Highly Insulating Windows

Christian KohlerWindows and Daylighting Research Group

Lawrence Berkeley National Laboratory

June 11, 2009

Windows and Daylighting GroupWindows and Daylighting Group

• 10-15 researchers dedicated to windows research. Mostly DOE funded.

• Engaged with industry since 1976• State-of-the-art user facilities for testing

and evaluation• Software used by over 8,000 users

worldwide

Performance IndicesPerformance Indices

• Key performance indices—U-factor

• Thermal resistance• Units Btu/hr-ft2-F• R-factor is inverse, U=0.2, R=1/0.2

= 5 hr-ft2-F/Btu

—SHGC• Solar Gains• Ranges from 0-1, higher means

more solar gains

—VT• Visible Transmittance• Ranges from 0-1, higher means

more daylight

Heat Transfer in WindowsHeat Transfer in Windows

Conduction

Radiation

Conduction

Convection

Low-e coatings

Special gas fillsMultiple cavities

Low conductance spacersBetter frames

Whole window metricesWhole window metrices

• Whole product vs center of glass

• Window components—Framing (structural)—Glazing (vision)

• Frame area can be 25% of totalarea

• NFRC and ENERGY STAR require whole product numbers

Highly Insulating Windows - rangeHighly Insulating Windows - range

Whole window U-factor

0.10 0.20 0.30 0.500.40

No heat transfer

Standard double-pane windows

Typical ENERGY STAR

windows

Highly insulating windows

0.35 = Northern ENERGY STAR

benchmark

Performance GoalsPerformance Goals

Heating Climates: static high solar, hi-R (U=0.1 Btu/h-ft2-F) can meet ZEH goals

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

70

75

80

85

90

95

100

105

110

115

120

125

130

135

140

145

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

SH G C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

U-f

act

or

Minneapolis, MN - Com bined Annual Heating and Cooling Energy (MBtu)

w indow s use energy

w indow s provide energy

Co

mb

ine

d A

nn

ua

l He

atin

g a

nd

Co

olin

g E

ne

rgy

(MB

tu)

doub le clear

low ga in low -e A r double ,h igh ga in low -e A r double

sing le clear

low ga in low -e A r trip le

target perform ance reg ion

typ ica l Energy S tar

m oderate ga in low -e K r trip le (acrylic center layer)

BenefitsBenefits

• Areas near windows are often places of great temperature variation and discomfort

• Conventional practice to avoid discomfort is to provide perimeter heating near windows

• Perimeter heat may not be necessary with highly insulating windows

Thermograms comparing a conventional dual-pane with a highly insulating window

LBNL / DOE ResearchLBNL / DOE Research

• Triple glazings– Develop lower-cost,

non-structural center layers

• Spacer interactions• High Performance Frames

– Collaboration with European researchers

– Focus on air leakage

2 sealed gas gaps at different temperatures and pressures

with standard glass, unit is thicker and heavier

low-e

thin glass or plastic held by spacer

spacer

low-e

only 2 paths for gas loss

Highly Insulating Frames

• Mostly driven by PassivHaus Institute in Germany

• 5 Windows being tested and simulated in Norway and US

• Verify performance with US rating criteria

Low-e storm windowsLow-e storm windows

• Pyrolytic Low-e coating (hard coat)

• Does not degrade in non-sealed cavity

• Identical installation cost to clear storms

SavingsSavings

• Whole house heating energy savings over a winter season in Chicago for new storms:—Clear storm windows 8-18%—Low-e storm windows 19-27%

• Estimated U-values:—Clear storm windows: 0.49 Btu/h-ft2-F—Low-e storm windows: 0.36 Btu/h-ft2-F

Cost effectiveness – Low-e StormsCost effectiveness – Low-e Storms

Total Window Cost

Annual Energy Savings

Simple Payback (yrs)

House 2- Low-E $1,738 $490 3.5

House 3- Clear $1,344 $111 12.1

House 4- Clear $2,661 $317 8.4

House 5- Low-E $1,738 $341 5.1

Thank You

Christian Kohler, CJKohler@lbl.gov

Windows and Daylighting Research Group

Lawrence Berkeley National Lab