Keeping Your Windows from Letting Heat Inside

During hot summer days, sunlight streaming in through windows can quickly heat up indoor spaces. Windows allow heat inside through three main mechanisms: conduction, convection, and radiation.

Conduction is the transfer of heat through direct contact with a solid material. Heat flows from the warmer side of a window to the cooler indoor side when there is a temperature difference between the outdoor environment and your home's interior.

Factors Affecting the Rate of Heat Transfer Through Windows

Several key factors impact how quickly heat enters through window glass:

• Temperature differential - The bigger the difference between indoor and outdoor temperatures, the faster heat transfers.
• Window size - Larger window surface areas allow more heat to pass through.
• Glazing type - Single pane windows conduct heat faster than double or triple pane.
• Frame materials - Metal frames conduct faster than wood or vinyl.
• Window coatings - Special low-emissivity coatings slow heat transfer.

Measuring Current Levels of Heat Gain

Before making window improvements, it helps to quantify existing heat gains. This allows proper sizing of air conditioning systems and aids in prioritizing windows needing upgrades. Performance testing under real weather conditions provides the most accurate data:

• Surface temperature meters record inner and outer glass temps.
• Heat flux sensors directly measure the heat flow rate.
• Infrared imaging maps surface temperatures.
• Custom testing chambers simulate environmental conditions.

Applying Window Films to Reduce Heat Transfer

Window films are thin plastic sheets that block and absorb heat passing through glass. Reflective, spectrally selective films limit radiant heat gain from sunlight. Low-emissivity films reduce conductive inward heat flow. Combining both reflective and low-E films on window provides improved insulation:

Reflective Films Limit Radiant Heat Gain from Sunlight

Reflective films block up to 97% of infrared rays and over 80% of visible light while maintaining sunlight visibility. This greatly cuts radiant heat entering through the window. Reflective films come in a variety of tints. A dark film with a lightweight metal coating provides good heat reflection yet maintains some light transmission. Vacuum deposition applies precise microscopically thin metal layers for excellent performance. While pricier, these films have longer warranties against cracking, peeling and bubbling.

Low-E Films Reduce Conducted Heat Flow

Low-emissivity or low-E films reduce conducted and convected heat flows through the glazing. The film has a special optically clear metallic coating allowing daylight passage while blocking long-wave infrared radiation. This resists heat flow into the room during cold weather while keeping interiors cool during hot conditions. Low-E films provide year-round insulation benefits. During colder weather, low-E films also cut heating costs by containing interior warmth.

Combining Films for Improved Insulation

Applying two separate films in tandem improves thermal resistance further. A reflective film blocks solar heat gain while the Low-E film stops conducted heat flows. The optimal approach is low-E film on surfaces 2 and 5 with the reflective film on surface 3. Proper installation ensures gapless bonding to the glass to prevent heat bypassing the film. Utmost care must be taken during application to avoid air pockets between film layers.

Using Window Awnings and Shades

Exterior window awnings and interior roller shades prevent solar radiation from striking and heating window glass. Both shading approaches work to block heat gain:

Exterior Window Awnings Shade Glass Surfaces

Outdoor awnings prevent sunlight from directly hitting windowpanes. With solar radiation blocked, the glass absorbs less heat energy. Window awnings keep around 65% percent of solar heat from flowing through windows into internal spaces. Deeply slanted awnings around 45 degrees provide better shade coverage on low afternoon sunlight. Manual or motorized retractable awnings allow custom positioning as needed.

Interior Window Shades Also Minimize Heat Gain

Indoor window shades and blinds also minimize heat gain after sunlight has passed through windows. Light colors reflect more solar energy back through the glass. Warm-colored shades absorb more heat, transferring excess warmth into the interior. Automated smart shades attuned with weather conditions and sun orientation provide optimal indoor temperature and lighting control with little effort.

Proper Shade Positioning Is Critical

For exterior awnings or interior shades to limit solar gain effectively, correct positioning is imperative. Outdoor shades should fully cover windows and be angled properly to account for seasonal sun angles. Indoor shades when fully drawn must completely cover windowpanes with sufficient overlap on all sides to prevent heat leaks. Poor shading allows intense beams to bypass protections and heat rooms.

Replacing or Upgrading Window Assemblies

Altogether replacing drafty single-pane windows with new energy-efficient windows offers excellent insulation against outside summer heat. Upgrading existing windows with additional panes, improved frames and high-tech coatings also enhances efficiency. Let's examine upgrade options to strengthen your windows against heat intrusion:

High-Performance Window Glass Types

Advanced multi-pane windows utilizing innovative glass materials and layered glazings minimize conducted, convected and radiated heat flows much better than regular single glass sheets while allowing light through. triple and quadruple glazing and aerogel-filled windows demonstrate very high insulation values.

Gas Fills and Special Coatings Restrict Heat Flow

Replacing air between glass panes with dense insulating gas fills slows conductive and convective heat flows. Adding specialized microscopically thin optical coatings reflects and blocks solar infrared and UV rays while allowing light to pass through. Soft low-E coatings on one pane and solar-rejective hard coatings on another provides exceptional insulation.

Improved Frames and Weatherstripping Seal Gaps

Old leaky window frames allow hot outdoor air to bypass window glazing through cracks and holes. Replacing with new reinforced frames having integral weatherstripping, tight-fitting joints and durable seals better prevents air leaks. Well-fitted windows inhibit convection loops around edges transporting heat through the frame directly into living spaces.

Professional Installation for Proper Weatherproofing

Proper window installation matters just as much as high-performance window products themselves for the window unit to deliver on insulation and efficiency. Employing professional installers ensures air sealing measures follow industry's best practices for impeccable weatherproofing, sealing all leaks leading in from the heated outdoors.

Working with Window Landscaping

Alongside window treatments, thoughtful outdoor landscaping around windows boosts heat defenses using the natural cooling abilities of plants. Trees, bushes and vines properly positioned right outside windows provide cooling benefits:

Strategic Tree Planting for Window Shading

Tall deciduous trees planted on window-facing sides block intense daytime sun exposure during hot summer months while allowing winter sunlight to filter through bare branches. Evergreen trees offer year-round screening from heat-bearing light. Lightweight shade cloths attachable to tree branches enables adjustable shading as needed.

Vines and Bushes Insulate Window Surfaces

Fast-growing vines mounted on front-side window trellises shield against solar heat influx while beautifying exteriors. Heat-tolerant bushes planted underneath form an insulating dead airspace keeping windows cooler in hot temperatures. Both vines and bushes discourage larger trees blocking scenic views.

Adequate spacing between vegetation growth and window surfaces lessens moisture buildup which can damage window frames and lead to mold if neglected. Frequent pruning of encroaching branches maintains the required ventilation gap for air to freely circulate. Any accumulated moisture also gets wiped down right away.

Together windows and adjoining landscapes cooperate to exclude summer warmth outside while welcoming winter sunshine indoors. With some thoughtful planning and periodic maintenance, your windows remain clear providing unhindered daylighting and views year-round.