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climates

Very Cold - A very cold climate is defined as a region with approximately 9,000 heating degree days or greater (65°F basis) or greater and less than 12,600 heating degree days (65°F basis).

Cold - A cold climate is defined as a region with approximately 5,400 heating degree days (65°F basis) or greater and less than approximately 9,000 heating degree days (65°F basis).

Mixed-Humid - A mixed-humid and warm-humid climate is defined as a region that receives more than 20 inches of annual precipitation with approximately 4,500 cooling degree days (50°F basis) or greater and less than approximately 6,300 cooling degree days (50°F basis) and less than approximately 5,400 heating degree days (65°F basis) and where the average monthly outdoor temperature drops below 45°F during the winter months.

Hot-Humid - A hot-humid climate is defined as a region that receives more than 20 inches of annual precipitation with approximately 6,300 cooling degree days (50°F basis) or greater and where the monthly average outdoor temperature remains above 45°F throughout the year. This definition characterizes a region that is similar to the ASHRAE definition of hot-humid climates where one or both of the following occur:

  • a 67°F r higher wet bulb temperature for 3,000 or more hours during the warmest six consecutive months of the year; or
  • a 73°F or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive months of the year.

Hot-Dry/Mixed-Dry - A hot-dry climate is defined as region that receives less than 20 inches of annual precipitation with approximately 6,300 cooling degree days (50°F basis)or greater and where the monthly average outdoor temperature remains above 45°F throughout the year.

A warm-dry and mixed-dry climate is defined as a region that receives less than 20 inches of annual precipitation with approximately 4,500 cooling degree days (50°F basis) or greater and less than approximately 6,300 cooling degree days (50°F basis) and less than approximately 5,400 heating degree days (65°F basis) and where the average monthly outdoor temperature drops below 45°F during the winter months.

Marine - A marine climate meets is defined as a region where all of the following occur:

  • a mean temperature of the coldest month between 27°F and 65°F;
  • a mean temperature of the warmest month below 72°F;
  • at least four months with mean temperatures over 50°F; and
  • a dry season in the summer, the month with the heaviest precipitation in the cold season has at least three times as much precipitation as the month with the least precipitation.

information

Building Science Insights are short discussions on a particular topic of general interest. They are intended to highlight one or more building science principles. The discussion is informal and sometimes irreverent but never irrelevant.

Building Science Digests provide building professionals from different disciplinary backgrounds with concise overview of important building science topics. Digests explain the theory behind each topic and then translate this theory into practical information.

Published Articles aare a selected set of articles written by BSC personnel and published in professional and trade magazines that address building science topics. For example, our work has appeared in Fine Homebuilding, Home Energy, ASHRAE's High Performance Buildings, The Journal of Building Enclosure Design and The Journal of Building Physics. We thank these publications for their gracious permission to republish.

Conference Papers are peer-reviewed papers published in conference proceedings.

Research Reports are technical reports written for researchers but accessible to design professionals and builders. These reports typically provide an in-depth study of a particular topic or describe the results of a research project. They are often peer reviewed and also provide support for advice given in our Building Science Digests.

Building America Reports are technical reports funded by the U.S. Department of Energy (DOE) Building America research program.

Designs That Work are residential Case Studies and House Plans developed by BSC to be appropriate for residential construction in specific climate zones. Case Studies provide a summary of results for homes built in partnership with BSC’s Building America team. The case study typically includes enclosure and mechanical details, testing performed, builder profile, and unique project highlights. House Plans are fully integrated construction drawing sets that include floor plans, framing plans and wall framing elevations, exterior elevations, building and wall sections, and mechanical and electrical plans.

Enclosures That Work are Building Profiles and High R-Value Assemblies developed by BSC to be appropriate for residential construction in specific climate zones. Building Profiles are residential building cross sections that include enclosure and mechanical design recommendations. Most profiles also include field expertise notes, material compatibility analysis, and climate challenges. High R-Value Assemblies are summaries of the results of BSC's ongoing High R-Value Enclosure research — a study that BSC has undertaken for the U.S. Department of Energy (DOE) Building America research program to identify and evaluate residential assemblies that cost-effectively provide 50 percent improvement in thermal resistance.

Guides and Manuals are "how-to" documents, giving advice and instructions on specific building techniques and methods. Longer guides and manuals include background information to help facilitate a strong understanding of the building science behind the hands-on advice. This section also contains two quick, easy-to-read series. The IRC FAQ series answers common questions about the building science approach to specific building tasks (for example, insulating a basement). The READ THIS: Before... series offers guidelines and recommendations for everyday situations such as moving into a new home or deciding to renovate.

Information Sheets are short, descriptive overviews of basic building science topics and are useful both as an introduction to building science and as a handy reference that can be easily printed for use in the field, in a design meeting, or at the building permit counter. Through illustrations, photographs, and straightforward explanations, each Information Sheet covers the essential aspects of a single topic. Common, avoidable mistakes are also examined in the What's Wrong with this Project? and What's Wrong with this Practice? mini-series.

Research Reports
Joseph Lstiburek, Christopher Schumacher

This report summarizes hygrothermal analysis of specific attics constructed in California. The analysis was done using historical experience, published work in journals and trade publications, current building code requirements and WUFI hygrothermal simulations to assess benefits and risks associated with insulating the roof decks in both vented and unvented configurations. The focus of this report is on modifying conventional, ventilated attics, constructed with impermeable roof shingles (with fiberglass batt insulation on the ceiling plane) by adding fiberglass batt (or netted fiberglass or netted cellulose or spray applied fiberglass) insulation to the underside of the roof deck (i.e. on the slope) while leaving the attic air space ventilated to outdoors.

Hot-Dry/Mixed-Dry
Research Reports
Armin Rudd, Kohta Ueno, Joseph Lstiburek

Unvented-cathedralized attics are known to be advantageous in hot-humid and hot-dry climates, whereby, exterior moisture would be excluded for hot-humid climates, and attic mounted air distribution systems would be inside conditioned space for both climates. Current work focuses on the performance and durability of unvented-cathedralized attics in hot-humid climates with both tile and asphalt shingle roofing. The advantages for the hot-humid climate are expected to be even greater than for the hot-dry climate.

Hot-HumidHot-Dry/Mixed-Dry
Research Reports
Armin Rudd, Joseph Lstiburek

Sealed attic construction, by excluding vents to the exterior, can be a good way to exclude moisture-laden outside air from attic and may offer a more easily constructed alternative for air leakage control at the top of residential buildings.

Hot-Dry/Mixed-Dry
Research Reports
Armin Rudd, Joseph Lstiburek

In cold climates, the primary purpose of attic ventilation is to maintain a cold roof temperature to avoid ice dams created by melting snow, and to vent moisture that moves from the conditioned space to the attic. In cooling dominated climates, the primary purpose of attic ventilation is to vent hot air, heated by solar gain on the roof, thus reducing the cooling load contribution from the roof. The magnitude of the roof cooling load contribution is often in the area of ten percent of the total cooling load for an occupied house.

Hot-Dry/Mixed-Dry
Research Reports
Joseph Lstiburek

Just about everyone in the building industry can be guilty of using building terms loosely, and a prime example is with attics, roof assemblies. You can vent single components, you can vent assemblies, and you can vent spaces. But doing one is not the same as doing the others.

Research Reports
Joseph Lstiburek

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. Vapor diffusion should be considered as a secondary moisture transport mechanism when designing and building roofs. Specific vapor retarders are often unnecessary if appropriate air movement control is provided or if control of condensing surface temperatures is provided.

Research Reports
Building Science Corporation

When constructing unvented roofs with asphalt shingles in hot humid climates, a vapor barrier must be installed between the asphalt shingles and the roof deck. Asphalt shingles store water from dew (condensation due to night sky cooling) and rain. Asphalt shingles are a reservoir for water and behave similarly to traditional wood shingles and cedar shakes in that regard. With traditional wood shingle and cedar shake roofs, a receptor for this moisture is typically provided. This receptor is an air space that can be accomplished in a variety of ways.

Research Reports
Kohta Ueno

This article was written to tie together and summarize the various papers on unvented conditioned cathedralized attics found on our website. We realize that there is a wealth of information, and much of it too detailed to understand or digest in a single sitting. Furthermore, building officials might not have the time available to carefully examine the many documents on the page; this is meant to summarize the main arguments, and provide pointers to where detailed information and measured data can be found.

Research Reports
Joseph Lstiburek

Unvented roof systems can be safely used in many different climates. In cold climates, insulating sheathing must be added exterior to the roof sheathing to prevent condensation on the underside of the roof sheathing.

Published Articles
Joseph Lstiburek

Understand when to vent your roof and when not to, and how to execute each approach successfully. Reprinted with permission from Fine Homebuilding, Aug/Sept 2011, pages 68-72.

Published Articles
Armin Rudd

By creating a path for air to move, structural vents are supposed to prevent the buildup of moisture in an attic. This article was first published in Builder Magazine, January 2006.

Published Articles
Armin Rudd

This article reports on field experience of unvented cathedralized (UC) attics in the U.S. Traditionally, in some regions of the country, slab on grade construction is a preferred mode of construction. Mechanical equipment for air conditioning and distribution ducts are usually located in the attic spaces to conserve space. Conventional construction involves providing insulation on the floor of the attic and venting the attic space to the outside leading to  loss in efficiency in operation of the equipment and through duct leakage. Insulating the attic roof itself and blocking of ventilation to the outside transfers the air and thermal energy controls from the boundary with the living space to the plane of the roof. The air distribution systems now fall within conditioned space, which increases their efficiency, durability, and maintainability. This article was first published in the Journal of Building Physics, Vol. 29, October 2005.

Guides and Manuals
Betsy Pettit, Ken Neuhauser, Cathy Gates
The purpose of this guide is to provide useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate....
Cold
Guides and Manuals
Building Science Corporation
This guide describes ways to meet the roof venting requirements as per building code while converting the attic into living space by moving the insulation to the underside of the roof deck between...
Conference Papers
Kohta Ueno, Joseph Lstiburek

 

Unvented roof assemblies can bring attic mechanical systems into conditioned space, negating ductwork losses. However, in previous work, unvented roofs with air-permeable (fibrous) insulation, instead of air-impermeable insulation (spray foams) have shown localized moisture accumulation at the ridge. This research is a test implementation of two unvented roof assemblies insulated with air-permeable insulation (netted and blown fiberglass or adhered fiberglass) in hot/humid climates. One test roof is located in Houston, TX and has asphalt shingles; the other is in Orlando, FL, with concrete tile; both are in Climate Zone 2A. Given that localized moisture accumulation and failures occurred at the ridge in previous unvented roofs, a diffusion vent (open to water vapor but closed to airflow) was installed at the highest points in the roof assembly to allow for the wintertime release of moisture. The diffusion vent is an opening at the ridge and hips covered with a water-resistant but vapor-open membrane. As a control comparison, portions of the roof were constructed as a typical unvented roof (self-adhered membrane at ridge).

Collected data indicate that the diffusion vent roof shows greater moisture safety and less wintertime moisture accumulation than the conventional, unvented roof design. The unvented roof had winter periods of 95%–100% rh, with other sensors indicating possible condensation; high moisture levels were concentrated at the roof ridge. In contrast, the diffusion vent roofs had drier conditions. In the spring, as outdoor temperatures warmed, all roofs dried well into the safe range (10% MC or less).

Hot-Humid
Conference Papers
Christopher Schumacher, Ed Reeves

Unvented cathedralized attic (UCA) assemblies, created by eliminating ventilation and by moving the thermal insulation and air barrier from the ceiling plane to the rafters, immediately below the roof deck, are increasingly common in low-rise residential construction in the hot-humid and hot-dry southern United States. Unvented cathedral ceilings (UCCs) are similar to UCAs with the exception that the interior finish is also installed on the underside of or between the rafters rather than on the underside of the ceiling joists or collar ties.  The test program described in this paper sets out to determine whether or not an assembly that meets the new IRC code requirements but is constructed without a vapor barrier and using an air impermeable, vapor permeable, low-density, open-cell sprayed polyurethane foam insulation can perform satisfactorily in the cold wet climates of Seattle, WA and Vancouver, BC (Zone 4C).

Building Science Insights
Joseph Lstiburek
Simple physics…complicated language“Code world” is an interesting place where seemingly convoluted language is used to express simple concepts in clearly complicated ways.  The reasons for this are...
Building Science InsightsNewsletters
Joseph Lstiburek
Sometimes the obvious is not so obvious. And sometimes the not so obvious becomes obvious. For example installing leaky ductwork1 in a vented attic is a pretty dumb idea (Figure 1). It leads to...
Building Science InsightsNewsletters
Joseph Lstiburek
For all that we know about roofs–which is a great deal–sometimes things can get confusing. I am more than partly to blame for that. I wrote a lot of the code language dealing with both vented and...
Building Science InsightsNewsletters
Joseph Lstiburek
“What we’ve got here is failure to communicate . . .”1In what is turning out to be an unfortunate turn of phrase the terms “unvented attics” and “unvented roofs” have entered the lexicon. A lot of...

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