Q. I want to replace an old picture window with a bay or bow window. My budget is tight. Which type is best and most efficient?

A. Old, large, single-pane picture windows— which were common in houses built many years ago—are extremely inefficient. Not only is there a huge heat loss in winter, and heat gain in summer, through the glass, there likely is no insulation around the frame inside the walls. If it faces west, count on drapes, furniture and carpeting being badly faded.

Five-section bow window with a vinyl frame
This five-section bow window with a vinyl frame has casement windows on either end for maximum natural ventilation.
Photo by Champion Window

A bow or bay window is sometimes called the poor man’s sunroom. It can provide some of the benefits of a sunroom at a lower cost. These include making your room appear larger, providing a seat under glass at the window and a good location for plants. Although it costs less than a small sunroom, an efficient bow or bay window is not inexpensive.

The basic difference between a bow and a bay window is the number of panels. A bow window is made of four or more narrow window panels, often
of the same width. Five windows is the most common configuration. Using more window panels creates a more circular appearance that many people find attractive. To minimize the cost, have only the windows on each end operable. This configuration provides adequate natural ventilation when the ends are open.

Bay windows are made from three window panels. The two angled side panels usually open. They usually are angled at 30 or 45 degrees. The fixed center window is similar to a smaller picture window with an unobstructed view of the outdoors. To better simulate a sunroom, a 45-degree bay window extends out further than a 30-degree window from the house wall. This provides more space for plants or a bench seat.

Bow and bay windows
Bow and bay windows are available with no-maintenance vinyl or aluminum exterior cladding, and a natural or painted wood interior.
Photo by James Dulley

When replacing a large picture window 10 feet wide or larger with a bay window, a 30-degree design is your best choice. It provides plenty of openness and an area for plants, but does not extend too far. A large triple-pane center window is quite heavy to be cantilevered out far from the

There is not a significant difference in the energy efficiency or durability of a bow or bay window. A bay window, particularly a 30-degree design, may
be slightly more efficient because there are fewer joints and seams to be sealed between the window panels. Also, wherever there is framing material and supporting lumber in the wall, there is less room for insulation.

As with any replacement window style, glass is the heart of the window. Select the most energy-efficient glass your budget allows, even if it forces you to cut back on the styling or trim options. Triple-pane glass is best, but at least select double-pane glass with a low-emissivity coating and inert gas. Select the proper glass for your area because the location of the low-emissivity coating varies depending on your climate. All new glass types reduce fading.

Since a bow or bay window protrudes from the wall, it is ideal for natural ventilation during summer to reduce your airconditioning costs. Select casement window style for the side or end windows. Double-hung windows, which generally do not provide good natural ventilation, are also acceptable on a 45-degree bay window because they protrude enough from the wall to catch gentle breezes.

In addition to high-quality glass, look for a bow or bay window that has thick insulation in the seatboard and the top. When building your own bow or bay window, use a minimum of 1-inch polyurethane rigid foam. This saves energy and improves your comfort near the window. Your plants will also appreciate it during the winter.

Unless you are handy with tools, it is generally better to buy an entire unit designed as a bow or bay window. This costs more than assembling one from scratch, but it will likely be stronger and more airtight. When designing and building your own, consider installing cable supports from above for one that extends out far from the wall.

To ask a question, write to James Dulley, Energy Report, 6906 Royalgreen Dr., Cincinnati, OH. 45244, or go to www.dulley.com. Copyright 2019, James Dulley