Construction Design-Service Cavity

This post was first published at www.greenbuildingadvisor.com.

There are many methods used to make a home airtight, it all comes down to one simple rule, continuity.  Continuity is easily attained when there’s nothing that penetrates the air barrier.  No electrical boxes, plumbing drains and vents or ducts that need to extend from inside a building’s envelope to the outside.  Of course, there are times when different things need to extend from inside to outside, like the need for an outside water faucet.  But there are also many occasions when different systems end up outside that don’t need to be outside.  For example, forced air heating and cooling ducts that leave the conditioned space of the home simply because there was no space to keep them hidden inside the home.  Planning a service cavity can help keep most mechanicals inside the building envelope.

What is a service cavity?  A service cavity is an intentional space between the finished side of a wall, ceiling or floor and the air barrier.  This space or cavity is designed to contain mechanical, plumbing, and electrical systems.  The service cavity might need to be large to contain heating and cooling ducts, other cavities might be small, simply to keep electrical wires inside.

The easiest way to keep most of these systems inside is to design the home in a way that the exterior structural sheathing, both on the walls, roof, along with whatever system is used for the floor as the main air control layer, Joe Lstiburek’s Perfect Wall.   Building this way makes the entire structure a service cavity, nearly everything stays inside.  This is a great way to build but there may be instances where you can’t or don’t want to use the exterior sheathing as the air barrier (my recent Barndominium project did not have structural sheathing).

Most homes in my cold climate combine the air control and vapor control layers to try to achieve continuity in the air barrier.  This type of air control strategy often uses a membrane attached to the “warm in winter” side of an exterior wall or ceiling.  (This assembly can include an interior vapor barrier and should not be used in hot and humid climates.)  Traditionally, the structure is framed with sheathing installed.  Plumbing, electrical and HVAC systems are installed, then insulating is completed with the air/vapor control installed next.  This creates several holes in the air control by electrical boxes, plumbing pipes and ducts.  Designing a simple service cavity in these wall and ceilings will keep the air control continuous.

Ceiling Service Cavity

There are a lot of systems that need to be in our ceilings.  Lighting, ventilation ducts and plumbing vents are a few of the more common systems poking holes in our ceiling air barriers.  Creating a space between the finished ceiling and air control layer will not only keep a lot of those systems inside the building envelope, but also protect the air barrier from damage.  I’ve used a few different methods.

The entire ceiling could be designed as a service cavity.  Utilizing a 10-foot-tall sidewall, for example, then installing a suspended ceiling at 9-foot would provide a 1-foot service cavity across the entire ceiling.  Usually, the cavity is framed using wood then covered with drywall or some other ceiling finish, but I have seen some older homes use manufactured suspended ceiling systems to hide the mechanical systems.

Designing a soffit to keep mechanical systems inside the building envelope can also be a good choice.  This is accomplished by lowering the ceiling in an area of a home to accommodate HVAC ducts and equipment and plumbing systems.  This type of cavity design is often installed in hallways and closets, but I have seen the design also used in tray and other decorative ceilings.

This dropped ceiling over a closet helped keep the air source heat pump line set inside the conditioned space of the home and simplified the connection to the wall mounted air handler head.

A plenum truss is a roof truss with a designed notch that can be large enough to contain not only ductwork, but also plumbing and electrical systems.  The air barrier follows the ceiling plane, then turns and follows the notch in the truss.  I typically line the notch with whatever product is being used for the air control on the ceiling, such as a smart vapor barrier, (Intello, Majrex, Membrain, etc…), or polyethylene sheeting, (still commonly used in my very cold climate), then line the cavity with plywood or OSB to give the trades places to fasten their systems.  After the systems are installed, the finished ceiling is constructed hiding the cavity chase.  When possible, I try to design the home so that bath fans, kitchen exhaust, and HRV and ERV ducts are able to utilize the space without the need to leave the building envelope.

Another type of service cavity I’ve used on a ceiling is a simple ceiling strap.  It is common in New England to attach 1x wood boards to the bottom of a truss or rafter cord, this technique is not common in my market.  The design I utilize includes a double bottom plate, then install a 2x wood strap to the bottom cord of a truss after the air/vapor control layer is installed.  The reason for the double bottom plate is so that the ceiling height remains standard with the 2x attached to the bottom cord of the truss.  I try to design the system so that all interior walls are built after the ceiling is strapped.  This 1.5-inch-deep cavity will allow most electrical boxes and unprotected wire systems (nonmetallic sheathed cable systems such as Romex) to remain inside the building envelope.  The reason for using 2x material instead of 1x is in the electrical code; NEC 300.4 (D) Cables and Raceways Parallel to Framing Members and Furring Strips.  The code states that wires must maintain a 1.25-inch clearance from the nearest edge of a framing member.  (This code is also found in chapter 38 of the IRC, table E3802.1.)  I had a conversation with my local electrical inspector who will allow the 2x framing member to meet this clearance requirement, he feels the 1x framing does not meet the intent of the code.

Wall Service Cavity

My home, which was built in 1952, used exterior stud bays as return air ducts.  Not the best idea when wintertime temperatures can be colder than -40°F and the cavity is not airtight.  Using a framed cavity as a forced air duct is no longer allowed.  There is a design where an exterior wall could be used as a service cavity (air handling ducts would need to be hard-ducted), by building a double stud wall.  The outer wall of this design is the water, air, vapor and thermal control layer.  The inner wall, which is completely inside the conditioned space, can be used as a service cavity.  Once any HVAC, plumbing and electrical systems are installed, additional insulation can be installed in the “inner” wall to further increase the resistance to heat flow.  There are several good articles on this design in both Fine HomeBuilding and Green Building Advisor.

Another wall service cavity design utilizes the same principles as ceiling strapping, horizontal 2x wood strapping.  Because of the depth of the service cavity, usually this design is for electrical systems, though I have seen water supply lines installed in the cavity in walls with high levels of cavity insulation.  Much like the ceiling strapping design, the walls are framed, insulated with an air/vapor control membrane installed.  The 2x strapping is then nailed to the wall framing with the strapping added last.  The electrical system design will often use metal 4-square electrical boxes with mud rings for outlets and switches.  We have also used gangable metal boxes when more than two switches are needed in one location.  It’s best if the electrician understands why they are installing their systems differently than the norm.  I’ve found a lot of the trades have no training on installing airtight systems.

Floor Service Cavity

Service cavities in floor systems are not an assembly I see often.  I can only recall one, and it was in a commercial building constructed in the early 1980’s.  The concrete slab was poured with 4-inch by 16-inch channels used for forced air distribution and an electrical raceway.  The structure was a single-story office building, and the design was clever, though I do question air quality using concrete ducts in contact with damp soil.

Something I do try to design into floor systems is keeping any utility systems, such as electrical, phone and communication systems from penetrating an exterior wall.  I try to have these systems enter the home under the floor in a slab on grade home or through a below grade wall in a home with a basement or crawlspace.  One less hole through the wall water/air/vapor control layers.  The new electrical code requiring an outside disconnect before the electrical service panel can complicate this a little.  I have also had pushback from the communication companies in my market, they would rather come through a wall with their fiberoptic cable than fish the wire through a conduit.  Both situations can be accomplished with some forethought and communication with the installing contractor.

Electrical and communication conduits placed under slab.

Service cavities to keep mechanicals systems hidden inside the conditioned space of a home are best designed during the planning stage.  I have hundreds of thermal images showing air leaking holes that were punched through the air barrier.  This has led me to the conclusion that it’s a better choice to not create a hole, rather than try to seal one whenever possible.  A service cavity is one solution.