Insulation is material that slows the movement of heat through building surfaces. It keeps buildings warm in winter and cool in summer, reducing energy use and improving comfort. Insulation works by trapping air in small pockets, because still air is a poor conductor of heat.

There are many types. Fiberglass batts are common in residential walls and attics. Rigid foam boards are used on exteriors and roofs. Spray foam fills gaps and seals air leaks. Cellulose, mineral wool, and cotton are alternatives with different environmental profiles.

Performance is measured by R-value, which indicates resistance to heat flow. Higher R-values mean better insulation. But R-value is not the whole story. Air leaks, thermal bridges, and moisture can undermine insulation performance. A well-insulated wall with a gap around a window still loses heat.

Insulation also affects acoustics and fire safety. Some materials are better than others at blocking sound or resisting fire. The choice depends on the application.