Windows are the facial features of your house. They make it come alive and give it grace -- or wit -- and character.

They're also date markers. While there's always been some variety available, the predominant styles are as specific to their era as clothing fashions. Some styles predominate in a particular area, or even a particular neighborhood -- the stained-glass transoms of East Baltimore, for instance, or the multipaned, double-hung windows of 18th century New England.

If your old house still has some original windows, they can provide valuable clues to the age and intended style of the house. Even the glass can be special; old glass has imperfections that reflect light differently from new.

The whole question of replacing windows has to be approached carefully, because changing any aspect can seriously damage the character of the house.

Sometimes changes are unavoidable, but some of the most common sins committed against old windows are:

*Changing curved tops to flat.

*Changing the number of panes.

*Changing the size of the opening, whether it's to make it larger or smaller.

*Changing the style of the window (for instance, to casements, from double-hung.)

*Eliminating exterior trim.

*Replacing an original feature, such as true divided panes, with something fake, like single panes with "snap-in" dividers.

Often such changes are made in the name of "modernizing," but the truth is, you can't make an older house look modern by changing one feature any more than you can make a grandmother look like a teen-ager by putting her in Reebok Pumps.

Serious rehabbers usually don't consider such changes -- at least not on the parts of the building that are visible from the street.

On rear elevations and other parts of the building that aren't so visible, however, change may be in order.

One of the problems with old houses is not enough light, especially in rear sections or in middle rooms, like those in many Baltimore row houses, where there might be a single window. In such a situation, decreasing window size -- no matter how cheap it is -- decreases both light and ventilation. In most areas, building codes stipulate a certain amount of light and ventilation per square foot of room size.

On the other hand, opening up dark spaces, if it's done appropriately, can really brighten up an old house.

An example is our current rehab project, where a solid brick back wall on the third floor had conveniently fallen down. Since this is the part of the house that faces alleys and rooftops but overlooks the harbor, we're replacing that wall with windows -- a lot of windows.

It hasn't been easy to decide, however, just what kind of windows to install. Even when you're confined by the size of the opening -- we can't make the windows bigger than the old wall -- the range of options is bewildering.

In addition, some of the old distinctions in the window industry are crumbling -- the boundary between "custom" and "stock" sizes, for instance.

The first step is to gather as much information as you can. Never buy windows in haste -- or you'll have plenty of long, hot summers and long, cold winters to repent.

Next: Sorting through the options.

Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.

If you have questions, comments, tips or experiences to share about working on houses, write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore, Md. 21278. Questions of general interest will be answered in the column; comments, tips and experiences will be reported in occasional columns.

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