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Originally Posted On: https://localglassrepairs.com/solar-heat-gain-and-loss-the-energy-performance-of-glass/
Solar Heat Gain and Loss: The Energy Performance of Glass
The invisible waves and particles that pass through your windows, providing natural light, also influence the temperature and, therefore, your comfort. Referred to as heat gain and heat loss, the two-way flow of thermal energy through your windows is an essential function of energy efficiency.
Why does this matter? Excessive heat gain in warmer temperatures can make your living room feel more like a sauna, requiring constant air conditioning. In cooler temperatures, significant heat loss can create drafts and leave you feeling chilled, forcing you to always have the heat running.
That’s where metrics like the solar heat gain coefficient (SHGC) can help you choose windows that optimize light and comfort. In this blog, Local Glass Repairs LLC’s experts introduce you to this metric and others to help you make informed decisions on the energy efficiency of your windows.
Understanding Solar Heat Gain Coefficient (SHGC) for Windows
The SHGC is a rating system that helps quantify how much solar heat a window allows to pass through. It uses a scale of 0 to 1, with 0 representing a perfect heat blocker and 1 allowing all solar heat to enter.
A window with a low SHGC rating (closer to 0) reflects or absorbs more solar heat, keeping your home cooler when it’s hot outside.
A window with a higher SHGC rating (closer to 1) transmits more solar heat, potentially providing some warmth in cooler temperatures.
Here’s how you can use this metric to install the correct windows for your home:
- Set energy efficiency goals: Determine your energy efficiency objectives. Are you aiming to reduce cooling costs or maximize passive solar heating? This decision determines your ideal rating.
- Consult energy ratings: Consider other window characteristics such as thermal insulation, visible light transmission, and frame materials. Balance these factors for both aesthetics and efficiency.
- Window orientation: Assess how each window’s orientation (north, south, east, west-facing) affects solar heat gain throughout the day and select SHGC ratings based on the results.
- Climate: Washington, for example, experiences a marine west coast climate characterized by mild temperatures and frequent cloud cover. Your ideal SHGC rating is based in part on climate.
U-Factor for Windows and Its Impact on Energy Efficiency
U-factor is another window rating system that measures how well a window unit insulates against heat transfer. A window with a low U-factor indicates superior insulation, keeping your home cooler in hot temperatures.
This metric is based on the thermal resistance (R-value) of each component that makes up the window unit. R-value considers factors like the type of glass (single pane, double pane, etc.), the gas between panes (air, argon, etc.). and any coatings applied to the glass.
SHGC and U-factor both measure window performance, but there are key differences. The latter primarily relates to insulation efficiency, indicating how well a window prevents heat transfer, while the former focuses on solar heat gain, showing how much solar radiation is allowed through the window.
Factors Influencing Heat Transfer Through Glass
If you hold up your hand to a window on a hot day and notice warmth radiating from the glass, heat is primarily transferred through the window by radiation. The sun’s rays warm the glass, which then re-emits that heat as infrared radiation.
There are a variety of window properties that influence this radiative heat transfer:
- Glass type: Regular clear glass conducts heat more readily than low-emissivity (low-E) glass. Low-E glass has a special coating that reflects infrared radiation, helping keep heat in or out depending on the season.
- Number of panes: Insulated glass units (IGUs), also known as double-paned or triple-paned windows, consist of two or three panes of glass separated by a spacer filled with air or inert gas. These additional panes and the trapped gas provide more resistance to heat flow.
- Glass thickness: Thicker glass generally offers better insulation than thinner glass due to increased conduction resistance. The exception might be if the thicker glass isn’t low-E., However, in some cases, thickness can come with diminishing returns after a certain threshold.
Thermal Bridging and Its Effect on Window Performance
Thermal bridging refers to areas in a building envelope (the barrier between interior and exterior) where heat flows more easily due to a change in material properties. In windows, this often occurs around the frame, where conductive materials like metal can create a direct path for heat transfer.
This process bypasses the insulating effect of the glass itself and can significantly reduce a window’s overall performance, inviting in the usual suspects—higher energy bills and discomfort for you and other occupants.
To mitigate the effects of thermal bridging around windows, consider these installation techniques:
- Install insulated window frames and sashes.
- Ensure proper insulation around window openings.
- Use thermal breaks in metal window frames to reduce heat transfer.
- Seal gaps and cracks around windows.
- Choose windows with low U-factor ratings.
Insulating Properties of Gas-Filled and Vacuum-Insulated Glass
Gas-filled and vacuum-insulated glass are both types of windows designed to improve insulation compared to standard single-pane windows. In both cases, the result is a more consistent indoor temperature.
In a regular window, the space between the panes is simply filled with air. Gas-filled windows replace the air with inert gases like argon, krypton, or xenon. These gases are denser than air and have lower thermal conductivity, which means they transfer heat less effectively and provide better insulation.
In vacuum-insulated glass, the space between the panes is evacuated, creating a near vacuum. Since there are minimal gas molecules present, heat transfer by conduction and convection is significantly reduced.
Evaluating Window Frames: Materials and Design
The material of a window frame can significantly impact its energy efficiency, affecting heat transfer and air infiltration rates.
Focus on insulation when you’re looking at frames:
- Material: Prioritize materials with low thermal conductivity. Vinyl, fiberglass, and wood are generally better insulators than aluminum, for example.
- Frame design: Look for frames with a thermal break. This is a non-conductive material like nylon or plastic inserted within the frame to create a barrier and minimize heat flow.
If you’re worried that prioritizing efficiency might compromise aesthetics, fiberglass, composite frames, and painted wood can all be great choices. They come in a variety of styles and finishes that you may find suitable for your home’s design.
Explore High-Performance Windows With Local Glass Repairs LLC
At Local Glass Repairs LLC, we account for heat loss and gain when we install your windows. This ensures optimal performance and potential energy savings for your home.
If you’re tired of constantly adjusting the thermostat or feeling drafts in certain rooms, call (206) 778-5763 to explore our high-performance windows.
