Sponsored by the Northwest Energy Efficiency Alliance | Created by the Integrated Design Lab | 20111

Net Zero Energy Homes CASE STUDY 5

PROJECT PROFILE

Sustainability Certifications Northwest Energy Star Certified

Montana St. Jude Dream Home

City/State Billings, MontanaClimate Zone ASHRAE CZ 6AHDD/CDD 7164/2466Completion Date Aug 2009Number of Stories 2Number of Bedrooms 3Lot Size 0.077 acresGross Square FootageConditioned Area 2,034 SFVolume 17,286 CFBuilder/Contractor McCall HomesArchitect Studio 4 ArchitectsEnergy ConsultantsDavid Hales - WSUCost to Build $286,000 / $140/sfOccupants 4

2.2 ACH50

Dbl Pane Windows U-Value 0.30

Raised HeelR-60 Roof

R-10 Slab on Grade

Air-Air Heat Pump / Electric Resistance Backup

R-30 2x6 Wall with Sprayfoam & Blown Fiberglass w/ 1 ext. foam

6 kW PV Array

LEDs and CFLs

67-82% Eff. ERV

Heat Pump Water Heater

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OVERVIEWGENERAL DESCRIPTION

The St. Judes Dream Home Giveaway fundraiser program in Billings, Montana was taken on by McCall Development. The builder integrated the house into the Josephine Crossing Subdivision, where the firm was already working with Studio 4 Architects on designing and constructing a neighborhood development. Both local and national businesses donated all of the labor, materials, and land for the project, allowing all of the money made from the houses raffle to go to St. Jude Childrens Research Hospital in Memphis, Tennessee. The 1,920 square foot two-storey house is a first in two ways; one it is the first project St. Judes has constructed in Montana, and two it is the first residential project in Montana to seriously aim for net zero energy.

ELEVATION

UPPER LEVELFLOOR PLAN

MAIN LEVELFLOOR PLAN

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PROCESS

-5.7-0.2-2.5-5.5-4.4

-18.3+18.7

+.4$-16

Annual Heating Demand (kBtu/SF/yr)

Annual CoolingDemand (kBtu/SF/yr)

Lighting/App EnergyElec (kBtu/SF/yr)

Modeled Annual Bill

Subtotal Energy (kBtu/SF/yr)

PV Production(kBtu/SF/yr)

Net Energy(kBtu/hr-SF)

Hot Water Energy(kBtu/SF/yr) and

Appliance Energy Gas (kBtu/SF/yr)

=

DESIGN METHODSThe architect originally designed the floor plan of the project as one of the standard units in the development, but then passed on all detail development responsibilities to the builder when net zero energy became the goal. McCall Development took an active role in designing all of the energy efficiency details of the home and worked in conjunction with the Energy Star Program and David Hales from the

David Hales used Energy Gauge to iteratively model different systems and envelope packages for the project. Multiple energy models were created and updated throughout the design process in an attempt to achieve the best performing house possible. A final as-built version of the energy model was not created and different window specifications were just one of a few changes that are not reflected in the final energy model. Hales explained some of the limitations of using Energy Gauge as the main software package for the energy

Washington State University (WSU) Extension Energy Program. The use of strictly donated materials for the project also affected which materials and systems were available to work with.

model. First off, the softwares built in libraries for the envelope assemblies are very limiting. For instance, one of the design modifications included an insulated concrete basement with a floating slab. However, when defining a floor assembly, Energy Gauge contains no input parameters for perimeter insulation. Hales also did not have much confidence in the ground heat transfer calculation of the software. In terms of HVAC systems modeling, Energy Gauge cannot really accurately model the performance of ductless mini-split heat pumps. A variable refrigerant

ENERGY MODEL STATS

MODELED END USESENERGY MODELING

PROJECT CERTIFICATIONSAside from the Northwest Energy Star for Homes label, the project did not choose to pursue any other types of certification programs given the philanthropic nature of the home.

dishwasher1%

clothes1%

refrigerator8%

lights9%

range4%

dryer7%

misc24%

space heatpump32%

heat pump H20 heater14%

heating31%

cooling1%

DHW14%

lighting/appliances

54%

volume system with its constituent part load penalties and capacities, along with variable speed fans, cannot be currently modeled by Energy Gauge. Lower temperatures can also be a problem for the modeled heat pumps, which should perform better in reality. Lastly, the projects heat pump water heater could also not be accounted for in the energy model, especially the systems effect on the ambient conditions. David Hales mentioned that other Energy Consulting fims such as Ecotope use SEEM or Energy Plus to accurately model these types of systems.

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ENVELOPEFLOOR ASSEMBLY

WALL ASSEMBLY

The project team executed a monolithic slab, their preferred foundation type when not including a basement, due to the fact that the system cost them significantly less than a crawlspace and still achieves high performance. A continuous 2 R-10 layer of rigid foam insulation was placed underneath the slab, where the original plan involved R-15, but only enough material for the 2 of insulation was donated. This underslab insulation serves as a capillary break and effective moisture barrier between the concrete of the slab and the soil underneath the foam. For the perimeter insulation, the edge of the monolithic slab used a product called Energy Edge, which also served as the formwork for the pour. This product is basically a 2 layer of EPS foam within a recycled PVC frame, which forms and insulates the slab while protecting the perimeter insulation. Typical perimeter foam insulation has to be covered to protect it from impact

and insects yet also has to be finished and look presentable. This product integrates the formwork of the slab, the perimeter insulation, its protection, and finish all into one product. The vinyl covering can be finished with a synthetic stucco coating after a PVC primer is applied. An additional 2 of foam runs around the top and exterior edge of the shallow spread footing, before extending outward 3 for added frost protection.

A 2x6 advanced frame stud wall with 1 of Dow styrofoam on the outside of 7/16 structural sheathing serves as the projects wall assembly. The seams of the foam are taped together to form a drainage plane behind the concrete fiberboard siding. The stud walls 6 cavity is then insulated with 3 of closed cell spray foam and blown-in fiberglass insulation for a total R-value of 30. The project team also considered just using fiberglass batts in the cavities with 2 (R-10) foam on the outside for increased insulation and thermal bridging performance. However, the team was concerned with the aspect of nailing the siding

through 2 of foam. In retrospect, they believed this could have been achieved with the use of furring strips, which would also create a cavity for a rain screen. Luckily enough, spray foam for the project was donated to make up for the reduced foam on the exterior, while also forming the primary air barrier for the assembly. The top and bottom plates of the wall were caulked and sealed, and spray foam in the wall cavity was installed after all of the mechanical penetrations were made. A Tyvek building wrap was used outside of the 1 of rigid foam board to serve as the moisture barrier in the assembly. The project only

MONOLITHIC SLAB ON GRADE R-10 UNDERSLAB R-8 PERIMETER

2X6 SINGLE WALL BLOWN IN FIBERGLASS AND SPRAY FOAM

1 EXTERIOR RIGID FOAM R-30 8 THICK

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ROOF ASSEMBLY

A raised heel truss system with a ventilated attic comprises the roof assembly for the project. Blown in fiberglass insulation reaches an R-60 value above the main spaces, while an R-60 fiberglass batt system had to be utilized above the bonus room of the house. Taped interior drywall boards serve as the primary air barrier of the roof assembly. The top plates of the walls were also caulked and sprayed with foam after the drywall was installed to further increase the air tightness of the intersection. The latex paint applied to the drywall serves as the vapor retarder for the wall and ceiling assemblies. To limit the penetrations through the air barrier, the team generally avoided canned lights in the top floor, but ended up putting four in the upstairs living room. Consequently, special air tight can light fixtures were used with CFLs rated for sealed fixtures because of their ability to handle higher ambient temperatures. Utility boxes were sealed with caulk or rated to be air tight by the manufacturer. Venting for the upstairs bathrooms travel through the walls and down through the floor joists, rather than up through the attic. Also, by using a ductless heat pump, all ductwork and subsequent penetrations were avoided.

administered one blower door test at the completion of the project, which resulted in a 2.2 ACH50; the project team expressed that they would have liked to seen a tighter envelope.

WOOD-VINYL COMP. FRAMES DBL PANE AVG U-VALUE .30 SHGC .20

R-60 RAISED HEEL BLOWN IN FIBERGLAS