A Modest New House Proves Green Doesn't Mean Expensive

Prescott, AZ

Oct 8 2008 By Rob Wotzak | 5 comments

General Specs and Team

Location: Prescott, AZ
Bedrooms: 3
Bathrooms: 2
Living Space : 1189 sqf
Cost (USD/sq. ft.): $55/sqf

Student labor was free

Builder: Yavapai College Residential Building Technology program; Tony Grahame, Director.
Architect/designer: Prescott-area Habitat for Humanity

Construction

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p>Foundation: slab on grade with XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. foam at edge (R-5)
Walls: 2x4 @ 24 in. o.c.; 1-in. XPS foam and dense-pack cellulose (R-19 total)
Roof: raised heel trusses; 8 in. foam sprayed to underside of roof deck (R-32, unvented).
Windows: double-pane, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor., argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. -filled (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., .33 to .35; U factor, 0.32 to 0.35, R-3)
Garage: attached; insulated and sealed from living space

Energy

Heating/cooling: 14-SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. AC system; 40,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. direct-vent gas furnace (92.0 AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. )
Water heating: solar domestic hot-water system, 40.9 sq. ft. flat-plate collector, 80-gal. storage tank with electric backup
HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. rating: 57 (5 stars)
Annual energy use: 40.8 MMBtu

Solar water heating: 40.9sq.ft. SunEarth flat-plate collector; Solaraide 80-gallon hot-water storage tank/electric backup heater

  • Roof overhangs optimally sized for window heights to allow summer window shading and winter sun entry for passive solar heating.
  • Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. interior and exterior lighting package with CFLCompact fluorescent lamp. Fluorescent lightbulb in which the tube is folded or twisted into a spiral to concentrate the light output. CFLs are typically three to four times as efficient as incandescent lightbulbs, and last eight to ten times as long. CFLs combine the efficiency of fluorescent light with the convenience of an Edison or screw-in base, and new types have been developed that better mimic the light quality of incandescents. Not all CFLs can be dimmed, and frequent on-off cycling can shorten their life. Concerns have been raised over the mercury content of CFLs, and though they have been deemed safe, proper recycling and disposal is encouraged. bulbs
  • Extremely tight building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. (103 cfm @ 50 pascals equivalent to 0.63 ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.)
  • Energy Star appliances
  • HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. requirements calculated and sized appropriately
  • Programmable thermostats
  • Sealed ductwork; duct blasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. leakage measured 14 cfm @ 25 Pascals

Water Efficiency

  • Low-flow toilets, faucets, and showerheads
  • Water-conserving dishwasher
  • All hot water taps within 30 ft. of hot-water storage tank

Indoor Air Quality

  • Balanced whole-house ventilation with MERV-10 and HEPA filtration
  • Moisture-mitigation techniques including damp-proof subslab, soil surface graded away from house, foundation drains around perimeter, plastic sheet under slab, wall system designed for drying to inside and outside, appropriately size HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. controls humidity
  • Garage pressure-isolated from living space
  • RadonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. venting
  • No carpets
  • Multiple-return grills provide pressure-balancing between rooms
  • All ductwork within conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort.
  • Spot ventilation in bathrooms

Green Materials and Resource Efficiency

  • Advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. techniques
  • House designed in 2-ft. increments to conserve materials
  • Cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. made with recycled paper
  • Composite decking contains recycled plastic
  • Cardboard and metal construction waste recycled

Certification

NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Energy Value Housing Award: gold
EnergyStar: qualified based on HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. score (57)

Typical Looking, Uncommonly Efficient and Healthy

This rather typical-looking single-story ranch house is very uncommon in a lot of ways. It's super energy-efficient, it has superior indoor air quality compared to most new houses, and it cost $55/square foot to build.

That it was so cheap to build flies in the face of conventional thinking about green building. How was this done? Tradeoffs. Fewer studs means more room for insulation. Roof overhangs and quality windows mean a smaller AC system. An unvented roof means the AC and ducts can go in the attic.

Make the site work without overdoing it
The cost savings began with the site: a substandard lot (thanks to flood-plain issues) was engineered to bring it up to code. While there was a high level of engineering involved, the lot was kept localized as much as possible — many existing trees and shrubs were spared. No toxic pesticides were sprayed before construction, and the house is oriented along an east-west axis to maximize southern exposure for day lighting, passive warming in winter, and solar collectors on the roof that heat the water for free.

Standard green details done well
Advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope., exterior foam insulation, and an unvented roof yielded an extremely tight envelope — .63 ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations., at no "extra" cost. A smart plumbing layout cuts the amount of time it takes for hot water to reach a tap, which saves water. Many of the techniques used in the design and construction of this house are old news: insulating a slab keeps it warm and dry; foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. on the outside keeps the framing warm and dry, so mold isn't likely to gain a foothold; and better windows are worth it.

Lessons Learned

Because of the small lot size, a standard septic leach field was not feasible for this project. Tony Grahame, director of Yavapai College's Residential Building Technology program, solved this problem with the Geotextile Sand Filter system made by Eljen.

A plastic grid covered in a special drainage fabric allows for more surface area, more volume, and more air infiltration than the typical bed of gravel. The result is that you get a leach field that will likely perform better while taking up 50% less space. Using this system was the only way to build a home on this particular site, but it would be valuable to anyone trying to keep site disturbances to a minimum.


Rob Wotzak is assistant editor at GreenBuildingAdvisor.com

Tags: , , , , , ,

Image Credits:

  1. Tony Grahame
  2. Toshi Woudenberg

1.
Mon, 01/19/2009 - 20:11

Checking on the true cost
by Carl Seville, GBA Advisor

This is certainly an admirable project and I very much appreciate the simplicity of the efficiency - fewer high priced/high tech products and lots of good, simple decisions. I would like to understand what the relative cost of this house if all the labor was included at market rates instead of donated student time. We need to see more projects like this completed with the structure of the marketplace.


2.
Tue, 01/20/2009 - 08:15

This project illustrates a
by John Brooks

This project illustrates a lot of good concepts that can be translated to any climate zone. Comfortable,Efficient and Affordable.


4.
Fri, 04/10/2009 - 03:05

Some good ideas...
by Michael Schonlau

This is a useful case study. Even though the labor costs and overall house size aren't the most useful comparisons, there are a lot of interesting ideas here. Some of the concepts I'm now thinking about are solar water heating (are flat panels a better long-term value vs evacuated tubes??), 2x4 (24" oc)/2x6 (24" oc) with good insulation vs ICF/SIP's, Geotextile sand filters instead of standard septic laterals, and unvented roof with ductwork in conditioned space (how does that work?). Thanks for posting this project! I'm getting some great ideas from the case studies.


5.
Tue, 10/27/2009 - 02:49

Home Remodeling
by Chris Builder

Thanks for this. Always looking for new approaches to green construction. The Geotextile Sand Filter system sounds like an impressive innovation. In my area lots are getting smaller and smaller as property prices rise, so any and all space-saving efficiencies are well appreciated. I'm going to check out the Eljen website right now! Thanks.


6.
Wed, 12/02/2009 - 22:00

40,000 BTU furnace?
by Derek Vander Hoop

It seems to me that a home with 2x4 walls filled with dense pack cellulose and 1" of XPS foam on the exterior, located in Arizona, that is only 1200 square feet would not need a 40000 BTU furnace, particularly if attention was paid to the air barrier. Oversized, perhaps? Having said that, it is hard to find conventional furnaces manufactured with a lower BTU rating.


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