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| ICBO | International Conference of Building Officials, responsible for The Uniform Building Code. |
| Ice Dam | Ice formed at the transition from a warm surface to a cold surface, such as along the overhang of a house. The build-up of ice is the result of ice or snow melting on the roof area over the warmer, living area of a building and then refreezing when it runs down and reaches the overhang. |
| Ignition Temperature | The minimum temperature at which a material will combust. |
| Impact Resistance | A roof assembly’s ability to withstand the impact from falling objects such as hail. |
| Impregnate | To saturate; in roofing, asphalt impregnated fiber glass roofing felts are fiber glass mats that have been completely permeated with asphalt bitumen. |
| Infrared Thermography | The use of an infrared camera to detect moisture in roof insulation. |
| Inorganic | Involving neither organic life nor the products of organic life; relating to compounds not containing hydrocarbon groups. |
| Insect Screen | Material used to inhibit an insects ability to enter a building through openings in a roof such as vents. |
| Insulation | Material used to help maintain a certain temperature in a building by reducing the flow of heat to and from that building. See also Thermal Insulation. |
| Intake Ventilation | The part of a ventilation system used to draw fresh air in. Usually vents installed in the soffit or along the eaves of a building. |
| Interlayment | A waterproof material usually installed between adjacent rows of wood shakes to help with the roof’s waterproofing characteristics. |
| Interlocking Shingles | Shingles that lock together to provide wind resistance. See also T-Lock. |
| Internal Pressure | Atmospheric pressure inside a structure that correlates to the number and location of openings and air leaks. |
| Inverted Roof Membrane Assembly (IRMA®) | A variation of the "Protected Membrane Roof Assembly" in which Styrofoam® brand insulation is used. IRMA® and Styrofoam® are registered trademarks of the Dow Chemical Company. |
| IRWC | Institute of Roofing and Waterproofing Consultants |
| ISANTA | International Staple, Nail & Tool Association |
| Isocyanate | A highly reactive organic chemical containing one or more Isocyanate groups. A basic component in Sprayed Polyurethane Foam systems and some polyurethane coating systems. |