A vapor barrier is any sheet or material that offers great resistance to the passage of water vapor. They are widely used in construction to avoid interstitial condensation.

At sea level, the atmosphere has an average pressure, depending on the units chosen, of 1013 millibars,[1]​ 760 mm Hg, or 101,325 Pa "Pascal (pressure unit)"),[2]​ and contains on average approximately 1% of water vapor in suspension.

But the amount of water vapor that the air can contain varies with temperature: at 20 °C, the maximum partial pressure of water is 17.50 mm Hg (2,333 Pa); the air, and the maximum amount of water that there can be. If the water vapor exceeds the maximum amount (100% relative humidity), the water vapor condenses and precipitates (dew point).

Imagine the following situation: In a house, in winter, a person is taking a shower. The temperature in the bathroom can easily exceed 20°C, and the relative humidity will probably reach 100%. However, outside, the temperature may be 0°C. Water vapor filters through the wall that separates the bathroom from the street, but as it passes through the thermal insulation, the temperature gradually goes from 20 °C to 0 °C, so the air can no longer contain so much vapor, and it condenses on the insulation, wetting it and causing it to lose its properties in materials such as rock or glass wool. This is not the case for cotton-based insulators. This is when the vapor barrier becomes necessary, to contain the passage of vapor towards the insulation, thus allowing it to remain dry.

Vapor barriers are not completely impermeable to water vapor: they are materials with low permeability to it. In practice, the concept of permeance is also used, which includes the thickness as well as the material's own permeability, since a sheet of the same material, but double thickness, will be doubly effective.

Vapor barriers can take a multitude of forms, since by definition, any material with very low permeability is considered a barrier. The NBE-CT-79 (recently replaced by the CTE "Technical Building Code (Spain)") considered vapor barrier any sheet with resistance to water vapor (Rv) greater than or equal to 10 MN*s/g., with an upper limit of 230 MN*s/g. Thus we have as common examples: