Insulating a Bay Window

This story shows you how to install a bay window.

Installing a Bay Window

The farther a bay window protrudes from the house, the more dramatic the effect and the more it opens a room. You can choose among units that protrude at 45-, 30-, or 10-degree angles; the latter is sometimes called a bow window. Individual windows may be fixed, double-hung, or casement.

A bay unit attaches like a standard flanged window but with some important differences. Some units also require support from above using cables that attach to framing members in the eaves or the wall above. Some require support from below using brackets or a knee wall. Some need both types of support. A smaller bow window may not need this type of support and can be installed much like a standard flanged window.

Plan how you will finish the top and bottom of the bay window. You may be able to purchase a ready-made roof or you may need to custom-build one. The bottom is usually easy to trim out, but you may choose to build a wall down to the ground. In these the roof and skirt are built on the ground and then installed.

Prestart Checklist

A day or two depending on trimwork

Tape measure, drill, hammer, level, stapler, caulk gun, screwdriver, flat pry bar, circular saw

Good carpentry skills

Cut the opening and check for square. You will need one or two strong helpers.

Bay window, exterior casing, 2x4s, shims, caulk, roofing felt or building paper, drip cap, flashing, plywood, roofing, finishing nails, staples, roofing nails, interior trim, casing nails, insulation

Insulating and finishing the exterior: Step 1

Attach the cable brackets. Make sure the brackets are roughly in line with the cable attachment points on the window. Screws should be driven deep into framing members, not just the siding.

Insulating and finishing the exterior: Step 2

While checking for level adjust the mounting hardware as needed. Raise the outside of the unit slightly so it is higher than level to allow for settling.

Insulating and finishing the exterior: Step 3

Fill the framing with insulation. Cut a piece of fiberglass insulation to fit on top of the window, then cut a slightly smaller piece to rest on top of the first piece. Fill the rest of the cavity with loose fiberglass.

Insulating and finishing the exterior: Step 4

Add the sheathing pieces cut earlier, fastening them with 8d cement-coated sinker nails or 2-inch screws. Finish it by adding flashing and drip edge, then roofing felt and shingles.

Insulating and finishing the exterior: Step 5

Prefab the skirt as well. The complexity of the joints makes it much easier to build the skirt on a work surface rather than on the window itself. The framing shown is made of 2x4s, which will allow for 31/2 inches of insulation under the window. Attach the plywood bottom. Attach trim to cover the framing.

Attaching the window: Step 6

Tack insulation under the window. Cut a piece of plywood to cover the bottom. Bore holes in the plywood so you can tighten the cable nuts later if needed. Attach the framing to the underside of the window using screws. If there will be a short wall down to the ground, pour a small concrete pad and add matching framing on the pad.

What If... You need knee braces?

If required, or if you like the look, add knee braces at the bottom. A decorative wood brace like this attaches using lag screws driven into wall studs. These were made from two pieces of 2x12, laminated with glue and cut to shape with a band saw. Readymade braces are available at most home centers.

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