Window Insulation Technologies

By Tim ShayChemical Engineering Student

Goal: Reduce our need of fossil fuels How?

◦ Renewable resources Solar Biomass Wind Hydro Geothermal

◦ Energy Efficiency Conversion losses Automobiles Heating/cooling

Current Energy Issues

Why care?

Heat flow follows Newton’s law of cooling

◦ U = Heat transfer coefficient◦ A = Area◦ ΔT = Temperature difference

Why Focus on Windows?

Low-end window U-factor = 1.20 BTU/(ft2hr°F)

Home wall U-factor ~ 0.05-0.10 BTU/(ft2hr°F) Windows can lose about 10-20 times more heat per area

The science behind the different technologies

Modeling Testing Quality Assurance Consumer view point

Main Points

Conduction◦ Glass

Gas fill Multi-plane windows

◦ Spacer Structure

◦ Sash Material Design

Convection◦ Weather Strip

Radiation◦ Low-E coatings◦ Heat mirrors

Ways that Heat is Transferred

http://www.uwsp.edu/cnr/gem/SustainableEnergy/energy-statistics.htm

Generic diagram of heat flow with various materials

Temperature Profile Through a Composite Wall

Transport Phenomena, Bird, Stewart and Lightfoot

◦ Fluids cause large change in temperature

◦ Reduces heat transfer

Thermal conductivities of gas greatly affects overall heat transfer◦ Can generally treat as being

stagnant

Gas Fills

Gases used to fill windows

Gas Thermal Conductivity (W/mK)

Air 0.03

Argon 0.016

Krypton 0.009

Gas inside isn’t stagnant◦ Movement increases

heat transfer Heating and cooling

causes convection◦ Taken into account in

FEA models

Modeling with Convective Loops

Hot

Cold

Spacer Type

http://www.vytexwindows.com/homeowners/stay_warm_keep_cool/duratex/

Spacers are what seal the gas in and separate the panes

Spacers come in many different designs and materials

Can have a large affect on◦ Strength of window◦ Conduction of heat

Temperature/pressure changes stress the system◦ Window deflection

Window Deflection

Concave ShapeNormal Shaped IG

unit

Cold Temperatures

High Atmospheric Pressure

Hot Temperatures

Low Atmospheric Pressure

Convex Shape

Intercept spacers◦ Can bend more◦ Let less heat through

Box Spacer◦ More rigid◦ More heat transfer

Allows more conduction

Comparison of Intercept and Box Spacers

http://www.gilkey.com/quality.html

Tri-pane is the highest generally seen in the market

Increases resistance◦ More surfaces to transfer heat through◦ Generally thicker

Multi-Pane Windows

http://www.americandreamwindow.com/doublehung/doublehung.htm

Made from various materials◦ Wood◦ Vinyl◦ Aluminum◦ Special materials

Andersen’s Fibrex Consumer choice also plays a key role in selection Each has it’s own conductivity

Sash Material

http://www.sunrisewindows.com/whyreplace.html

Non solid frames have complex structures◦ Designed to maximize efficiency and for strength◦ Makes use of heat transfer between barriers

Sash Structure

http://www.gilkey.com/quality.html

http://commons.wikimedia.org/wiki/File:5-chamber_plastic_window_profile.JPG

Modeling of the Sash

Air flow will find any open gap between the exterior and interior◦ Create tight seal

Problems◦ Wear and tear

Extensive testing performed◦ 10,000 cycle test

Weather-Stripping

http://www.hometips.com/buying-guides/window-weatherstripping-options.html

The sun emits radiation to the Earth◦ This radiation is generally classified two ways for

window manufactures Solar heat gain Visible light

Light has three options on surfaces◦ Transmittance◦ Absorbance◦ Reflection

Radiation

Coatings of silver and metal oxides can be used to help reflect heat◦ More layers reflects more heat◦ Can go up to 3 layers

Low-e Coatings

Placed on second face◦ Sheltered from

elements◦ Most efficient location

Coating other surfaces◦ Must be resilient to the

elements◦ Must be safe

Tri-pane windows allow more surfaces to apply coating to

Application of low-e Coatings

http://www.taiwanglass.com/en/products/flatglass/processed/leg.html

Effects in Winter and Summer

http://arizona.inetgiant.com/Mesa/AdDetails/LOW-E-WINDOWS-SINGLE-OR-DOUBLE-PANE/7886272

Various Levels of Coatings

http://www.efficientwindows.org/lowe.cfm

More heavy coatings will prevent transmission of solar heat, but also visible light

Heat Mirrors

http://www.buildinggreen.com/live/index.cfm/2010/11/10/Heat-Mirror-for-Structural-Sealant-Glazings

Operates the same as low-e coatings

Made of a thin clear film

Also operates as a divider◦ Creates 2 gaps◦ Decreases conduction

Pricier

Negative Side Affects

The Vdara hotel in Las Vegas

Hotel Vdara “Death Ray” Sunbathers have

been severely burned in as little as 10 minutes

Reflected light is becoming an increasing issue

Affecting buildings that already exist

http://www.amusingplanet.com/2010/10/vdara-hotel-death-ray.html

Low end window U factor ~ 1.2 High end window U factor ~ 0.20 Example

◦ 1 window that is 10ft2

◦ ΔT = 70°F◦ ΔQ = ΔU*A*ΔT

700 BTU/hr◦ Over a 1 day span

16,800 BTU/day = 0.168 Therms/day 1 Therm ~ 1 $

Overall Affects