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Costs to Modify a New House Design 

for Net-Zero Source Energy

It requires an extra investment to achieve a net-zero source energy house, but of course, utility bills are very low.  How much extra cost?  For this grid-connected, demonstrated net-zero energy house, the following extra costs that totaled $22080 were incurred beyond the cost of the standard production new house.  All of these costs were for professionally installed systems, and included all material, labor, and permit costs.  (The only exception was the window shades that were installed by the owner.)  These costs were for additions beyond the minimum code requirements of IECC (International Energy Conservation Code) 2006, which are significant in this climate zone 6 environment. The solar systems were top-of-the-line quality.  

ItemInitial Cost -
after rebates
Cost Savings,
1st Yr.
Years to Pay Back
3.15 kW Solar PV System$5451$585*7
Solar thermal hot water$4293$8426
High solar gain double-pane windows**$265$674
Add additional window (40"x60") on south side to increase solar gain$410$1320
Remove outside door in MBR from standard plans-$405$290
R-38 to R-60 in attic***$1120$2128
2" of XPS (extruded polystyrene) on outside of walls$5000$4042
R-14 to R-19 on crawl space walls$390$6.5029
1" XPS under crawl space, 2" XPS inside footings$1489??
Lasco air boxes around electrical outlets$214??
Insulated cellular shades with side seals$1875$2235
Heat recovery ventilator$1978$9615

* The cost savings for the solar PV system are computed as if the total electricity generated were used by the house, which would be true for most families.  In fact, for this house, only a little more than 50% of the generated energy is used by the house, with the rest sold back to the utility at the wholesale rate.  The retail cost of electricity is about $0.10 per kWh, while the wholesale cost is about $0.03 per kWh.  
**  The extra cost for the high solar gain windows that were used on the S, E, and W sides of the house was computed for the incremental cost for high solar gain coatings but applied to the same type of standard windows used by the builder.  There was some concern about the high solar gain windows meeting the maximum U-value code requirement, plus the owner wanted wood rather than vinyl windows. The triple-pane, wood-clad windows actually used in the house were more expensive (about $6500) than the value shown.
***  Raised heel trusses
that were used in this house and these allow full-height insulation all the way to the edges of the ceiling and over the side walls, and these trusses make the code standard R-38 instead of R-49 for the attic insulation.  Therefore, R-38 was used as the baseline insulation depth.  The heights of the heel trusses and soffit baffles were extended to allow full insulation height for the added insulation.    

The following caveats and comments should be noted.  
  • Other modifications were made to the house design before construction that did not incur extra charges but were important to the solar systems, including 
    • changing the roof at the back of the house from a gable roof to a hip roof so that an overhang could be added to shade the high solar gain windows in the summer time.   In addition, this hip roof provided an ideal orientation and location for the solar thermal system. Overhang calculations were performed at a modest price ($10) using a shareware program at:
    • changing the garage roof from a hip roof to a gable roof with one side facing south-south-east so that there was sufficient area to mount the solar PV panels.
  • The cost of the 3.15 kW solar PV system including installation and permit fees before rebates from the federal government and the local utility was $18337.
  • A larger PV system would likely be required to achieve net-zero source energy for the house if it were inhabited by more than one person.
  • The cost of the solar thermal system including installation and permit fees before the tax rebate from the federal government was $6133.
  • The solar thermal hot water system is not well instrumented, so the cost savings for that system are an estimate.
  • First year cost savings were computed based on a natural gas cost of $0.772 per CCF ($0.705 per therm), and an electrical cost of $0.0997 per kWh.  The yearly inflation of energy costs was assumed to be 4.7% per year, based on the actual cost of natural gas in Colorado over the period from 1989 to 2010.
  • The total cost for window coverings was used for these calculations, but some type of window covering is often required for privacy and cosmetic purposes, independent of any energy conservation goals.  The cost savings shown do not include savings due to shading during the summer since the house does not have air-conditioning, but the window coverings are key to reducing unwanted solar gain during the summer, and keeping the house cool without additional cooling.    
  • The payoff periods would be shorter for lower amounts of insulation.  Each additional cm or inch of insulation costs the same, but the benefits decrease asymptotically toward zero.
  • All of the energy savings and payoff periods are specific to this location with its characteristic solar insolation, weather, etc.  For example, in parts of the world with lower solar insolation, the solar PV system would have a longer payback time.