(A SCEB Success Story) By Lynn Wallis SCEB Home Owner
Like many who might be reading this, as our retirement years approached, we began to think about what we wanted to do in retirement and where we might want to do it. We resolved the “where” first as we re-discovered Las Cruces, a place we first visited in the early 1970’s. We ended up purchasing a two acre lot in 2003 with the expectation we just might build the “retirement home of our dreams” someday.
And so it worked out that we relocated to Las Cruces in 2005 and began our due diligence about what we wished to build and who would do the building. As you might know, there are a lot of good builders in Las Cruces. In the end, personal chemistry, design compatibility and finances all played a role as we settled on Mario and Pat Bellestri of Soledad Canyon Earth Builders. We liked the houses they’d built a lot, we were impressed with their design and build processes, their attention to our goals and their pricing was fair.
Custom homes are, well, custom homes. Folks have very different goals and a good custom builder will work to accommodate those goals. And so it was with SCEB’s. Three of our items where:
- A home designed for retirement and aging.
- Low interior and exterior maintenance.
- Economical operation – energy efficiency. (A core SCEB goal anyway.)
For a variety of reasons, we ended up with a larger home than we expected. We have a bit over 3,000 SqFt heated and cooled. Rooms, halls and baths are large enough to accommodate a wheel chair resident comfortably and we have room for long term guests if needed. The lot is hilly and only supported a two car garage, but it is large and there is a separate shop/storage room. Both the garage and shop are cooled with an evaporative cooler.
The house is located in Miners Ridge east of Las Cruces in the Organ Mountain foothills at about 5,000 ft elevation. The additional 1,000 ft of elevation over the valley causes our daytime temperatures to be 3-4 degrees cooler and nighttime temperatures to be that much warmer…except in the winter when we are often much warmer than the valley on calm nights due to an inversion layer.
Our lot and location in Southern New Mexico dictated many of our “solar” choices. This far south, at this altitude, with 320 plus sunny days a year, solar design means “controlling” heat gain. The Las Cruces climate is “cooling” dominated, so winter heat gain was not a major priority.
Our lot dictated a west facing house with a north-south long axis. In the end, our design maximized the morning light and easterly view to the mountains and minimized the summer heat gain with limited south and west windows. Further, the garage and shop were located on the west face of the house, buffering much of the house proper…especially given they are cooled with an evaporative cooler set to about 82 degrees in the summer.
Our Design for Energy Efficiency
Our design is based on executing the fundamentals well and incorporates only two, may be three un-common features.
- Control air flow.
- Insulate well.
- Use efficient HVAC equipment.
- Use efficient appliances.
Controlling Air Flow
We achieved this by.
- We used a form of massively thick rammed earth walls that are 18 inches thick. No air movement through them!
- We used good quality windows and doors. The windows are all casement style that seal tighter when the wind blows. They are dual pane, inert gas filled with a tint suited to minimizing solar gain. The doors have the same type of glass panel and have triple latches to maximize their seal.
- We used a spray on foam ceiling insulation on the underside of the roof deck and parapet walls. This served to completely seal the roof from air movement, provided the necessary insulation and sealed the HVAC ducts inside the heated-cooled envelope.
- Finally SCEB’s and their sub’s did a great job with sealant at all points in the home.
Air infiltration is controlled so well that provision was made for fresh air replacement in the HVAC design.
We achieved this by.
- Good quality windows and doors.
- Providing additional insulation on the outside of the rammed earth walls.
- The spray foam roof insulation.
- Attention to detail.
R-values for the walls and doors certainly meet code, but you can easily find homes with higher R-values. But real world results tend to vary based on execution. Our real world results reported below speak for themselves.
Use Efficient HVAC Equipment
It is common local practice to use “package” heating and cooling units located on the typical flat roof. You might want to think about all that as you design your home, but that is what we have.
Typical practice is to install units with a gas or propane heater and a standard AC cooling unit. In the winter the heater runs and in the summer the AC runs with all the equipment being on the roof and the air ducts running in the ceiling above the insulation. A large home will have two or more such units installed.
We did several things a bit differently.
- The package units we have are dual fuel heat pumps. That is to say that heating is accomplished primarily with a heat pump and when the temperature is below 32 degrees, backup heat is supplied by a propane heater, not electric resistance as is typical for heat pumps. AC is provided as normal.
- Our units are rated 14 SEER for cooling and 3.0 COP for heating. The 14 SEER rating is 1 higher than the 13 minimum at the time of building and the COP of 3.0 is considered pretty good.
- Our ceiling duct work is well taped and zip tied to minimize duct leaks.
- We specified return air ducts at floor level in all bedrooms as a comfort issue.
- And our ceiling duct work is all within the insulated ceiling space.
- One of the package units has a “fresh air system” installed to provide fresh air.
- Both units have “air cyclers” installed to allow the fan to run on an automatic schedule for comfort and fresh air intake.
Cooling works pretty much as you would expect from a well designed system. Given the massively thick 18 inch walls, the house warms slowly and the AC starts running later in the day than you would expect and runs well after dark in the cool of the evening bringing the walls back down. The result is a very comfortable home.
Given the mild winter climate and warm winter inversion layer specific to our location, the heat pumps provide 85% or more of the seasonal heating. Our best estimate is that heat supplied by the heat pumps costs only 40% as much as propane heat at the register given recent propane and electric rates.
The garage and shop are cooled by a MasterCool evaporative cooler. We are generally successful in keeping that southwest facing area at 82-85 degrees, but the unit does work hard on a sunny 105 degree day with two hot cars just pulled in.
Use Efficient Appliances
Our appliance choices included the use of propane for water heating, clothes drying, grilling and cook top. All the rest are electric and Energy Star rated…but nothing special really. That is with the possible exception of the water heater.
We have a tank-less model that supports multiple users and are very pleased with it. We estimate it costs about half to operate compared to a traditional unit. The heat lost from the tank is not heating the garage 24/7 year around.
Otherwise, we use efficient lighting, but we have way, way too many TV’s, cable boxes, computers, printers and chargers plugged in to be considered really “green”.
There are two permanent residents with occasional guests. We have blinds and we use them to control the sun as needed morning and evening, winter and summer. We are energy conscious, but not obsessive. In the summer we cool to 77 daytime and 75 evening. In the winter we heat to 70 daytime and 68 evening. If we’re hot, we cool a bit more, if we’re cool, we heat a bit more. We don’t suffer. We do not have a pool or spa.
The temperature buffering effect of the massive walls has a marked effect on internal temperature stability. The air temperature can certainly move around, but the structural temperature moves very slowly. It makes for a very comfortable home. For the technically inclined, we estimate that each degree change in the structure (walls and floors) requires 2 million BTU’s.
Results – Did We Achieve our Goal?
We think we did pretty well.
For the most recent 12 month period results are as follows:
- Electric consumption was 12,480 KWH’s or 1,040 KWH’s per month costing about $146 per month.
- Propane consumption was 106 gallons or about 9 gallons per month costing about $22 per month.
- Total fuel cost was about $168 per month for heating, cooling, cooking, water heating clothes washing and drying and grilling. For the house and garage both.
If heating and cooling alone are considered:
- Electric consumption for heat pump-AC operation is estimated to be 3,490 KWH’s for 12 months or $41 per month.
- Propane consumption for back-up heating is estimated at 78 gallons or $16 per month.
- Total heating and cooling costs are estimated at $676 for the year or $57 a month.
All other household energy consumption dwarfs heating and cooling running about $111 a month! We’re very happy (and comfortable) with the results!