Tony Sivori wrote:
If anyone here has any experience about increased air conditioning
costs after going to a darker roof, I would like know how much your
bill went up (or didn't).
Executive summary:
1) proper venting and a ****-load of insulation can (or will) make
shingle-color irrelavent in terms of heat-load to the home's A/C system
2) consider using a radiant barrier underlayment under the shingles -
regardless of shingle color
3) I suspect that shingle color DOES play a role in shingle life-span
(darker shingles will get hotter, probably will have shorter life)
4) I've paid more attention to the homes in my area (great-lakes area)
with light-colored asphalt shingles, many of which have trees nearby,
and I see little to no evidence of staining / discoloration. The extent
to which this staining is assumed to happen might be over-estimated.
A study (reproduced at the very bottom of this post) gives these
conclusions:
- The greatest influence on roof temperature is geographic location.
- The direction a roof faces has the second greatest influence on
average roof temperature
- The color of roofing materials influences the mean temperature
of a roof system slightly less than direction
- Ventilating the area under a roof deck reduces the average
temperature 0.5 degrees Celsius (about one-third the influence
of the direction or color and one-thirty-sixth the influence of
geographic location). Even with wind assistance, ventilation
reduces average roof temperature about half as much as using
white rather than black shingles.
- Within the ranges studied, slope has the least influence on average
shingle temperature.
Remember, that study is looking at roof temperature, not ambient air
temperature inside the attic. They conclude with this statement:
"Many shingle manufacturers provide warranted products that are
widely distributed and are of many colors and exclude from
warranties those shingles installed on unventilated decks. This
exclusion has no justification; the variations in geography,
direction and shingle color have greater influences on average
temperature than the degree of ventilation."
The first study that I'm reproducing (immediately below) is focused on
attic air temperature and household heat-load.
===================
Monitored Summer Peak Attic Air Temperatures in Florida Residences
http://www.fsec.ucf.edu/en/publicati...6-98/index.htm
The field data was from a variety of residential monitoring projects
which were classified according to intrinsic differences in roofing
configurations and characteristics. The sites were occupied homes spread
around the state of Florida. There were a variety of different roofing
construction types, roof colors and ventilation configurations.
The ambient air temperature was obtained at each site by thermocouple
located inside a shielded exterior enclosure at a 3-4m (10-12 foot)
height. The summer 15-minute data from each site were sorted by the
average ambient air temperature into the top 2.5% of the observations of
the highest temperature. Within this limited group of observations, the
average outside air temperature, attic air temperature and the
coincident difference was reported.
Attic air temperatures vary considerably depending on roofing type,
color and ventilation.
It is worth noting that light gray or "white" asphalt shingles have a
measured solar absorptance of approximately 75% as opposed to true
reflective roofing systems which have absorptances less than 30% (Parker
et al., 1993B). Dark gray shingles have a solar absorptance of about
90%.
http://www.fsec.ucf.edu/en/publicati...es/attic-5.gif
Figure 5. Influence of asphalt shingle color on measured average attic
air temperature at two adjacent Homestead, Florida sites (HO2 and HO5)
during the summer of 1996.
Conclusions
Although the 21-house data set is not large enough to comprise a
statistical sample, it does suggest some important influences on summer
attic design temperatures in Florida. Light roof colors, deck-mounted
radiant barriers, added attic ventilation and tile roof construction
were all shown to reduce peak attic air temperatures. A simple
characterization of the collected data would be as follows:
Table 6
Comparison of Coincident Attic to Ambient Design Temperature Difference
Case / Attic Design Temperature
Shingle roof soffit vent only / Ambient + 35F (19.4C)
Shingle roof soffit and ridge venting / Ambient + 22F (12.2C)
Shingle roof Radiant Barrier soffit vent only / Ambient + 25F (13.9C)
Tile roof / Ambient +10F (5.6C)
White roof / Ambient -1.5F (0.8C)
A simple calculation illustrates the importance of controlling the peak
attic air temperatures measured in this study. As example, consider a
residence on a a peak summer day at 95F served by a three ton cooling
system with a sensible capacity of 27,000 Btu/hr and an EER of 9.0
Btu/W. The assumed residence has a 1,800 square foot ceiling with R-30
attic insulation.
Supply ducts typically comprise a combined area of ~25% of the gross
floor area (see Gu et al. 1997, Appendix G, and Jump and Modera, 1994),
but are only insulated to R-4. With the peak attic temperatures for a
shingle roof with poor ventilation estimated at 130F, and 75 maintained
inside, a UA dT calculation shows a ceiling heat gain of 3,300 Btu/hr.
With R-4 ducts in the attic and a 57 air conditioner supply temperature,
the heat gain to the duct system is 8,212 Btu/hr if the cooling system
ran the full hour under design conditions-- more than twice the ceiling
flux. However, the magnitude of both ceiling and duct heat gain is 43%
of the air conditioner's design sensible cooling load. Thus, attic heat
to ceiling and attic to duct heat gains are a major portion of the
design cooling load for residences.
In the example the attic related gains are also responsible for a 1.28
kW increase in peak air conditioning electric demand. As a contrast,
with a white roof system, the estimated attic air temperature would be
93.5F, with a total ceiling and duct heat transfer rate of 5220 Btu-- a
reduction of 6,300 Btu/hr and a drop in electrical demand of 700 W if
the system was at design capacity with the dark roof.
==================
See also:
http://www.newton.dep.anl.gov/askasc...9/eng99520.htm
http://findlayroofing.com/blog/roof-...e-temperature/
===============
(the following material does not consider shingle-color in terms of
imacting shingle-lifespan)
http://www.findlayroofingnc.com/reso...me-temperature
Shingles Follow The Rules - Black is Hotter Than White
No one questions if black shingles get hotter than white shingles, but
many people have wondered what the temperature difference really is.
Shingle manufacturers have been especially interested in answering this
question, and many have performed tests to determine the number.
Their test conditions and results vary, but the temperature difference
between a white and black shingle, when exposed to the same amount of
sunlight, is generally stated to be from 20° to 40° F.
Without a doubt, 40° F is a big difference. But keep in mind that this
number is likely taken from a small number of specimens, and the testing
was done under very specific conditions. The point is, these numbers
aren't good enough to suggest you race out to buy all the white shingles
in town today.
Every Shingle Color Stays Cool With Good Roof Ventilation
Test data is useful information to have, but the most important data
comes from actual rooftops and homes. And the real rooftop data clearly
shows that shingle color is a small player when it comes to shingle
temperature.
Any professional roofing contractor will tell you that choosing a
shingle color should be based on personal preference more than on
temperature concerns. Proper roof ventilation is the real key to keeping
your shingles cool and making them last as long as possible. There are
countless dark-colored roofs across the country that are still going
strong after 15 years, to prove the point.
What About The Temperature Inside My North Carolina House?
With good roof ventilation, the impact of shingle color on roof and
attic temperatures becomes somewhat insignificant. The presence of
insulation in an attic makes shingle color practically irrelevant, in
terms of interior room temperatures.
Put another way, there is no need to worry about temperature effects
when choosing a shingle color for your home. Instead, you should work
with your roofing contractor to make sure that your roof has the ideal
amount of ventilation.
By installing the proper combination of ridge and soffit vents, for
example, your can keep your North Carolina roof cool during even the
hottest of days. A cool roof keeps your shingles cool, and allows them
to achieve their longest possible life. At the same time, you will be
keeping your home cooler all summer long.
================
What's the value of ventilation?
A study of asphalt shingles demonstrates ventilation may not be as
important as other variables
http://www.professionalroofing.net/a...2/feature2.asp
The topic of asphalt shingles splitting and cracking has received much
attention lately. Asphalt fiberglass shingles have been experiencing
vertical splits, as well as horizontal splits in exposed tabs. These
dislocations, called thermal splits, are the subject of a great deal of
litigation, including class-action lawsuits. The splits are not
associated with quality of installation. Rather, the splits occur in
shingles where self-sealing adhesive firmly adheres the shingle tabs and
a shingle's tear strength is low or inadequate to withstand a thermally
or mechanically induced load.
Whenever asphalt fiberglass shingle manufacturers are faced with
thermal-splitting problems, one excuse they usually offer is that the
area under a roof deck was not ventilated properly.
This excuse is offered not because there is any evidence of a
cause-and-effect link between thermal splitting and ventilation but
because shingle warranties (all the shingle warranties listed in NRCA's
2002-03 Steep-slope Roofing Materials Guide) specifically exclude
warranties in the case of "inadequate attic ventilation." This is based
on the premise that shingles applied to decks over unventilated attics
will be unacceptably hotter than shingles applied to decks over properly
ventilated attics and have significantly shortened service lives as a
result of the increased temperature.
Lawyers say impractical or unreasonable contract or warranty provisions
may not be supported by court decisions. The following information
reveals results from a study we conducted that investigated the
reasonableness of the "inadequate attic ventilation" exclusion in
warranties.
Some parameters that can influence roof temperature are geographic
location, color, exposure orientation, slope and degree of attic
ventilation. We report the means (averages) of the maximum and average
annual temperatures of the roofing materials for each combination of
these parameters.
(lots of material describing a multi-city study not reproduced here)
Conclusions
The following conclusions are based on data from our numerical model:
* The greatest influence on roof temperature is geographic location. The
mean roof temperatures for Miami and Green Bay, Wis., for example,
differ by 18 degrees Celsius.
* The direction a roof faces has the second greatest influence on
average roof temperature (in excess of 1.44 degrees Celsius in the east
through south-to-west range studied, but the real difference is greater
because other directions, such as north, will be cooler).
* The color of roofing materials influences the mean temperature of a
roof system slightly less than direction (1.45 degrees Celsius average
for these parameters).
* Ventilating the area under a roof deck reduces the average temperature
0.5 degrees Celsius (about one-third the influence of the direction or
color and one-thirty-sixth the influence of geographic location). Even
with wind assistance, ventilation reduces average roof temperature about
half as much as using white rather than black shingles.
* Within the ranges studied, slope has the least influence on average
shingle temperature.
Many shingle manufacturers provide warranted products that are widely
distributed and are of many colors and exclude from warranties those
shingles installed on unventilated decks. This exclusion has no
justification; the variations in geography, direction and shingle color
have greater influences on average temperature than the degree of
ventilation.
However, ventilation should be recommended to remove the small quantity
of moisture in a roof system; it can prolong the life of a wood deck
even if it does not extend the life of shingles.
Carl G. Cash is a principal with Simpson, Gumpertz & Heger Inc.,
Arlington, Mass., and Edward G. Lyon is a senior staff engineer with
Simpson, Gumpertz & Heger.