Incandescent lamp - A lamp employing an electrically charged metal filament that glows at white heat.
Indirect-gain system - A passive solar system in which the glazing is separated from the living space by a heat storage wall that may be masonry or containers of water.
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.
Insolation - The amount of sunlight falling upon a surface, usually measured in Btu's per square foot per hour or Btu's per square foot per day.
Insulation - Material used in a building's walls, ceiling, floor or roof to hinder the flow of heat.
Interior finish - Any coverings that cover the interior walls of a house. Examples are drywall, paneling, etc.
Internal gains - Sources of heat within a building that are not part of the heating system, for example, people, animals, lights and appliances.
Inverter - A device that converts direct current to alternating current, either mechanically or with solid state circuitry.
Isolated-gain system - A passive solar system in which the collector, storage and living space are all physically separated from one another.
Jack post - A type of structural support made of metal, which can be raised or lowered through a series of pins and a screw to meet the height required. Basically used as a replacement for an old supporting member in a building.
Jambs - The top and sides of a door, window, or other opening. Includes studs as well as the frame and trim.
Joint compound - A synthetic-based formula used in combination with paper tape to conceal joints between drywall panels. Also see taping.
Joists - Horizontal framing members that support a floor and/or ceiling.