Clear Thinking About Windows

Spring Training, 2016

Jim Larsen

April 25, 2016

Code references

whole window

Window Thermal Components

Outdoors Indoors

#1 #2 #3 #4

Glass/Frame

Interface

Frame

Window

Therm

Edge of Glass

(2.5 inches)

Center of Glass

Aluminumy = 0.82x + 0.42

Alum TBy = 0.79x + 0.22

Clad Woody = 0.69x + 0.18

Wood or Vinyly = 0.63x + 0.14

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55

Win

do

w U

-Fac

tor

IG Glass U-Factor

Operable Window U-Factor vs. Glass Properties

Sunlight on a Window

Solar Heat Gain Coefficient (SHGC) is sum of direct transmission and inward flowing absorptance

Tinted Glass SHGC = 0.52

#3 surface low-E is hot in sunlight

Window Solar Heat Gain

Frame areas can vary from 10% to

40%

X

X

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

Win

do

w S

HG

C

Glass SHGC

Window SHGC as a Function of Frame Area

15% Frame 25% Frame 35% Frame

The Rule of Panes

• Single Pane ~ R1

• Double Pane ~ R2

Window U 0.55 – 0.80 with Double Pane

More Panes

• Single Pane ~ R1

• Double Pane ~ R2

• Triple Pane ~ R3

• Quad Pane ~ R4

Window U ~ 0.35 with Quad Pane and Non-Metal

Frame

Less Panes with Low-E

• Single Pane ~ R1

• Double Pane ~ R2

• Triple Pane ~ R3

• Quad Pane ~ R4

• 2 Pane + Low-E ~ R3

• 2P + Low-E + Argon ~ R4

Quad Pane Equivalent

Low-E blocks(thermal) radiation

Argon slows conduction

Warm edge reduces conduction losses

Non-Metal Window U ~ 0.302 Pane+ Low-E+ Argon

+ Warm Edge

Back to the Future: More Panes?

• Single Pane ~ R1

• Double Pane ~ R2

• Triple Pane ~ R3

• Quad Pane ~ R4

• 2 Pane + Low-E ~ R3

• 2P+Low-E+Argon ~ R4

• Triple w/2 Low-E + Argon ~ R8

Triple Pane Challenges

• Glass cost about 2X double pane

• 2X IG seal failure risk

• Weight (sash balance)

• Thickness• Vertical sliders (single & double hung) are about 2/3

U.S. window market

Last Step for Double PaneAdd 4th Surface Low-E = R5

Low-E units are warm on winter night. Clear glass is cool.

Traditional Options for Solar Control Glass

Tinted• Adds color to “body” of glass

• Absorbs sunlight and re-radiates to exterior

Reflective• Mirror appearance

• Reflects sunlight out

Clear Window SHGC0.60

The Solar Spectrum

45%3% 52%

2P Clear: Window SHGC ~ 0.6

2P Tinted: Window SHGC 0.4 - 0.5

Today’s Option for Solar Control Glass

Tinted• Adds color to “body” of glass

• Absorbs sunlight and re-radiates to exterior

Reflective• Mirror appearance

• Reflects sunlight out

Spectrally Selective Low-E• Clear glass appearance with solar control

• Reflects solar infrared (invisible to human eye)

High Solar Gain Low-E: Window SHGC ~ 0.5

Low-E Coatings - Circa 1980’s

• 2 processes: Pyrolitic (hard) or Sputtered (soft)

• Pyrolitic low-E coatings are high solar gain and applied to clear glass

• Sputtered low-E coatings are silver based, must be insulated or laminated, are available in wide range of solar gain levels. Only high solar option at this time.

High solar gain low-E is hot in sunlight (#3 surface)

High Solar Gain Low-E

Summer

Transmission SHGC Temperature

#2 Surface 0.56 0.62 87°F

#3 Surface 0.56 0.68 101°F

Medium Solar Gain Low-E: Window SHGC ~ 0.3

Window SHGC ~ 0.30

2 Pane+ Medium Solar

Gain Low-E

| Low-E | Clear GlassMed Solar | High Solar

Low Solar Gain Low-E: Window SHGC ~ 0.2

*

*

*

Unobtanium*

Comfort Science?

55

Metabolic Rate

Comfort @ 70°F

Clothing Insulation

Standard Office Conditions

1. Activity Level

2. Adaptive Clothing

Can a Better Furnace Offset Cold Windows?

No Comfort Science Needed………

Extra Layers Help!

Cold Weather Analysis

• Assume large window (patio door) and close proximity (3’ away)

Cold Weather Analysis

• Assume large window (patio door) and close proximity (3’ away)

• Limit increase in PPD to 10% over baseline

– Cold threshold set to 56°F for “average” roomside surface temperature

MRT Analysis (window size & proximity)

2P Clear @ 0°F = +12% in PPD

Cold Weather Analysis

• Assume large window (patio door) and close proximity (3’ away)

• Limit increase in PPD to 10% over baseline

– Cold threshold set to 56°F for “average” roomside surface temperature

• Calculate hourly roomside glass surface temperature for no wind conditions(use weather data file: 8760 hours)

Cold Weather Analysis

• Assume large window (patio door) and close proximity (3’ away)

• Limit increase in PPD to 10% over baseline

– Cold threshold set to 56°F for “average” roomside surface temperature

• Calculate hourly roomside glass surface temperature for no wind conditions

• Hour counts as Discomfort if window temperature falls below threshold

Glazing

Layers U-factor SHGC VT

1 Single Metal 1.29 0.73 0.69

2 Double Metal 0.83 0.65 0.63

3 Double Tinted Metal 0.83 0.54 0.47

4 Double Low-E, Hi SHGC yes Metal 0.65 0.58 0.61

5 Double Low-E, Med SHGC yes Metal 0.64 0.38 0.56

6 Double Low-E, Lo SHGC yes Metal 0.63 0.26 0.49

7 Double Metal, Thermal Break 0.60 0.62 0.63

8 Double Tinted Metal, Thermal Break 0.60 0.51 0.47

9 Double Low-E, Hi SHGC yes Metal, Thermal Break 0.42 0.55 0.61

10 Double Low-E, Med SHGC yes Metal, Thermal Break 0.42 0.35 0.56

11 Double Low-E, Lo SHGC yes Metal, Thermal Break 0.41 0.23 0.49

- Double 4th Low-E Hi SHGC yes Metal, Thermal Break 0.38 0.51 0.60

- Double 4th Low-E, Med SHGC yes Metal, Thermal Break 0.37 0.34 0.54

- Double 4th Low-E, Lo SHGC yes Metal, Thermal Break 0.37 0.23 0.49

12 Single Non-metal or metal clad 0.88 0.64 0.65

13 Double Non-metal or metal clad 0.52 0.57 0.59

14 Double Tinted Non-metal or metal clad 0.52 0.47 0.44

15 Double Low-E, Hi SHGC yes Non-metal, thermally improved 0.29 0.50 0.57

16 Double Low-E, Med SHGC yes Non-metal, thermally improved 0.28 0.31 0.52

17 Double Low-E, Lo SHGC yes Non-metal, thermally improved 0.27 0.20 0.4621 Double 4th Low-E Hi SHGC yes Non-metal, thermally improved 0.25 0.46 0.56

22 Double 4th Low-E, Med SHGC yes Non-metal, thermally improved 0.24 0.30 0.51

23 Double 4th Low-E, Lo SHGC yes Non-metal, thermally improved 0.24 0.21 0.46

18 Triple Low-E Hi SHGC yes Non-metal, thermally improved 0.20 0.41 0.50

19 Triple Low-E, Med SHGC yes Non-metal, thermally improved 0.19 0.28 0.45

20 Triple Low-E, Lo SHGC yes Non-metal, thermally improved 0.19 0.18 0.37

Frame

Values for simulation2016 EWC

window # Glazing Type Argon Gas

Low-E History

• 1980’s High Solar Gain

I’m too damn hot!

Low-E History

• 1980’s High Solar Gain

• 1990’s Medium Solar Gain• No more field complaints

• 2000 Low Solar Gain• Low-E now in building code from Muskoka to Miami

• Mandate for low-solar in South

• Northern users beware of overheat with high solar

“Hot” Window Analysis

• Assume occupant is in direct sunlight

• Limit increase in PPD to 10% over baseline

– Solar gain < 70 Btu/hr·ft²

Window Comfort Research

WarmCool

+1 +2 +3

Cold Cool

Slightly

Neutral

Slightly

Warm Hot

-3 -2 -1 0

“Hot” Window Analysis

• Assume occupant is in direct sunlight

• Limit increase in PPD to 10% over baseline

– Solar gain < 70 Btu/hr·ft²

• Hour counts as Discomfort if transmitted solar exceeds threshold

– Calculate gain on 4 orientations (N, E, S, W)

Summary

• “Cold” discomfort is climate specific• Northern latitudes require lower U-Factors due to colder

extreme winter ambient temperatures

• Risk of “Hot” discomfort exists in all climates• Solar radiation levels similar across North America

• Orientation specific

• Beware overheat risks as building performance improves