Condensation in Attics: Are vapor barriers really the answer?

Abstract

Calculations of water vapor flow through walls and ceilings are frequently based on the permeabilities of building material, and implicitly assume that most of the vapor transport takes place by diffusion. In order to determine the validity of this assumption, a model for vapor transfer between two spaces by diffusion and convection is first developed. Measurements in a number of houses reveal that, for normal construction practices in U.S. wood frame houses, the transfer of water vapor from living space to attic is almost entirely by air movement. “Kraft” paper “vapor barriers”, frequently attached to batt insulation, do not effectively hinder air (or moisture) flow into attics. Even after numerous cracks and openings in the attic floor, below the insulation, were plugged, significant air flow from living space to attic remained, and evidence of condensation of water vapor within the attic was observed. In new housing, a continuous sheet placed between the ceiling and the framing of the attic floor should effectively hinder air and moisture flow. In existing housing, where it may not be possible to install a continuous sheet under the attic insulation, adequate opening for ventilation with outside air should be provided to prevent moisture build-up and condensation.