Air-Sealing Tips for Efficiency That Lasts // Part 1: Start on the Drawing Board
Start on the Drawing Board
We founded 475 High Performance Building Supply with one primary goal: being a resource for professionals in North America who want to build airtight. Over the years we’ve worked with everyone from tiny-house enthusiasts in Southern California to the firms building the world’s largest Passive House in New York, and we’ve learned endless lessons on how to integrate strategies for long-lasting airtightness. The bulk of what we’ve learned is applicable to all buildings, regardless of size, shape, use, or type. Thanks to a deepening understanding of its value, and to advances in the energy codes demand for airtight building is growing. In this four-part series, we’re laying down our most important recommendations for ensuring building efficiency and comfort by employing sturdy airtightness strategies.
The Red Line
Getting the details of construction right and choosing the most appropriate materials is the first and most important step in making an airtight assembly. That’s why, in this series, we’re beginning on the drawing board. The vast majority of problems related to the air barrier are problems that can be eliminated with some forethought and smart sequencing. So begin by doing the red-line exercise.
Even when it seems like a waste of time, the red-line exercise is well worth the effort. All you need for this exercise is your project details and a red pencil. In plan and section, draw one continuous red line around the building envelope to represent your primary air barrier. Every turn your pencil makes is a transition and needs to be planned for. Circle those transitions. Now sketch out the simplest means you can think of to connect the air barrier at those points. This process will save time and materials, not to mention headaches. And the details you come up with at this stage can be integrated into plans and projects yet to come. Think of this exercise as an investment in future buildings, not just the one at hand.
The earlier in the design process you do this exercise, the better. The discoveries you make can affect decisions on everything from window frames to plywood selection to the footprint of the structure itself. Ideally your continuous red line will not be intersected by interior wall framing, or even joists and rafters. Identifying problems at this critical stage can prevent an air barrier catastrophe on the job site.
This may sound intimidating at first. We know that pushing standard construction toward high performance gets complicated. But there are always ways to simplify the techniques and whatever the challenge may be, you are not the first. If you’re ever in a bind, there are resources widely available that you can turn to.
Continuous Means Continuous
In regard to the air barrier, Chris Corson of EcoCor High Performance Buildings is fond of the phrase Continuous means continuous, and so are we. When it comes to the air barrier, it’s best to stay out of the way and keep continuity simple. Continuous does not mean that the air barrier stops at every window buck; it does not mean continuous up to the rafters; and it does not mean that the air barrier stops and starts at every bay. Continuity is important because a small linear crack is the equivalent to a giant hole in the assembly. At best, a gap in the air barrier will lead to problems with comfort and a reduction in insulation value. At worst, it can allow moisture to penetrate the assembly, leading to mold and rot, and the attendant risks to health and safety.
When preparing a building for a blower door test, we can often get a good sense for how well it will perform from a quick visual inspection of the air barrier. Beyond the obvious gaps, we look for clean right angles and straight lines for tape application. We look for indications that connections are obviously improvised. In a retrofit situation, we look to see if floorboards have been cut back to allow continuity between floors.
We will expand on this subject in greater detail in part 2 of the series, but to illustrate the point, early adopters of airtight building BLDGtyp design-build firms of Brooklyn masterfully plan for continuity between rooms. They apply small strips of airtight smart vapor retarder before the interior wall framing begins, rather than taping at every interior wall connection. These preapplied strips can be attached to the rest of the air barrier membrane with a single piece of flat tape, making the job exponentially easier.
Plan for Intentional Holes
Every penetration through the air barrier, every break in the red line, needs to be carefully considered. The reason we painstakingly review these issues in the design stage is to limit the quantity of these intentional holes in the air barrier. Even after simplifying penetrations, you will still have windows, pipes, and plumbing. Everything inset into the wall, from electrical sockets, light switches, light fixtures, or vent grilles, must be considered in relation to the air control layer.
Dan Hines, of Habitat for Humanity of Washington, D.C. applies this design principle to his strategy of bundling penetrations into a single PVC tube. This eliminates the complication of detailing each individual pipe, wire, and insulated heat pump line set.
Windows, doors, and skylights are our largest intentional holes. It is critical to have a solid plan for air barrier connections to fenestration. Apart from the sheer linear distance of rough opening at play, connections at windows are a fundamental (not to mention noticeable) driver of occupant comfort. And comfort is perhaps the most common reason why homeowners choose to build high performance in the first place.
The Service Cavity Is an Air Barrier's Best Friend
Nothing will simplify the air barrier design more than including a service cavity in the details of your wall assembly. The service cavity is formed by running battens (aka furring strips) horizontally, inboard of the interior air barrier. We’ve seen 2 x 2, 2 x 3, and 2 x 4 battens used to make these service cavities. This creates an area between the drywall and the air barrier, making the electrician’s life easier, and reducing the number of pipe and wire penetrations. We consider drywall to be a sacrificial layer; it will be bumped, broken, and bruised over years of occupancy. For airtightness to last, that gap is crucial.
The service cavity can also help to insulate the building. It can be lined with rigid insulation or (our favorite) sheep wool batts. Also, if the primary insulation is dense packed, the battens will help to support its weight, especially at ceilings.
We know that service cavities are rare in the United States, and that it may be an uphill battle to get them included in the design plan. That said, no other detail is more likely to support long-term airtightness. Furthermore, a service cavity cannot be installed “later”; it must be included right from the drawing board. A primary example of why airtightness begins in the design stage.
To learn more, go to www.foursevenfive.com.
Wrapping It Up
The first step in creating an air-tight building envelope is to do the red-line exercise. The next step is to incorporate airtight drawing into the architectural set. The inclusion of airtight detail sets provides an overall context for the air barrier, and by highlighting critical junctures it makes for a more-accurate design. It gives you a road map for airtightness success—a road map that all started with one red line.
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