which are notoriously problematic to flash and seal.
Air barriers are a system of materials that work together to prevent conditioned air from escaping from a building. They are used to increase the energy efficiency of buildings.
When used, air barriers are part of the building envelope.
Air barriers can be required by code depending on location and new versus existing construction.
Why are they needed?
It's all about controlling condensation and moisture management within the building envelope. So, while air barriers are part of the building envelope, vapor retarders are used within the envelope. Condensation within a roof or wall system can be very damaging to a building's structure, attracting mold if it doesn't dry out in a reasonable time, and causing damage to interior finishes. The dew point is frequently within a wall or roof system, therefore humid air must be prevented from reaching that point. Otherwise condensation will occur. Vapor retarders can be used as part of a larger strategy to minimize condensation.
What's the difference between a vapor retarder and an air barrier?
Vapor retarders are used to prevent diffusion of moisture vapor. However, they also help prevent humid air from reaching a cold surface inside the building envelope. So, by definition, they do act as barriers to vapor and air movement. But their intended purpose is not to improve energy efficiency, and they do not have to be part of the building envelope.
It is considered bad practice to install materials that totally block moisture diffusion on the opposite sides of a building assembly, such as a vapor barrier at the deck level within a roof assembly. Roof membranes are vapor impermeable and any moisture that does get into the assembly, either during construction or from within the building, needs to be able to get back out. Therefore, vapor retarders should be used because they have some moisture permeability or they shouldn't be installed completely tightly.
Where are they installed?
For the roof system designer, vapor retarders should be inside of where the dew point is (e.g. on the warm side in winter.) This schematic shows a typical roof assembly with a fully adhered membrane over two layers of insulation. The vapor retarder can be installed anywhere between the top of the roof deck and just below the dew point.
Some roof system designers suggest putting the vapor retarder over the first layer of insulation. This makes excellent sense, because it reduces the amount of damage that can occur during removal and replacement of the roof if the vapor retarder is applied directly to the deck.
When are they needed?
The decision on whether or not to add a vapor retarder to a roof assembly is normally based on risk. As noted in a previous article on moisture, it is important to consider building use as well as location. For many situations, good roofing practice is usually sufficient to minimize or prevent moisture related issues within a roof assembly. So always using two layers of polyiso insulation with staggered joints is a good approach to reducing air flow up into the assembly. In addition, fully adhering the roof membrane prevents the membrane from billowing/fluttering in windy situations and drawing air up into the assembly.
For the lowest risk, and for those buildings that will have a higher than normal interior humidity, installing a vapor retarder is a good option.
What can go wrong?
As noted earlier, no part of a building structure should be sandwiched between two vapor impermeable materials. Any moisture that does get in will not be able to escape. While roof membranes are there to prevent precipitation from getting into the building, there are scenarios when it can occur. Here are three examples:
- During construction, if rain or frost is present at any time during membrane installation. When the roof gets closed up, that moisture will not be able to escape if there is a tightly sealed impermeable vapor retarder further down in the assembly.
- If a leak occurs in the roof membrane. Again, that water will not be able to escape if there is a tightly sealed impermeable vapor retarder further down in the assembly.
- If a vapor retarder has been installed but not sealed around penetrations in a system with a mechanically attached membrane. When the membrane billows/flutters in high winds, humid interior air can be drawn up around penetrations into the roof system. Any subsequent condensation will likely take a long time to dry back down.
- Vapor retarders are used within roofing assemblies to control moisture migration. They block air flow, provided they are tightly sealed around penetrations and the perimeter.
- Vapor retarders always need to be placed on the warm side of the dew point.
- It is suggested that vapor retarders are best combined with fully adhered assemblies.
In future articles we'll cover the specifics of terminating roof membrane air barriers to wall system air barriers, and discuss more details about vapor retarders.