Passive solar house by Denise Aughtman in San Miguel de Allende Mexico (C) Daniel FriedmanLow-Budget Passive Solar House
Design Details

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This article describes the construction and energy features of a low-budget passive solar home constructed in San Miguel de Allende Mexico.

We describe cost and design trade-offs made by the owner-builder Denise Gelpi Aughtman, a doctor of oriental medicine and acupuncture who practices in Mexico. The home is functional, occupied, has extremely low operating costs, and like most solar home projects, Aughtman has more improvements in the works.

Our page top photo shows windows admitting morning solar gain in this nearly-balanced-energy passive solar home in San Miguel de Allende, Guanajuato, Mexico.

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Case Study: a Modest Passive Solar Home Combines Beauty, Low Cost, and Minimum Energy Consumption

Passive solar home interior (C) Daniel Friedman, Denise AughtmanLike many homeowners who are attracted to homes designed to make use of renewable energy, Denise Aughtman, the owner-builder of the home described here could not approach her project with a fat wallet.

The modest passive solar design house described in this article provides a real-world example of a working solar home built on a shoestring and operated on a thread. Sited with a beautiful view of the mountains above San Miguel de Allende, and located a few kilometers outside of town, this modest home needed to provide shelter, beauty, and economy without breaking the bank.

The passive solar design Aughtman used to lay out and construct the home combines utter simplicity, very low cost, effective solar gains providing light and warmth during cold weather, very efficient use of water and electricity, and the beauty of open space inside the home, and views of the mountains over San Miguel de Allende in central Mexico.

The sense of smallness of the overall size of the home when first viewed from outside is promptly lost on entering the structure: Aughtman designed a single large room divided into kitchen, dining, living room, work, and sleeping areas by furnishings or bookshelves (photo at left). A single partition wall was constructed to provide privacy for the bathroom.

The structure is designed to permit a later addition of a second story, accessed by a separate stairwell in an attached tower.

Passive Solar Design Simplicity - Thermal Mass

Thermal mass for a passive solar home (C) Daniel FriedmanThe home is located on a high plain at approximately 6200 ft. in a climate that combines warm daytime temperatures with cool nights. Constructed of plastered masonry walls and roof on a poured concrete slab set just above grade, the home's thermal mass absorbs heat during warm hours and returns it to the occupants during chilly mountain nights.

Our weather table below shows that in the climate of this passive solar home, annual average low and high temperatures are 52-77degF. During hot weather, the night-cooled masonry walls and roof help keep the house cool during the day.

During cooler months, passive solar gain to the building's masonry roof and walls provides warmth for nighttime heating.

Adjustment to the amount of solar gain from single-glazed windows around the curved-wall front of the home is provided simply by opening or closing curtains to admit or stop sunshine onto the tiled floor of the building's interior.

Additional lighting for areas of the home most distant from the glass window and door wall are lit by fixed glass glazing in the upper walls and glass block skylights.

Aughtman's budget did not include radiant floor heating for this passive solar home; she reports that the home is adequately warmed during cool weather by the solar-heated masonry roof, walls, and when needed (open the curtains), the tile floor set directly onto the 6 to 12-inch thick concrete floor slab.

See THERMAL MASS in BUILDINGS for details about using thermal mass for building comfort.

Potable Water Supply & Rainwater Collection at a Passive Solar Home in Mexico

Rainwater holding tank for a passive solar home (C) Daniel Friedman

Water supply for home use as well as agriculture is critical in dry climates such as the Southwestern U.S., Mexico (the location of this home) and many other parts of the world. Our weather table below shows that in the climate of this passive solar home, rainfall varies from about 3 mm in the most dry month (February), to 129 mm in the wettest month (June).

While the home is supplied water from a nearby community well, Aughtman elected to make use of several designs to collect rainwater and to make simple use of graywater at her home.

See DRINKING WATER EMERGENCY PURIFICATION for details of various methods, including emergency methods for purifying drinking water.

See WATER TREATMENT EQUIPMENT CHOICES for more information on water treatment equipment.

Rainwater is collected from a near-flat rooftop and channeled to a large fiberglass holding tank - the blue tank in our photograph, (above left).

Piping also permits directing water into this tank from a well-fed cistern located atop the concrete block tower). The tower's height provides water pressure to the building.

Currently water is taken out of the bottom of this tank by a simple tank drain valve and hose attachment; to supply this water upwards to the building plumbing fixtures or perhaps to the cistern, a small electric pump will be installed. The tower and a ramp connecting the tower's second floor to a future second story addition on the main house were un-finished at the time of our photograph.

Onsite Waste Disposal - Passive Solar Home Septic System in San Miguel de Allende

Sink graywater used for irrigation (C) Daniel Friedman Sink graywater used for irrigation (C) Daniel Friedman

Graywater from the kitchen sink and bathroom shower/tub and sink as well as from a laundry is routed outside to a simple surface channel cut into the soil where it waters ornamental plants, fruit trees, and a vegetable garden.

While a professionally designed graywater system makes use of filtering, Aughtman's budget did not provide for a graywater holding tank, filtration, and a pump. Instead she makes sure that soaps used in the home are low-nitrate, low phosphate, biodegradable products. Washing machine drainage is passed through a simple external laundry filter to remove lint.

[Note: in the U.S. and many other jurisdictions, discharge of raw graywater to the ground surface is prohibited by local building and sanitation codes.]

See GREYWATER SYSTEMS for design and maintenance details for graywater treatment/disposal systems.

Cesspool for blackwater at a passive solar home (C) Daniel Friedman

Blackwater, sewage from the home's toilet is routed to a 1000 gallon site-built cesspool located on the property.

The volume of wastewater to be processed is low because of the separation of graywater for onsite irrigation use.

See TANK PUMPING FREQUENCY TABLE for general septic tank pumping frequency; those septic tank cleaning intervals would be significantly extended for this home of a single occupant, occasional guests, and graywater separation.

For detailed information about cesspools, see CESSPOOLS or go to SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR to access our septic system design, construction, inspection, and maintenance site.

Composting Food Waste at a Passive Solar Home

Cesspool for blackwater at a passive solar home (C) Daniel Friedman

In keeping with the energy conservation and recycling spirit of the Aughtman passive solar home, Denise has included a compost facility in a corner of her property (photo at left).

Because of the seasonally very dry climate in San Miguel de Allende, Aughtman covers the compost pile with plastic to improve its decay-conversion to mulch for use on her vegetable garden, fruit trees, and flowers.

Passive Solar Design for Average Temperatures and Rainfall in San Miguel de Allende, Mexico

Simply put, when outdoor temperatures vary roughly the same amount above and below the human comfort zone (65 degF to 70 degF) then in principle, a passive solar home should need almost no additional energy source to provide heating and cooling.

Our table of average weather conditions in San Miguel de Allende (below) shows that in that climate, while daytime highs are not much above the comfort point of 78 degF., nighttime lows average 26 degF below that number: solar heating of a thermal mass in the building is wanted for night time indoor comfort. (This information with other weather comments is also at WEATHER in SAN MIGUEL de ALLENDE).

At THERMAL MASS in BUILDINGS we describe a second home in this climate and we report that its typical morning low temperature in the coldest months falls to about 61 degF inside that home when no heating source is provided other than the daytime-warmed thermal mass of the building's masonry walls and roof.

Weather Conditions, San Miguel de Allende, Guanajuato, Mexico1

  Temperature Range Amount Below/Over 78 degF Rainfall
January 44-71 degF / 7 - 24 degC 34/-7 .5 in / 13 mm
February 46-73 degF / 9 - 25 degC 32/-5 .1 in / 3 mm
March 49-78 degF / 11-27 degC 29/0 .2 in / 5 mm
April 54-82 degF / 12-29 degC 24/4 .8 in / 20 mm
May 57-85 degF / 14-31 degC 21/7 1.2 in / 30 mm
June 58-81 degF / 14-27 degC 20/3 5.1 in / 129 mm
July 57-78 degF / 13-24 degC 21/0 4.7 in / 119 mm
August 57-79 degF / 13-23 degC 21/1 4.6 in / 117 mm
September 56-77 degF / 14-24 degC 22/-1 4.7 in / 119 mm
October 52-76 degF / 12-24 degC 26/-2 4.7 in / 119 mm
November 49-74 degF / 9-23 degC 29/-4 .6 in / 15 mm
December 46-70 degF / 8 - 21 degC 32/-8 .4 in / 10 mm
Annual Average 52-77degF 26 / -1  


1. Other San Miguel de Allende weather estimates including average high and low temperatures and rainfall vary slightly around these temperature and rainfall estimates. On average the warmest month in San Miguel de Allende is May, the coolest month in San Miguel de Allende is January, and the wettest month in San Miguel de Allende is July.


Continue reading at PASSIVE SOLAR PERFORMANCE PROBE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or lso see SOLAR ENERGY SYSTEMS our solar energy home page, and SOLAR HOUSE EVALUATION.

At CISTERN CONSTRUCTION GUIDE we illustrate a combination of rainwater collection and graywater collection, diversion, re-use on a home built in an area of water scarcity.

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