Cool roofing: a really hot topic.

For millennia shrouded in history, man has had a roof overhead, be

it made of stone, twigs, ice, thatch, slate, wood, asphalt, tin, steel,

painted metal, tile, and various and sundry materials. All served, to

one degree or another, to keep out the elements and maintain the

interior of the dwelling at a livable temperature. For the vantage point

of those who are living in the first decade of the Third Millennium

A.D., not much has changed–many, perhaps all, of these materials are in

use somewhere in the world today, and their basic purpose is essentially

unchanged. Even when limited to modern building materials in general use

throughout the industrialized world, and with particular emphasis on

painted metal, the topic is still huge and interesting–and getting more

interesting by the day. In the United States alone, approximately 20

billion square feet of new and replacement roofing are installed every

year, and a new dimension is being added to the traditional requirements

placed upon roofing: energy conservation. Not just energy conservation

for any given building or structure, but energy conservation on a local,

regional, and national scale.

[ILLUSTRATION OMITTED]

It is easy to imagine standing on the roof of a house or building

on a sunny summer day and feeling the waves of heat rising from the

roof. During the hot summer months, the surface temperature of a roof

can be considerably hotter than the surrounding air temperature, perhaps

by as much as 40[degrees]F or more. Almost anyone in the construction

industry can affirm that a roof surface–especially a dark-colored

one–is absorbing a certain amount of radiant energy from the sun. The

roofing material cannot dissipate the heat completely, and it therefore

heats up until it reaches an equilibrium temperature, where the rate of

heat loss equals the rate of heat gain. There is more, however. Some of

the energy absorbed by the roof will make its way into the building

cavity. In the summertime, and in those areas of the country where air

conditioning is prevalent, the greater the amount of energy transferred

into the building, the greater is the amount of cooling required. More

cooling equates to greater demand for electricity, and–as this demand

increases–more electrical power generating facilities are required.

This “vicious circle” is exacerbated by the fact that

electricity cannot be stored. Enough electricity needs to be available

for the greatest peak demand period, or shortages will be experienced.

Since electricity is principally created by burning something (oil,

natural gas, coal), more electricity equates to more pollution. Is it

any wonder, then, that both the Department of Energy (DOE) and the

Environmental Protection Agency (EPA) are interested in ways to slow the

constantly increasing need for electricity, which should, in turn, lead

to a resultant improvement in air quality?

The problem with this “hot roof” phenomenon, however,

doesn’t stop there. A “hot roof” creates the need, not

just for more “cooling” in any given building, but for greater

cooling capacity, on a regional basis. To reduce the heat flow into the

building cavity, additional insulation will help to reduce cooling

demand. As the roof heats up and reaches its equilibrium temperature,

however, it is releasing some of this thermal energy to the surrounding

environment. This creates localized heating, and is commonly called the

“heat island effect.” Leave the wooded park in August to pick

up some items at the local “big box” and you will feel the

heat island effect as you enter the urban area, with a paucity of trees

and preponderance of concrete. This heat island effect leads to elevated

air conditioning use, increased air pollution (due to more consumption

of fossil fuels to meet this increased energy demand), and the formation

of smog, which increases with higher temperatures. The heat island

effect may have devastating consequences. In July 1995, it is estimated

that between 500 and 700 people died in the city of Chicago during a

heat wave. (1,2) Certainly the daytime–and nighttime–temperatures

during those few July days were high, but the heat island effect added

many degrees to the urban Chicago area. It is natural, therefore, that

the EPA and National Laboratories (3) are interested in addressing heat

island issues from the perspective of improving air quality.

No single approach or technology can be expected to overcome these

issues, but “cool roofing” is rapidly becoming recognized for

the benefits it provides to all players in the roofing and construction

industry. Research by the National Laboratories, as well as other

institutions, points to the importance of cool roofs in lowering the

energy demand of buildings and mitigating urban heat island effects.

Cities and states across the country are taking note of this development

through the incorporation of cool roof requirements into their building

codes or the initiation of cool roof incentive programs. As a result,

the concept of cool roofing is currently one of the hottest topics in

the building and construction industries, as well as in those related

industries, such as paints and coatings, which supply them with products

and services. What, however, is “cool roofing,” why is it of

such interest, and how can coatings contribute to making regular roofs

into “cool roofs”?

WHAT IS COOL ROOFING?

In a nutshell, cool roofing is any roofing solution that reduces

energy demand and mitigates the heat island effect. Cool roofing may be

achieved in a number of ways. Simply altering the color of the roofing

material to a lighter color will lower the roof temperature. Asphalt

shingles can be formulated in light colors, flat roofs that may utilize

a layer of black tar can be coated with a number of field-applied

coatings which are bright white, and–while not always practical or even

possible–a roof may be “vegetated,” where grass, ivy, and

other low-maintenance ground cover is grown on the roof surface. The

most important aspect of cool roofing, regardless of what kind of

roofing system is being considered, is that the radiative properties of

the uppermost layer of the roof control all of the radiative properties

of the entire roofing system. This is where coatings technology enters.

A one-mil factory-applied coating exerts as much influence on the

radiative properties of a metal (or any other) roof as a 20-mil

field-applied roof coating, or a 100-mil roof membrane. When it comes to

cool roofing, the “coating is king.”

Benefits of cool roofing are clear and virtually universal in hot,

sunny climates, particularly with regard to the use of air conditioning,

which can be decreased, if cool roofing is installed. One rule of thumb

suggests that cool roofing is generally cost-effective in the U.S. at

locations found in Zones 1, 2, and 3, shown in Figure 1. In colder

climates, the same roofing performance characteristics that reduce solar

heat gain (thus cooling load) are also at work during the heating

season, meaning that additional energy is required to heat a “cool

roof” building. The emphasis on cool roofing, however, is placed on

cooling issues, rather than heating issues, because it is estimated

that, in the U.S., we consume three times as much electricity for

cooling as we do for heating. An easy-to-use calculator is available on

the Oak Ridge National Laboratory website (4) that can be used to

evaluate the savings potential for an individual building, on a

“year-round” basis, using calculations based upon its

location, type of roof, air conditioning system, and local (sometimes

seasonal) power rates.

[FIGURE 1 OMITTED]

For existing buildings in air conditioning-dominated climates,

application of cool roofing offers three important benefits affecting

overall building performance:

* Reduction in air conditioning energy costs

* Improvement of the building occupants’ comfort

* Possible extension of the life of the roofing materials

For new buildings, cool roofs can reduce the air conditioning

tonnage in which the owner must invest.

From the perspective of the power grid, utility and power supply

planners are chronically concerned about peak power demand–a demand

driven substantially by air conditioning loads in commercial and

residential buildings in nearly every region of the country (not just

the Sunbelt). Most people quickly recognize that reduced peak power

demand means fewer power plants, and the substantial investments that

they require. What is sometimes underestimated–at least until the

massive outage event in August, 2003, in the northeastern quadrant of

the U.S. and southern regions of Canada near Niagara Falls and as far

north as Hamilton, Ontario–is the fragile nature of this grid and the

need to minimize peak demands. In some states, cash rebates in return

for the use of cool roofing are used as a promotional strategy both by

state agencies and utility companies to help create greater awareness of

cool roofing benefits and encourage building owners to install cool

roofs. Many utility groups charge premium rates during peak demand

periods, so the ability to reduce demand for electricity during peak

periods is beneficial for virtually everyone.

KEY PARAMETERS: SOLAR REFLECTANCE, THERMAL EMITTANCE, AND AGING

Light-colored surfaces, in general, are fairly good reflectors of

solar radiation, regardless of composition. So are shiny metallic

surfaces. No surface, however, is a “perfect” reflector–and

certainly not the real-world surfaces that we commonly find on roofs.

When sunlight strikes a roof, the laws of physics dictate that whatever

solar energy is not reflected by the surface must be absorbed by it (or

pass through it, a case that will be ignored in this discussion, since

roofing materials are virtually always opaque). The absorbed energy

presents a problem, because it has to go somewhere. There are three

possibilities for heat flow:

* Conductive heat loss is the transference of heat into the

building envelope via surface-to-surface contact.

* Convective heat loss occurs as a liquid or gas (moving air and

wind in the case of roofing) absorbs some of the heat energy.

* Radiative heat loss occurs as the roofing material itself

radiates some of its energy back into the atmosphere.

Most roofing materials have reasonably similar

temperature-dependent conductive and convective heat transfer properties, and the latter variable is more a function of wind speed

than anything else. The key properties for roofing materials, therefore,

are their reflective properties (which affect the roof temperature) and

their ability to re-radiate the heat to the sky.

In the roofing industry, the Total Solar Reflectance (TSR) is the

key value that describes the extent to which a material reflects all

wavelengths of energy, not just the infrared region of the

electromagnetic spectrum. The ability of a material to emit energy by a

radiative process is known as its Thermal Emittance (TE). Whereas TSR

can be rather intuitively related to the color of a material, perhaps

based on our life experience with car interiors and dark-colored roofs

(e.g., dark colors are good absorbers, and light colors are good

reflectors), TE follows no such simple rule. TE is a basic material

property, and, in the roofing market, most materials in commercial use

have TE values ranging from 0.75-0.90. For example, a TE value of 0.80

means that the surface being measured radiates 80% as effectively as a

perfect emitter, which would have a value of 1.0 (100%). As a general

rule, TE is not color dependent. Black painted metal has about the same

TE as white painted metal, as long as the two coatings have the same

resin chemistry, are applied at the same thickness, and are coated onto

the same substrate. The exception to this rule is bare, unpainted metal

roofing, which typically exhibits TE values in the range of 0.10-0.30

(10-30%). Materials with high TE values emit more heat than shiny metal

materials at the same temperature. If a material cannot radiate heat as

effectively, it will be hotter when compared to materials that are good

radiators of heat. Although mathematically related under certain

conditions, it makes greater sense, for practical purposes involving

building materials technology, to consider these two radiative

properties–total solar reflectance and thermal emittance–to be

separate and independent of each other.

[ILLUSTRATION OMITTED]

Specifications for cool roofing will dictate TSR and/or TE values.

At this point in the evolution of cool roofing, most of the emphasis is

being placed on flat roofs, where the TSR requirements are between 0.65

and 0.70, minimum (depending upon the code authority or municipality).

These values essentially limit the color of coatings to white,

off-white, and some very light colors. This restriction, of course,

makes sense. Light colors reflect a great deal of solar energy, whereas

dark colors absorb a great deal of the energy. Cool roofing

specifications usually require a thermal emittance minimum as well, and

this will be discussed in more detail later in this article.

ASTM methods well suited for determining solar reflectance on

roofing materials include:

* E 1918 Standard Test Method for Measuring Solar Reflectance of

Horizontal and Low-Sloped Surfaces in the Field

* C 1549 Standard Test Method for Determining Solar Reflectance

Near Ambient Temperature Using a Portable Solar Reflectometer

* E 903 Standard Test Method for Solar Absorptance, Reflectance,

and Transmittance of Materials Using Integrating Spheres is appropriate

for laboratory measurements of small samples

* C 1371 Standard Test Method for Determination of Emittance of

Materials Near Room Temperature Using Portable Emissometers

* E 408 Standard Test Methods for Total Normal Emittance of

Surfaces Using Inspection-Meter Techniques

* ASTM E 1980 Standard Practice for Calculating Solar Reflectance

Index of Horizontal and Low-Sloped Opaque Surfaces represents a new

term, Solar Reflectance Index (SRI), which combines both TSR and TE into

one value

Over time, the performance characteristics of most materials and

systems degrade through normal wear and tear. Radiative properties of

roof surfaces are no exception. For cool roofs, dirt accumulation and

biological growth can cause performance degradation, as can simple wear

due to exposure to the elements, including UV radiation, rain, high

humidity, film erosion due to wind-borne abrasive materials, and so

forth. Generally, researchers have found that most performance drop-off

involving radiative performance occurs in the first several years, after

which time the performance stabilizes. It is imperative, therefore, that

manufacturers of roofing materials understand not only the initial

radiative properties of their materials, but also these same properties

after several years of exposure has taken place.

[ILLUSTRATION OMITTED]

SLOPE MATTERS

In general, roofing and roofing materials are characterized as

being “low-slope” or “steep-slope.” Low-slope is

generally considered in the industry as a roof with a

“rise-over-run” of two feet in 12 (“2:12”) or less.

(Flat roofs are de facto considered to be in the “low-slope”

category). Steep-slope would be any roof with a rise-over-run

relationship of greater than 2:12 (>10[degrees]). In general,

steep-slope roofs are far more common in residential applications,

whereas low-slope roofs are more common in commercial applications,

although this is certainly not a hard-and-fast rule. Another significant

difference between low-slope and steep-slope roofing has to do with

aesthetic considerations. Generally, low-slope roofing cannot be seen

from ground level, so the appearance of such roofing is not a major

factor. Steep-slope roofing, however, is easily visible–and “looks

count.” As a result, manufacturers of steep-slope roofing products

invest considerable resources addressing aesthetic issues. Asphalt

shingles are now commonly supplied with dimensional characteristics and

variegated color schemes. Painted metal roofing comes in thousands of

different colors, shapes, sizes, and textures. Concrete tile, clay tile,

and cedar shake roofing are all designed with performance and aesthetics

in mind. Because of the different requirements for steep-slope vs

low-slope roofing–i.e., steep-slope roofs are typically designed as

water-shedding systems, and low-slope roofs are designed to form a seal

against water penetration–different materials are typically used in

their composition and construction.

NATIONAL INITIATIVES PROMOTING COOL ROOFING

The environmental community, and those concerned with

“sustainability,” recognize the significant benefits of cool

roofing. In the U.S. Green Building Council’s Leadership in Energy

and Environmental Design Rating System[TM] (LEED), (5) cool roofing

comes into play in two different ways:

* Qualified cool roofing materials receive a direct credit under

the “Sustainable Sites” category, in recognition of the

benefits of cool roofing in mitigating heat island effects.

* Cool roofing can help the buildings to attain energy efficiency

levels needed to obtain points in the “Energy and Atmosphere”

section. This credit recognizes the environmental benefits of reduced

electric power consumption.

LEED represents an incentive-based initiative that promotes

energy-efficient design in all aspects of building construction. Roofing

is just one of the components of a building that can be used to maximize

the effective use of energy.

The U.S. EPA’s Energy Star[R] program was begun in 1992 to

provide guidance to consumers on a wide array of products. A key goal of

the Energy Star Program is to assist consumers in identifying specific

products, within any general class of products, which are more

energy-efficient than the others. For any given product, Energy Star

chooses an appropriate rating system (with substantial input from its

manufacturing and other partners) and the level of performance Energy

Star performer. Consumers thus know that products bearing the Energy

Star label, whether computers, kitchen appliances, air conditioning

units, or roofing materials, are efficient performers.

For roofing materials, the Energy Star program (6) has been

available since early 1999. Energy Star focuses exclusively on the solar

reflectance of materials, including both initial values and values

obtained following three years of weathering. Manufacturers self-report

reflectance values, although they are required to use an appropriate

ASTM test method for determining reflectivity, as previously described.

Three-year aged values are determined by field measurement of actual

roofs (chosen by the manufacturers) that have been in place for at least

three years. The numerical average of multiple samples from different

regions of the country determines the final three-year value. Energy

Star also allows the use of “test farm” samples (coupons),

provided that the samples are [greater than or equal to]24 sq. in. in

area and exposed at a weathering facility which is in compliance with

the requirements of ISO/IEC 17025: 1999 General Requirements for the

Competence of Testing and Calibration Laboratories. Energy Star permits

washing of samples prior to testing. Again, these aged reflectance

values are self-reported by the participants. For low-slope materials,

Energy Star qualification requires an initial total solar reflectance of

at least 0.65 (“65%”) and a three-year value of at least 0.50

(“50%”). Note: A solar reflectance value of 0.65 is basically

a white material. For steep-slope materials, initial and

“aged” values of 0.25 and 0.15, respectively, qualify. The

allowance of substantially lower TSR values for steep-slope roofing

recognizes the broad appeal of–and desire for–colored roofing (i.e.,

not a white material) in applications where the roof is visible to the

casual observer. As of January 28, 2004, there were 625 products listed

as Energy Star qualified roof products.

These different slope characteristics, combined with the wide range

of materials used in roofing–from cedar shakes to asphalt shingles;

from tile to painted metal shingles; from built-up roofs to various

singleply systems and many others–make “one-size-fits all”

measurement and rating systems tricky at a practical level, but, after

years of effort, the extant methods seem to handle most situations quite

well.

NEW BUILDING CODE REQUIREMENTS FOR COOL ROOFING IN CALIFORNIA

With an eye toward increasing the energy efficiency of buildings,

state and municipal code bodies are increasingly requiring more

effective roof radiative performance, and California has taken the lead.

The California Energy Commission (7) (CEC) recently adopted the 2005

language for the state’s building energy efficiency standard, Title

24. This update will have a very significant impact with regard to the

use of cool roofing on low-slope, non-residential buildings. In essence,

this change in the building code requires cool roofing for all

significant roofing jobs for low-slope, non-residential buildings,

including “greenfield” new construction and re-roofing jobs.

(It is estimated that there are three times as many re-roofing jobs as

there are new installations.) In other words, in the State of

California, any job involving roofing, and requiring a building permit,

falls within the scope of this change. The final effective date for the

2005 update is not yet determined, but a date sometime in the latter

half of 2005 is a reasonable estimate for now. This language can be

found on the California Energy Commission’s Title 24 website. (8)

Before discussing California any further, it is sensible to step

back and ask if genuine benefit comes from these energy efficiency

initiatives. The best quantitative answer to this question comes from a

study done by the Natural Resources Defense Council (NRDC). In their

work, Energy Efficiency Leadership in California: Preventing the Next

Crisis, the graph shown as Figure 2 was developed to demonstrate the

total effect of the overall efforts that have taken place in California,

over the past few decades, to minimize energy consumption.

This figure demonstrates a number of interesting things. The first

is that, in 1977, the per capita consumption of electricity in

California was close to the per capita consumption of the other 49

states. The 1974 energy crisis was a wake-up call for California. The

State launched a large number of initiatives to slow the growth in

electrical power demand, and the graph clearly shows a lower slope for

California after 1977 than for the rest of the U.S. Roofing, of course,

is only part of the equation. To achieve the results shown in Figure 2,

greater utilization of high-tech window technology, enhanced levels of

insulation, development of energy-efficient appliances, and air

conditioners, etc., was necessary.

[FIGURE 2 OMITTED]

California has had much experience with the concept of a State

Energy Code. Specifications for windows and air conditioning have been

codified for some time. In the case of roofing materials, the CEC had no

desire to create its own specified department to collect and disseminate

radiative data relevant to materials proposed for use within the state

to comply with the Energy Code. Instead, it provided the seed money to

create the Cool Roof Rating Council (CRRC, see below), and has

designated it as the sole supervisory entity for the purpose of

maintaining a credible rating system for radiative properties of roofing

materials.

Per Title 24, alternate code compliance methods are available for

those situations where a cool roof is not feasible for some reason.

Under one compliance method, it is possible to “trade off”

other (improved) building efficiency measures against roofing materials

that fall short of the solar reflectance and thermal emittance

requirements for cool roofing. For example, assuming that compliant

roofing is not possible, more insulation or less window area may be

accepted in exchange. There are other issues associated with Title 24

compliance, but which fall outside of the scope of this article. Readers

interested in learning more are advised to visit the CEC website for the

details.

In future years, California is likely to consider cool roofing

requirements for residential low-slope and steep-slope roofs as well as

for nonresidential steep-slope roofs. As is the case with Energy Star,

it is likely that the cool roofing definition in such cases will be

different from the current nonresidential, low-slope requirements.

COOL ROOF RATING COUNCIL (CRRC)

Formed in 1998, the CRRC (9) was established with a commission to

set up a fair, accurate, and credible rating system for the radiative

properties of roofing materials. It requires that values be established

for both solar reflectance (TSR) and thermal emittance (TE). As with

Energy Star, the CRRC requires initial and three-year aged values.

Unlike Energy Star, the CRRC system does not define “cool” in

terms of any particular performance level; it simply establishes a

system to collect credible radiative data (both initial and aged), and

maintains a directory of rated products with their rated properties. The

CRRC rating program began on September 1, 2002; as of February 2004, the

CRRC Directory of Rated Products has listed 143 products.

In another significant departure from the Energy Star program, the

CRRC protocols require the use of independent, third-party,

CRRC-accredited testing laboratories to establish rated values,

utilizing appropriate ASTM test methods. Manufacturers cannot

“self-report” performance values. The CRRC system also

requires that a portion of rated products be subjected to random testing each year, to assure system integrity.

[ILLUSTRATION OMITTED]

Under the CRRC requirements, “aged” values are determined

by examining exposure data generated at three designated “test

farms” representing hot/humid, hot/dry, and “Midwestern”

exposure conditions. All rated products are exposed at all three test

farm sites in exactly the same manner and then tested, after three

years, using the same ASTM test methods that were used to establish the

initial values. The CRRC protocol, unlike the Energy Star protocol, does

not permit washing of samples prior to the aged test measurements. The

CRRC aged testing program component was scheduled to begin in March

2004.

The Energy Star program for cool roofs is well designed to

reinforce its primary mission of encouraging consumers to choose

high-performance roofing products for their buildings, on a voluntary

basis. The CRRC system, on the other hand, is more focused on

maintaining a technically rigorous rating system for use by various

parties who may wish to use the CRRC system to assure conformance to

performance levels that they choose. The two organizations maintain a

close and productive working relationship. U.S. EPA and DOE (sponsoring

organizations of Energy Star) assisted in the establishment of the CRRC,

and are well-represented on its Board of Directors.

ASHRAE AND OTHER STATES

Currently, the American Society of Heating, Refrigeration, and

Air-conditioning Engineers (ASHRAE) (10) has established two energy

efficiency standards–90.1 for commercial buildings and 90.2 for low

rise residential buildings. The current ASHRAE standard requires a

minimum total solar reflectance of 0.70 and a minimum thermal emittance

of 0.75 when tested in accordance to ASHRAE’s listed test methods.

ASHRAE 90.1 and 90.2 are referenced by 37 states in their energy codes

and are also included in the 2003 version of the International Energy

Conservation Code (IECC). Other jurisdictions, notably the states of

Arizona, Florida, Georgia, and the City of Chicago have either adopted

or are considering adoption of cool roofing standards for inclusion in

their building codes.

[ILLUSTRATION OMITTED]

COOL ROOFING–SOUND CONCEPT, EXCELLENT TIMING

The concept of cool roofing is based upon the sound premise that

whenever energy can be saved, power generation will either be contained

or decreased, and the environment will subsequently be improved. The

roofing market is a $10+ billion market, and many opportunities exist

for coatings to contribute to the success of this market, both

aesthetically and with regard to energy conservation. There are new

challenges facing today’s coatings chemists, but there are also new

formulating options available to them as well, and responsible and

productive use of all of the available tools at our disposal can have

profound effects on the use of energy and the subsequent improvement of

the environment. Can reflectance be increased on an entire

customer’s standard product line of a rainbow of colors? New

pigment technology now exists that helps to answer this question. Can a

thin-film coating take advantage of a metal substrate’s inherently

high solar reflectance, while at the same time maximizing the thermal

emittance, which is an area where bare metal suffers in this cool roof

environment? How about “smart” coatings that change from dark,

heat-absorbing colors when the air temperature is cold (i.e., during the

winter months), to a white or light heat-reflecting color during the hot

summer weather? Such coatings would have heat-absorbing properties when

heating is required, and heat-reflecting properties when cooling during

the summer is required. The thin film or coating on a roof–presumed by

so many people to provide only aesthetic value–controls all of the

radiative properties of the entire roof. This is truly a new era for

coatings technology, where a newly-recognized set of material

properties–solar reflectance and thermal emittance–provides

opportunities for all of us to improve the environment and minimize

energy demands.

References

(1) http://www.sws.uiuc.edu/atmos/statecli/General/1995Chicago.htm.

(2) Klinenberg, Eric. “Heat Wave: A Social Autopsy of Disaster

in Chicago,” 2002.

(3) http://eetd.lbl.gov/Heatlsland/.

(4) http://www.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm.

(5) http://www.usgbc.org/leed/leed_main.asp.

(6) http://www.energystar.gov/index.cfm?c=roof_prods.pr_roof_products.

(7) http://www.energy.ca.gov/.

(8) http://www.energy.ca.gov/2005_standards/rulemaking/documents/index.html.

(9) http://www.coolroofs.org/.

(10) http://www.ashrae.org/.

by David A. Cocuzzi and George R. Pilcher Akzo Nobel Coatings Inc.*

*P.O. Box 489, Columbus, OH 43216-0489.

http://www.thefreelibrary.com/Coolroofing:areallyhottopic.-a0123240952

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