We broke ground in early April 2014, which in Colorado is like playing Russian roulette with the weather. I anticipated we would need about eight months to complete the project before the winter cold and storms would prevent contractors from getting to the site. Favorable weather prevailed, and we were able to meet that timeline.
The house is single-level, with two bedrooms and two baths, and a rectangular shape for optimal passive solar gain. A floating slab foundation was selected (with a thermal break from the stem wall). The wall design differed from the main living area and the garage (a choice I would regret later). The main living area is a double wall—an advanced framed 2-by-6 exterior wall separated by a 3-inch space from a 2-by-4 interior wall—while the garage walls are an advanced framed 2-by-6 wall with 1.5 inches of EPS foam on the exterior.
Meticulous attention was given to air-sealing and minimizing penetrations through the building envelope. Knowing that proper sealing is as important as good insulation, I spent many hours in the evenings foaming and caulking gaps. I performed two blower door tests—one before the drywall was installed to identify missed penetrations and the second when the house was complete. There were a few electrical penetrations I had missed, but the majority was improper sealing of the plastic vapor barrier in the main ceiling area by my framers. The final blower door test resulted in a reading of 161 CFM at 50 pascals, much better than the related Passivhaus standard of 280 CFM.
The slab is insulated underneath to R-20 (5 inches of high-compression-strength EPS) and the external side of the stem walls were covered with 1.5-inch EPS board for another thermal break. The main living area walls have a weighted value of R-45.4 provided by 3 inches of closed-cell spray foam and 9 inches of dense-pack cellulose fill. The garage area walls are R-25.3 with 5.5 inches of dense-pack cellulose and 1.5-inch EPS foam board on the external face. The attic is R-60 from 18 inches of blown-in cellulose.
We built the walls 24 inches above the ceiling level, which allows full ceiling insulation depth all the way to the edges of the walls. I also incorporated dropped ceilings in northern sections of the house which simplified wiring, HVAC, and plumbing runs while minimizing penetrations. We choose fiber cement board on the exterior for wildfire protection and ability to install it ourselves to cut costs.
All the doors are insulated, even the interior ones, for thermal and acoustic separation. For optimal temperature management, the two exterior doors both open to a buffer room that is separate from the main living area. Triple-locking exterior doors increase air sealing and security.
All windows are fiberglass-clad, triple-pane units, but air-filled, instead of gas-filled (given the elevation gain, gas fill would have been lost in transit). There are few windows on the east, west, and north sides; and most windows are non-operable (“fixed”) for the best insulation. The majority of operable windows are awning type, which offer the best air-sealing. Casements were used only where required for emergency egress.
Rolling exterior insulated metal shutters by Rollac were installed for all windows. The economic justification was their additional insulation of about R-1.2 to R-2 to the thermally weakest spaces in the walls, providing security when we are away, and providing wildfire protection. To further decrease the risk of fire damage, all external surfaces are non-flammable.
The standing-seam metal roof met our requirements for ease of maintenance, wildfire protection, and effective rain harvesting. Most roof pitches in Colorado are 6:12 or greater to shed snow, but that was too steep to maximize rainwater harvesting. A 3:12 pitch was selected and snow guards added to “hold” snowfall until it melted and could be captured. Continuous soffit and ridge ventilation help prevent ice damming. A 3-foot overhang provides seasonal shading for the windows.
During the winter months, the average night temperature inside the main living area is in the high 60s to low 70s. Over an average winter night (anywhere from the teens to single digits), that space will lose 3°F to 4°F by morning. In general, with the house temperature “charged” with a couple of days’ worth of winter sun, it can sustain three days of cold temperatures in the low teens and no sunshine before dropping into the very low 60s.