I-beam– a beam with an I or H-shaped cross-section. The horizontal elements of the “I” are known as flanges, while the vertical element is termed the “web”. I-beams are usually made of structural steel and are used in construction and civil engineering.

I-joist– An engineered wood joist, the cross section of which resembles the letter I.

Incandescent lamp– A lamp employing an electrically charged metal filament that glows at white heat.

Index– The interest rate or adjustment standard that determines the changes in monthly payments for an adjustable rate loan.

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.

Interpolation– In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points.

IBC– International Building Code

IEBC– International Existing Building Code

IFC– International Fire Code

IRC– International Residential Code

Irregular Buildings– Per IRC 2015, R301. A building or portion of a building shall be considered be irregular where one or more of the following conditions occur:

  1. Where exterior shear wall lines or braced wall panels are not in one plane vertically from the foundation to the uppermost story in which they are required.

Exception: For wood light-frame construction, floors with cantilevers or setbacks not exceeding four times the nominal depth of the wood floor joists are permitted to support braced wall panels that are out of plane with braced wall panels below provided that.

  • Floor joists are nominal 2 inches by 10 inches (51 mm by 254 mm) or larger and spaced not more than 16 inches (406 mm) on center.
  • The ratio of the back span to the cantilever is not less than 2 to 1.
  • Floor joists at ends of braced wall panels are doubled.
  • For wood frame construction, a continuous rim joist is connected to ends of cantilever joists. When spliced, the rim joists shall be spliced using a galvanized metal tie not less than 0.058 inch (1 .5 mm) (16 gage) and 1-1/2 inches (38 mm) wide fastened with six 16d nails on each side of the splice or a block of the same size as the rim joist of sufficient length to fit securely between the joist space at which the splice occurs fastened with eight 16d nails on each side of the splice; and
  • Gravity loads carried at the end of cantilevered joists are limited to uniform wall and roof loads and the reactions from headers having a span of 8 feet (2438 mm) or less.
  1. Where a section of floor or roof is not laterally supported by shear walls or braced wall lines on all edges.

Exception: Portions of floors that do not support shear walls or braced wallpanels above, or roofs, shall be permitted to extend not more than 6 feet (1829 mm) beyond a shear wall or braced wall line.

  1. Where the end of a braced wallpanel occurs over an opening in the wall below and ends at a horizontal distance greater than 1 foot (305 mm) from the edge of the opening. This provision is applicable to shear walls and braced wall panels offset in plane and to braced wallpanels offset out of plane as permitted by the exception to Item 1.

Exception: For wood light-frame wall construction, one end of a braced well panel shall be permitted to extend more than 1 foot (305 mm) over an opening not more than 8 feet (2438 mm) in width in the wall below provided that the opening includes a header in accordance with the following:

  • The building width, loading condition and framing member species limitations of Table P602.7(1) shall apply; and
  • Not less than one 2 x 12 or two 2 x 10 for an opening not more than 4 feet (1219 mm) wide; or
  • Not less than two 2 x 12 or three 2 x 10 for an opening not more than 6 feet (1829 mm) in width; or
  • Not less than three 2 x 12 or four 2 x 10 for an opening not more than 8 feet (2436 mm) in width; and
  • The entire length of the braced wall panel does not occur over an opening in the wall below.
  1. Where an opening in a floor or roof exceeds the lesser of 12 feet (3658 mm) or 50 percent of the least floor or roof dimension.
  2. Where portions of a floor level are vertically offset.


  • Framing supported directly by continuous foundations at the perimeter of the building.
  • For wood light-frame construction, floors shall be permitted to be vertically offset when the floor framing is lapped or tied together as required by Section P502.6.1.
  1.  Where shear walls and braced wall Unes do not occur in two perpendicular directions.
  2. Where stories above grade plane partially or completely braced by wood wall framing in accordance with Section P602 or cold-formed steel wall framing in accordance with Section P603 include masonry or concrete construction.
  1. Where this irregularity applies, the entire story shall be designed in accordance with accepted engineering practice.

Exception: Fireplaces, chimneys and masonry veneer as permitted by this code.

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.

Interest – The cost paid to a lender for borrowed money.

Interior finish– Material used to cover the interior framed areas of walls and ceilings.

Irrigation– Lawn sprinkler system.