Reducing carbon emissions

We are a reasonably typical family of four (with two elementary-school age boys) in a 2400 sq. ft. (240 m^2) wood frame house in a small city residential neighborhood of single-family dwellings. Nevertheless, we've been able to take several actions to help reduce our environmental impact.

I wouldn't push anyone into taking all these actions, and living in Boulder certainly helps in several ways, but I feel that many of these steps could be taken by a lot of people.

This is just a first start at this WWW page. I'll try to flesh it out with more specifics (brands/models/numbers/photos) when I get a chance.

Reducing vehicle use

A huge impact on our energy consumption was simply the choice of where to live. We paid somewhat (perhaps 50%) more to live in town where there are trees to shade the house in the summer. (So we don't use air conditioning.) However, this also places us within a mile and a half of my office, my kid's school, and stores. (Actually, one kid travels further, using a school bus for reasons beyond our control.) We are less than three miles from downtown. Thus, walking and biking are always options, but car use is minimal even when we drive.

We also have selected reasonably energy-efficient cars. Both are Volkswagen wagons at the moment, but we've never (in 25 years) owned a vehicle that got less than 30 mpg.

Reducing home carbon emissions

We've cut the emissions of our house by about half by:

Upgraded our natural gas furnance from ~60% efficient (~1977) to 96%. We considered changing to a geothermal heating/cooling system, but given the inefficiency of electricity production, this wouldn't have resulted in less carbon use just for heating. (It makes a huge difference when offsetting air conditioning!) ($4600)
Installing 3kW of photovoltaic solar panels to offset (predicted) about 110% of our electric load. (This is a bit more than is economical, but the system we purchased was not available in a size at 100%.) The system has 14 SPR-220 (220W) panels and an SPR3300x (3.3kW) inverter, all from Sunpower. We went through a local solar PV contractor (Namaste) since our utility -- Xcel energy has made the permitting process so cumbersome (and because I'm always hesitant to put holes in my roof!) [Xcel clearly did not want to create this program -- only recently have you even been able to find it from their home page -- but was forced to by Colorado voters.] ($13,000)

Over 3 years, our system has now produced over 11 MWh and we are about even with the grid. We would have produced a bit more, but in Spring 2008 I accidentally kicked apart one of the connectors while cleaning snow off the panels and didn't realize it for almost 2 months:( Our first year, Xcel somehow calculated a surplus of 0.602 MWh and sent us a check for $20.91 (at 3.414 cents/kWh). However, they also reset our "0" reading to the same 0.602 in the middle of winter (near our system's anniversary). Since we are net consumers this time of year, Xcel then billed us for 0.145 MWh use (at 7.462 cents/kWh), so we had to pay them back $10.82 for electricity we produced!!! The second year's check was for $5.91. If we had installed our system in any other season, we wouldn't have this double-billing. This isn't fair, but was in our original contract that we were forced to sign...
The output rate of our system is shown below.

I have added to this graph a curve which represents a prediction based on the actual panels, their actual orientation, the actual shading of our roof from neighbor's trees, cloud-free input of 1000 W/m^2, assuming 800 W/m^2 direct and 200 W/m^2 diffuse radiation. I then multiplied this prediction by 0.55, which appears to match our output. Thus, we are getting 55% of clear sky values, which presumably mostly is due to cloud cover, though some is due to less than perfect efficiency of the system, and snow covering the panels from time-to-time. I've also added a dashed line showing what we have produced if I hadn't broken the connector. The long term trend seems to be getting worse, though winter 2009/2010 was much cloudier than normal and chilly, so our furnace kept on using electricity.
We have to admit that with a pre-solar electric bill of only $300/year, it will take over 40 years to pay back the photo-voltaic system! (Assuming that future electric rates approximately follow interest rates.) Thus, even with rebates this project isn't really cost effective. Nevertheless, $13K was affordable for us.
Replaced front door (some thin wood) with a fiberglass one from Therma-Tru. ($750)
Replaced old metal-frame sliding door (with compromised seals in double-pane panels) with one with a wood frame and new double glass from Hurd. ($1400)
As mentioned above, our house has trees that shade in the summer. We also have an attic fan to reduce summer heat. Finally, the house has a "daylight basement" which keeps reasonably cool even on the hottest (100F+) summer days. Thus, we haven't needed air conditioning.
Finally: We replaced 10 single-pane(!)+storm windows with double-pane windows ($5200). (I looked into simply replacing the panes with double-pane inserts, but the glass guy thought our sashes wouldn't support the additional weight. I also thought about replacing just the storms, but my WWW reading -- including the EPA site -- wasn't optimistic that this would help much.)

Our electricity bill is now $80/year (just fees) and gas ~$800/year.