The Service Cavity: Making Airtight Construction Easy
We know that the importance of airtightness is second only to keeping the rain out. This is because airtightness greatly effects indoor air quality, thermal comfort, and energy efficiency - while protecting the enclosure from moisture damage (see here). Given these pretty convincing reasons, what should our airtightness target be, and how does a service cavity help us achieve it?
The challenge to reach recommended airsealing results – 3.0ACH50 in the new building codes or 0.6ACH50 to meet Passive House requirements - can be readily overcome with good strategies. One key strategy, relatively new to the U.S. construction market but gaining ground, is to install a service cavity - or “installationsebene" in German. A service cavity containing electrical and plumbing services greatly reduces the number of penetrations through the control layers. So instead of hundreds of switches and lights puncturing the control layers and potentially compromising performance, the liability is minimized and greater robustness is assured.
The service cavity is formed by running 2x2 battens (or 2x3 on edge) horizontally inboard of the air and vapor control layer. This makes the electrician's life easier, as they no longer have to drill through a large number of studs to wire the outlets and switches. The back of the service cavity can be made airtight with an intelligent vapor retarder (INTELLO Plus or DB+), or structural sheathing (preferably plywood) - in both cases taped with TESCON Vana at all seams and edges.
The service cavity can also be employed in a gut renovation of a Brownstone, or other buildings with structural brick walls. We offer plenty of service cavity details mixed in with our free CAD details and High Performance Historic Masonry Retrofits downloadable ebook. The airtightness of brick and mortar when the blowerdoor is creating a pressure difference of 50Pa is surprisingly low. To construct an airtight layer, first offset a 2x4 at sufficient depth from the brickwork for insulation purposes, then INTELLO Plus along with a service zone to the inside of this assembly. In this way a verifiable airtight layer can be constructed quickly, especially in comparison to coating the brick wall with a liquid applied membrane.
The larger stud walls behind the membrane should be filled with blown-in fiberous insulation after installation of the INTELLO Plus, or batt fibrous insulation can be installed beforehand. The blown-in holes can be quickly and easily repaired with Tescon Vana 150 (as shown below). The building is now ready to be blowerdoor tested, which should be early enough in the construction phase to correct any mistakes and get the building as tight as a Passive House / Enerphit project.
After all services are installed and before installing the drywall, optional fibrous insulation or rock wool can then also be installed in that cavity.
Of course you would want to make sure that during this period no new holes are made in the airtight membrane. Instruct plumbers, electricians, and other trades that might carry a utility knife to not act like Zorro! After the insulation and drywall is installed, the airtight layer behind the service cavity will be protected from nails to hang pictures, screws to hang shelves , etc. Even wires for phones, cable, and internet can be easily installed without compromising the airtightness of the building for years to come.