‘Walls Need to Breathe’ and 9 Other Green Building Myths
The Energy Nerd plays Whack-a-Mole with some common misunderstandings in home building
Just for fun, I’ve rounded up ten oft-repeated statements that are either half-truths or outright falsehoods. I’m sure some readers will disagree with my conclusions; if you’re one of them, don’t hesitate to post a comment.
Green building myth #1. New York City is an environmental nightmare
This myth has been debunked many times, most recently by author David Owen, in his New Yorker article titled “Green Manhattan.” In fact, the average resident of Manhattan uses much less energy, and has a much smaller carbon footprintAmount of carbon dioxide and other greenhouse gases that a person, community, industry, or other entity contributes to the atmosphere through energy use, transportation, and other means. , than the average American. Compared to a resident of New York City, the average suburban American is wearing carbon clown shoes.
Owen wrote, “Most Americans, including most New Yorkers, think of New York City as an ecological nightmare, a wasteland of concrete and garbage and diesel fumes and traffic jams, but in comparison with the rest of America it’s a model of environmental responsibility. By the most significant measures, New York is the greenest community in the United States, and one of the greenest cities in the world. … The average Manhattanite consumes gasoline at a rate that the country as a whole hasn’t matched since the mid-nineteen-twenties, when the most widely owned car in the United States was the Ford Model T. Eighty-two per cent of Manhattan residents travel to work by public transit, by bicycle, or on foot. That’s ten times the rate for Americans in general, and eight times the rate for residents of Los Angeles County.”
Myth #3: Energy-Efficiency Retrofits
Myth #4: One Air Barrier or Two?
Myth #5: Blower-Door-Directed Air Sealing
Myth #6: Understanding R-Value
Myth #7: Air Conditioner Basics
Myth #9: Radiant-Floor Heating
In a separate article, Owen explains why the residents of Manhattan are so much greener than Vermonters.
Green building myth #2. Walls have to breathe
Bored readers may move on to the next item; I know that this is a tired old argument. But the “walls have to breathe” statement still keeps popping up, so I’ll take this opportunity to whack it back into its hole.
As I’ve written elsewhere, mammals and birds breathe to oxygenate their blood. Walls don’t have any blood that needs to be oxygenated, so they don’t need to breathe.
Walls have different needs from people. Although walls don’t need to breathe, they do need to be able to dry out when they get wet. And a building’s residents need fresh air.
Most people who are hung up on the “walls have to breathe” idea seem to like straw-bale walls covered with plaster or clay; evidently “walls that breathe” need to be vapor-permeable. (I think. But you had better ask the people who say “walls have to breathe” to be sure.)
But it’s possible to build a high-performance wall that is, for all intents and purposes, vapor-impermeable and airtight. Consider a well-sealed wall that is sheathed with foil-faced polyisocyanurate insulation. Almost no water vapor or air can get through the wall, so most people would agree that the wall doesn’t “breathe.” But it can perform quite well, as long as it can dry on both sides of the polyiso. (This could be accomplished by installing rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. siding on the exterior of the polyiso, and permeable materials on the inside of the polyiso.)
Of course, the occupants of any building need fresh air. That’s why buildings have operable windows and mechanical ventilation systems.
Green building myth #3. Renovation is less expensive than new construction
Whether this is true depends on the extent of the intended renovation. If you’re aiming for a high-performance home, you’re probably embarking on a deep-energy retrofit. My advice: double-check your bank balance, and good luck.
If you live in a drafty house with poorly insulated walls, a poorly insulated ceiling, a damp basement with no insulation, and old windows, all you need is everything.
A remodeling contractor will be glad to help you. He knows how to build what you need, and he’s done it many times. The only problem with your job is that your existing house is in the way. That’s why the work will cost more than new construction.
Green building myth #4. Spray polyurethane foam creates an air barrier
If you install spray polyurethane foam in your wall and ceiling cavities, you don’t have an air barrier; you’ve just got one component of an air barrier.
Here are just a few of the potential air leakage sites that need to be addressed before your foam-insulated house has an air barrier:
- The crack between the foundation and the mudsill.
- The crack between the mudsill and the rim joist.
- The crack between the subfloor and the bottom plates.
- The crack between each pair of doubled studs.
- The crack between the top plates and the second-floor rim joist.
- The cracks around the perimeter of your attic access hatch.
Green building myth #5. Caulking the exterior of a house reduces air leakage
For the last 30 years, newspaper advice columnists have been telling homeowners that a good way to seal air leaks in a wall is to caulk any cracks on the home’s exterior. This is bad advice, for two reasons: first, because the most significant air leaks in a typical home are located elsewhere, and second, because exterior caulk can do more harm than good.
A caulk gun in the hands of an overenthusiastic homeowner can be a dangerous weapon. It’s not unusual to see caulk installed where it doesn’t belong — for example, blocking drainage at the horizontal crack between courses of lap siding, or blocking weep holes in windows.
If you want to improve the airtightness of your house, put away the caulk gun and ladder. Instead, get a few cans of spray foam and head for the basement or attic.
Green building myth #6. R-value tests only measure conductive heat flow
Of the three heat-flow mechanisms — conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., convection, and radiation — radiation is probably the least understood. Sensing an opportunity, some marketers of radiant barriers, reflective insulations, and “ceramic coatings” take advantage of common misconceptions to promote their products.
An oft-repeated falsehood is that “R-value measures only conductive heat transfer.” In fact, R-values include all three heat transfer mechanisms.
The most common method of testing a material’s R-value is ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. In this test, a technician measures the thermal resistance (resistance to heat flow) of a specimen of insulation placed between a cold plate and a hot plate.
To understand how all three heat transfer mechanisms are involved, consider the flow of heat across a fiberglass batt. Heat wants to flow from the hot side of the fiberglass batt to the cold side. Where individual glass fibers touch each other, heat is transferred from fiber to fiber by conduction. Where fibers are separated by an air space, heat is transferred from a hot fiber to a cooler fiber by radiation and by conduction through the air. In ASTM C518 tests of fiberglass insulation, air movement within the fiberglass batt (convective heat transfer) is rare, although the test captures the phenomenon when it occurs.
For more information on R-value testing, read “Understanding R-Value.”
Green building myth #7. Air conditioned homes don’t need a dehumidifier
In a hot humid climate, air conditioners make a home more comfortable by lowering the temperature of the air (sensible heat removal) and by dehumidifying the air (latent heat removal). When the thermostat detects that the indoor air temperature is too warm, the air conditioner switches on; when the thermostat is satisfied, the air conditioner switches off.
While the equipment is operating, some dehumidification occurs. However, the ratio of latent heat removal to sensible heat removal is a function of equipment design and weather conditions; it is out of the control of the homeowner.
When an air conditioner runs flat out for hours at a time, it’s usually pretty good at dehumidification. But in an energy-efficient house with low-solar-gain windows, the typical air conditioner runs for fewer hours. Although the equipment easily cools the house, it may not lower indoor humidity levels to comfortable levels.
As reported in the January 2003 issue of Energy Design Update, researchers in Houston were called to investigate high levels of indoor humidity plaguing a group of energy-efficient homes participating in the US Department of Energy’s Building America program. They discovered that “improvements in window performance and envelope tightness … lowered the buildings’ sensible cooling loads to the point that existing air conditioners [were] unable to handle the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates..” The recommended solution: each house needed a stand-alone dehumidifier in addition to a central air conditioner.
As homes continue to be built to higher energy standards, the need for supplemental dehumidification is likely to increase in hot humid climates along the Gulf Coast and elsewhere in the Southeast. Stand-alone dehumidifiers are a fairly inexpensive solution to the problem. Unlike an air conditioner, a stand-alone dehumidifier continues to lower indoor humidity until the desired set point is reached. The downside: a dehumidifier adds heat to the house. As long as the house has a properly sized air conditioner, however, this shouldn’t be a problem.
Green building myth #8. Efficiency rating labels on furnaces account for all types of energy
The Annual Fuel Utilization Efficiency(AFUE) Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. (AFUEAnnual Fuel Utilization Efficiency. Widely-used measure of the fuel efficiency of a heating system that accounts for start-up, cool-down, and other operating losses that occur during real-life operation. AFUE is always lower than combustion efficiency. Furnaces sold in the United States must have a minimum AFUE of 78%. High ratings indicate more efficient equipment. ) number on a furnace or boiler label does not include any accounting of electrical energy. As a result, an apparently efficient appliance with a high AFUE may still be an electrical hog.
The AFUE number is a laboratory rating of an appliance’s efficiency at burning natural gas, propane, or oil. The calculation accounts for typical chimney losses, jacket losses, and cycling losses, but not electricity use.
A gas furnace has several electrical components, including the furnace fan (by far the biggest electrical load), an igniter, a draft inducer, and controls. Oil furnaces include an oil pump, an oil burner motor, perhaps a power vent unit, and a furnace fan. The AFUE gives no clues concerning the power draw required to run these electrical components, which varies from appliance to appliance.
Most furnace fans draw between 500 and 800 watts, with an annual electricity use that averages about 500 kWh per year. Furnace fans account for 80% of the electricity used by furnaces, so total furnace electricity use averages about 625 kWh per year. If a homeowner operates the furnace fan continuously, either to improve air mixing or to satisfy an electronic air cleaner, annual electricity use is much higher. Since inefficient furnace fans produce waste heat, they are particularly problematic in cooling climates.
If you’re buying a new furnace, look for one with a blower powered by an electronically commutated motor (ECM). Such motors use significantly less electricity than conventional permanent split capacitor (PSC) motors.
Green building myth #9. In-floor radiant heating systems save energy
Proponents of in-floor radiant heating systems often claim that such systems save energy compared to conventional heating systems. The idea is that people living in homes with warm floors are so comfortable that they voluntarily lower their thermostats, thereby saving heat.
The only problem with the theory is that no reputable study has ever shown it to be true, while at least one study has disproved it. Canadian researchers visited 75 homes during the winter to note where the homeowners set their thermostats. The 50 houses with in-floor radiant heating systems had thermostats set at an average of 68.7°F, which was a little bit higher than the thermostat setting at the 25 homes with other types of heat delivery (either forced air or hydronic baseboard), which averaged 67.6°F. Since homes with radiant floors don't have lower thermostat settings, the researchers concluded that “there will generally be no energy savings [attributable] to lower thermostat settings with in-floor heating systems.”
Other radiant floor proponents have suggested that homes with radiant floors can have lower boiler temperatures compared to homes with baseboard units. This factor, however, would be responsible for only very minor energy savings, if any. It has also been suggested that homes with radiant floors might have reduced infiltration compared to homes with forced air heat. While this is certainly possible, high infiltration rates are best solved by addressing air-barrier problems at the time of construction.
Radiant floors, like baseboard radiators, are a heat-distribution system. When it comes to heat distribution, a BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. is a Btu. The overall efficiency of a hydronic heating system is basically governed by the boiler; the distribution equipment plays only a minor role in system efficiency.
Finally, it should be noted that a home with a slab-on-grade radiant floor heating system may have higher heat loss to the ground that would a home with a forced-air heating system — a factor that might lower rather than increase the radiant heating system’s overall efficiency. The best way to counteract this problem would be to increase the thickness of insulation under the slab.
Green building myth #10. Green building helps save the environment
New construction, like virtually all economic activity, has a detrimental effect on the environment. To build a new house in the Northeast, trees must be cut on the building site and a foundation must be dug. More trees are cut to supply lumber, and cement factories must burn fuel to produce the necessary cement. And so on.
Unless you’re engaged in environmental restoration — for example, decontaminating the soil at an abandoned industrial site so that trees can once again be established there — doing nothing is always better for the environment than developing land and building new homes.
If it’s important to build a new building, of course the construction should be done with environmental sensitivity. But I propose we give a green building award every year to any American who didn’t build a house. These people are true green heroes!
Portions of this article first appeared in the Journal of Light Construction.
Last week’s blog: “Saving Energy With Manual J and Manual D.”
Photo credit for photo on home page rotator: Peter Lenardon
- Daniel Morrison
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