Window Heat Gains & Losses

Introduction

Windows are special feature of the thermal barrier of a house. Their unique feature of letting the sunshine in and special challenges they bring make it worth considering separately. For a more complete understanding of the dynamics of the thermal barrier, see the articles for improving thermal performance.

Discussions in this article include

• Current State of Windows
• Contributing Issues to Window Heat Transfer
• Diagnosing Windows
• Window Characteristics
• Customer Performance Targets for Windows
• Improving Windows
• Post Window Installation Inspection

Challenges. Windows are a significant source of heat loss in cold climates and solar heat gain in warm ones. We use windows to provide natural light, ventilation, and look outdoors. These features make it difficult to limit heat transfer while preserving light and view.

Avoid Replacement. From a cost-effectiveness point of view, replacing windows is nearly last in terms of other priorities unless they are being replaced for other reasons besides energy.

There are most cost-effective ways for handling windows by using window treatments and awnings.

Window Performance Goals. Regardless of the window improvement approach, the goal is to reduce heat loss or solar gain—maybe both.

Current State of Windows

Air leakage problems are between the window and rough opening for it
Single pane windows lose 2-3 times more heat than dual-pane windows
Single pane windows allow more condensation other types
Metal frame windows conduct more heat that other types

Contributing Issues to Window Heat Transfer

Compared to other home performance concerns, the behavior of window installers is least suspect in terms of energy performance. Most of the issues are purely physics. It’s up to consumers to understand window performance, because we’ll get what we pay for (usually).

Beware of getting more window than you need as there are cheaper ways of achieving your objectives, with significantly better performance, at less cost.

Home Energy Audit: Diagnosing Windows

Tip: Diagnosing window performance depends on window characteristics.

The process for reducing improving window performance follows this path:

• Interview Homeowners
• Conduct Visual Assessment
• Select Window Options
• Manage Indoor Moisture Levels
• Install Window Treatments and Windows
• Post Window Installation Inspection

Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.

As they relate to windows, the following questions are asked during the homeowner interview
Are there any hot or cold rooms?
Do you notice any condensation, such as on windows?
Are there any mold or mildew issues?
Do you feel any drafts?
How are your energy bills: comparatively high, medium, or low?
Is there anything else you feel is relevant?

Answers to these questions help us evaluate the system in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.

Conduct Visual Assessment. An investigation of the windows is done as part of the assessment of the thermal barrier. Windows are vitally important to the way heat is transferred in and out of the house.

Window Assessment. The extent of the visual assessment is determining the characteristics of the window and whether they are intact. Any shading for and orientation of the windows is also observed.

Noted Properties. The interesting characteristics are the number of panes, type of glass, material the frames and sashes are made of, and the style of window. Other features may be noticed too.

Window Treatments. Besides the window itself, certain types of window coverings are considered as part of the wall, such as storm windows, solar shades, and awnings or porch roofs.

Window Condition. It’s important for windows to function correctly. Sashes need to fit tight when closed. Seals holding window panes in place need to be intact.

Recommendations

Unless new windows are desired, recommendations are made to use window treatments to limit heat loss or reduce solar heat gain where needed.

Select Window Options. Unless the windows are in poor shape or being changed for some other reason other than energy performance, the preferred choice is to improve the windows in a cost effective way.

There are ways of reducing thermal transmittance (U-factor) and controlling solar heat gain. The benefit is that some or all of the window treatments are available even when the windows are ever replaced.

First, the focus needs to be on reducing heat flow through the window. The way to do that is add a second window pane, possibly in the form of a storm window.

Storm Windows. Storm windows can be installed inside or outside.

Exterior Storm Windows. Exterior storm windows are similar in quality, as long as the frames are metal. Plastic one must have the ability to withstand temperature extremes and ultraviolet light. The choice depends on desired features, such as air tightness, openable sashes, and weeps holes

Interior Storm Windows. Interior storm windows are designed with an effective air seal around the perimeter to prevent warm, moist indoor air from condensing on the primary window. Panes made of plastic have a slightly higher R-value than glass.

Window Insulation. Interior window shades, drapes, and shutters are very effective for insulating windows and improving comfort. They are expensive, but cost less than replacement windows.

Insulating walls, ceilings, and floors and installing storm windows should take priority over using window insulation.

Solar Screens & Shades. Sun screens reduce solar heat gain through the window. Solar screens reportedly block 99% of UV light striking the window, which reduces fading in half resulting from heat and visible light.

The claims are that solar shades & screens do a better job than window films and awnings. A lot of commercial websites claim these benefits, but studies and white papers were not quickly found using Internet search tools.

Window Films. Window films compete with solar shades & screens. Similar issues with the search engines preclude finding objective sources of information.

Awnings and Overhangs. Awnings and overhangs work well, especially on south facing windows. Properly designed, in winter solar heating can be maximized while blocking the sun in the summer.

Landscaping. Trees and shrubs are awesome for blocking solar heat gain. Just be sure the roots don’t impact on the foundation and branches don’t touch the house in any way. Landscaping can also benefit walls and ceilings too.

Replacement Windows. Many window treatments, especially awnings and overhangs, as well as landscaping work regardless of the window installed.

However, when window upgrades or replacements are needed, the thermal features of windows need to be considered. The most important ones are thermal transmittance (U-factor) and solar heat gain coefficient (SHGC).

Don’t forget the most important features of a window is to let light in and to be able to see out, so maximize visible light transmittance (VT) too.