Conventional wood-framed walls perform many functions. Exterior walls are supposed to support the roof load, resist racking, and provide insulation. They must also provide space for routing electrical cables and (in some cases) plumbing pipes or even ductwork. If the walls are built properly, they should also include an air barrier.
Timber-frame manufacturer Tedd Benson has criticized our conventional approach to building walls. He calls these multipurpose walls “entangled,” and he proposes a new approach to wall building — one that .
According to Benson, “Wires and fixtures should be separated from high-performance wall and roof systems whenever possible. [At Bensonwood Homes,] we add a mechanical chase layer to our walls always, and to our roof panels when there is a heavy mechanical demand in that area. Otherwise, we have dedicated chases at the building peak, at the eave and other strategic locations when necessary. The point is that after sealing a building…, we need to also make changes and upgrades easy and provide a path other than one that violates the integrity of the envelope.”
There are many ways to disentangle utilities from the other functions of a wall. Approaches range from timber-framing, to PERSIST construction, to the use of Larsen trusses, to a variety of techniques adopted by Passivhaus builders in Europe. One element shared by many of these techniques is the service cavity.
What is a service cavity?
A service cavity (installationsebene in German) is a “wall-within-a-wall” — a secondary wall on the inside of an exterior wall. It can be framed conventionally, using vertical 2x4s or 2x3s, or it can be created by installing horizontal 2×2 or 2×3 strapping. The main purpose of a service cavity (also called a “service core”) is to provide room to run wiring, plumbing, and ductwork.
Most proponents of service cavities recommend the installation of an air barrier between the service cavity and the wall insulation. This air barrier can be a flexible membrane — for example, a “smart retarder” membrane — or rigid sheets (OSB or plywood) with taped seams.
Ceilings, like walls, can be equipped with a service cavity. For example, at the Freas house in Olympia, Washington, workers from the Artisans Group framed a ceiling service cavity with 2×4 ceiling joists that hang from the structural joists. The two joist assemblies — an upper (structural) one and a lower one for services — are separated by an OSB air barrier.
If you are a purist on the issue of disentanglement, you won’t fill your service cavity with insulation. Rather, the service cavity will remain empty of insulation for the life of the building, to facilitate the work of electricians when the home is remodeled.
On the other hand, some builders go ahead and insulate the service cavity once electrical and plumbing rough-in work is complete.
You’ll have to give up some interior space, but there are benefits
There are two main drawbacks to installing a service cavity:
- A service cavity entails additional cost in materials and labor, and
- A service cavity takes up interior space.
Weighed against these drawbacks are several advantages:
- A service cavity provides a way to locate a durable air barrier on the interior side of the wall, where it belongs.
- Since most or all of the wall insulation is on the exterior side of the service cavity, plumbing pipes are protected from freezing.
- Installing the air barrier on the exterior side of a service cavity is easier than the Airtight Drywall Approach, because there is no need to seal around electrical boxes.
- If the air barrier is located on the exterior side of a service cavity, it will be less likely to be damaged by homeowner alterations.
- If it is detailed properly, an air barrier on the exterior side of a service cavity can also function as a vapor retarder.
- If you use plywood or OSB as the interior air barrier, the air barrier can provide shear bracing (which some types of exterior sheathing, like thin fiberboard, may not be able to provide).
Five key questions
There are many ways to build a service cavity. If you want to include a service cavity in your next home, you’ll need to answer five questions:
- Should the service cavity be framed with vertical studs or horizontal strapping?
- Assuming your walls have double or triple framing, which wall is the bearing wall?
- Where do you want your air barrier: at the outside sheathing layer, directly behind the service cavity, or at both locations?
- What’s the best air barrier material: OSB, plywood, or a flexible air-barrier membrane that comes in a roll?
- Should the service cavity include insulation?
Vertical framing or horizontal framing?
For decades, many builders in New England and Canada have been installing 2×3 horizontal strapping on the interior side of exterior walls. (On the Breaktime forum at the Fine Homebuilding website, some posters referred to this type of wall as a “Mooney” wall, until other posters objected to one builder taking credit for a long-established practice.)
Although the two main reasons that builders install horizontal strapping are to interrupt thermal bridging and to create a thicker wall that holds more insulation, many builders install a polyethylene air barrier behind the horizontal strapping. This allows them to run wiring without worrying about patching holes in the polyethylene air barrier.
When it comes to framing a service cavity, many European builders prefer horizontal strapping to vertical studs. , notes that service cavity framing can be either vertical or horizontal. “This wall-inside-a-wall can be a structural 2Ã—4 wall in lightweight construction,” Buisman wrote. “Or you can run the 2Ã—2s (or 2Ã—3s on edge) horizontally when the structure is on the exterior. This makes the electrician’s life pretty easy as he no longer has to drill through a large number of studs to get the outlets where they are needed.”
Albert Rooks of Olympia, Washington, another distributor of European building materials, wrote, “Try splitting a 2×6 to create the service cavity — 2 5/8 inch is just enough space and 2x6s (for us) are the best buy. Less interior space is lost to the cavity. Also: It takes some willingness for adventure, but think of framing the service cavity with split 2x6s horizontally rather than vertically. It’s faster to wire and it’s really only to hang the drywall. Once you’re just adding furring strips, there is no reason that they need to be vertical. They are only a nailing plane for the drywall.”
John Brooks, an architect from Dallas, Texas, points out that horizontal strapping “helps reduce thermal bridging and convection in the cavity.”
Lucas Durand of Neebing, Ontario, is including a service cavity in his new house. He wrote, “I will be installing 2x3s horizontally for my service core. Faster wiring (not boring holes) was what really appealed to me.”
A service cavity protects the air barrier
According to Patrick Haak, a sales representative for Swiss tape manufacturer Siga, a service cavity “protects the air barrier against penetrations caused by installation or later by the occupants. The cost difference for this additional layer is usually not significant.”
When it comes to installing an air barrier on the inside of a wall, many North American builders are most familiar with the . While the ADA is a time-tested method of building a tight wall, some Europeans worry that drywall air barriers are vulnerable to abuse. : “After the insulation and drywall is installed, the airtight layer behind the service cavity will be protected from nails to hang pictures or screws to hang shelves; and even wires for phones, cables and Internet, etc., can be easily installed without compromising the airtightness of the building for years to come.”
But in the eyes of some North American builders, the European worry about the vulnerability of ADA air barriers is overstated. According to Matthew Amman, a builder in Ashland, Oregon, “I understand the service cavity idea, but I don’t think it guarantees any less possibility of air barrier compromise. … Remodeling is due mostly to aesthetics, and judging Passivhaus buildings on this, these buildings will be remodeled for sure. This will most definitely interrupt the service cavity and the whole wall in general, but the whole idea that a building will be airtight is false, no matter how you build it. I haven’t seen any building with an ach of 0, and these tests are performed with the building in a state of closure that I am sure will not remain in daily use. As far as I concerned, any cutting and patching done in drywall is then taped and mudded, thus air sealing it once again. To imply that Passivhauses in America will not be remodeled, thus air barrier temporarily broken, is utterly ridiculous.”
However, Albert Rooks is quick with a counterargument. “I disagree about ADA being a good solution because it is repairable,” Rooks wrote. “The point of it being repairable is a bit foolish. If you’re building big thick cavity walls in cold climates, a service cavity is certainly the way to go. Put the pressure barrier away from the finish surface so that it doesn’t need to get repaired. … The ‘repairable ADA scenario’ requires an understanding by a homeowner that something needs to be repaired — a very unlikely occurrence. Whereas putting the issue out of reach is certainly more reliable. I of course have sympathy for the cost of adding a service cavity. These things have to move by progression and it’s tough to add cost.”
Air barrier material choices: OSB or a membrane sold in a roll?
Several European manufacturers sell air barriers that come in a roll. Some of these are exterior air barriers; in the U.S., these products are usually called water-resistant barriers (WRBs). Other products are interior air barriers; in the U.S., these products are usually called “smart retarders.”
Floris Buisman’s company, Four Seven Five, sells an interior air barrier called Intello Plus. , “The larger stud walls behind the [air barrier] membrane should be filled with blown-in cellulose after installation of the Intello Plus, or any fibrous insulation can be installed beforehand.”
However, most North American builders who include service cavities install a layer of OSB directly behind the service cavity. Once the seams between the OSB are taped, the OSB serves as an air barrier. According to GBA reader Jan Nielsen, “After spending all this effort to get a super tight envelope, the last thing I want is a little electrical or plumbing work to compromise the envelope and cause moisture damage over time, so I really like the idea of OSB or plywood behind a service cavity.”
Albert Rooks points out that OSB can double as a vapor retarder. He wrote, “OSB is around 0.7 perm. This is enough to stop large amounts of interior vapor penetrating the cavities. And it’s still just vapor-open enough to let things dry to the interior when the levels are higher in the cavity than the interior. It’s a failsafe feature.”
Should a service cavity be insulated?
When the PERSIST construction method was developed in Canada in the 1960s, stud bays were always left uninsulated. The stud walls held up the roof and provided a space for wiring, but they were not intended to be filled with insulation.
Many Passivhaus builders have adopted the PERSIST approach to service cavities. However, others like to see the cavities filled with insulation. Garth Sproule, a GBA reader from Canada, wrote, “Maybe the best way to stop convection in the service cavity would be to throw in some cheap batt insulation before drywall — boosts the total R-value a little as well.”
Another fan of insulated service cavities is Albert Rooks, who wrote, “I like adding insulation into the service cavity. It’s an available cavity that is ready to fill. … To take the idea a little farther, for the Passive House designer, if they don’t insulate the cavity, it counts as treated floor area in the PHPP. [Editor’s note: Floris Keverling Buisman disputes Rooks’s statement; see posted comments below for more information on this topic.] Not that it’s all about Passive House, it’s just illuminating that it counts either as insulated assembly, or as heated floor area. It’s not something that can be set aside and ignored. It has a value in either retention or consumption of the heating/cooling load.”
What about buildability?
It’s easy enough to draw a complicated wall sandwich on a piece of paper. If a designer wants, a wall can include three stud walls — two walls to define the insulated portion of the wall, and a third wall as service cavity — with an OSB air barrier in between.
However, in addition to adding up the cost of such a complicated wall, it’s important to consider buildability. Whether the air barrier consists of OSB or a flexible membrane, seams will need to be taped, and the air barrier will need to be sealed at the top and bottom. Insulation must be installed in a way that doesn’t interfere with the air-sealing steps. Before deciding on a wall assembly, a designer has to think through each step required for insulation and air sealing.
While OSB is a durable air barrier, many cellulose installers prefer to blow cellulose behind a membrane like InsulWeb — because poking the InsulWeb provides real-time feedback on cellulose density. If InsulWeb is used, how will it be attached to the studs? Will the wall’s air barrier be installed before or after the cellulose is installed?
According to Albert Rooks, the air barrier seams can be taped before the service cavity wall is raised. “Place OSB on the backside of the interior stud wall and tape it. Tilt it up onto gaskets (Resource Conservation Technology).”
Since there are many ways to build a service cavity, no universal rules apply except this one: the wall must be buildable.
Weighing the pluses and minuses
Many experienced designers of superinsulated houses have decided that service cavities make sense. According to Thorsten Chlupp, a builder in Fairbanks, Alaska, the service cavity framing “is now part of your conditioned space and is utilized as your utility chase — electrical, mechanical, and plumbing, which opens many more options on how you can run your utilities. It is your structural building core. It can be insulated if additional R-value is needed and the 1/3 to 2/3 insulation ratios in-between you pressure boundary is maintained. I actually think 40/60 is still a safe ratio. The insulation is wrapped around the exterior of the structure and keeps everything warm and is protected with a weather barrier.”
Chlupp continued, “The key in both assemblies is the placement of the airtightness layer within the assembly, where it can be continuous and (most importantly) protected. If you place your poly/air/vapor layer on the inside frame face you just can’t get there. Even if you did the best job (and hardest) to air seal you be surprised to see how fast this can deteriorate a few years with homeowners taking over. And you fight your installations of electrical, mechanical and plumbing — now, today and in the future. An installation layer is simply invaluable. Combine this with a foolproof pressure boundary and you got a system which just works.”
According to Garth Sproule, service cavities have both pluses and minuses. “I really like how using a service cavity makes it so easy to create an air barrier that is easy to build and requires no maintenance,” he wrote. “It also allows rewiring and re-plumbing without disturbing the pressure boundary. The costs, however, are not insignificant. Apart from the obvious costs of the materials and labor involved, we must not forget that we are giving up significant interior floor space. This unusable space must still be conditioned the same as the rest of the interior. For example, on a 30′ x 40′ building with a 3.5-inch service cavity, we have to give up about 40 square feet of floor area.”
Taking all these factors into consideration, Lucas Durand concludes that service cavities make sense. “I think the issue of the extra cost of a service cavity is a little overblown,” he wrote. “The loss of interior space is a fact. But what wall system doesn’t involve a compromise of some type?”
Last week’s blog: “Fukushima’s No-Entry Zone.”