Musings of an Energy Nerd

The Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. program has been in trouble since 2012, when the Environmental Protection Agency rolled out a new version (Version 3) of the program’s requirements. Many builders found the Version 3 requirements so onerous that they dropped out of the Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. program. The number of new homes that have been certified under the Energy Star program has fallen 36% in recent years — from 130,305 in 2011 to 83,897 in 2015.

A very small percentage of U.S. homes are off the electricity grid — far fewer, for example, than in Africa. That said, North American designers of off-grid homes often end up posting questions on GBAGreenBuildingAdvisor.com.

To help this subset of builders avoid common design errors, I’ll share what I’ve learned from living in an off-grid house for 42 years.

Let’s say you walk into an unheated room. The air temperature is only 50°F. There is a wood stove in the room, but the wood stove is unlit.

Your body is at about 98°F. Since your body is warmer than absolute zero, it is radiating heat in all directions. (All objects that aren’t at absolute zero emit infrared radiation). The wood stove is cool — it’s at 50°F. Still, the wood stove is also radiating heat in all directions.

The CO2 emissions associated with the burning of natural gas are less than the CO2 emissions associated with burning an equivalent amount of coal. Because of this fact, natural gas is seen by many policy makers as a “clean” alternative to coal.

In the last few years, however, climate activists have been pointing out two worrisome facts: (1) methane (natural gas) is a potent greenhouse gas — about 80 times more potent than CO2 (on a mass basis) during the first 20 years after the methane is released; and (2) our nation’s natural gas pipelines are much leakier than some people thought.

An earth tubeVentilation air intake tube, usually measuring 8 or more inches in diameter and buried 5 or more feet below grade. Earth tubes take advantage of relatively constant subterranean temperatures to pre-heat air in winter and pre-cool it in summer. In humid climates, some earth tubes develop significant amounts of condensation during the summer, potentially contributing to indoor air quality problems. is a buried ventilation duct. The idea behind burying ventilation ducts — the ducts conveying fresh outdoor air to a building — is that the soil surrounding the ducts will warm the ventilation air during the winter and cool the ventilation air during the summer.

Earth tubes can work, as long as:

• the duct is installed in a climate with useful soil temperatures,
• the duct has a large enough diameter,
• the duct is long enough,
• the duct is buried deep enough,

The familiar NFRC sticker found on most new windows sold in the U.S. includes a number in the upper left-hand box labeled “U-factor.” For many homeowners and builders, an encounter with this sticker represents their first exposure to U-factor.

Once people understand that a window has a U-factor, they might learn that walls and ceilings can have a U-factor, too. At that point, confusion may begin.

An 6-kW photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) system can be installed for about $18,000 in many U.S. locations. With a 30% federal tax credit, the system costs the homeowner only $12,600 — or even less if utility or state rebates are available.

This 6-kW system will produce about 8,000 kWh per year in Boston (worth about $1,600) or 10,300 kWh per year in Phoenix (worth about $1,230). That’s a lot of electricity.

Let’s say you’ve owned your house for several years. Your growing interest in energy efficiency brought you to the GBAGreenBuildingAdvisor.com web site. GBA taught you that it’s a great idea to install rigid foam on the exterior side of your wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , as long as (a) the rigid foam is thick enough to keep your sheathing above the dew point during the winter, and (b) your walls don’t have any interior polyethylene.

UPDATED on August 11, 2017 with modeling data for the Wolfe Island Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates.

Longtime readers of GBAGreenBuildingAdvisor.com know that I get frustrated by exaggerated energy savings claims. A glaring example is the statement that “a Passive House building uses 90% less energy than a conventional building.” A variation on this claim: “A Passive House building uses 90% less energy than a code-minimum building.”

At the recent BuildingEnergy 17 conference in Boston, there were at least two presentations that touched on double-stud walls. John Straube, a professor of building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. science at the University of Waterloo in Ontario, used his presentation to raise a warning flag, noting that “these walls will work if everything works — if there aren’t any defects — but they don’t work if there is something wrong.”