Alternative University

Environmental Design

Residential Homes

Energy-Efficient Home Design & Construction

Modern energy-efficient homes use very little energy. The houses are well insulated, preventing heat loss in winter and preventing heat gain in summer. There are no chimneys or wood stoves for winter heating. Fresh air is provided with a heat recovery ventilator (HRV – also called an energy recovery ventilator, ERV, explained below).

“while some parameters, such as a house’s compactness or its southern-facing orientation, had a relatively low influence on its energy efficiency, others such as its standing and level of insulation and ventilation equipment saw a greater influence in improving the house’s performance. In particular, super insulation and HVAC equipment lead to a 90% decrease in heating demands, in comparison to what would be required if simply following the standard building code.”
— 
Vladimir Jovanovic, Energy-efficient Building Design in Southeast Europe, p. 112

These homes have comfortable temperature throughout the house, instead of fluctuating temperature gradients.

Sharply reducing the energy use saves money, increases comfort, and helps to protect the environment.

Figure 1:  Passive house retrofit in Europe. The roof is being raised to add insulation. The gable wall will be extended to appear like the roof is not thick.

No Internal Combustion

Energy-efficient houses are well-sealed. A key feature of effective sealing of the building envelope is that there is no venting other than through a heat recovery ventilation (HRV) system.

That means there are no fireplaces, no wood stoves, and no gas appliances. Sharply reduced heating load eliminates the need for fireplaces and wood stoves, which require houses to be leaky.

A house gets colder in winter if it has a fireplace or wood stove, because those devices expel air out of the house, requiring new cold air to enter the house through leaks in walls, doors and windows.

Likewise, gas appliances, including gas water heaters, and gas cooking stoves and ovens, produce combustion emissions that pollute the house air and require venting which causes the house to leak, making the house much colder in winter and much hotter in summer.

Highly efficient electric appliances replace all gas appliances, improving air quality indoors and outdoors.

Figure 2:  Electric oven and range stovetop.

Figure 3:  Built-in electric oven and range.

Figure 4:  Cooking on an electric induction range.

Heat Recovery Ventilator (HRV)

A heat recovery ventilator (HRV – also called an energy recovery ventilator or ERV) is a ventilation system that is built into your house and circulates fresh air from outside. It blows stale air from inside the house to outside, and brings in fresh outside air, while transferring the heat and humidity from the expelling air to the incoming air.

Figure 5:  Heat Recovery Ventilator (HRV), also called Energy (Enthalpy) Recovery Ventilator (ERV).

Figure 6:  Air circulation schematic, with HRV in the basement.  [Wiki]
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HRVs work like central air conditioners or heaters, in the sense that electricity is required to power fans. And HRVs also have filters that need to be changed or cleaned periodically, like central air conditioners and heaters.

However, an HRV has a very important difference compared to legacy AC or heating systems: it uses much less energy.

Figure 7:  Heat recovery ventilator (HRV) in attic of a model home. Wire holes through the attic floor are sealed to prevent air leaks.
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Figure 8:  Close-up of a heat recovery ventilator (HRV) with side cover removed.  Click for more pictures of HRVs.

The incoming air may be preheated or precooled before entering the HRV, for example passing through underground ducting, to absorb heat from the ground in winter, and release heat to the ground in summer.

Energy-efficient heat pumps may also provide additional climatic conditioning for the house, such as radiant floor heating in the winter.

Figure 9:  Example diagram of a heat pump system that transfers heat to/from the ground.

Even if conventional electric heating and cooling systems are also used, they can be sharply reduced in size, substantially lowering both installation costs and operating expenses.


Air Tightness

For a building to be energy efficient, it must be air-tight. In new construction, this is accomplished by carefully sealing all of the building envelope layers during construction, according to Passivhaus o standards (also referred to as Passive House or Passive Home).

Certification by the Passivhaus Institute is optional. You can verify meeting Passivhaus standards by conducting a blower test of the home. If the house has less than a certain number of air exchanges for a given time interval, it is well-sealed.

Figure 10:  Blower test on a house in France.  A fan blows air in or out through an exterior door opening, with all other exterior doors and windows closed. Air pressure is measured to calculate number of air exchanges per hour. To meet passivhaus standards, the blower test is conduction with air flow in each direction (both into and out of the house).

Figure 11:  Blower test on a house near Atlanta, Georgia.

Homes that do not pass a blower test can still be sealed by hiring an aerosol spray operation, conducted in an empty house (new home or remodel) — which in combination, with a blower test that blows air into the house, migrates the special aerosol into outward air leaks whereupon the aerosol hardens to plug the leaks.


Insulation

Another hallmark of energy-efficient construction, besides being well-sealed, is that the buildings are very well insulated. This combination, of high insulation and air-tightness, sharply reduces heating and cooling loads.

One way to achieve high insulation values is to use structural insulated panels (SIPs), consisting of thick foam insulation sandwiched between building boards. The geometry of each SIP assures high strength.

Figure 12:  Installing SIP walls in Atlanta. Click for more pictures of SIP construction.

SIPs may be used as walls, roofs, floors, and foundations. For an example of using SIPs as a roof, see the JLC article listed below about a passive house built in California. In that case, SIPs were used for the roof, while spray-in cellulose insulation was used in extra thick walls (referred to as double walls).

Figure 13:  Example of double wall framing in Colorado.

Figure 14:  Another example of double wall construction, viewed from inside the house. Outer sheathing is coated oriented strand board (OSB). The inside of the outer sheathing is coated with 75 mm (3 inches) of closed-cell spray foam. The remaining cavity will be filled with blown fiberglass insulation. Mantell-Hecathorn Builders, 2017.
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Other possible ways of insulating walls include installing insulating foam blocks:

Figure 15:  Installing foam blocks on a multi-story passivhaus building in Germany.

Figure 16:  Foam blocks ready to install on new passivhaus townhouses for soldiers on a US Army base in Germany. U.S. Army Corps of Engineers photos by Carol E. Davis. [USACE]

Figure 17:  Installing second layer of foam blocks, with non-overlapping seams to reduce thermal bridging o.

Figure 18:  Installing foam blocks around a corner.

Figure 19:  Townhouse building of the preceding photographs completed.

Foundations

Foundations for energy efficient homes are usually constructed of insulated reinforced concrete, with barrier membranes to reduce insect and moisture infiltration.

Figure 20:  Concrete slab on grade will be poured over foam insulation boards that will be placed and sealed on the ground.

Figure 21:  Insulated Concrete Form (ICF) blocks hold reinforcement rods (“rebar”) that concrete will be poured into, to form a foundation footing wall. The foam forms stay in place after construction, to provide insulation on both sides of the concrete.

Figure 22:  ICF corner with external barrier already attached to the outside of the ICF blocks at ground level. A floor slab will be poured in the background, over closed-cell spray foam insulation.

Figure 23:  Drainage tubing around the inside (not shown) and outside (shown here) of the foundation footings will help reduce moisture infiltration into the foundation.

ICF blocks can also be used to build entire walls, not just foundation and basement walls, but that is not recommended because other types of walls are more efficient above the foundation and basement.

Metal flashing to prevent termites is installed between the concrete foundation and stud walls.

Foam panels may be used instead of ICF blocks to make foundation walls thicker, provide more insulation, and use fewer seals (for less thermal bridging).

Figure 24:  Concrete foundation forms using rigid foam insulation panels instead of ICF blocks. The foam will remain after construction, to insulate both sides of the concrete. Kalamazoo Valley Habitat for Humanity, 2018.
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Windows

Energy-efficient homes use windows that have three or four panes of glass, with inert gas (instead of air) filling the space between the panes of glass, and thermal breaks to reduce thermal bridging through the window frame.

Figure 25:  Triple-pane hinged window, with argon gas between the panes, and low-emissivity coating to slow heat transfer. Other gas fills, that are more expensive than argon, allow the glass panes to be positioned closer together (not shown). Evolutionary Home Builders, 2013.
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Inert gas between glass panes may be argon as shown here, or argon mixed with other gases to allow somewhat closer spacing of panes without the full additional cost of only using other gas.

A hinged window (shown above) has better sealing than sliding windows when closed, and allows more of the window area to provide air flow when open (reducing the need for extra window area).

Transparent films or coatings may be used between panes to reduce infra-red heat transfer. For more information, see: Energy Efficient Windows o

Energy efficient windows have been common in Europe, and are now starting to be manufactured in the United States.

Windows have less insulating value than walls, providing incentive for windows to be medium sized instead of very large.

Before the advent of LED lighting, when lighting was more expensive with less options, there was incentive to make windows larger. That is no longer necessary, because lighting now uses much less energy and is higher quality.

The use of larger windows for providing interior lighting was called daylighting.

“the case for daylighting design must transform to one that elevates its potential benefits not in energy consumption, but rather in occupant health and well-being.”
— 
Elizabeth Donoff, “Daylighting in an LED World”, Architectural Lighting, 2018.

Daylighting can still play a role in design, for example as accent lighting and for emergency lighting (power outages), but will play a much smaller role than before, because some LED features provide superior health benefits than daylighting, for example ability to control glare and color, and provide more uniform illumination. New luminaires and architectural designs to incorporate them will be developed to better harness the potential of LEDs — an important new area of opportunites.

Figure 26:  Energy-efficient windows and light emitting diode (LED) luminaires.

Another reason to avoid extra-large windows, besides substantial energy waste costs, is to prevent breakage from climate change.

Climate change is driving up wind speeds in storms in all climate zones.  Increasing occurrence of wild fires means more debris from fire-generated winds will strike houses. The larger a window is, the more susceptible it is to breakage that allows the storm or fire into the house.

Figure 27:  The outer pane of this window broke under radiant exposure from a neighbor’s house that had ignited in a 2007 southern California wildfire. [UCCE]

Summer Cooling

Energy-efficient homes need to implement shading of windows to prevent solar heat gain in summer. This may be accomplished with shading devices or shade trees.

Examples of shading devices include awnings o and window overhangs.

Figure 28:  Awnings on a round passivhaus building in Germany. Some of the windows are open for summer cooling. Air intake vent for HRV is shown on right (vertical pipe sticking out of the ground). [Wiki]

Figure 29:  Types of window overhangs.

Figure 30:  Window overhangs on a house in Puerto Rico. Projections shade the windows, while the door is unshaded. Energy-efficient windows will last longer if shaded from direct summer sunlight. [UPR]

Figure 31:  Window overhangs made of recycled wood.

When planting shade trees where there are cold winters, consider deciduous trees that lose their leaves in winter to allow winter solar gain. Examples of small to medium size deciduous shade trees include Emerald Sunshine Elm o and Redpointe Maple o.

Avoid planting trees that have high resin content, like pine trees, since they are much more flammable.  And avoid planting shrubs that are woody or sappy, like manzanitas and star jasmine.  Plant Ceanothus (California Lilac) o instead.  No plants are fire-proof, but if you are going to plant trees or shrubs, grow plants that are fire-resistant.

“Firefighters often refer to ornamental junipers as little green gas cans. During a wildfire involving homes, embers can smolder undetected under ornamental junipers. The junipers can then ignite and burn intensely after firefighters have left the area.”
— 
Fire Adapted Communities (see References below)
Figure 32:  Prevent branches from growing to within 3 meters (10 feet) of a building. [FEMA]

In fire-prone areas, besides growing plants that are fire resistant, exterior walls should be fire resistant, for example coated with stucco o.

Figure 33:  Applying stucco to the exterior of a building in Italy. U.S. Army Corps of Engineers photo by Mark Nedzbala.

References

Aerosol Sealing of Building Envelopes (WCEC)

Double Wall Framing (Steven Winter Assoc.)

Passive House in Wildfire Country (JLC)

Panelization / Passive Building (Builder)

Prefab Passive Houses (Architect magazine)

Phoenix Haus (Colorado Prefab)

I-joist Cellulose Insulation Framing (JLC)

Window Types and Technologies (Energy.gov)

Windows: Heat loss & Heat gain (UK)

Fire Adapted Communities (Nevada)

Construction in Wildfire Zones (FEMA)

Fire-Resistant Landscaping (CalFire)


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2021–Sep–22  20:27  UTC