How much should seams between sheets of vapor retarder be lapped?

The correct answer is that seams between sheets of vapor retarder should be lapped 6 inches. This 6-inch overlap is crucial in ensuring an effective barrier to moisture, helping to prevent condensation within the building envelope. Proper lap dimensions help maintain the integrity of the vapor retarder by ensuring that moisture cannot easily penetrate through gaps or seams. A 6-inch overlap is widely recognized in building codes and best practices as a standard for effective vapor control.

When using vapor retarders, ensuring that seams are adequately overlapped is essential for maintaining a continuous barrier against water vapor movement. If the overlap were smaller, such as 2 or 4 inches, it might not provide enough assurance against moisture infiltration, leading to potential issues like mold growth or damage to building materials. Meanwhile, an 8-inch lap, while potentially providing a greater barrier, is generally seen as unnecessary and may add complexity to the installation without providing substantial additional benefit. Thus, adhering to the standard of a 6-inch overlap is both practical and effective for this application.