http://www.buildingscience.com Documents that have been recently added to the website, with the most recent first for each document type. daily 1 2010-02-15T02:42:46Z http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies An edited version of this Insight first appeared in the ASHRAE Journal. Sometimes in order to do the right thing you have to do “a workaround.” I love the “I-Codes.” But they can drive you crazy. I love them because most of the time they are right. In fact almost always they are right. But, every now and then . . . No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z BSC Article http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies Attics or roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone (Map 1). The choice of venting or not venting is a design and construction choice not a requirement determined by the physics or by the building code. The model building codes allow both vented and unvented roof assemblies. The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice. No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Link Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies Two seemingly innocuous requirements for building enclosure assemblies bedevil builders and designers almost endlessly: keep water vapor out, let the water vapor out if it gets in. No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Link Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies Low-permeance vapor barriers are widely used on the interior of wall and roof systems in large parts of North America. Many codes and standards imply or even state that low-permeance vapor barriers should be used in all cold regions as well as many moderate climate zones. The influence of vapor barriers on the hygrothermal performance of wall and roof systems is a function of exterior climate, interior climate, solar absorptance, rainwater absorption, and the vapor and thermal resistance of all of the layers in the system. In many practical situations, a low-permeance vapor barrier will not improve hygrothermal performance and may in fact increase the likelihood of damaging condensation or trap moisture in the system. This paper will examine the role of vapor barriers on hygrothermal performance with the aid of simple and transparent diffusion calculations supported by measurements from full-scale natural exposure monitoring. The phenomenon of summertime condensation, the drying of roofs and walls, and multiple vapor barrier layers will be explored. The importance of properly assessing both the interior and exterior climate will be discussed. Vapor diffusion control strategies will be presented. No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Link Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate, the characteristics of the materials that comprise the assembly and the interior conditions. Vapor barriers are not typically intended to retard the migration of air. That is the function of air barriers. No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Link Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Inner Content Container http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies Roofs can be designed and constructed to be either vented or unvented in any hygrothermal zone. Air barrier systems are typically the most common approach, however, air pressure control approaches are becoming more common especially in cases involving remedial work on existing structures. No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Link Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z File Content Inner Proxy http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Inner Content Container http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies No publisher Joseph Lstiburek vapor permeance ASHRAE moisture-Vapor Permeance relative humidity doc-Insight moisture-Physics of Moisture Control vapor barrier 2012-05-14T20:50:40Z Image Content Inner Proxy