Air-Sealing Tips for Efficiency That Lasts // Part 2: Continuous Means Continuous
This is part 2 of a series that describes how to air seal the most difficult parts of buildings.
Chris Corson, of EcoCor High Performance Buildings, likes to say that when you’re thinking about an air barrier, “continuous means continuous” —and that means no gaps, with no exceptions. The air barrier must be continuous to be effective, and continuity should be simple—it’s best to stay out of its way. Keep the red-line exercise in mind, and the fewer transitions, the better (see part 1 of this series, “Start on the Drawing Board,”). Even in difficult areas like window bucks, floor and roof joists, and stud bays, a continuous air barrier is indispensable for ensuring building performance, because a small gap in airtightness amounts to a large hole in the building assembly. Gaps, at best, cause drafts and reduce the effectiveness of insulation. At worst, gaps allow moisture loading of the assembly, which increases the possibility of mold and rot—leading to health risks and decreasing the building’s structural integrity.
Transitions are inevitable and require careful planning and attention. Before you conduct a blower door test, visually inspect the air barrier. This will give you a good sense of building performance, and where the visible gaps are. Every connection, juncture, and surface transition is a possible site of failure, and should be inspected for continuity. Above all, remove structural elements from the air barrier’s path, allowing for continuity in the simplest way possible.
Gaps Can Be a Major Problem
Wetting through an air leak in gypsum board can be up to 90 times greater than wetting by diffusion over the course of a heating season (see Figure 1). This fact underscores the critical importance of air barrier continuity: Gaps in the air barrier can compromise building performance. A continuous, interior air barrier ensures that conditioned air remains within the structure, reducing the energy demand associated with heating and cooling. The air barrier also prevents moisture loading of the insulation and interior assembly, which can cause health and structural problems as we said above. By allowing unconditioned exterior air to infiltrate into the conditioned space, and allowing moisture to accumulate in the assembly, small discontinuities in the air barrier can add up to big problems.
Thoughts on Spray Foam as an Air Barrier
Spray foam has been billed for years as the way to meet the dual goals of airtightness and insulation. But despite its widespread use, foam is not a reliably long-lasting air barrier. Think again about that ever-important red line. Foam is inherently discontinuous—it stops at every stud bay, every joint, and every penetration. Building components move, expand, and shrink over time, but because it is rigid, foam cannot stretch as the building moves, or as a wire wiggles back and forth. Instead, it cracks or pulls back from stud bays, leaving holes in the air barrier. Because foam is difficult and expensive to remove, it's almost impossible to repair once it is damaged. And its vapor impermeability prevents drying, which can cause moisture to accumulate in the enclosure.
When it comes to foam, we say less is best. We can deliver healthier, safer, longer-lasting, and better-performing buildings without it. Because superior, continuous, vapor-intelligent air barriers are now readily available, we’re seeing a growing wave of professionals shifting to foam-free building. We research and write frequently about foam use in buildings, and we encourage you to check out our blog (https://foursevenfive.com/foam-fails/) for a full discussion on the subject.
Where Continuity Gets Complicated and How to Deal
Transitions between stories, rooms, materials, and building components can be the most difficult places to ensure solid air barrier connections. Here are some of our approaches to dealing with these challenges.
Going from Room to Room
Although drywall can theoretically be airtight, it simply doesn’t work as a continuous air barrier. Drywall can’t bridge continuously from room to room, meaning that every transition between rooms or from wall to floor constitutes a gap. We refer to drywall as a sacrificial layer, because it will be subjected to wear and tear by occupants; it becomes increasingly unviable as an air barrier over the lifetime of the building.
We recommend that an interior air barrier be attached to the studs before interior framing begins, ensuring that it is continuous from room to room. Once the barrier is attached to the studs, interior framing can be constructed inboard of that. This ensures a linear and continuous air barrier containing within it all the interior components of the building. This calls for designers and builders to practice proper staging of building elements, and to know? when they have the best opportunities for access.
Going from One Floor to the Next
Floor joists and beams should also be contained within the air barrier and shouldn’t penetrate your red line. Making sure that these beams are within the barrier requires advanced planning and smart construction sequencing, but it can really help to ensure continuity.
One simple method of keeping joists within the air barrier is to wrap them with an airtight exterior membrane, such as vapor-open Solitex Mento, before they are set, and to lay this membrane over the floor before the second-story wall is built. After the wall goes up, this membrane can be attached to the interior air barrier, ensuring continuity from one floor to the next. The reason for using a vapor-open membrane at this point is that different wall materials have different vapor profiles. The ideal is to have a vapor-open exterior and a vapor-variable interior—and because these joist pockets are closer to the assembly exterior, a vapor-open membrane makes the most sense. For projects that involve many interior-to-exterior zigzags, there are membranes available that can do double duty, allowing for the use of a single material that snakes between floors.
Another method of ensuring air barrier continuity is to use a single linear interior air barrier along the inside of the service cavity, and to support floor joists with a ledger board within the cavity (see Figure 2). This mitigates the need to wrap the membranes meticulously around the floor joists, and reduces the number of transitions in the air barrier.
Going from One Material to Another
Different materials react differently to temperature and moisture, expanding and shrinking at different rates, and some materials adhere to one another better than others. When two different materials meet, the risk of a compromised air barrier increases significantly. Planning for resiliency at these junctions is key to ensuring air barrier integrity.
From joists and rafters to wall. Contega PV fleece tape can be used to make airtight connections between plaster walls and wooden joists or rafters. Thick layers of plaster are less prone to cracking, and will ensure the effectiveness of the connection. For connections to smooth masonry that will not be plastered, effective taping will be sufficient.
From concrete slab or subfloor to wall. Buildings move over time, so use materials that will allow enough slack in your air barrier to easily accommodate the amount of movement that can occur between the two planes. This holds true for both interior and exterior connections. We advise using adhesive caulks that dry flexible, such as Contega HF. Use primer to prepare a substrate for surfaces that are porous or unstable.
Our only exception to this rule concerns connections from one vapor-closed material to another—for example, a connection between a concrete slab and a subslab vapor barrier. Such a connection may not allow adhesives to cure. In these situations, we use tape made for subslab poly, such as COMPEGO.
From concrete to structural support. Contega Solido SL tapes will adhere to a wide variety of materials, such as membrane, metal, and concrete, and can be further reinforced with Contega HF adhesive. For exterior sealing of joists to masonry, Contega Solido EXO should be used.
Not only must the windows themselves be airtight but the transitions between the window frames and the rest of the barrier must be airtight as well. There are many ways to ensure continuity of the air barrier at the window frames, but it is key to ensure that the membrane is continuous into the window openings and around the frames (see Figure 3). Once the window has been placed into the frame, all window seams should be taped to the continuous air barrier with airtight, flexible tape. Pre-made corners facilitate installation and ensure a robust air barrier.
Air Tight Windows
To learn more, go to www.foursevenfive.com.
Continuous Means Continuous
The best way to ensure a continuous air barrier is by providing the simplest, most straightforward path, limiting the intrusion of structural elements and the number of transition points. And a single air barrier system is best; attempting to combine elements of different systems introduces more transitions and increases the likelihood of gaps. To quote Thoreau, Simplicity, simplicity, simplicity.” Your design can be frittered away by detail; or, with smart planning, you can step out of the way and allow a single, continuous air barrier to do its job, and ensure that your building performs well for years to come.
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