The beautiful drama and architectural interest bay windows add to an average room makes their time-intensive installation process well worth it. This how-to explains every step and will help you properly install and insulate bay windows.

January 26, 2019

Bay windows are one of the more complicated window styles to install, but their impact is well worth the effort. This type of window protrudes from the house while also letting in lots of light. And 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.

Expect to spend a day or two installing and insulating the window. Before you begin, brush up on your carpentry skills, cut the opening, and check for square. You will also need one or two strong helpers.

Bay Window Design Ideas

What You Need

  • Tape measure
  • Drill
  • Hammer
  • Level
  • Stapler
  • Caulk gun
  • Screwdriver
  • Flat pry bar
  • Circular saw
  • 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

Step 1: Attach Cable Brackets

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.

Step 2: Adjust Hardware

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.

Step 3: Fill with Insulation

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.

Step 4: Install Sheathing and Shingles

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.

Step 5: Build the Skirt

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.

Step 6: Attach to Window

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.

Tips for Framing the Roof

Though small, the roof over a bay window has all the components of a standard roof. Framing is made of 2x4s or 2x2s. Start by attaching the pieces against the wall, then cut the rafters to the required angles and install them. Attach 1/2-inch plywood sheathing, add a metal drip edge at the bottom, and cover with roofing felt. Install the shingles and then slip a piece of metal drip cap under the siding and over the shingles.

How to Add 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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