Passive Solar Construction Costs for Sun Plans

Because we are not familiar with building costs in areas other than our immediate vicinity, we do not provide cost estimates with our house plans, but we do provide some guidance for our custom design services. We have also recently constructed a passive solar home of our own and being familiar with the construction process in other areas, we can pass on the following general information regarding costs that can be applied to most of our designs.

Some passive solar plans are more complicated than ours so the costs may not be the same, but we pride ourselves in designing within conventional construction methods as much as possible in order to keep the costs down. Please note that this information does not address active solar energy systems, such as solar water heating or photovoltaic panels, as these must be addressed by professionals in active solar design.

Be sure to look at the "complexity" column on the far right of our house plan list page. We rate them either "simple', "average", or "complex," and a very rough estimate is that each adds about 10% more than a simpler plan of the same s.f., but only your builder can say for sure.

Design and Planning Costs

An often overlooked part of construction costs is the upfront planning and design costs including architectural and engineering as well as things such as professional estimating, preliminary home energy rating, etc. In the architectural profession, with a custom designed home with full architectural and engineering (A&E) services (see Chapter 5 in our book on more about the many options), design fees can be between 10-20%. At Sun Plans, with our limited service custom design work, we recommend a planning budget of 6% including your local structural engineering fees. That fee is approximately what you would be paying a Realtor if you were instead buying an existing home. If you are able to use one our designs from our web site, you might be able to reduce the fees to as low a 2% (house plan price and local structural engineering fees.) For a $300,000 home that could be a design budget of $6,000. (Total design fees/costs for A&E for custom changes, by us or others, to a pre-designed plan might be 4% or $12,000, while a new custom design might be $18,000 with our limited services and much more with full service architectural and engineering where much more services are provided.)

Planning Time

A related subject to cost is planning time. We recommend that you have your plans at least 2 months prior to starting construction to allow for the minimum reasonable time to include engineering. Six months would probably be a more reasonable time frame to allow for busy seasons such as the spring, work loads of engineers and builders, and complex homes. (See our house plan list for which we have rated "complex".) Some clients like a year esp. if they will be doing some of their own subcontracting and/or material buying. Engineering analysis esp. in areas with strong seismic activity, high winds, heavy snow, and unstable soils may lead to additional changes being required to the plans. It is better to allow more than less time to have the additional changes made either locally or through us if mutually agreeable. (We often have a consulting engineer available to do the work for an additional charge, but even he states that a local engineer is usually more experienced with your local conditions and therefore will most likely do the engineering quicker and for lower fees.)

Sun-Tempered Design (Less South Glass)

A sun-tempered house plan has fewer south-facing windows than regular passive solar homes, and therefore requires no additional thermal mass in the home to maintain comfortable indoor temperatures. Those would be homes that have the net s.f. of south glass between 5% and 7% of the heated floor area. Our same house plan list mentioned above shows the % of south glass for each plan. These homes have approximately the same number of windows as in any standard home, but most of them are on the south. Some of our house plans are intentionally designed this way at the owner's request. Also, it can be difficult to get a lot of south glass in large one-story homes where the house is deep from north to south due to the owner's requirement with views or room sizes. If you select a house plan that has integral mass, but find that you do not want it for several reasons, then we can modify the house to have less glass and less mass through our Custom Changes services.

There are no extra costs associated with the sun-tempered aspects of the home. However, there can be a few extra costs associated with the general energy efficiency of the structure depending upon what is standard in your area. The energy penalty averages about 10-20% over a passive solar home with more south glass.

An Example for a $300,000 s.f home: : You might spend an additional $9,000 (3% of construction costs) on cellulose insulation, caulking, better windows, and a high-efficiency heating and cooling. A 2100 s.f. house on a 30 year mortgage at 6%, the mortgage payment would increase by $54 per month to borrow this additional $9,000. At $.10 per kilowatt-hour, the total energy bill might average $75 per month for a energy conscious family of 3. The estimated monthly energy savings might be at least $75/ month. When you subtract the $54 extra mortgage payment, you have a positive net cash flow the first month you move in! ($75-54=$21) and that only increases annually! (These figures can vary widely across the country so that is why a home energy rater should work along with your builder. Together they can assist you with detailed costs in your area.)

As a rule, an energy-efficient home will save at least 30% on your energy bill. Sun-tempering the home can boost this savings by approximately another 30% for a total of 60% depending upon where you live. This savings applies to cooling as well since sun-tempered design also keeps out the sun in summer.

Passive Solar Design (Most of our House Plans)

Our passive solar house plans are suitable for moderate to cold climates and Canada. Passive solar homes have more south-facing windows than sun-tempered homes, and therefore require additional thermal mass to maintain comfortable indoor temperatures.

You would need approximately 50 s.f. extra of windows for every 1000 s.f. in the home. (That's about 4 extra windows, and good windows will cost about $400 each, so say an extra $2/s.f. of construction costs.) You would also need approximately 300 s.f. of thermal mass on the floors or walls for each 50 s.f. of additional windows. Adding this mass (and finished flooring of tile or brick) over a wood-framed floor is approximately $6/s.f. to $12/s.f., which averages another $2-4 per square foot of total heated area.

This is where slab-on-grade foundations can help with construction costs, since the mass and floor are one, and therefore there are no additional costs. There are construction methods that allow for slab-on-grade construction even in very cold climates.

The extra construction costs for the passive solar features (extra windows and thermal mass) would be approximately $4-7/s.f. or 3-5%. When you add the general energy efficiency upgrade mentioned above under the sun-tempered home, the total costs for energy efficiency and passive solar construction would be 6-8%. Your energy savings with the passive solar version can be expected to increase at least another 10% over the sun-tempered version. A 10% savings in colder climates will save more energy than a 10% savings in milder climates.

Summary

On average throughout the U.S., a sun-tempered or passive solar home can result in energy savings from 65-75% when combined with general energy-efficient construction. This savings is compared to the average home being built today. The additional construction costs associated with energy-efficient construction and our sun-tempered or passive solar design are 3% to 8%. Perhaps add 10% as a rule. Compare this to energy and construction costs in your area to see the benefits.

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