Windows provide more than a view of the neighborhood: They're really important parts of your home's protective envelope. Designed to let in light and air, they also hold the elements at bay. When a window begins to fail—to warp, rot, leak or simply refuse to open and close—the costs in terms of both energy loss and aggravation quickly add up. While new windows are no one's favorite way to spend money, without them you'll eventually spend the equivalent in higher heating and cooling costs—and you'll still have bad windows. The good news is that there are more choices in do-it-yourself window replacement today than ever before.

The Kit Alternative. The simplest replacements are 100 percent vinyl window kits. Here, you remove all but the external frame—the window casing—and have a complete replacement insert custom made to fit the opening. The replacement consists of an insulated vinyl casing with double-hung vinyl windows already installed. To reduce costs, you can order the package with single-hung windows, in which only the bottom window opens. Vinyl windows are also available with low-E glass for reduced heat infiltration.

The next step up is a wooden sash kit, which often comes with vinyl-clad external components. The vinyl exterior is rotproof and never needs painting, while the interior components resemble a standard window. These sash kits do not come preassembled, however. Instead of sliding in a complete insert, you fit new window case components and then install the sashes. You'll pay a little more, but you'll get a tight window with a conventional appearance.

Casement window kits are also available for a little more money. All the usual add-ons, like mullion grilles, screens and low-E glass, are available with these replacements. But while mullion grilles are external on double-hung windows, they are often sandwiched between the glazed panes of casement windows.

Doing The Work To determine the size of the window you'll need, first remove the interior trim with a small pry bar. Then, measure the old window and the rough wall opening and find a close replacement. A new window of the same general size as the old will often be slightly shorter, owing to a different sill design.

1--Use a small pry bar to remove the window trim from the inside. Proceed slowly and carefully if you plan to reuse the trim.	2--Use a sharp utility knife to cut through the caulk between the trim and the siding around the old window.

With the replacement window on site, go outside and cut through the caulk bead between the old brick mold and the siding. .Then, use a cat's paw to pull the casing nails along the outer edge of the old brick molding

3--Pry out all the nails along the outer edge of the brick molding by driving a cat's paw under the nail heads.	4--To free the window, tap it loose from the inside with a hammer and block of wood. Strike the casing in several spots.

In most cases, removing the casing nails won't immediately free the window. With a helper outside to steady it, go back inside and loosen the window by tapping with a hammer and block of wood . When it breaks free, grip the window along each side and lift it out

5--As the window loosens, have a helper support it from the outside. When it's free, grip it on both sides and lift it out.	6--To remove the old tin drip cap, slide a reciprocating saw blade between the tin and sheathing and cut through the nails.

If the old window had a drip cap along its top, remove it with a reciprocating saw fitted with a hacksaw blade. Just slip the blade between the metal cap and the wall's sheathing and slice through the nails that hold it in place

In many cases, you'll also need to cut some of the siding from the edge of the window opening, especially if the original window wasn't centered. This cut is not for the trim boards, which you'll make later, but just to clear a space for the nailing flange. With a circular saw set to a depth of about 1 in., cut out about 1 1/2 in. around the opening

7--To make room for the new window's nailing flange, cut away about 1 1/2 in. of siding from the opening's perimeter	8--To raise the new window slightly, nail 3/4-in.-thick blocks into the lower corners of the framed opening.

To center the shorter new window, nail 3/4-in. blocks into the lower corners of the framed opening. Then, lift the new window into the opening for a test fit. Press the nailing flange firmly against the sheathing, centering the window from side to side as best you can, and tack it in place on both sides with 1 1/4-in. roofing nails. These nails have large, 3/8-in. heads that won't pull through the slots in the nailing flange. Remember that this is just a test fit, so don't sink the nails in all the way.

9--Set the new window in place for a test fit. Press the flange tightly against the sheathing and center the window.	10--Use roofing nails to temporarily hold the flange against the wall sheathing. Don't drive the nails completely in.

With the window in place, check that it's centered and plumb—a 4-ft. level works best here. Framed window openings are seldom level, so you may need to shim the bottom on one side. The best place to shim is between the casing and the spacer blocks you nailed into the corners. When you have the window centered and plumbed, mark the sheathing around the nailing flange for future reference.

11--Use a 4-ft. level to plumb the window. Shim the casing if needed, then mark the flange position on the sheathing.	12--Lay the trim against the window and mark the siding to be cut away. Leave a 1/4-in. gap between the trim and the window.

This is also a good time to mark the siding cut for the 4-in.-wide trim. Just hold a length of trim against the window and mark the siding along the outside of the board on all sides . Because windows expand on hot days, allow 1/4 in. between the trim board and the window when marking these cuts. Remove the window to cut out the excess siding along these lines.

Before installing the window permanently, slide a new drip cap under the siding along the top. Nail it through the siding with galvanized nails. With the drip cap in place, reinstall the new window, carefully aligning it with the marks from the test fit, and nail it in place. Place a nail about every 8 in. around the nailing flange. Then, trim the window with the 4-in. stock. Start with the top board, then the bottom and finally the sides. Use 8d galvanized nails to fasten the trim.

13--Install the top and bottom trim boards first, then the sides. Secure the trim with 8d galvanized finishing nails.	14--Caulk joints between the trim, siding and window with a high quality, paintable silicone caulk. Then, prime and paint trim.

Carefully apply a bead of paintable silicone caulk to every seam, including the joints between the siding and trim, and the trim and window. Be sure to caulk the seam between the siding and the drip cap, as this joint can trap water and ruin the siding. Finish by priming and painting the trim.

Because newer window casings with their vinyl sash guides can spread apart at the center—and because spreading sides can allow the windows to fall out—manufacturers now recommend shimming the casings on each side of the opening. This can be tricky, because if you over-shim, you'll interfere with the operation of the window. Nailing the shims in place through the sashes can also pose problems.

To keep the windows from spreading during shipment, each window has a nylon ribbon that holds the casings together. This band can serve as a guide when shimming the casings. Just slide tapered shims between the casing and the framed opening, one pair on each side. When the shims contact both surfaces, but just before they loosen the nylon ribbon, you'll have the right amount of shim. Cut the shims to size and slide them back in place with a dab of construction adhesive on each side.

15--To shim between the casing and side framing, use tapered wooden shims with a dab of construction adhesive on each.	16--Use a putty knife to loosely pack fiberglass insulation between the window casing and the wall's framed opening.

You'll also need to insulate between the window and the framed opening. The easiest approach is to pack fiberglass insulation into the joint. Don't overpack the joint—insulation needs air pockets to work. Another approach is to partially fill the gap with expandable foam. Again, don't overdo it. Use a minimal expansion foam and stop before it expands past the inside edge of the window casing.

To complete the job, install the trim on the inside. Start by mitering a length of trim 1/4 in. longer than the top header casing. Nail this piece in with 1/8 in. of casing showing on each side, then do the same for the bottom of the window. Finish by mitering and nailing in the side pieces. If you're using a hardwood trim, you'll need to bore pilot holes for the nails to keep the wood from splitting. Use 3d finishing nails, placing one about every 12 in. Then set the nails and fill the holes.

17--Finish by trimming the window casing on the inside wall. Secure the mitered trim with 3d finishing nails. Set nails and fill.

Widening The Opening If you want a wider window, you'll need to reframe the window opening. There are several ways to approach this work, but here's an easy, low-tech method that nearly anyone can manage.

The first step is to cut out the wallboard around the opening. You'll need plenty of working space, so cut well beyond the actual opening and remove the insulation from the stud spaces. Use a reciprocating saw with a hacksaw blade to cut through the nails that hold the header to the king studs. Follow by cutting the nails that secure the studs to the top and bottom plates and those that secure the wall sheathing to the studs.

18--Use a reciprocating saw with a hacksaw blade to cut nails securing the old header. Do the same for the studs and sheathing.	19--Build a temporary support wall to take the load when you remove the old header. Shim it to make contact with the ceiling

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Before removing these structural members, build a small temporary wall to use as a ceiling support. A three-stud, 26- to 36-in.-wide support will usually suffice. Set it in place about 18 in. in front of the window opening and shim it underneath until it makes firm contact with the ceiling. Then, remove the structural members you've cut free. When framing a new opening, make it 1 in. larger than the new window in both directions.

Window headers are supported by a two-stud configuration at each side. The outside or king stud is a full-length stud that's nailed to the top and bottom plates. The inside or jack stud is shorter and supports the header and the weight of the roof. The length of the jack stud is determined by the desired height of the window. The height of a new window on a retrofit usually matches the height of the surrounding windows. In our case, the tops of the other windows are 11 in. down from the ceiling. Allowing 3 in. for the double plate at the top of the wall, we have about 8 in. of header space to fill. And because our window would be less than 4 ft. wide, 2 x 8 header stock would provide enough support. A wider opening may require a larger header, so check with your local building inspector or code authority before you start the job. Cut the jack studs to length and nail them in place against the king studs, one on each side

20--Nail the jack studs in place against the king studs on each side. Window height determines the length of the jack studs.	21--Set the new header atop the jack studs and nail it in place through the king studs. Then, remove the temporary support wall.

To build the header, cut two 2 x 8s to fit between the king studs and rest on the jack studs. Then, cut a piece of 1/2-in. plywood to sandwich between the 2 x 8s. Apply construction adhesive to the first 2 x 8 and set the plywood into this adhesive. Apply adhesive to the plywood, place the second 2 x 8 on top and nail the assembly together with 12d nails driven from both sides.

Set the header on the jack studs and nail through the king studs. Frame in the bottom of the window opening and cut away any excess siding and sheathing.