Envelope Design Concept

We’re back! A lot has been going on but we are committed to writing on a more regular basis. We’ve actually started foundation work but I’d like to describe our envelope design concept and then follow up with the actual site work in the next post.

To start with I think it is helpful to understand the envelope design—one of  the most important aspects of our energy efficiency. I’ve spent a lot of time researching green building and high performance houses and visiting informative sites such as Green Building Advisor, Fine Home Building and Building Science Corporation. It’s amazing how much information is out there and it can easily turn into a journey of ‘one more click’ over many hours. This is one of the reasons that I decided to sign up for Marc Rosenbaum’s class on designing a Zero Net Energy Home. What better way to learn about a subject than to be guided by a widely recognized subject matter expert who has already ‘walked the talk’ and can provide the speed tips and practical knowledge based on experience. One of the most important criteria of a good design is implementing a good air barrier to enable a tight building envelope. It was during Marc’s class that I started focusing on using a 12 inch double wall construction for CreekSide Net Zero. Below is the sketch that illustrates the initial design concept that Todd Campbell and Ty Allen at New Energy Works started with as they developed the detailed drawings.

If you’re not into technical details then here’s the summary:

Sub-slab insulation: R15.6

Foundation walls: R24

Walls: R41

Ceiling: R60

Infiltration target: 0.6 ACH50 (air changes per hour at 50 pascals)

How do we get to these values?

Sub-slab insulation: R15.6, 4 inches of EPS foam board (2 – 2 inch thick layers with staggered seams) under 4 inch concrete slab

Foundation walls: R24, 2 inches of Thermeze EPS on inside of 10 inch poured concrete wall (R3.9/ inch), 4 inches of mineral wall board (R4 / inch)

Walls: R41, 12 inch double stud wall with a continuous layer of Intello Plus variable vapor membrane on outside surface of inside wall, 8.5 inches of dense pack cellulose (R3.5 / inch) in wall cavity outside of Intello, 3.5 inches of damp spray cellulose in inside wall cavity

Ceiling: R60, Vented roof, 18 inch I beam with 16.5 inches of dense pack cellulose (R3.5 / inch)

Infiltration target: 0.6 ACH50, careful attention to air sealing, primary wall air barrier is the exterior Zip board sheathing with all seams taped, primary ceiling air barrier is Zip board sheathing with all seams taped mounted to the underside of the I beams

Design concept sketch:

Envelope Concept

The technical info:

The walls: The double wall consists of two 2×4 inch walls separated by 5 inches to provide an all important thermal break between the two wall surfaces. A typical 2 x 6 wall framed 16 inches on center with R19 insulation actually has an overall R value of only about R15 due to the thermal bridging that occurs every 16 inches where the wall studs are located. Using a double wall construction these thermal bridges are eliminated with the open space between the walls.

Filling the entire wall with dense pack cellulose at R3.5 / inch results in an overall R value of R40. However, one of the concerns with the double wall construction is the risk of condensation building up and increasing the moisture content on the inside of the exterior sheathing.

Initially, it seems counter intuitive to think that the thicker the wall the less likely there would be a problem. However, because the wall is thicker and there is less heat escaping, the exterior sheathing is actually colder and thus any vapor that contacts it is more likely to condense. As a result, it becomes very important to control air and vapor movement through the wall. With our double wall design the primary air barrier will be the exterior Zip board sheathing with taped and sealed seams. This design does not address the potential for warm interior air and vapor that will condense if it comes into contact with the cold sheathing. This is where the team from Airtight Services out of Marion, NY made a significant recommendation to modify the design during one of the initial design reviews. The company is owned by Matt Johnson and his two primary co-workers are Matt Bowers and Bill LaBine. They’re certified Passive House consultants and they are very knowledgeable in building science. Their recommendation (which we’re implementing) was to install a continuous vapor retarder membrane called Intello Plus on the outside surface of the inside wall. Pro Clima Intello Plus is a smart vapor retarding membrane that has varying vapor diffusion depending on the season. If the Intello was installed directly behind the drywall it would be susceptible to cuts and tears as the drywall and electrical were being installed. By installing it on the outside surface of the inside wall we’ve effectively created a service cavity within the inside wall that can be filled with damp spray cellulose after the electrical and mechanicals are complete.

During the design process we had a few design enclosure meetings with both the New Energy Works and Airtight Services teams to discuss the implementation details for the envelope design. It was pretty cool to see the collaboration and building on ideas to improve and modify the design to enable NEW to pre-build the walls and ship them to the site in sections.

Here’s a photo of the first prototype double stud wall that the NEW team built to help prove the concept. Left to right in the photo: Todd Campbell, Brad Hall, Bryan Bleier, me (I didn’t get the black jacket memo) and Ty Allen

NEW Double Wall

The Ceiling: I focused on using I beams in order to get enough depth to get to an insulation R value of R60 with an air channel above the insulation to enable a cold roof. When doing the calculations with Marc’s thermal bridge calculator it turns out that we need an 18 inch I beam when sheathing is installed between the beams on the bottom of the upper flange to provide the air channel. Another key component of the energy design is that the primary air barrier will move from the exterior sheathing on the walls across the top sill plate to Zip sheathing which will be installed inside the house on the bottom of the I beams. No can lights for this energy efficient house!

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