| IEQ | Indoor Environmental Quality - includes IAQ, thermal comfort, daylighting, etc. |
| IESNA | Illuminating Engineering Society of North America. |
| I-Joists/I-Beams | Wooden members made of two pieces of solid lumber or laminated veneer lumber (LVL) held together with a web of OSB. Substituted for wide dimensional lumber as a support for a ceiling or floor. |
| Impreg | Wood in which the cell walls have been impregnated with synthetic resin so as to reduce materially its swelling and shrinking. Impreg is not compressed. |
| Incipient Decay | The early stage of decay that has not proceeded far enough to soften or otherwise perceptibly impair the hardness of the wood. It is usually accompanied by a slight discoloration or bleaching of the wood. |
| Incising | A pretreatment process in which incisions, slits, or perforations are made in the wood surface to increase penetration of preservative treatments. Incising is often required to enhance durability of some difficult-to-treat species, but incising reduces strength. |
| Increment Borer | An auger like instrument with a hollow bit and an extractor, used to extract thin radial cylinders of wood from trees to determine age and growth rate. Also used in wood preservation to determine the depth of penetration of a preservative. |
| Indoor Air Quality | The nature of air that affects the health and well being of building occupants. |
| Indoor Air Quality (IAQ) | Indoor air quality. |
| Infiltration Basins & Trenches | Pond or trench designed to encourage subsurface infiltration of water through temporary storage. Usually require pre-treatment to remove sediment and oil. |
| Infrared Emittance | The ability of a material to reradiate infrared radiation in the range of 5 - 40 micrometers. A perfect emitter has a value of 1.0. Most building materials have an emittance of approximately 0.9. Clean, untarnished metals have low emittance values. |
| Integrated Design | An approach where the design of each system takes into account and balances the design of other systems. Often an interdisciplinary approach, integrated design should begin at the earliest stage of a project with a guiding set of principles. |
| Intensive Green Roofs | Deep soil roof, with irrigation system that is capable of supporting a wide diversity of plants and habitats. |
| Intergrown Knot | A knot whose rings of annual growth are completely intergrown with those of the surrounding wood. |
| Interior Plywood | A general term for plywood manufactured for indoor use or in construction subjected to only temporary moisture. The adhesive used may be interior, intermediate, or exterior. |
| Interlocked-Grained Wood Grain | Grain in which the fibers put on for several years may slope in a right-handed direction, and then for a number of years the slope reverses to a left-handed direction, and later changes back to a right-handed pitch, and so on. Such wood is exceedingly difficult to split radically, though tangentially it may split fairly easily. |
| Internal Stresses | Stresses that exist within an adhesive joint even in the absence of applied external forces. |
| Interphase | In wood bonding, a region of finite thickness as a gradient between the bulk adherend and bulk adhesive in which the adhesive penetrates and alters the adherend's properties and in which the presence of the adherend influences the chemical and/or physical properties of the adhesive. |
| Intumesce | To expand with heat to provide a low-density film; used in reference to certain fire-retardant coatings. |
| Isotropic | Exhibiting the same properties in all directions. |