While most roofing professionals naturally focus on the roof system and roof leakage, there’s another type of leakage that is at least as important – air infiltration.
You’re probably familiar with the term vapor retarder (or vapor barrier), and you recognize that vapor retarders need to be installed on the warm (high vapor pressure) side of a construction to slow the diffusion of water vapor into the thermal insulation. In colder climates, the position of the retarder needs to be toward the interior of the building because the predominant vapor drive is toward the colder outside. In the air-conditioned south, or in buildings that function as coolers or perishable refrigeration, the retarders need to be toward the exterior side of the construction.
We also have recognized that most buildings don’t require retarders at all. In fact, detractors of retarders point out that roof insulation has a tolerance for some water, and water that does accumulate during the colder months can “self dry” during the following summer months if there’s no retarder in the way. They also point out that, if the roof does leak, the retarder may delay discovery of the roof leak until damage is widespread.
That situation may be changing. With more and more cool roofs, e.g. high albedo surfacing, being installed, summer drying may not be adequate to return the insulation to a dry condition before the wetting cycle starts all over again.
We also know that air leakage can carry far more water vapor into the roof system than moisture diffusion through a retarder1. (Reportedly 30 to 200 times more.) In Maxwell Baker’s Roofs, Design, Application and Maintenance2, Baker estimates that a one-perm vapor retarder might pass 5 pounds of moisture per month (per 100 square feet), but that a 1-square-inch hole in that barrier could pass 80 pounds of water in the same time period (under conditions of 5 cfm airflow and 0.1 inches of water pressure differential).
Since there’s so much attention now focused on energy conservation, we need to emphasize the consequences of this insulation getting wet. The energy loss through wet roof insulation can be increased by 70 percent over dry3.
Organizations, such as the Air Barrier Association of America (ABAA), have been organized to further the understanding of the fundamental nature of air barriers. The AABA points out that:
Vapor barriers are not to be confused with air barriers. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material.
Vapor barriers or vapor retarders are intended to control the rate of diffusion into a building assembly. As a vapor barrier, it will control the rate of moisture flow where it is placed; therefore, the vapor barrier doesn’t have to be continuous, doesn’t have to be free of holes, doesn’t have to be lapped, doesn‘t have to be sealed, etc. A hole, for example, in a vapor barrier simply means that there will be more vapor diffusion in that area compared to the other areas where the vapor barrier is intact.
Water vapor permeance is measured by the amount of water that will work its way through a material. This is normally reported in ng/(Pa•s• m2). Many areas of the country require a vapor barrier that has a maximum water transmission rate of 60 ng/(Pa•s2).
Much work is being done and much discussion is being held on whether vapor barriers should be used at all – and, if they are used, what the water transmission rate should be. There’s discussion on the need to allow buildings to dry. Keep in mind that, during this time period where vapor barriers are being discussed, the local building code must still be met, which may demand barriers even when building science would indicate that they should not be used.
Water vapor may be transported by air leakage, but this is dealt with by installing a proper air barrier. Vapor barriers are intended to be installed on the warm side of the insulation. The air barrier, by contrast, can be placed anywhere in the roof cross-section.
There are also a wide variety of materials that can be used to form an air barrier, such as:
- As a roof system, SPF is highly impermeable, but as an air barrier, it also forms tight flashing seals at pipe penetrations, curbs, and walls. SPF can also be used under the deck at junctions of walls and roof deck, and other penetrations through the roof deck and insulation. This is especially useful for tightening up existing buildings.
- Self-adhering flexible membranes are widely used in waterproofing and as an underlayment for steep roofs where eaves icing is probable. Since neither hot asphalt nor torching is necessary, they’re suitable for sealing difficult-to-get-at locations.
- Liquid membranes will seal continuous surfaces, they require reinforcement to bridge joints and gaps in the substrate.
- Un-adhered membrane systems, such as ballasted and mechanically fastened single-ply systems, don’t make good air barriers because air can freely migrate laterally. In addition, if the mechanically fastened roof system billows in a windstorm, it serves to “pump” air. Windstorm resistance of ballasted and mechanically fastened systems is greatly enhanced by installing a separate air barrier.
- Condensation is regularly observed on the underside of un-adhered systems in cold weather. (Because the thermal insulation used in these systems generally consists of closed-cell foam, this may not be a fatal defect.)
- In metal roof systems, a vapor retarder is commonly laminated to the interior face of glass fiber batt insulation. Integrity of the retarder can be improved by overlapping the plastic film and sealing it with tape, but does not address air movement at walls and penetrations. Because many metal buildings have many sources of air leakage, wintertime internal humidity may never build up to the point where condensation is a factor. As energy codes call for much higher R-values and tested U-values, we may see more condensation problems appearing. The solution should be to mandate tested air barrier systems in such structures.
- In nonstructural (hydrokinetic) metal systems, it’s easy to create an airtight barrier on top of the OSB/plywood deck (usually using self-adhering membranes).
Roof Air Barriers4
The roof membrane can be considered an air barrier since it’s designed to withstand wind loads if it’s fully adhered or hot- or cold-mopped. Mechanically fastened and ballasted roof systems, because they displace and momentarily billow or pump building air into the system, don’t perform the required functions of containing air without displacement. In these cases, another air barrier must be selected in the system. Either a peel-and-stick air and vapor barrier on the inboard side of the roof system (inter conditions and weather dependent), or taped gypsum underlayment board beneath the insulation can be used in a system with adhered under layers of thermal protection board and insulation. Those layers must be designed to withstand maximum wind loads without displacement. Because of the critical importance of continuity with the wall air barrier, a pre-construction on the air barrier system must include the trades involved in the air barrier system, such as the wall air barrier subcontractor, the window subcontractor, the sealant subcontractor, and the roofing subcontractor, to discuss the connection between the roof air barrier and the wall air barrier, as well as the sequence of making an airtight and flexible connection. It’s also important to ensure that the materials being joined together are compatible.
Penetrations into roof systems, such as ducts, vents, and roof drains, must be dealt with, perhaps by using SPF (or other sealant) or membranes to air tighten these penetrations at the targeted air barrier layer.
How do we know if an existing building has air leakage problems? We have a number of powerful tools:
- While this column usually refers to infrared thermography (IR) as a tool to detect wet thermal insulation, airflow will also carry heated air. IR does not “see” the warmer air, but detects the surfaces that the warm air has impinged upon.
- ASTM E-779 describes a procedure for pressure testing a building. This will indicate whether the building can meet a desired level of “tightness.” Usually, this is defined as less than 0.02 L/s•m2 @ 75Pa (<0.004 cf/min/ft2 @ 0.3 in H2O).
- A smoke plume is directed at suspected air leakage points. The smoke source can be as small as a pencil or large enough to fill a sizable roof using “theater” smoke.
- For existing buildings, it’s worthwhile examining abandoned rooftop equipment to see if it has been properly sealed off. It may be necessary to remove the units, install replacement decking, and tie a new air barrier to what’s already there.
