You can't gauge the truth of an opinion by the frequency of its
repetition. Exhibit A: Builders' opinions concerning energy,
insulation, heating, and cooling. A fair percentage of these
beliefs — derived in part from product marketing,
obsolete recommendations from "experts," and oft-repeated tales
heard at lumberyards — prove upon examination to be
half-truths or outright misconceptions. But like Whac-a-Mole
pests, they just keep popping up.
To set the record straight, this article will strive to clobber
the pesky moles one more time.
"Window replacement is a cost-effective
way to save energy."
Replacing old single-pane windows with new double-pane low-e
units certainly saves energy. But the cost is so high —
and the amount of energy saved is so low — that window
replacement is almost never cost-effective. Depending on the
climate and the window cost, the payback period for replacement
windows can be as long as 20 or 30 years.
According to calculations posted on an Energy Star program Web
site, installing new double-pane low-e windows in a typical
2,000-square-foot single-story house that previously had
single-pane units will result in annual energy savings of $125
(in a mild climate like California's) to $340 (in a severe
climate like New England's). If the old windows had storms, the
savings drop to $20 to $70 per year. Exact mileage may vary,
but anyone who expects that window replacement will have an
energy payback needs to be prepared for a very long wait.
The most cost-effective window retrofit measure is the
installation of low-e storm windows. Although many storm-window
suppliers are unfamiliar with the product, low-e storms can be
ordered. Suitable glass with a pyrolitic (hard-coat) low-e
coating is available from most glass distributors. According to
a recent study, the payback period for installing low-e storm
windows on older houses in Chicago averaged just 4.3
"Housewrap is an air
When housewrap was first marketed to builders in the 1980s,
manufacturers touted its benefits as an air barrier. The
marketing campaigns were so successful some builders still
believe that "housewrap" and "air barrier" are synonyms.
In fact, the most important function of housewrap is as a
water-resistive barrier (WRB). Installed between siding and
sheathing, a WRB is designed to stop rain that sneaks past the
Housewrap can reduce air leakage between sheathing panel edges
somewhat, especially if the housewrap seams are taped. But the
cracks between wall sheathing panels don't account for much of
the air leakage in a typical home; the big air leaks are
Air leaks occur in many locations, from the basement to the
attic. For example, leaks are common between the top of a
concrete foundation and the sill plates, between the subfloor
and bottom plates, and around attic access hatches. Significant
amounts of air can also leave a house through electrical boxes
in partition walls, by traveling up the stud bays and into the
attic through cracks between the drywall and the partition top
plate. All of these leaks — and many others — need
to be addressed before a builder can brag about the tightness
of a home's air barrier.
"Interior vapor retarders are a good way
to prevent wet-wall problems."
Northern builders tend to overestimate the importance of vapor
retarders. Worries about vapor-retarder placement are often
misguided, since wet-wall problems are usually caused by
wind-driven rain or deficient air barriers, not vapor
diffusion. Most of these baseless worries concern either the
foam sheathing (sometimes vilified as a "wrong-side vapor
retarder") or the lack of an interior vapor retarder.
By keeping wall cavities warm, properly specified and installed
foam sheathing actually reduces the chance of condensation
inside a wall. And interior polyethylene can be safely omitted
from walls — even in cold regions of the country —
as long as kraft-faced insulation is used. Almost all walls are
free of vapor diffusion problems, in part because even painted
drywall provides a fair amount of resistance.
According to the 2007 Supplement to the International Energy
Conservation Code, polyethylene vapor retarders are not
required in any location in the U.S. In northern climates
(Marine Zone 4, as well as Zones 5 through 8), the code
requires that walls include an interior vapor retarder; either
kraft facing or polyethylene is acceptable.
"It's good to omit vapor retarders in
ceilings, to provide a way for moisture to leave the
Some cold-climate builders believe that, while vapor retarders
are useful on walls, they should never be installed on ceilings
"because you have to let the ceiling breathe, so that moisture
can get out of the house." This interesting misconception
contains several wrong-headed notions wrapped up in a single
Most attics include ventilation. In theory (although not always
in practice), attic ventilation can help remove high levels of
humidity that might otherwise condense on the cold roof
sheathing. However, attic moisture problems usually indicate
the existence of two flaws: a wet basement or crawlspace, and a
ceiling with air leaks.
Ceilings were never intended to be "moisture-relief valves" for
homes. Ideally, a ceiling should be as airtight as possible, to
keep warm, humid indoor air from reaching the attic. In cold
climates, the ceiling should include a vapor retarder (for
example, kraft facing or vapor-retarder paint) on the
warm-in-winter side, to limit vapor diffusion through the
High indoor humidity during the winter — usually
indicated by condensation on windows — is rare in most
homes. When it occurs, the solution is to increase the rate of
mechanical ventilation. If the home lacks a whole-house
ventilation system, a simple remedy for dripping windows is to
leave bath exhaust fans on for 24 hours a day until the
moisture problems go away.
"In-floor radiant heating systems save
Proponents of in-floor radiant heating systems often claim that
such systems save energy compared with conventional heating
systems. The idea is that people living in homes with warm
floors are so comfortable they voluntarily lower their
thermostats, thereby saving energy.
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°. This was actually a little bit
higher than the thermostats at the 25 homes with other types of
heat delivery (either forced air or hydronic baseboard), which
averaged 67.6°F (see Notebook, 12/01). The researchers
concluded, "There will generally be no energy savings due to
lower thermostat settings with in-floor heating systems."
Other radiant-floor proponents have suggested that homes with
radiant floors have lower boiler temperatures compared with
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 with 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
Radiant floors, like baseboard radiators, are heat-distribution
systems. When it comes to heat distribution, a Btu 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 lose more heat to the ground
than a home with a forced-air heating system would — a
factor that might lower 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.
"Caulking the exterior of a house reduces
Newspaper columnists often suggest that leaky walls can be
improved by filling cracks on the exterior of a house with
caulk. This is bad advice, for two reasons: First, most
significant air leaks are located elsewhere; and second,
exterior caulk can do more harm than good.
A caulk gun in the hands of an overenthusiastic builder can be
a dangerous weapon. It's not unusual to see caulk where it
doesn't belong — for example, blocking drainage at the
horizontal crack between courses of wood lap siding, or
blocking weep holes in windows.
If you want to limit infiltration in a leaky house, put away
the caulk gun and ladder. Instead, get a few cans of spray foam
and head for the basement or attic.
"Efficiency rating labels on appliances
account for all types of energy."
Neither the annual fuel utilization efficiency (AFUE) number on
a furnace or boiler label nor the energy factor (EF) used to
rate gas water heaters includes any accounting of electrical
energy. As a result, an appliance with a high AFUE or EF number
may still be an electrical hog.
An appliance's AFUE is a laboratory rating of its efficiency at
burning natural gas, propane, or oil. The calculation accounts
for typical chimney, jacket, and cycling losses — but not
A gas furnace has several electrical components, among them 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
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 percent 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 meet
the needs of an electronic air cleaner — annual
electricity use is much higher. Since inefficient furnace fans
produce waste heat, they are particularly problematic in
To reduce energy consumption, look for a furnace with a blower
powered by an electronically commutated motor (ECM). Such
motors use significantly less electricity than conventional
permanent split capacitor (PSC) motors.
A gas water heater's EF includes thermal standby losses but not
electrical power usage. Studies have shown that power-vented
water heaters draw between 100 and 200 watts for an average of
84 minutes per day (about 76 kwh per year); high-use families
have water-heater run-times of up to 240 minutes per day (about
219 kwh per year).
Although annual electricity use attributable to power-vented
water heaters is relatively low, one Canadian researcher
concluded that "it appears that the power-vented water heaters
deliver very little energy savings when you factor in the use
of the power-vent motor" (Energy Design Update, January
"Spray polyurethane foam is a vapor
This is a half-truth. Closed-cell spray foam — also
called "2-pound foam" because it has an average density of 2
pounds per cubic foot — is an effective vapor retarder.
Installed at a thickness of 21/2 inches, closed-cell spray foam
has a permeance of only 0.8 perm.
On the other hand, open-cell spray foam (average density, 1/2
pound per cubic foot) is not a vapor retarder. Installed at a
thickness of 3 inches, open-cell spray foam has a permeance of
about 16 perms, making it fairly permeable to water
When installed directly against wall or roof sheathing in a
cold climate, open-cell spray foam needs to be protected on the
interior side with a vapor retarder. In most cases, painted
drywall provides enough vapor resistance to avoid
However, when open-cell spray foam is installed in a cold
climate between rafters to create a so-called "cathedralized"
attic, the roof sheathing can accumulate moisture. Though rare,
this problem is most likely to occur in homes with elevated
indoor humidity. The solution is to cover the attic side of the
insulation with a vapor retarder — vapor-retarder paint,
"Air-conditioned homes don't need a
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 Energy Design Update (January 2003), researchers
in Houston were called to investigate high levels of indoor
humidity plaguing a group of energy-efficient homes
participating in the U.S. 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 load." 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 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 setpoint is reached. The downside: a dehumidifier adds
heat to the house. But as long as the house has a properly
sized air conditioner, this shouldn't be a problem.
"R-value measures only conductive heat
Of the three heat-flow mechanisms — conduction,
convection, and radiation — radiation is probably least
understood by the average builder. Sensing an opportunity, some
marketers of radiant barriers, reflective insulations, and
"ceramic coatings" take advantage of this common misconception
(that R-value is a measure of conductive heat transfer alone)
to promote their products. But in fact, R-values include all
three heat-transfer mechanisms.
The most common method of testing a material's R-value is ASTM
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 one by radiation and by conduction
through the air. In ASTM C518 tests of fiberglass insulation,
air movement within the fiberglass batt (that is, a convective
loop) is rare, although the test captures the phenomenon when
Since R-value measures the resistance of a material to all
three heat-flow mechanisms, it remains a useful way to compare
insulations and to judge the performance of insulation
Once insulation is inserted into a wall, however, the
performance of the insulation is affected by additional factors
that aren't measured by R-value testing. While R-value testing
measures the effects — if any — of convective loops
with a tested sample, it can't be expected to account for air
leakage through a wall caused by wind or other pressure
differences acting on a defective air barrier. A leaky wall
assembly insulated with fiberglass batts will not perform as
well as the same wall assembly insulated with spray foam with
the same R-value; but the difference in wall performance is due
to the spray foam's ability to reduce air leakage rather than
to a difference in R-value between the two materials. The fact
that some insulations are more porous than others does not
imply that R-value tests are misleading.
To obtain the best performance from fiberglass insulation, the
Energy Star Homes program now requires most
fiberglass-insulated framing cavities (including knee walls) to
be enclosed by air barriers on all six sides. If builders pay
attention to airtightness, fiberglass insulation can (at least
in theory) meet the performance expectations that the R-value
label promises. Nevertheless, in the real world, builders who
use fiberglass are unlikely to reduce air leakage enough for a
fiberglass-insulated wall to perform as well as a wall
insulated with the same R-value of cellulose or spray-foam
"Radiant heat passes right through conventional
The idea that conventional (mass) insulation products allow
radiant heat to pass right through them — that "mass
insulation is transparent to radiant heat" — is a scare
tactic used by some marketers of radiant barriers. The
misleading claim leads some builders to falsely conclude that
radiant heat can travel like radio waves right through a deep
layer of attic insulation, with the only solution being a layer
of aluminum foil.
Radiant heat travels through air (for example, from an open
fire to nearby skin) or a vacuum (for example, from the sun to
the earth). It can't travel through a solid material like
concrete. If sunlight warms a concrete patio, the heat travels
to the ground below not by radiation but by conduction; in
other words, the concrete is first warmed by the sun (by
radiation), and then the warm concrete gives off some of its
heat to the soil below (by conduction). In this example, there
is no radiant heat transfer directly from the sun to the
A microscope reveals that most insulation products consist of
fibers or pieces of material surrounded by air. If one side of
an insulation blanket is exposed to radiant heat energy, most
of the radiation ends up hitting a fiber or speck of material
in the insulation layer, heating up that fiber. The warm fiber
can then reradiate some of the absorbed heat to an adjacent
fiber, as long as that adjacent fiber is at a lower
When radiant heat hits one side of an insulation blanket, only
a tiny percentage of that radiant heat is "shine-through"
radiation — that is, radiation that manages to miss all
of the fibers in the insulation blanket and emerge unscathed on
the other side of the blanket. "With insulations like
fiberglass or cellulose, radiation can be absorbed by one piece
of material and then reradiated," explains David Yarbrough, an
insulation expert and research engineer at R&D Services in
Cookeville, Tenn. "There is very little shine-through radiation
with any of these materials."
The fact that heat flows through a layer of insulation, usually
by a combination of two or three heat-transfer mechanisms, does
not mean the insulation isn't working. Although insulation
doesn't stop heat flow, it slows it down considerably; the more
insulation, the lower the heat flow.
How much heat flows through an uninsulated ceiling into a
1,000-square-foot 32°F attic? Assuming that a 72°F
house has an uninsulated drywall ceiling — that is, a
ceiling assembly with an R-value of 2 — the heat flow
across the uninsulated ceiling is 20,000 Btu per hour.
If insulation is added until the ceiling assembly has an
R-value of 38, the heat flow is reduced by 95 percent, to 1,052
Btu per hour.
Martin Holladay is the editor of Energy