They let in the light and keep out the wind. They allow in a breath of fresh air when it's hot and stuffy.

They can, however, cause problems when they leak and let in the rain.

“They” are your home's windows.

Window replacement ranks as one of the 10 most popular home improvements. Year after year Americans spend millions of dollars replacing windows in an effort to reduce energy costs, improve comfort and preserve clear contact with the great outdoors.

If you have those old-fashioned-style wood windows that slide up and down (double-hung), and you are considering an upgrade, you are in luck. These windows are the easiest and least expensive to modernize. Old-fashioned wood double-hungs are the least energy-efficient type of window. On top of that, the weight and pulley mechanism that facilitates their operation is difficult to repair and maintain. The worst thing about older double-hung windows is their slide-by design. They let in more air than they keep out.

When we were boys it was our job to perform regular maintenance on the windows in our home. We regularly had to re-rope the weights, lubricate the pulleys and wax the frames to make sure that they didn't stick or grab. We learned early in life that the window we didn't want was the kind operated with ropes and pulleys. Maybe that's why sliding windows became so popular.

Several years ago we discovered that a replacement window was available for double-hungs that included a modern tracking system, fully weather-stripped frames and a tilt-out feature that made cleaning easy.

The most serious problem with window replacement is leaking. The frame of a window has a waterproof connection with the wall. If the frame is removed, the waterproof connection is destroyed. Creating a new waterproof union is time-consuming and expensive. Therefore, replacing a window without disturbing its frame has several advantages.

With metal windows, the frame is too small to hold new, thicker double- or triple-pane glass. Older metal frames are barely wide enough to hold one sheet of glass let alone two or more. When a metal-frame window is being replaced, we suggest removing the wall covering around the opening and reinstalling the new window in the same way as the original one was installed. With stucco this means lots of hammering, chiseling and stucco patching.

On the other hand, wood window frames are enormous. There is plenty of room to utilize them in a new configuration that includes modern insulated window panels. If the wood frame is in good condition, reuse it and replace only the window panels. There is another consideration. Many wood window frames are quite individual in appearance. Replacing them with a plain vanilla upgrade could destroy the appearance of a home.

A window should provide light equal to 10 percent of a room's floor area and enough ventilation to equal 5 percent of its floor area. For example: if a room is 100 square feet in area the window(s) must provide 10 square feet of light and 5 square feet of ventilation.

First, remove the old windows, side trim, weights and pulleys. Then make any necessary repairs to the frame. This, by the way, is an opportune time to paint the frame as well.

The type of replacement assembly we like the most involves the installation of new side tracks into the existing frame. They include a mechanism that replaces the rope-and-pulley system and provides an excellent weather seal between the new windows and the old frame. The windows are then individually mounted into their tracks.

Another type of replacement assembly involves the use of windows in a pre-assembled frame. The outside of the new assembly...frame and all...fits inside the old wood frame. This type of window is easier to install. We once retrofitted an entire home in a single day. The problem is that the actual opening of the new window is substantially reduced with this type of replacement. The new frame takes up quite a bit of the original opening. Under certain conditions this can be a serious problem. The building code requires the inside clearance of a bedroom window to be a certain size. Installing a frame inside of a frame can reduce the clearance.

A REMINDER
It is important to remember that our high comfort level associated with reusing wood frames is based on the fact that they are conducive to the kind of retrofit we have suggested. By the same token, we feel that metal frames are not. With metal, we suggest complete removal and replacement.