August 17, 2016

How To Stay Airtight Without A Service Cavity

Intro / Caveat

As you may know, 475 recommends you use a service cavity on the interior of high performance, airtight building envelopes. We’ve enumerated the values of service cavities many times, in many ways. So to be clear, our stance on the subject remains firm: service cavities are the best case scenario, and the recommended detail for any airtight assembly. With that out of the way…..

How To Stay Airtight Without A Service Cavity

It’s really quite simple: every penetration through the air barrier needs to carefully considered. When you don’t have a service cavity, the number of these penetrations may add up. We’ve covered our best practices for the major penetrations that cannot be avoided: Windows, and Existing Joists. Most everything else you’ll come across is probably inset into the wall, whether that be electrical sockets, light switches, light fixtures, or vent grills. For all these elements, the air barrier will just need to go around it, dodging each switch box.

Since junction boxes and “airtight” light fixtures are not airtight, there are a few ways you could do this. One way is to wrap around each and every fixture with the INTELLO, like a Christmas present. Since we can already hear the groan that last sentence is producing, you can stop calculating the hours of labor that would take now. We have two options for airtight boxes that have considered this problem already: the INSTAABOX from Pro Clima, and the LESSCO Box from Low Energy Systems Supply Company, Inc. Functionally they’re similar, but each have unique features to help you deal with different situations. We list a few notable differences between the two boxes below.

The installation of these boxes goes as follows:

  1. Attach the airtight boxes to the stud along with the utility box (or other penetration)
  2. Poke a hole through the side to connect wires
  3. Tape off the wires as they enter the box (see this video tutorial)
  4. Apply INTELLO Plus to the wall
  5. Tape the lip of the airtight box along with any INTELLO Plus seams

IMG_3353 intellomashpee
 

Airtight box options

 

INSTAABOX:

  • Size of box interior: 10.2” x 5.1”
  • Box depth: 2.2”
  • Lip size: 1.2”
  • Does not need to be mechanically fastened to the stud
  • Made of flexible material – wire holes can be punched
    small, so the flexibility helps the box stay airtight.
  • Vapor permeable with class III vapor retarding properties

LESSCO Box:

  • Size of box interior: 6.5″ x 7.5″
  • Box depth: 2.9”
  • Lip size: 1.2”
  • Must be mechanically fastened to the stud
  • Made of rigid material
  • Vapor-closed

Given the differences in dimension, shape and material, you have a choice over which type of box will be best for your particular application. Flexibility vs rigidity, and the 2.2″ vs 2.9″ depth tend to be the two biggest factors that determine which way people go. If the INTELLO is to be pressed just behind the drywall, the INSTAABOX will be too shallow for most switch boxes.

However, there is an option for creating a reliable gap between the INTELLO and drywall without using a service cavity: FastWeb Strips. When using a vapor barrier or dense-pack netting, some installers like to staple back the material on to the sides of the studs to give an extra 1/4″ gap. This is sometimes called “inset stapling” or “lip stitching” (let us know if you’ve heard another term in the comments below). FastWeb Strips offer a high performance version of this method. One solid strip can span the length of the stud, held in place by a few screws. The strips hold INTELLO against the studs (wood or metal) but adds an extra 1/4″ space behind the drywall. See the video below for an introduction.

Gaskets Galore

Pipe and cable connections ROFLEX and KAFLEX, INSTAABOXThis one is quick and easy: you’ll need to seal every wire and pipe each time they pass through the air barrier. This probably means you’re taking advantage of our bulk rates on ROFLEX and KAFLEX gaskets. [ROFLEX is for pipes, KAFLEX is for wires]

We get glowing reviews about the amount of time and energy these gaskets save in the airsealing process. You just slip them on as you install the pipe or wire. Done. Friction fit. They’re EPDM rubber, so each gasket is still airtight if they move around. We suggest you hand your electrician and plumber a box or two and just have them slip them on as they work. When the air sealing team comes by, it takes just a few pieces of tape for a simple airtight seal.

Summary

In short, airtight walls can be done very well with FastWeb Strips, INSTAABOXes and LESSCO boxes. It just may take more labor to attend to each switch box and light fixture individually than simply strapping the wall with 2×3’s. What can’t be changed in a wall without a service cavity: holes in the drywall for hanging pictures or anchoring shelves will likely be a hole in the INTELLO. That’s a primary reason why service cavities will always be our top recommendation.

no-service-cavity

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