Ashtree Passive House
By the Numbers
|Location||Boise, Idaho 83712|
|Size||2845 SF (iCFA), 3250 SF (gross)|
|Construction||VB light-frame wood construction|
|Walls||2x6 structural wall w/ spray-applied cellulose
OSB (air barrier)
8 inch site-built z-girts (2x2 w/ cont. OSB web) w/ spray-applied cellulose
Rainscreen (1x3 battens w/ stucco or fiber-cement board siding)
|Roof||2x4 service cavity
OSB (air barrier)
Manufactured trusses w/ 18” loose-fill cellulose
Standing seam metal roof.
Frost-protected shallow slab
Heating & Cooling:
Frame: Zola Thermo uPVC
Glazing: Zola Triple Improved
Chris & Ann Vonde
727 N. Ashtree Way
Boise, ID 83712
Scott Yribar (Vaughn Yribar Architecture)
442 W. Thatcher St.
Boise, ID 83702
Steve O’Connor (Enerform, LLC)
317 W. Boise Ave.
Boise, ID 83706
Skylar Swinford (ESCO)
1619 N. Phillippi St.
Boise, ID 83706
Gabe Border (Gabe Border Photography)
2007 S Latah St
Boise, ID 83705
The Ashtree Passive House is a single-family residence located in Boise, Idaho. The purpose of the project was to create a beautiful and energy-efficient home for a young couple and their one-year old daughter. The home is 3250 gross square feet with a two-car garage. The goal of the project was to create a warm, contemporary home that would take advantage of the views and provide a connection to the outdoors. At the same time the clients set out to meet the Passive House building standard. The project was completed in January 2016 and will be Idaho’s first PHIUS+ Certified Passive House.
The project site was largely conducive to traditional passive-solar strategies in that the desired views were generally facing south. However, overheating due to large areas of south-facing became one of the largest design challenges. Another challenge was utilizing a slab-on-grade on the sloped site.
Design Solutions – Site
The project was sited to take advantage of views to the south. The north-south axis is oriented 7 degrees east of due south. This arrangement allowed the fixed shading devices (canopy & roof overhangs) to provide effective shading during the hottest summer months yet allow for plentiful solar gains during colder months. The house was also positioned on the site to allow for a flat pad to be created. This required a significant amount of excavation through hard basalt but provided a solution to utilize a slab-on-grade, the optimal way to insulate the floor.
Design Solutions – Envelope
The envelope was designed to allow for traditional 2x6 framing at 16 inches on center, a method easily understood and executed by the local trades. The required structural OSB sheathing was used as the air barrier. All seams of the sheathing were taped using Siga Wigulv. The base of the walls was simply taped to the exterior edge of the slab, which sits on a 15 mil vapor barrier. A fluid-applied membrane was used on the exterior side of the slab before two layers of 3 inch EPS insulation were attached. The OSB air barrier was continued on the underside of manufactured trusses by means of an 8 inch strip of OSB installed on the top plate and taped before the trusses were set. A site-fabricated z-girt consisting of offset 2x2 with a continuous web of OSB was then attached to the exterior side of the walls. Both the structural wall cavity and the exterior z-girt cavity were filled with spray-applied cellulose. The roof was insulated with 18” of loose-fill cellulose in a vented attic space. An interior service cavity was added to the underside of the second-floor roof to provide space to run the Zehnder Comfotube ducts for the HRV. A critical design challenge was control of unwanted solar gains. This was largely accomplished with by generous roof eaves and a fixed shading canopy on the south side. On the large south windows exterior roller shades were utilized.
Design Solutions – MEP Systems
Both heating and cooling are provided by two Mitsubishi ductless heat pump units. A 12,000 Btu/h unit is located on the south side of the lower level and a 9,000 Bth/h unit is located on the south side of the upper level. The City of Boise Building Department required a heat source to be present in each bedroom so three electric resistance baseboard heaters were installed to satisfy this requirement. They also provide a backup heat source during an extremely cold period when the temperature drops below the operating range of the miniplits. Natural gas was available at the site so hot water is provided by an HTP Phoenix Light Duty condensing gas water heater with a 50-gallon tank. Ventilation is provided by a Zehnder ComfoAir 350 HRV. A 6kW PV array was added a year after the project was complete. The system was sized to offset site electricity usage, but not natural gas. A monitoring system is in place and it will be interesting to see how production versus total site energy will compare after a year or so.
The project was built by a new team that had not previously executed a new-build project. This was both a challenge and an advantage. It was a challenge because general project management had to be learned as the project progressed. But it also was advantageous because the team didn’t have any preconceived notions about how things should be done. The team was open to all of the assembly details and was willing to take on tasks that other builders might think were two tedious or expensive. There were many lessons learned but the team was uncompromising in their efforts to execute correct, quality details. The house was not completed in record time but the attention to detail at all stages of the project resulted in a very successful outcome.
The rating process for the project went exceptionally smooth. I performed a pre-insulation blower door test after the OSB sheathing was air-sealed and the windows and doors were installed to verify the integrity of the air-barrier. The initial result was well below the project’s target of 0.6ACH50 and essentially no bypasses in the air-barrier were evident during the testing. After the project was completed, I performed a final blower door test and the project had 78 CFM of leakage which equates to 0.20ACH50. This result was especially impressive because this project utilizes a kitchen exhaust hood with a dedicated makeup air system. The exhaust and makeup air duct were each sealed with mechanical dampers that clearly performed well during the final testing.
From the Homeowner
The large windows are our favorite part of the design. They take advantage of the views of the Boise foothills and our backyard garden. The house promotes a feeling of connection to the outdoors in every season.
Idaho experiences a fairly wide range of temperatures. Lows in the winter can be consistently below freezing and highs in the summer often top 100 degrees. When we decided to build a passive house we were uncertain how it would react to these swings in temperature. We have found, however, that the house is consistently comfortable inside. During the spring and fall our minisplit heatpumps are rarely on, yet the temperature in the house fluctuates only a degree or two throughout the day. With proper management, the house remains equally comfortable during the extreme temperatures of winter and summer. In the summer, we reduce the heating load in the house by eliminating use of the dryer, shading west and south facing windows, and opening windows to bring cool morning air into the house. Keeping interior doors open to promote the circulation of heated and cooled air throughout the house is also essential. These management techniques are easy to do and have no significant impact on our lifestyle. The house is also extremely quiet and always feels and smells fresh because of the constant supply of outside air. Overall the quality and livability of the house are exceptional.
Building a Passive House has taught us that reducing our energy use does not require major changes to the way we conduct our daily lives. Smart management of the house is essential to making it perform at its best, but the design of the house and the high quality of the building envelope makes it easy to reduce our energy use without sacrificing our lifestyle.