More On The Building Envelope

Lots of folks have been keen to know what our wall and roof assemblies - the building envelope - are made of. Well, wonder no more, friends.

For context, the building envelope, the collection of assemblies that separate the inside from the outside, is arguably the most important part of any home. It’s the primary barrier against the elements and the key to comfort, durability, and efficiency. As we tell our clients, you can always switch out your light fixtures over time, but you only build your enclosure once so make it a good one. For the Spring Street Passive House, we landed on a set of assemblies that balance performance, energy efficiency, carbon, occupant health and comfort, and cost.

Walls

Our wall frame assembly is a 10-inch double-stud wall with two 2x4 wall studs. This creates a large cavity that we can completely fill with dense-pack cellulose with an R-value of 37. Cellulose insulation is made from recycled newspaper with very low embodied carbon. The air gap between the doubled studs eliminate most of the thermal bridging. The result is a highly-insulated, vapor-open wall system that can dry out if it ever gets wet. 

We settled into this idea after a few rounds of deliberation. A single 2x8 stud wall would pass code and possibly be enough for PHIUS certification, and is a simple, standard wall frame that carpenters in the area are accustomed to. While the Gorge experiences cold temperatures, winters are comparatively mild with average lows in the 30s. We hoped this  wall framing choice could be place to save costs. However, after to talking with our friend and colleague Skylar Swinford who recently installed a double-stud wall in one of his projects we reconsidered. Skylar explained that the double stud wall gives benefits that greatly outweigh the increased cost, which he reasoned would be minimal. We took the idea to our builder Brent Foster who agreed that additional 2x4s and larger top plates plus the extra few inches of insulation would be a minimal material cost increase, and his crew could get this done efficiently. This approach would increase R-Value of the wall without needing to apply exterior continuous insulation. We love this as an example of how increased performance can sometimes be achieved with minimal cost increase, given the right information at the right time.

Our architect, Scott Witt, applied his mastery in high performance enclosures and completed the rain screen assembly design. From inside to outside it consists of:

1. Natural lime plaster or Mineral-based paint on sheetrock

2. 2x4 “extra” stud to extend insulation cavity

3. R-37, 9-¼” cavity fill cellulose insulation

4. 2x4 structural stud wall (with 2x10 top plate)

5. Sheathing

6. Solitex Mento 1000 Vapor Open Weather Resistive Barrier, fully adhered

7. Ventilated rain screen (¾” furring strips)

8. Weathered steel (aka Corten) sheathing, 22 gauge

Roof

Due to building height limits our architect Scott Witt had to really lean into his creative process to fit a high performance roof assembly into the building. 

Spray Foam Question. There were several times during design when we considered the benefits of spray foam to solve some of the challenges of this compact roof. Spray foam insulation has grown in popularity in residential construction the past few decades. We really didn’t want to use foam if we could avoid it. We’ll do a whole post with thoughts on a foam free house. So we found ourselves again relying on the expertise of Scott to solve the challenge of the roof assembly without the use of spray foam. 

From inside to outside it consists of:

1. Natural lime plaster or Mineral-based paint on sheetrock

2. Air Gap?

3. R-80, 20” cavity fill dense pack cellulose insulation

4. Water and Air Barrier, Solitex Mento Plus over 2x rafters

5. Vented rain screen (2x4 battens)

6. Sheathing ⅝” CDX 

7. Vapor open fully adhered underlayment

8. 1x4 Wood Lath

9. Metal Roof