Q: When I’m building an insulated header, should I place the insulation on the inside or the outside of the assembly?
A: Steven Baczek, a residential architect from Reading, Mass., who specializes in designing durable, low-energy homes, responds: As with many questions regarding building techniques, the answer to this one is, it depends. Before we talk about insulation, we must acknowledge that a header’s primary duty—transferring loads around window and door openings—is structural. Without that structural factor, the header is pretty useless no matter where you place the insulation. But we should also acknowledge that in many situations, headers aren’t a necessary structural ingredient in the wall framing. That’s a topic worthy of its own discussion.
For the sake of this discussion, however, let’s say that we’re building a header for a 2x6 wall—so we’re working with 5 1/2 inches of width. Also, let’s say that we’re using dimensional lumber to make a header for a 3-0 door opening. For the structure of the header, we need two 2-bys, nailed together on a code-approved schedule from both sides. A horizontal 2x6 is also needed under the header for nailing the sheathing outside and the drywall inside. The double 2-by header leaves a 2 1/2-inch space for insulation, and most builders use 2-inch rigid foam and leave a 1/2-inch air space. But here is where the strategies vary.
For the best building performance, placing the insulation on the outside keeps the header warm and puts the thermal break on the outside of the header (see illustration, above). Note that in this scenario, the sheathing cannot be nailed directly to the header. Alternatively, the typical treatment of placing a layer of 2-inch rigid foam on the inside of the 2-bys also provides a thermal break for the header. However, when using this method in cold climates, it’s important to seal the edges of the foam; otherwise, condensation may form on the cold, outer face of the insulation (against the 2-bys) and, over time, lead to rot.
When we use a double 2-by header on the high-performance houses that we build in high-wind areas, we always put our structure to the outside, which allows us to nail the sheathing directly to the structural header. The insulation installers can fill the space on the inside of the header with the same cavity insulation that they use in the stud bays. With this configuration, we usually provide air-sealing for the entire wall at the sheathing.
