Air-Sealing Tips for Efficiency That Lasts // Part 4: Protect Your Air Barrier

May 24, 2017
Summer 2017
A version of this article appears in the Summer 2017 issue of Home Energy Magazine.
Click here to read more articles about Building Envelope

This is part 4 of a series that describes how to air seal the most difficult parts of buildings.

A service cavity and vented rainscreen promote durable airtightness.

Unless you intend to live like Thoreau, rising with the sun and heating your cabin with a small iron stove, your building will need utilities. But supplying utilities means putting holes in your building assembly—and as we’ve emphasized throughout this series, holes are the enemy of air barrier continuity. So what’s the best way to plan for these incursions into your air barrier without compromising its performance?

For indispensable windows, doors, and utilities—those must-have penetrations—smart planning, good sequencing, and excellent craftsmanship make all the difference. We’ve recommended flexible gaskets and airtight tapes to seal penetrations and connect them to the rest of your air barrier, which works wonders for preserving continuity. But the priority is always to reduce the number of holes; even if you have a great way of sealing penetrations, it’s better not to have penetrations in the first place.

We recommend building a service cavity, constructed inboard of your interior air barrier, and a vented rainscreen, constructed outboard of your weather-resistant barrier (WRB), to preserve the longevity of these layers. In addition to vastly reducing the number of penetrations in your air barrier, these cavities greatly decrease the likelihood that your barrier will be accidentally damaged or penetrated in the years to come—ensuring a robust, long-lasting air barrier that will do its job throughout the life of your building.

The Service Cavity: You’ll Thank Us Later

Although you may need only a single penetration to run water and electrical lines into your building, you may need many more penetrations to distribute water and electricit throughout the building. When you consider every faucet, every outlet, every gas line, you could easily end up with a hundred holes in your air barrier, each one a possible site of leakage. (That’s yet another reason why we advise against using drywall as an air barrier. Not only is drywall discontinuous from room to room, but it can be further compromised by utility penetrations.)

A service cavity constructed inboard of your air barrier helps solve this problem. It offers you a way to distribute utilities throughout the building without multiplying the number of holes in your air barrier, and it makes it much easier to install fixtures and utilities in the future, without worrying about penetrations. After the interior air barrier is installed, the service cavity is constructed by running horizontal battens that will ultimately support your drywall. This creates a space between your drywall and your air barrier in which utilities can be split and distributed, dramatically reducing the number of penetrations needed. It also separates the air barrier from the drywall, which will be cracked, screwed, drilled, and otherwise abused by the building’s occupants. This bit of distance between the two layers is the best insurance against any accidental penetrations in the future.

At the Association for Energy Affordability in the Bronx demo, horizontal battens attached inboard of the air barrier form the structural basis of the service cavity.

To construct the service cavity, we typically recommend attaching 2 x 2 or 2 x 3 battens, or furring strips, to the building studs after the interior air barrier is installed. Horizontal battens are better, because utilities are more often distributed horizontally than vertically, eliminating the need to drill service holes through studs. They also provide counterpressure to blown-in cellulose insulation. This helps to optimize density and distribution of the insulation and maximize its performance. And the 90° angle between the studs and the battens minimizes thermal bridging. After electrical wires and water supply piping have been constructed, gypsum wallboard (GWB) can easily be attached to the furring strips. For even better thermal performance, the service cavity itself can be insulated, increasing the overall R-value of your assembly.

The service cavity is a great example of a smart, affordable, simplifying solution that serves multiple purposes. It distributes utilities without unnecessarily penetrating the air barrier; it protects the air barrier from the wear and tear interior walls are subjected to; it provides an opportunity to add more insulation; and it ensures your air barrier’s health and longevity. But you won’t build one by accident, or stumble on the idea near the end of the construction process. Building a service cavity requires a bit of advance planning—but it makes distributing utilities while also protecting your air barrier infinitely easier, and it protects you from having to individually tape every outlet, every faucet, and every supply pipe (see Figures 1 and 2).

So plan to build yourself a service cavity. You’ll thank us later.

Creating a Service Cavity

Creating a Service Cavity
Figure 1. The service cavity creates a space for utilities to be distributed throughout the building interior. It minimizes penetrations of the air barrier, which is inboard of the service cavity.

Ideal Service Cavity

Ideal Service Cavity
Figure 2.Another look at constructing the ideal service cavity. The interior air barrier is applied over the studs and insulation, horizontal battens are layered over the air barrier, and GWB is applied as a finish over the service cavity, safely set away from the air barrier.

Vented Rainscreens: Protecting Your Weather-Resistant Barrier

We’ve emphasized the importance of your vapor-intelligent, airtight interior air barrier throughout this series. But it’s also important to protect your vapor-open WRB on the building exterior to ensure the long-term performance of your assembly. A vented rainscreen serves much the same purpose as the interior service cavity—it creates a space between the WRB and the building siding, thereby protecting your WRB from damage and ensuring its performance for years to come. It’s also vastly preferable to exterior sheathing in terms of vapor intelligence, as it facilitates drying of your assembly through your vapor-open exterior membrane and WRB.

Rainscreen Details

Rainscreen Details
Figure 3. Details showing exterior vented rainscreens and interior service cavities, along with the double air barrier system we strongly recommend for airtight, high-performance wall construction.

Vertical strapping is applied over the exterior air barrier to construct the vented rainscreen. Horizontal boards attached to the vertical strapping can either serve as the rainscreen, or serve as an attachment point for another layer.

Vertical battens are applied outboard of the WRB, which will serve as the attachment for the vented rainscreen.

Yes, your WRB is weather resistant—hence its name. But to ensure its longevity, you need to protect it from nature’s harshest elements, including bulk water intrusion and wind impact. A vented rainscreen does just that. The cavity is constructed by laying vertical strapping over the WRB and attaching it to the building studs, creating an attachment point for exterior weathering boards and a space into which water can evaporate out through the exterior membrane. This protects your WRB from bulk water intrusion, wind impact, and UV radiation, and maximizes the drying potential of this vapor-open layer.

This vented rainscreen can also serve as a sort of service cavity for utilities. Although exterior utility distribution may be less extensive than interior distribution, you will probably have a hose, lights, and outlets on the building exterior. So a service cavity that facilitates their distribution without increasing the number of penetrations in the assembly is ideal.

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We strongly advise using this dual-membrane system, consisting of a vapor-intelligent or vapor-retarding interior air barrier and a vapor-open, weather-resistant exterior air barrier. We use the Pro Clima system, with Intello Plus as the interior air barrier and Solitex Mento as the exterior air barrier in most cases. This ensures the best air barrier performance and a vapor-intelligent assembly, providing the many advantages that we explain above. e. It also ensures that insulation is held between two airtight layers, which vastly improves its performance—like putting a windbreaker over a thick wool sweater.

A service cavity in the building interior and a vented rainscreen on the exterior will ensure the long-term performance of your air barrier and insulation (see Figure 3). (We also recommend these approaches for your roof. Building these cavities does entail a bit more planning and deliberate sequencing, but it will pay big dividends in the long run.

From tiny houses in California to the largest Passive House in New York City, 475 High Performance Building Supply is a trusted source of free knowledge for building professionals, and an easy-to-use platform for purchasing unique, high-quality European high-performance building envelope solutions—all provided exclusively by 475, and with next-day delivery. We at 475 compile details of air barrier solutions, and have even published our favorites in a downloadable series available in eBook or CAD detail form.

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