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Materials & Assemblies, Project Spotlights

Project Spotlight – INTELLO Plus with “Integrated Service Cavity” in Leicester VT

May 7, 2015

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.

IMG_6866        IMG_6860A 
At windows, INTELLO Plus was wrapped and taped into framed openings. Before windows went in, a strip of INTELLO was added to connect the inner and outer walls and contain the blown-in insulation.

So that’s one way to build double stud walls with an integrated service cavity.  Read on to see how they continued the INTELLO Plus up across the ceiling.

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!).

For this roof, two 10′ (double-wide) and one 5′ strip of INTELLO Plus running lengthwise covered the width. Seams were taped and strapping was added to help support the weight of the cellulose and keep bulging to a minimum.

Congratulations to the hardworking folks at SEC and VIA for a job well done! We’ll report back soon with updates on the blower door test and more details.

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

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6 Responses to Project Spotlight – INTELLO Plus with “Integrated Service Cavity” in Leicester VT

  1. George Carvill May 11, 2015 at 6:13 am #

    Please reply…
    It is not clear to me how the cellulose gets blown in. Does the Intello get holes punched in it or what?

    • Oliver May 11, 2015 at 9:26 am #

      Yes, holes are punched in the airtight Intello Plus layer so that nozzles can be inserted for densepacking with cellulose or other blown-in insulation. Once complete, the airtight layer is reconstituted with tape/patches.

  2. James May 19, 2015 at 10:01 pm #

    Interesting details. For the 70% – 30% rule does any insulation outside the exterior sheathing and weather barrier count toward the 70%? Keep up the great articles.

    • Oliver May 20, 2015 at 9:35 am #

      As long as there are no vent spaces or other gaps, all exterior insulation would be taken into account for this calculation.

  3. Bobby May 24, 2015 at 11:13 am #

    Hi, I have been designing an extension to an existing building in Brooklyn and we are specifying the INTELLO Plus with “Integrated Service Cavity”. However, due to proximity of other buildings, the exterior walls must be 2-hour rated — so 2 layers of gypsum wall board each side of the stud per approved UL designs. In these situations, would you recommend we still apply the Intello to the inside of the stud and then 2 layers of wall board over that, then a service cavity? I worry about additional unintentional penetrations into the membrane once it’s burred that deep into the wall. If you know of any case studies on this type of assembly yet or feedback on if another direction should be considered — this would be appreciated. I have already been working closely with 475’s High Performance Historic Masonry Retrofits Book on the project so far. Thank you!

    Bobby Johnston, AIA
    Co Adaptive Architecture

  4. Floris June 5, 2015 at 3:43 pm #


    In general, screws from the drywall that penetrate the INTELLO are not a concern – this because if the drywall is clamped by the screw/batten, it won’t be able to leak.

    However, if drywall is attached directly to studs then the outlets in that wall should be installed in airtight boxes (LESSCO or INSTAABOX) so they can be sealed back to the INTELLO.

    if homeowner were to hang some pictures with long screws etc at a later point in time. This could would make a small hole in the INTELLO. Because of the large inward drying reserve of INTELLO, this is ok and one of the reasons to use a intelligent/smart vapor retarder (and not a fixed one), since it does allow unintentional (unanticipated) moisture from leaks and diffusion to dry inward.

    Let us know if you have any other questions.

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