Getting to Zero

Confluence Architecture was instrumental in turning an old 1970 energy hog home into a an efficient, low energy use residence.   The project qualified for $12,000 in  grants from CORE and $7,092 in rebates from Holy Cross Energy.      The home is fully electric and uses air-source heat pumps for heating and cooling.  It is projected to be net-zero. Confluence was responsible for planning, architecture and energy modeling on this home.






Snowmass Village Energy Code Requirements

Town of Snowmass Village Community Development website here…

2015 International Energy Conservation Code (IECC) and REOP adoption…

Ordinance No. 2 2011  adoption…

Town of Snowmass Village CFA Maximum
Tier I 1000-3000 HERS 75
Tier II 3001-5000 HERS 70
Tier III 5001-10000 HERS 65
Tier IV 10000+ HERS 60

Link to Colorado Energy Conservation Code Hub for; Aspen, Basalt, Carbondale, Eagle County, Pitkin County, Town of Telluride and the Town of Mountain Village

Town of Mountain Village

As of August 20, 2020…

2018 International Energy Conservation Code

Special provisions for exterior energy use.

Permit fee discounts available for nonexistent or offset exterior energy uses.

Link to Town website and adoption language…

CFA TOMV yes snowmelt no snowmelt
< 3600 Tier I prescriptive prescriptive
3601-7000 Tier II 2018 HERS 60 2018 HERS 60
7001-13000 Tier III 2018 HERS 55 2018 HERS 60
13001+ Tier IV 2018 HERS 50 2018 HERS 60

Link to Colorado Energy Conservation Code Hub for; Aspen, Basalt, Carbondale, Eagle County, Pitkin County, Town of Snowmass Village and the Town of Mountain Village

Colorado Energy Conservation Code Hub

One stop for energy code information on the multiple jurisdictions of the Roaring Fork Valley, Telluride and Mountain Village.

I will try to keep this list current.


Jump to Aspen Colorado’s energy conservation code compliance page…


Jump to Basalt Colorado’s energy conservation code compliance page…


Jump to Carbondale Colorado’s energy conservation code compliance page…

Eagle County

Jump to Eagle County Colorado’s energy conservation code compliance page…

Pitkin County

Jump to Pitkin County’s energy conservation code compliance page…

Town of Snowmass Village

Jump to TOSV’s energy conservation code compliance page…

Town of Telluride

Jump to Town of Telluride Colorado’s energy conservation code compliance page…

Town of Mountain Village

Jump to TOMV energy code compliance page…







How much solar PV do I need for my electric vehicle?


There is much to like about Electric Vehicles (EVs). There is much to like about Solar Photovoltaic power collection (PV) too. But when you put the two together, something extra happens… You can now think of your solar system as being paid off, not by offsetting the cost of the electricity that runs your house, but by offsetting the cost of the gas you’re not buying.

Below, I am going to list the real-world drive data from our two company cars, a Nissan Leaf EV and a Prius Prime PHEV (Plug-in Hybrid Electric Vehicle). Sorry hybrids, you don’t get to play this game. Then I’ll list our real-world solar array data and calculate the value of the electricity we have harvested from our PV array. And answers some important questions; How much PV do I need to offset driving an EV or PHEV?

3.24 kW PV array:

Number of panels:                           12

Power per panel:                              344 watts

Annual production:                         4,128 kWh

Cost after rebates:                          $5,796

Thermal solar panels on lower roof, PV array on upper roof

Thermal solar panels on lower roof, PV array on upper roof

The calculations will vary, of course, from place to place and car to car and driver to driver. These cars live in Carbondale, Colorado. The climate is pretty good for EVs; not too hot, not too cold. Typically highway driving punctuated by small towns. The road is never level, up valley, down valley, up the hill, down the hill. We run Blizzak winter tires for about five months, and Michelin Energy-savers the rest of the year. I’m no engineer; there is much rounding and estimating used in these calculations. For projections I will use these fuel values:

Street value of a kWh:                    $.11 (summer 2019)

Street value of a gallon of gas:    $2.60 (summer 2019)

Total annual value of electricity produced by PV based on utility cost of kWh:

Total:                                                $520

Confluence company cars at local EV event.

Confluence company cars at local EV event.

2013 Nissan Leaf EV, 24.0 kWh battery

Miles driven annually:                     13,000

Miles/kWh: (average)                      4.4 miles/kWh

Total kWh consumed:                      3,022

Percentage of public charging:      10%

kWh provided by PV:                       2,720

Leaf consumes:                                 66% of our production

Leaf consumes:                                 8.0 panels

Miles per panel:                                1,463

Confluence company cars at local EV event.

Confluence company cars at local EV event.

2018 Prius Prime PHEV, 8.8 kWh battery

Miles driven annually:                      19,000

Hybrid ratio:                                       50% EV : 50% ICE (Internal Combustion Engine)

MPG:                                                    80.5

Gallons of gas:                                   118

Cost of gas:                                         $307

Miles/kWh:                                        5.4 miles/kWh

Total kWh consumed:                      1,713

Percentage of public charging:       10%

kWh provided by PV:                       1,542

Prius consumes:                                 37% of our production

Prius consumes:                                 4.5 panels

Miles per panel:                                 2,111

Total annual value of electricity produced by PV based on gasoline offset:

If 22,500 miles were driven in a gas drive vehicle (e.g. 2005 Subaru Outback, 32 MPG)

Gas offset EV portion of Prius:      $772

Gas offset Leaf:                                 $1,056

Total value of gas offset:                 $1,828 (703 gallons)


Every solar PV panel you put on your roof will push your EV 1,500-2,000 miles.

So, the bottom line is that our PV system makes us $520 worth of electricity annually, not bad. It would pay itself off in about 11 years at that rate. But when we put that electricity into an EV, it saves us from buying $1,828 worth of gas over our old ICE car, which translates to a 3.2 year payback!





Prince Creek Home Breaks Ground





Confluence is excited to announce that Prince Creek Home (outside of Carbondale Colorado) is framing. This home is a modern reinterpretation on an existing ranch home foundation. It will be net zero! To achieve net zero the home uses SIPS, good foundation insulation, heat pump heating system, proper window location and shading and PV.

The underutilized U-factor alternative?

I review quite a few residential IECC (International Energy Conservation Code) submittals, and I would estimate that three-quarters of them are submitted as a straight-up prescriptive submittal. That’s when the table below is followed, without deviation. There is nothing wrong with this approach, but if a little flexibility is required, then leave the R-values behind and look at assemblies as U-factors, that can be morphed and traded around.

Table R402

The U-factor alternative (2015 IECC R402.1.4) is a very powerful and useful method, but I don’t see it get used much.

Table R402 U-factor alternative

I think it can be useful to use a chart like the one below to see building assembly alternatives by U-factor. PDF link…  U-factor alternative assemblies

For instance; can I substitute OVE (Optimum Value Engineering) or Efficient Framing for CI (Continuous Insulation) in zones 6 & 7. The Prescriptive compliance alternative would have at least R5 CI installed on the exterior of the above grade walls. The U-factor alternative says; use any wall with a U-factor of .045 or better. So, at a glance, from the list, I see that I could substitute R3.6 CI (i.e. 1.5” ZIP insulated sheathing) for the R5 CI and bump up the cavity insulation number to R23 and build the wall with efficient framing techniques. Don’t like CI at all? Then substitute an efficient framed wall with the cavities foamed solid to R36. Don’t like CI or efficient framing? Then you could use a 6” SIP, ICF or straw bale. Check the total U-factor of your specific assembly, it could vary from the U-factors on the list by a couple of thousands. Here is a super-good online wall calculator for R-values and U-factors including checks for moisture control.

If you still don’t like the choices that the U-factor alternatives gives, then it is time to move up to the Total UA Alternative, AKA RESchecks (2015 IECC R402.1.5). Often, projects get bumped out of the prescriptive path alternative because the insulation can’t easily be provided in a particular location. Then the Total UA Alternative could be used, because it can trade-off different assemblies. For instance, slab edge insulation, often hard to do at a door threshold, patio or deck attachment or behind stone veneer. The uninsulated slab edge can be “traded” for surplus U-factors on completely different assemblies anywhere in the project.

If you still don’t like the choices that the Total UA Alternative gives, or still having trouble reaching the code threshold, then it is time to go fully custom with the Simulated Performance Alternative (2015 IECC R405) or the Energy Rating Index (ERI) Compliance Alternative (2015 IECC R406). Both alternatives can checked by the software at the same time, but the ERI Alternative is more powerful, because it take into consideration low infiltration rates, high efficacy lighting, appliances and renewable energy sources. The only certified ERI program currently is the HERS Rating.

Please contact us if we can help you comply with the energy code in the smartest possible way.

Link to Colorado Energy Conservation Code Hub for; Aspen, Basalt, Carbondale, Eagle County, Pitkin County, Town of Snowmass Village, Town of Telluride and the Town of Mountain Village

Energy & Sustainability Services

Recently more jurisdictions have adopted the 2015 IECC or the IgCC and we have been helping several architects & designers with energy and sustainability code compliance.

Go custom! You don’t have to follow the recipe. Make the energy code work for your project.

Farm out the energy work to Confluence. We will be responsible for any or all of these, bring value to the performance of the building, and take the load off of your hands:

  • Energy code compliance
    • Infiltration (blower door) testing
    • Assembly UA trade-off (calculation software)
    • Total UA trade-off (REScheck or HERS Rating)
    • Performance path compliance (REScheck or HERS Rating)
    • ERI path compliance (HERS Rating)
  • Code compliance/optimization and Construction Documents
    • Ventilation calculations
    • Sealed crawlspace & ventilation details
    • Continuous Insulation details
    • Back-ventilating siding and attachment details
    • Efficient framing details
    • Fenestration flashing details
    • Radon mitigation details
    • Thermal and pressure envelope delineation
    • Vapor retarder specifications
    • Air-sealing details
  • Local/municipal Green/Efficient Building Checklists
    • Carbondale, Basalt, Town of Snowmass Village, Telluride, Mountain Village
  • Above-code/Net Zero design and certification
    • LEED, Passive House, HERS Rating, etc.

Very Large Residential and Commercial Infiltration Tests

Confluence can perform very large residential and commercial infiltration tests, or blower door tests.


We set up eight fans in three doorways for this test at Habitat For Humanity’s new ReStore warehouse In Glenwood Springs, Colorado. Although, not the biggest test we have conducted. That honor goes to a 56,000 square foot house built for a Saudi Arabian Prince in the Starwood neighborhood near Aspen, Colorado.