Energy Code Terminology

• Solar Reflectance - the ability of the surface to reflect solar radiation away from that surface

• Infrared Emittance - the ability of a material to emit or radiate heat energy that builds up in the material.

• Cool Roof Color - A cool roof color is required for a conditioned building and it must meet the energy code Cool Roof finish values for Solar Reflectance and Infrared Emittance. These values will vary based upon the energy code requirements being met.

• Maximum Required U-Factor – The energy codes are lowing the established maximum required U-Factors for building construction. These new requirements will result in increased R-Values

• Conditioned Buildings - For a conditioned building, to get a building permit, a Building Envelope Energy Calculation will be required showing that the building is energy efficient.

A building is conditioned if it has a system with heating capacity of more than 3.4 Btu/hr-ft2 or cooling capacity of more than 5 Btu/hr-ft2

.

The new energy requirements and terms are being added to the energy codes.

Cool Roofing Terminology (cont’d)

• Solar Spectrum - radiation energy originating from the sun, including ultraviolet, visible and near infrared radiation.

• Conduction - the passing of heat through a roof material into the layer in contact directly beneath the surface

• Convection - the heating of the air that passes over a hot surface

Energy Balance on Roof Surface

Re-Emitted Energy

Reflected Radiation

TotalSolar

Radiation

Absorbed Energy

Net Heat Flux Into Roof

Convection

Roof Surface Layer

Solar Energy Spectrum

• Ultraviolet (UV)– 3% of total energy– responsible for

sunburn

• Visible (VIS)– 40% of total energy– visible light

• Infrared (IR)– 57% of total energy– felt as heat!

Cool Roofing Testing Methods

• ASTM E-903 - Reflectance (Laboratory)

• ASTM E-1918 - Reflectance (field test for variegated surfaces)

• ASTM C-1549 - Reflectance (portable device)

• ASTM E-408 - Emittance (Laboratory)

• ASTM C-1371 - Emittance (portable device)

Cool Roof Requirements

• High reflectivityHigh reflectivity– Initial readingInitial reading 65% 65%– ASTM E-903-96 orASTM E-903-96 or

ASTM E-1918-97ASTM E-1918-97 OROR

Minimum Solar Minimum Solar Reflectance Index (SRI)Reflectance Index (SRI) of of 75% 75%

– ASTM E-1980 for ASTM E-1980 for moderate wind conditionsmoderate wind conditions

High emissivityHigh emissivity 80%80% ATSM E 408-71 ATSM E 408-71

(1996)(1996)

SOLAR ENERGY SPECTRUM

Net Heat Flux into Roof

solar It Reflected

solarIt Absorbed)

IItt

Total SolarIrradiation hair(tair-ts) IRR

Net InfraredRadiation

with R=(Ts4-Tsky

4 )

Convection

solar AND IR ARE BOTH VERY IMPORTANT

Measuring Solar Reflectivity

1.0

.75

.50

.25

0

Least Efficient

Most Efficient

ReflectedAbsorbed

•Solar Reflectivity describes an object’s ability to REFLECT solar radiation away from its surface.

•It is measured in the UV, IR and visible light wavelengths and therefore should not be confused with gloss/sheen which is based solely on visible light reflection

•Don’t confuse reflectivity with GLARE!

Total Solar Reflectance

A measure of reflectivity of an object over the entire spectrum of sunlight that hits the Earth’s surface, weighted by the intensity of sunlight at each wavelength.

Solar Reflectance

• Solar reflectance describes an objects ability to REFLECT solar radiation away from its surface

• It is measured in the UV, IR and visible light wavelengths and therefore should not be confused with gloss/sheen which is based solely on visible light reflection

• Don’t confuse reflectivity with GLARE!

Gloss/Sheen vs. Solar Reflectance

• GLARE is a CONCERN for commercial metal roofs• Glare is the reflection of sunlight that can impair vision and

create an ANNOYANCE• This becomes CRITICAL around airports (impaired vision of

pilots) and in tightly built residential neighborhoods• The GLARE of a coated surface is controlled by the SHEEN• Low Sheen = Low Glare

Gloss/Sheen and Solar Reflectance are totally independent properties and do not have an affect on

one another.

A change in gloss and sheen does not change solar reflectance and vice versa.

Sunl

ight

Sunlight

Low IR Reflectance

High IR Reflectance

Low IR Reflectance = Higher Temperature High IR Reflectance = Lower Temperature

Low IR Pigments High IR Pigments

The same color roof can be cool or hot depending on the pigment in the roofing!!!

Suns Energy 6% Ultraviolet 52% Visible 42% Infrared 100%

Same Color Roof

UV

- V

isibl

e - In

frar

ed UV

- Visible - Infrared

The same color roof can be cool or hot depending on the pigment in the roofing!!!

Suns Energy 6% Ultraviolet 52% Visible 42% Infrared 100%

Energy Star (High Slope) (over 2:12 roof pitch) 25% Total Reflectance (Initial) 15% after 3 years

Sunlight

Low IR Reflectance

High IR Reflectance

Low IR Reflectance = Higher Temperature

High IR Reflectance = Lower Temperature

Low IR Pigments

High IR Pigments

UV - Visible - Infrared

UV - Visi

ble -

Infr

ared

UV

Visible

Infrared

UVVisi

bleInfra

redCareful selection of weatherable IR

reflective pigments can produce dark, aesthetically pleasing colors that meet Energy Star roofing requirements.

•Lower cooling costs

•Reduced Heat Island Effect

•Potential for for longer life cycle due to lower temperature

Same Color

Measuring Infrared Emissivity• Efficiency of a surface ability Efficiency of a surface ability to emit heat by to emit heat by

radiationradiation; the ratio of the radiant energy ; the ratio of the radiant energy emitted by a surface to that emitted by a emitted by a surface to that emitted by a blackbody at the same temperature. blackbody at the same temperature.

• Decreasing emittance may lead to increased Decreasing emittance may lead to increased energy use.energy use.

• Values are expressed from 0 to 1.0Values are expressed from 0 to 1.0• Same type of values as solar reflectanceSame type of values as solar reflectance

0 .25 .50 .75 1.0

Least Efficient Most Efficient

80% Cool Roof Requirement

Roof Surface Temperature

Infrared Emittance Has a Lesser Impact Than Solar Reflectance

(Example for Air Temperature of 98° F) Source: LBNL, ASTM D1980

solar reflectance emittance temperature (F)0.70 0.75 124

0.70 0.90 122

0.55 0.75 1390.70 0.75 1250.80 0.75 115

T = 2° F

T = 10 - 14° F

Color Selection

• Light and medium shades usually meet the 0.25 requirement

• Dark colors can usually be reformulated to meet the 0.25 requirement

The Primary Energy Concern is the Heat in the Building

• Dark materials ABSORB MORE HEAT from the sun.

• When those dark surfaces are roofs, some of the heat is TRANSFERRED INSIDE

• When that happens, the Urban Heat Island Effect becomes a factor.

The Problem – Metal Stability

• Metal Expansion and Contraction:– Standing seam applications are subject to

DISTORTION by heat– The temperature variations create

MOVEMENT within the standing seam leg, WEAR, and OIL CANNING

• Oil Canning:– The STRESS in the platform angles of the

LEG causes distortion, exasperated by heat

absorption

Cool Coating Benefits

• Heat is REFLECTED away from buildings• All the advantages of greater reflectivity can be

had WITHOUT SACRIFICING COLOR CHOICE• Dramatically increases the reflectivity of medium

to dark colors to such a degree that the product will meet the ENERGY STAR specifications for STEEP SLOPE Cool Roofs

• Smog is REDUCED when environmental

temperatures are reduced

Impact on Temperature and Energy Consumption

• Rule of Thumb:– FOR EVERY 1% INCREASE IN ROOF

REFLECTANCE, TEMPERATURE DECREASES 1°F• example: Improving reflectance from 10% to 50%

lowers surface temperature 40°F– FOR EVERY 10% INCREASE IN ROOF REFLECTANCE,

COOLING/HEATING ENERGY COSTS DROP 2¢/ft2 (warm climates)

Per LBNL: based on DOE 2 model, LBNL models, ORNL calculator, EnergyPlus model

Roofing Material Thermal Properties

Metal (unpainted) 0.60-0.80 0.04-0.10

Metal (painted and granular coated)0.10-0.75 * 0.75 +

Comp Asphalt Shingles 0.05-0.25 0.90

Modified Bitumen 0.05-0.25 0.90

Built Up Roofing 0.05-0.80 0.90

Concrete/Clay Tile 0.20-0.70 0.90

White Single Ply Membrane 0.70-0.80 0.85 +

Source: ORNL and LBNL

Emissivity is generally high in coatings and paint films, but very low in unpainted metallic surfaces

InitialSolar Reflectance Infrared Emittance

* depending on color

Roof Surface Temperature

Infrared Emittance Has Less Impact Than Solar Reflectance

(Example for Air Temperature of 98° F) Source: LBNL, ASTM D1980

solar reflectance emittance temperature (F)0.70 0.75 124

0.70 0.90 122

0.55 0.75 1390.70 0.75 1250.80 0.75 115

T = 2° F

T = 10 - 14° F

Urban Heat Island Effect • Urban Heat Island - a built environment wherein the

large proportion of dark surfaces such as asphalt paving and dark roofs absorb solar radiation and radiate the heat back into the atmosphere causing higher ambient temperatures and higher pollution levels

• Urban areas are 6-8 °F warmer than suburbs (Dark pavements, dark roofing and less vegetation)

• Roof surface temperature has an effect on the following items: (lower temperature = less smog, less pollution,

lower peak energy demand)• High reflectance/emittance = low surface

temperatures

Thermal Image of Washington, DC

Hottest Surface Colored in Red

Heat is a catalyst for smog

The air in Los Angeles is noticeably cleaner during winter, yet the number of cars on the road is approximately the same as in summer.

Smog Formation

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

Knoxville Phoenix Chicago

City

White Membrane Unpainted Galvalume Aluminum Coating

R-15R-5

Knoxville Phoenix Chicago

Sav

ings

, $/f

t² p

er y

ear

Roof Energy Savings

White Membrane or White Painted Metal

Unpainted Galvalume

Aluminum Coating

Heating Degree Day• The sum of the degree days for heating,

using a common base of 65°F, used with other factors to evaluate the energy requirements of a heating season.

• Example for HDD Base 65º FFor any one day, when the mean temperature is less than 65 °F, there are as many degree-days as degrees F temperature difference between the mean temperature for the day and 65°F. Annual heating degree-days are the sum of the degree-days over a calendar year.

3600 HDD Isothermal Line

Building Envelope

• One of most important factors in designing energy-efficient buildings

• Strongly affects heating and cooling loads (HVAC Energy)

• Investment in insulation or energy-efficient windows can result in smaller HVAC systems to help pay for the better envelope

Envelope Compliance Methods• Prescriptive

• Trade-off

• Energy Cost Budget (Whole Building)

Prescriptive Method

• Specified for Location– ASHRAE 90.1 – 16 Climate Zones

• Insulation for Opaque Components– Maximum U-Factor or Minimum R

• Roofs, Walls, Floors

• Fenestration– Maximum SHGC and U-Factor

• Windows, Skylights

Roof Umax 0.065

Trade-Off

• Offers flexibility• Thermal performance of one envelope

component can fail to meet prescriptive requirement as long as other components perform better than what is required

• Proof of compliance is more involved– Overall Heat Loss– Overall Heat Gain

NAIMA Prescriptive Solutions

Screw Downw/R-19

U=0.098

SSRw/R-19

U=0.065

SSR2 Layers R-19

U=0.046

ASHRAE = 0.065