Did you know that there are building code rules and regulations about how much natural light and natural ventilation must be present in each and every room of a home?

Before your building department approves a house or an addition or even a window change, certain light and ventilation rules must be met. Closets, bathrooms, hallways and garages don't fall under these guidelines.

Here are the rules:

NATURAL LIGHT

A given room must have a window or glass door that is sized to equal at least 10 percent of the floor area of the room. So, if a room is 10 x 12 feet, then the room area would be 120 square feet and the minimum size of the window in that room would have to be12 square feet, for example.

Pretty simple really. For every 10 feet of floor area you need one foot of natural light area.

VENTILATION

Figuring out ventilation is even easier. Natural ventilation must equal 5 percent of the floor area or exactly half the requirement for that of natural light. In our natural light example above the window also would qualify for minimum natural ventilation if half of it was operable.

Although handled as separate issues, you can see that natural light and natural ventilation actually have a very specific relationship to one another as far calculation rules are concerned.

The required amount of light is double that of natural ventilation.

Why is this so important? Simple, natural light cuts down on the need for artificial illumination and therefore can save energy. In the winter warming sunlight can cut on heating costs.

As for ventilation, keep in mind that Mother Nature's breath reduces mildew and mold. And unless you live in downtown Los Angeles, outside air is usually quite medicinal (lots of oxygen) and refreshing.

Of course, the rules get a little more complicated as you apply them throughout the house.

For example: Adjacent rooms may be grouped together for light and ventilation calculation purposes, but the size of their shared doorway must equal at least 50 percent of the area of the wall space that separates the rooms.

And these guidelines just cover the minimums _ there are also rules about maximums.

You see, this is why architects get so much money.

If you have a question, simply bring the details to your local building department and ask what you can do before you spend the money doing it wrong.

For more home improvement tips and information visit our Web site at www.onthehouse.com or call our listener hot line at 1-800-737-2474 (ext 59).