The design of this two-story home in Boulder, Colorado, adhered to timeless passive solar principles
Back in the early 1980s, I built a 1,480-square-foot passive solar home in Boulder, Colorado. There have been substantial improvements in active solar equipment since then. But today’s passive solar design principles are quite similar to the principles preached in the early 1980s. Solar energy provides almost all the winter heating and hot water for this home. The cost savings I achieved by eliminating a central heating system were invested in thermal mass, additional insulation, and better glazing, so no additional construction costs were incurred compared to conventional construction.
Solar heat with electric backup
Construction began in mid-1982, and I moved in a year later. During the following two years, I worked full-time while completing interior work and landscaping.
Most of the home’s winter heating requirements are met by the home’s south-facing windows. The solar heat is stored in the high-mass floor (a tile-covered concrete slab) and is retained by a superinsulated building envelope. Two passive solar (batch-type) hot water tanks provided almost all the home’s domestic hot water. Electric radiant heat and a small electric hot water tank serve as backup.
Conductive heat loss, air infiltration, and solar gain calculations were done for each separate room as part of the design process. Winter heating bills were approximately $25/month (in 2014 dollars) in a cold (5,600 heating degree day) but sunny winter climate. Utility costs are so low that the cost of a heat pump or central heating system can’t be justified.
Temperatures stay in a comfortable range
The main living area’s south-facing window wall, helped by the high-mass floor and the superinsulation, resulted in a typical winter day indoor temperature fluctuation of 68°F to 78°F. After an overcast day, temperatures dropped to 65°F by the following morning, without using backup heat. Very rarely, after two or more successive overcast winter days, temperatures dropped to 63°F. The lowest recorded indoor temperature was 59°F; this temperature occurred once in five years, after multiple overcast days with no backup heat used.
During the coldest period experienced in five years, outdoor temperatures stayed below 0°F for five days, dropping at night to -15°F. Yet indoor temperatures in the main living area on the first floor climbed into the low 70s and dropped to about 65°F by early mornings, with no backup heat used.
Upstairs in the bedrooms, backup heat was used overnight only on the coldest nights in occupied bedrooms. Typical winter day indoor temperature fluctuations were from 65°F to 82°F. Each of the the three upstairs bedrooms has a south-facing sliding glass door leading to a common deck that shades the windows below during the summer.
A simple design
The home is a 20’ x 40’ two-story home with an attached garage on the north side, resembling a Colonial or saltbox design. The home’s foundation consists of concrete grade beams at the perimeter surrounding a slab on grade. The roof is framed with trusses. The cedar siding and cedar shingle roofing conformed to the style required by the neighborhood Architectural Control Committee.
The main living area on the first floor has an open floor plan with a kitchen, family room, and dining areas. The stairway is U-shaped. There is a doorway to the garage, and a short hallway with a laundry area and half bath.
Upstairs are three bedrooms facing south, one study facing north, and a full bathroom. The east side was designed for a later addition. An addition could add an airlock entrance, a formal living room, and a dining room, and could expand the master bedroom and add another bathroom.
Passive solar features
Active solar systems use collectors, pumps, fans, controllers, or other equipment that use and store energy to operate.
Direct-gain passive solar heating has three necessary components:
- Significant south-facing glazing to capture winter sunlight. Each habitable room or space needs south-facing windows or skylights that measure about 25% of the floor area. The windows must get unobstructed sunlight from at least 10:00 a.m. to 2:00 p.m. during winter months.
- Superinsulation and tight construction to minimize heat losses. South-facing glazing alone cannot capture enough heat to keep a building comfortable in a cold climate if it the building is poorly insulated or leaky.
- Thermal mass to soak up solar heat during the day and then radiate that warmth back at night when the sun no longer shines.
Examples of thermal mass include concrete floor slabs, brick walls, large water tanks or tubes, stonework or tile that is inside the building’s thermal envelope (to retain stored heat). Air, wood and cloth do not hold enough heat to keep a home comfortable for an extended period of time. Thermal mass is necessary to reduce indoor temperature fluctuations. Otherwise temperatures in the afternoon become too hot, and temperatures before the sun rises become too cool. Thermal mass absorbs heat when air temperatures rise above the temperature of the thermal mass, and give off heat when the surrounding air temperatures fall below the temperature of the thermal mass.
The south-facing glazing on the first floor includes an 8-foot-wide Andersen sliding glass patio door with a frame of vinyl-clad wood. The double-pane glazing is rated at about R-2. Flanking each side of this sliding door are two site-built 4’ x 6’ fixed double-glazed windows, also rated at about R-2. (See Image #3 for a view of the first-floor window wall.)
This glazing provides about 140 square feet of south-facing glass for the 640-square-foot first-floor area. That ratio is a little less than the recommended rule of thumb (providing south-facing glass equal to 25% of the floor area). Typical conductive heat loss, infiltration and solar gain calculations were used to estimate the required glazing area.
Three windows (a 4’ x 5’ double-casement window and a 3’ x 2’ awning window) face west, toward the mountains, while a single window (a 3’ x 2’ casement) faces north for cross-ventilation purposes.
Each of three second-floor bedrooms had a similar Andersen 6-foot-wide patio door facing south, providing access to a common 4-foot-deep balcony that spans the south side of the home. The ratio of south-facing glass to floor area for the bedrooms is about 20% for the two larger bedrooms and 27% for the smallest bedroom. The bedroom on the south-west corner of the house also has two small casement windows facing west toward the mountains.
The second-floor deck shades the first-floor glazing during the summer months, when the sun is mostly overhead at midday. The bedroom patio doors were shaded by a roof overhang. During the winter, when the sun remains lower on the horizon, the second-floor deck and roof overhang do not shade the south-facing windows at all. Sunlight reaches deep into the home, reaching a maximum penetration of over 13 feet on the winter solstice (December 21).
From late November through January, a substantial amount of sunlight can shine through the south-facing window wall onto the tiled slab floor (the thermal mass) from 10:00 a.m. to 2:00 p.m., storing heat. From late May through July, almost no sunlight shines through the south-facing glazing.
Batch-style solar thermal collectors with integral tanks
Two batch-style solar hot water collectors with integral tanks were installed on the roof.
â€¢ The tanks are insulated except for the glazing area.
â€¢ The tank inside each unit is covered with a nickel selective-surface that minimizes radiation of heat at night.
â€¢ The tanks were plumbed in series to minimize the chance that the incoming cold water would mix with the hottest water.
â€¢ A temperature/pressure relief valve located on the roof releases hot water if the water temperature reaches the boiling point, which was never observed.
â€¢ A tempering valve indoors was installed to assure that any excessively hot water is mixed automatically with cold water to prevent scalding.
These two solar hot water tanks provided sufficient hot water year ’round for two residents, except on winter mornings, when the water is only lukewarm. By afternoon (and through the evening), the solar-heated water is hot again, without using any backup water heating. A small backup electric hot water heater could be turned on to boost the temperature of the solar hot water tank output on winter mornings if desired.
The attic over most of the home was insulated to R-60 with fiberglass batts. A ridge vent and an attic fan controlled by a thermostat were used to ventilate the attic during the hottest summer days. The fan seemed to help reduce summer heat in the upstairs bedrooms, but no measurements were made to verify this result.
The access hatch to the attic was a drywall rectangle attached to a stack of polyisocyanurate foam board to increase its insulation value. Folded polyethylene sheeting was used to inhibit airflow through the hatch edges.
The second-floor study located over the southern half of the garage area had an unvented insulated sloped roof; the polyisocyanurate insulation was rated at R-27. This room had an operable skylight facing north. The room gained considerable heat during summer; an exterior cover on the skylight helped reduce the solar gains. This room remained unused except for storage.
Floor and foundation insulation
The most critical component to insulate was the tiled concrete slab floor in the main living area; the slab’s thermal mass was an important part of the passive solar design. The foundation’s grade beams had 2-inch XPS foam board (R-10) outside the wall; the above-grade portion of the insulation was protected by aluminum flashing.
Additional vertical XPS foam layers were installed between the grade beams and the slab; these insulation layers ranged from R-35 to R-50. The slab is protected from the outdoors by a total of R-45 (and in some inaccessible places to R-60).
Superinsulation and air sealing details for the walls
All exterior walls were insulated with R-19 fiberglass batts between the 2×6 studs (installed 24 inches on center). The siding consists of ¾-inch cedar shiplap.
â€¢ The north and west walls were sheathed with 1-inch-thick foil-faced polyisocyanurate. Diagonal steel bracing was used to prevent racking.
â€¢ Since the south wall had more glazing than opaque wall area, and since the south wall was often heated by solar radiation, the wall was sheathed with plywood.
â€¢ The east wall was designed for a future addition. It was sheathed with full sheets of ½-inch plywood at the corners and ½-inch foil-faced polyisocyanurate elsewhere.
â€¢ Wall corners and intersections were insulated with XPS foam board cut to fit between the studs.
â€¢ Window and door headers were all doubled 2x12s with a 2x nailer to finish out the interior bottom of the header; the rest of the interior area of the headers was insulated with 2-inch-thick foil-faced polyisocyanurate.
â€¢ Sheathing seams were taped with duct tape to create an air barrier.
â€¢ Each insulation bay in the walls had a bead of canned spray foam along the edge of the studs and plates where they met the sheathing to limit air leakage.
â€¢ A layer of polyethylene was installed on the interior side of all exterior walls, followed by drywall installed with attention to air sealing.
â€¢ Electrical boxes were place on interior walls whenever possible. On exterior walls, electrical boxes had a foam gasket and child safety plugs on any unused outlets to limit air leakage.
â€¢ Plumbing was kept out of exterior walls.
Plumbing: reducing heat loss and wasted water
The plumbing system was designed and installed to minimize heat losses and wasted water.
â€¢ Almost all of the home’s plumbing was located in compact 10-foot-wide, two-story, vertically stacked plumbing partitions. Shorter piping lengths for the hot water supply pipes minimized installation costs and reduced wasted water and the time spent waiting for hot water to reach sinks or appliances. The first-floor partition divides the kitchen from the bath and laundry on the first floor. The second floor bathroom is directly above the half bath and laundry area. All fixtures were located close to these plumbing partitions. The small backup electric water heater was hung from the joists above the clothes washer. A tempering valve to prevent scalding from the solar-heated water and other valves were located near the ceiling above the clothes dryer.
â€¢ Hot water pipes and the backup hot water tank were insulated.
â€¢ The pipes leading to the roof-mounted solar collectors were wrapped with thermostatically controlled heat tape and were heavily insulated.
â€¢ The holes where pipes penetrated the top plate of the plumbing wall were sealed with foam.
Soundproofing and extra thermal mass
Interior non-load bearing partitions were soundproofed.
â€¢ Partitions were insulated with fiberglass batts
â€¢ Double layers of drywall on some partitions added additional thermal mass as well as noise reduction
â€¢ The gap between the drywall installed on partition walls and the subfloor was caulked to reduce sound transmission
â€¢ Interior doors are sold-core pine
Pounding on the walls and blasting music can hardly be heard on the other side of these soundproofed walls. The second floor was insulated to reduce sound transmission as well. Although insulation was used for soundproofing, it did isolate all upstairs rooms from one another to enable some rooms to remain unheated. In practice, however, this had no use, because of the abundant passive solar heat. There was no easy way to measure the contribution of the extra thermal mass from installing two layers of drywall on some of the walls.
Window shades and insulated shutters
All windows in the home had pleated window shades and home-made insulated shutters.
Most shutters in the home were typically only used during snowstorms or heavily overcast periods. Bedroom shutters also help keep the bedroom dark.
Garage insulation and doors
In addition to the sliding patio doors, the home had three other exterior doors. An R-5 insulated steel door led from the home to the garage. The garage had a weatherstripped wood door leading to the outdoors, and a 16-foot-wide overhead garage door insulated with an inch of polyisocyanurate.
The 2×4 garage walls were sheathed with plywood and finished with cedar siding. On the interior of the garage, the studs were insulated with R-11 fiberglass batts and finished with drywall.
The garage foundation wall was insulated on the exterior with XPS for a few feet from its intersection with the house foundation to reduce heat loss from the house.
Bedrooms had a double layer of drywall on some partitions for a little extra thermal mass, but the extra drywall is inadequate to keep temperature fluctuations low enough for comfort. Bedrooms would get into the low 80s during typical sunny afternoons, and drop to about 65 degrees by dawn. On the coldest nights, we would use backup-electric radiant heat in the ceiling during the night. On most winter nights, we would air out the bedroom to cool it off before going to bed.
This direct-gain passive solar home has a 4-inch concrete slab finished with ceramic tile as the main thermal mass on the first floor. Underneath the slab is a layer of concrete blocks, turned on their side to create air channels through the block that run in the north-south direction below the slab. A thermostatically-controlled blower can pull warm air from the first floor ceiling, down through an interior wall to reach the layer of concrete block. Air is driven through the concrete block channels and back into the first floor at two floor vents at the southwest and southeast corners of the floor.
This system was recommended by a consultant to increase the ability to store heat extracted from the warm air during sunny winter afternoons. In practice, this extra heat storage seems unnecessary. Running the blower also added the odor of concrete to the home air, and increased the possibility of radon gas contamination in the home’s air. A radon test after the home had been completed was negative. However, the block may provide more thermal mass and temperature stabilization than the concrete slab floor alone.
The design includes a thermal break between the concrete slab and the foundation wall. Vertical XPS insulation was installed on the interior of the foundation wall, up to the top of the foundation (equal to the top of the slab). The floor tile which covers the slab does not extend all the way to the concrete foundation wall. Oak plywood spans about 10 inches from the tile near the outside edge of the slab, over the top of the XPS insulation, to the portion of the top of the foundation wall not covered by the exterior wall. (See Image #6 for wall, foundation, and slab construction details.)
No horizontal insulation was installed below the slab and the concrete blocks under the slab. The distance from the slab down below the concrete grade beam and back up to the ground surface was the distance that would put any cooling or heat from the ground to be out of phase with the current season. It would take six months for the summer heat or winter cold to reach the slab. So heat would arrive in winter, and cooling in summer. In addition, the sand and dirt provided insulation and thermal storage itself. Ground temperatures of about 50 degrees could be brought to 70 degrees. In practice, there appeared to be no noticeable problem with a lack of insulation below the slab.
To prevent the tile floor and concrete slab from cracking, and to prevent moisture uptake, a number of precautions were taken:
â€¢ The disturbed earth was tamped down with rented tamping equipment
â€¢ The slab area was left to settle for months before the concrete slab was poured
â€¢ A few inches of sand was installed above the earth
â€¢ A polyethylene vapor barrier was installed over the sand
â€¢ Concrete blocks placed sideways were installed over the sand for additional thermal mass, and to provide channels under the slab to circulate warm air for additional heat storage
â€¢ ¾” plywood was placed over open air channels along the north and south edges of the concrete block layer (to prevent poured concrete from filling the air channels)
â€¢ Polyethylene sheeting was installed over the concrete block and plywood-covered areas before the slab was poured
â€¢ The 4-inch concrete slabs which covered most of the first floor area (approximately 17’6” x 29’6”, inside the foundation walls and interior foam board insulation) was divided into three sections. One section covered most of the 13’6” x 22’ main open living area; another section covered the kitchen, bath and laundry, and a third section was under the U-shaped stairs and part of the dining area. If cracks did appear, they would occur under the kitchen breakfast bar, the stairway, or the dinette table area, rather than randomly in the middle of the floor area. No cracks appeared.
â€¢ Drain tile was installed at the base of the concrete grade beams, on the exterior. The drain leads to two dry wells.
Free air conditioning from the thermal mass
A number of people who visited this home on a winter afternoon have remarked, “If the house is this warm in the winter, it must get really hot in the summer!” Some who visited in the summer at midday said, “If it’s this cool inside the house in the summer, it must be freezing in the winter!”
How could a home’s interior temperature work counter to the seasons? That’s the beauty of a well-designed passive solar home.
â€¢ During the winter, the sun remains low on the horizon during midday, streaming light and providing significant heat through the large south-facing windows.
â€¢ During the summer when it gets hot outdoors, the noon sun is overhead, beating on the roof, not the south-facing side of the house. (This house has R-60 attic insulation, plus an attic fan and ridge vent to air out the attic. So it stays relatively cool inside during hot summer days.)
â€¢ During the morning, the sun beats on the east side of the house. (This house has no east-facing windows and well-insulated walls.)
â€¢ In the afternoon, the sun beats on the west side of the house. So this house remains cool in summer until the sun hits those west-facing windows. (Better plant deciduous trees to shade those west-facing windows during summer!)
The added bonus that keeps the downstairs area feeling like it is air-conditioned is the thermal mass of the tiled concrete slab floor. At night, by opening up the small second-story window in the hallway, cool air floats down the stairs and settles on the slab, cooling it off. During the hot day with windows closed, the slab keeps the room cool as it soaks up the heat to keep the indoors cool until late afternoon. When the sun beats mercilessly through the west-facing windows, the indoor temperatures climb into the high 70s. That’s not “air conditioned,” but it is still better than the 90 degrees outdoors.
The user experience
What’s it like to live in a home that provides very warm temperatures on cold winter afternoons? What’s it like to have bright sunlight streaming in during the dead of winter? And shade and cool temperatures at midday in summer when outdoor temperatures climb into the 90s? And to get this … for free?
Most people would want some or all of these features — especially those who do not like the cold or dark winters (like me) or who have Seasonal Affective Disorder (SAD). Is there anyone who would prefer higher utility costs? It is irrational that we don’t use superinsulation and passive-solar design principles in more homes today. By eliminating your central heating system and putting that money into insulation and better windows, you get so much in return. Just orienting your home toward the winter sun makes a big difference.
Aside from savings on utility costs and on thermal comfort, there are other potential advantages to building a passive solar, net-zero-energy, Passivhaus, or superinsulated and tight home:
â€¢ Homes like this have the potential to be sustainable, improve air quality, and reduce greenhouse gas emissions contributing to global warming.
â€¢ Homes that do not rely on fuel oil or electricity as the main source of heat and hot water are more resilient. You face lower risks of supply disruption and utility cost escalation.
â€¢ A passive solar or solar tempered home helps people keep in touch with the seasons and daily cycle of light and dark, warm and cold.
â€¢ I designed and built this home myself, which was a lot of work and financial stress, but one of the greatest accomplishments of my life.
What would I do differently next time? In my most recent design for a superinsulated home designed for a cloudy, cold climate, some specifications were different:
• Use even higher insulation levels (R-80+ ceiling, R-40+ walls and floor, R-5 windows)
• Eliminate leaky sliding glass doors and only use triple-pane, low-e, argon-filled casement and fixed windows
• Reduce the west-facing glazing to almost none, which is easy with a long east-west axis footprint
• Use updated air and moisture sealing products and techniques
• Install an HRV
• Don’t use as much thermal mass for a cloudy climate, especially the concrete block active hot air storage system
Recommendations based on this experience:
• Choose a lot based on location, location, and location — and also solar access and maybe south-facing views
• Orient the home and most of the windows to face south, with a longer east-west axis
• Make the time and investment for massive insulation, tight building materials and practices, and the best quality materials
• Use kitchen layout standards for sufficient space on the sides of the refrigerator, cooktop, sink, and ovens
• Install lots of electrical outlets and convenience features
• Learn to do what you don’t know from experts, books, the Internet, and educational events
• Take the risk and build your own house
Bob Opaluch has had a lifelong interest in energy-efficient homebuilding, passive solar design, and designing and building furniture. He designed and built the passive solar home in Boulder, renovated a house in Massachusetts, and designed and built dozens of furniture pieces for friends and family. He recently led a course in Sustainable Architecture for Lifelong Learning Collaborative, an adult ed organization in Providence, R.I. Bob has degrees in philosophy and applied mathematics from Brown University, and MA and PhD degrees in psychology from UCLA. He was a psychology professor for five years, and a usability and design engineer for 20 years. Bob is a divorced single parent. His son started college this year.
General Specs and Team
Foundation: Perimeter concrete grade beams surrounding a slab on grade
Vertical insulation at perimeter of foundation: R-45 XPS
Wall construction: 2x6 studs, 24" o.c.
Wall insulation: R-19 fiberglass batts plus either ½-inch or 1-inch polyisocyanurate on the exterior
Siding: Diagonal 3/4-inch shiplap cedar boards
Ceiling insulation: R-60 fiberglass batts
Windows: Double-glazed sliders, casements, and awnings
Roof framing: Roof trusses (unconditioned attic)
Roofing: Cedar shingles
Domestic hot water: Two batch-type solar thermal collectors with electric resistance backup
Space heating: Passive solar design with electric resistance backup
5600 heating degree day climate
Plumbing core was designed to keep pipe runs short
Alternate Energy Utilization
Renewable energy equipment: Two passive (batch-type) solar thermal collectors