Air barrier strategy, redundancy, and responsibility
As a HERS rater I get to regularly talk to designers and builders about air barrier details. There are a lot of misconceptions about where and how the air barrier should be established. A building’s air barrier can seem abstract because it is comprised of many air impermeable materials that have multiple functions and are installed at different times during construction. Yet it is a very real component of the building that will fail, sometimes with major consequences, if it is not continuous and contiguous. A little leakage is often acceptable, yet a significant amount of leakage will compromise the quality of the building and may result in failing energy code.
There are three general approaches to establishing an air barrier and as long as vapor transport is considered, redundant air barriers are a very good thing.
- On the exterior of the assembly. Example- Taped house wrap and/or continuous insulated sheathing in a wall or hot roof system.
- In the middle of the assembly. Closed cell spray foam or other impermeable insulation materials such as closed cell rigid boards. (densepack cellulose is not an air barrier!)
- To the interior of the insulation. Airtight drywall approach by sealing the sheetrock/framing intersections and all outlet and other service penetrations through sheetrock.
What air sealing approach is the best?
The short answer is that it doesn’t matter, as long as there is a strategy and the entire project team is aware of what needs to be done. I prefer the exterior insulated sheathing approach as it is often the most straight forward to execute and to convey to builders because the penetrations are easiest to target (windows, hood vents etc.) In this system the rigid insulation works with the wrap that flashes window openings for a successful air barrier. The added continuous R-value of this assembly often results in a very comfortable living space.
The popular Zip system- integrated sheathing, air, and water resistant barriers- has proven to work as a good air barrier when installed correctly. House wrap and plywood wall sheathing, on the exterior of a ‘typical’ framed wall are adequate air barrier materials, but in reality, there are usually many tears and gaps so this approach is not sufficient. Air and (hopefully not) bulk water can find it’s way into the wall insulation assembly and through openings in the drywall.
For high performance building, redundancy is smart.
For example, even with a tight layer of continuous wall insulation (aka insulated sheathing), it is a good idea to implement cost and time effective aspects of the air-tight drywall approach as a redundant measure. This is actually required, in part, by the Energy Star and Mass Save New Construction rebate programs. AIrsealing details can easily fall through the cracks (pun intended)
Who is responsible for this elusive building Component?
The designer and builder should be responsible for speccing and executing the air barrier. The air barrier can be traced on the plans as a continuous red line in section views: If your pen lifts, then it is not continuos. Some insulation companies offer air sealing or guaranteed air barrier performance, which takes the load off the builder. Yet other are clueless so the GC’s carpenters or the GC him/her self is responsible for doing the detailed caulking or foaming- often while thinking of a thousand other things.
If there is a HERS rater on the project they should be tasked with assessing and inspecting the air barrier at various stages of the building process- including during the preliminary energy modeling phase, before construction starts. As a HERS rater I always discuss these details with the builder as early as possible. My hope is that the question, ’what is your air barrier strategy?” receives less perplexed looks as air barrier design and execution reaches the mainstream.