I-Beam - A steel beam with a cross section resembling the letter "I". It is used for long spans as basement beams or over wide wall openings, such as a double garage door, when wall and roof loads bear down on the opening.

I-Joist - Manufactured structural building component resembling the letter "I". Used as floor joists and rafters. I-joists include two key parts: flanges and webs. The flange or from of the I joist may be made of laminated veneer lumber or dimensional lumber, usually formed into a 1 ½" width. The web or center of the I-joist is commonly made of plywood or oriented strand board (OSB). Large holes can be cut in the web to accommodate duct work and plumbing waste lines. I-joists are available in lengths up to 60 feet long.

Incandescent Lamp - A lamp employing an electrically charged metal filament that glows at white heat. A typical light bulb.

Infiltration - The passage of air from indoors to outdoors and vice versa; term is usually associated with drafts from cracks, seams or holes in buildings.

Inside Corner - The point at which two walls form an internal angle, as in the corner of a room.

Insulating Glass - Window or door in which two panes of glass are used with a sealed air space between. Also known as Double glass.

Insulation Board, rigid - A structural building board made of coarse wood or cane fiber in ½- and 25/32-inch thickness. It can be obtained in various size sheets and densities.

Insulation - Any material high in resistance to heat transmission that, when placed in the walls, ceiling, or floors of a structure, and will reduce the rate of heat flow.

Interior Finish - Material used to cover the interior framed areas of walls and ceilings.