One of the most popular high performance walls is the double stud wall. Its appeal lies with the fact that it creates a good thermal break between two simple-to-build wood frame walls. It can also be economical, even before considering long-term energy savings. The open cavities are typically dense-packed with blown-in cellulose or fiberglass for insulation values ranging from R-30 to R-50 (effective). Depending on climate zone, these are insulation levels that can get your project a long way toward Passive House levels of performance.
As the popularity of these walls has grown, so has concern about their moisture management. 475 High Performance Building Supply has looked extensively at the hygrothermal behavior of double stud assemblies. Read our blog post “Keeping Sheathing Dry in High-R Double-Stud Walls – a WUFI study” to review how these types of walls flirt with danger – if not risking failure altogether – if steps are not taken to address condensation at the cold sheathing. As we demonstrate, the sensible, foam-free solution is to provide robust and resilient interior airtightness and smart vapor control with INTELLO Plus (or DB+).
It is also best practice to protect this all-important interior air and vapor control layer. 475 recommends adding a 1.5″ deep service cavity – typically using 2x lumber on its side. While that is code compliant for use with shallow electrical boxes, the industry preference is to run electrical wiring in 2×4 or 2×6 stud walls. In some cases, electrical contractors will only work with standard service cavities. Clearly there is a demand for high performance double stud walls that still provide a full service cavity. Moreover, architects and homeowners who want to maximize interior space are looking for ways to trim down wall depth.
The solution can be found in our set of Double Stud construction details (a free download at 475 CAD Details). We call it the “Integrated Service Cavity” – as opposed to the more conventional (and space hogging) “Inboard Service Cavity”. As shown in drawing 2b, the airtight membrane goes on the back side of the inner stud wall instead of the front. That frees up the inner stud wall for electrical and other services, while keeping the services separate from the air control layer. As with any service cavity, the space can be insulated after the wiring is complete – usually with mineral wool batts. For optimal vapor control when insulating the service cavity, we recommend double stud gaps greater than 3″ to follow the 70:30 rule of exterior to interior insulation.
How to build an Integrated Service Cavity with INTELLO Plus
SEC Inc, in partnership with architect Jean Terwilliger, now of Vermont Integrated Architecture, went all in on a double stud wall with integrated service cavity. Impressively, they pulled it off during the legendary Winter of 2015 – in a part of Vermont that didn’t see temperatures creep above freezing for more than two months. Here’s how they did it.
First, they planned carefully for construction through the winter. That meant getting weathered in as soon as possible. So they first framed and sheathed the load-bearing outer stud wall and second floor TJIs to create an interior work zone. They elected to finish the INTELLO Plus installation on the first floor – to make that space comfortable for ensuing work – before moving upward. Had construction occurred in the warmer months, construction sequencing would have presumably proceeded differently.
After the load-bearing outer stud walls were up, they framed sections of inner stud wall on the floor, stapled INTELLO Plus on the back side, and tilted the assemblies up and into place. They used the 10′ wide (Double Wide INTELLO Plus) product to span the height and length without any joints. That left flaps at the top, bottom and sides for making tape connections with TESCON Vana to adjoining sections. Even though the inner studs will hold back the densepack insulation, we recommend stapling every 4″-6″ o.c. to keep the INTELLO taut and secure. This project had a lot of short wall segments. While longer spans may have required less taping overall, the shorter spans were easy to maneuver.
At corners, INTELLO flaps around the end studs were trimmed and taped neatly for airtightness. Likewise, flaps from under the bottom plate were connected flexibly and durably to the subfloor with CONTEGA HF caulk adhesive. Note the full service cavity for all electrical and other needs. KAFLEX Mono, KAFLEX Post and other gaskets were used to make the limited number of airtight connections outward through the INTELLO Plus.
INTELLO Plus for an airtight truss roof
INTELLO Plus was also used at the interior of the scissor truss roof for airtightness and smart vapor control. Here again, thinking ahead a couple of steps helped simplify the airtight details. Strips of INTELLO Plus were placed strategically to span across interior framed walls. Light fixture boxes and other penetrations were carefully taped or gasketed. At gable end walls, strips of sheathing – inner surface flush with truss – were added for strong edge connections (see below). Also note that INTELLO Plus was applied at the front side of the upper gable end walls – without any service cavity. (While not best practice, the airtight layer is less likely to be compromised in these upper walls – assuming the electricals up there were dealt with in an airtight manner!).
The recipe for this project’s airtightness and ventilation:
- INTELLO Plus (standard and double wide) for interior airtightness and vapor control
- TESCON VANA for flat seams on airtight membranes, window connections
- CONTEGA HF for interior connections from INTELLO Plus to concrete subfloor
- TESCON PRIMER RP to optimize caulk and tape connections to concrete
- UNITAPE PATCHES for reconstituting airtight layer at blow-in holes
- LUNOS e² for decentralized, ductless heat recovery ventilation
- ROFLEX and KAFLEX airtight gaskets