Alternative University

Environmental Design

Residential Homes

Energy-Efficient Home Design & Construction

Modern energy-efficient homes use very little energy. The houses are air-tight and well insulated, preventing heat loss in winter and heat gain in summer. Use of extra-large windows is avoided, and there are no chimneys or wood stoves. Fresh air is provided with a heat recovery ventilator (HRV – explained below).

These homes have constant comfortable temperature throughout the house, instead of variable temperature gradients. Sharply reducing the energy use saves money, and helps to protect the environment.

Figure 1:  Energy-efficient houses under construction in Kansas.

Figure 2:  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

A hallmark of an energy-efficient house is that it is air-tight. A key feature of air tightness is that there is no venting other than through a heat recovery ventilation (HRV) system. That means 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 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, must also be vented to expel combustion emissions, causing a house to leak, making the house much colder in winter and much hotter in summer. Highly efficient electric appliances replace all gas appliances.

Figure 3:  Electric oven and range stovetop.

Figure 4:  Built-in electric oven and range.

Heat Recovery Ventilator (HRV)

A heat recovery ventilator (HRV) is a ventilation device 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:  Air circulation schematic, with HRV in the basement.  Click on image to view larger. [Wiki]

An HRV works like a central air conditioner or heater, in the sense that it requires electricity to power a fan, and it has filters that need to be changed or cleaned periodically. However, it has a very important difference compared to AC or heating systems: it uses much less energy.

Figure 6:  Heat recovery ventilator (HRV) in attic of a model home. Paper cup on right shows scale (HRVs are not very big). Wire holes through the attic floor are sealed to prevent air leaks. Click to view larger.

The incoming air may be preheated before entering the HRV, for example passing through underground ducting to pick up ground heat.

Figure 7:  Close-up of a heat recovery ventilator (HRV) with side cover removed.  Click for more pictures of HRVs.

HRVs only work in houses that are air tight.


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 air-tight.

Figure 8:  Blower test on a house in France.  A fan blows air in 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.

Figure 9:  Blower test on a house near Atlanta, Georgia.

Homes that do not pass an initial 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, migrates the special aerosol into leaks whereupon it hardens to plug the leaks.


Insulation

Another hallmark of energy-efficient construction, besides being air-tight, 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 wood or hemp panels. The geometry of each SIP assures high strength, with the foam functioning like webbing of I-beams.

Figure 10:  Installing SIP walls in Atlanta.

Figure 11:  SIP home construction in Denver.

Figure 12:  SIP roof in Hood River, Oregon.

SIPs must be sized at the factory, not altered in the field, requiring careful design planning.

Figure 13:  SIP manufacturing plant.

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 14:  Example of double wall framing in Colorado.

Other possible ways of insulating walls may include insulating foam:

Figure 15:  Inserting flexible foam blocks between rigid foam blocks on a multi-story passivhaus building in Germany.

Figure 16:  Wall insulation construction in rural Iceland. [Wiki]

Not requiring wood to heat rural homes allows Iceland to replant its forests.

“wood was used for fuel until as late as the 1940s, both for cooking and heating the new wood frame and concrete houses, which were colder than the sod homes that Icelanders lived in before… Some birch forests are [now] popular recreation areas and they are recognised as remnants of an ecosystem that once covered much of Iceland. They also act as sources of forest-related plants, animals and fungi to colonise afforestation areas.”
— 
“Forestry in a Treeless Land”, Icelandic Forest Service, 2017 [pdf]

Windows

Windows have much less insulating value than passive house walls, providing incentive to keep the windows medium sized, not too 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, since lighting has become much cheaper and higher quality.

The use of larger windows 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).

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, for example wiping out corn fields in Iowa, increasing the size and force of hurricanes, etc.  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. Extra large windows (“picture windows”) are especially prone to this problem.


Summer Cooling

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

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. A pine tree planted near a house would experience explosive combustion during a wild fire, breaking windows and introducing fire into the house through the broken windows.

For additional fire prevention, 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.


References

Passive House in Wildfire Country (JLC)

Panelization / Passive Building (Builder)

Prefab Passive Houses (Architect magazine)

Double Wall Framing (Steven Winter Assoc.)

I-joist Cellulose Insulation Framing (JLC)

Fire-Resistant Landscaping (CalFire)


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2020–Oct–26  16:49  UTC