Vented Attics & Vented Crawl Spaces
(From: Building Science Corporation - Joe's Top Ten :List of Dumb Things to Do in the South)

"My definition of an unvented attic is an attic where there are no vents and where the attic insulation follows the slope of the roof sheathing thereby including the attic space within the conditioned building enclosure.

The rationale for venting attics in the South is to "flush" heat. The dominant heat transfer mechanism in an attic is radiation. Venting attics will not "flush" radiation. The air change in a perfectly built and vented attic (code 1:300 ratio) results in an average air change rate of 3 to 6 ach. At this attic air change rate there is approximately a 2 to 3 percent reduction in heat transfer to the conditioned space through the vented attic as compared to an unvented attic insulated to the same level. This assumes an airtight ceiling and no ductwork in the attic and certainly not leaky ductwork in the attic. The moment ductwork (assumed airtight in this instance and insulated at R-6) is installed in a vented attic, the balance changes. There is approximately a 5 to 7 percent increase in heat transfer to the conditioned space as compared to my version of an unvented attic. This is due to conductive heat gains through the surface of the ductwork and air handler now located in a "hostile" location (a hot, vented attic), rather than inside a 75°F conditioned space (the "house"). The moment leaky ductwork is installed in a vented attic there is approximately a 25 percent increase in heat transfer to the conditioned space. Of course this does not happen if you have airtight ducts and an airtight ceiling (then the penalty for venting the attic is only 5 to 7 percent as previously noted).

Now, if you locate the ducts within the conditioned space and also build an airtight ceiling, this is approximately 2 to 3 percent more efficient than my version of an unvented attic. I never said that this wasn't the most energy efficient way to do it. Of course when is the last time you saw ductwork below an attic ceiling coupled with an airtight attic ceiling? Builders put things in attics because they don't leave any room in the house for the ductwork and air handler. If they continue to do this, then venting attics is a dumb idea.

So much for the energy concerns. Now lets talk moisture. What? Are you all crazy? The air outside is hot, humid and disgusting. And you want to bring this into an attic where it can diffuse through the vapor barrier-less attic insulation and get to the cold, air conditioned ceiling? What were we thinking! Before it gets there it will see those cold R-6 insulated ducts, fittings, etc. and drip all over. Give me a break. Venting attics in the South was dreamed up by some disgruntled Yankee pissed about the Civil War and wanting to get even. Be sure when insulating at the roofline in humid climates to follow moisture control principles as you would with any insulated wall so that the roof assembly is self-drying (www.buildingscience.com/resources/walls/exterior_sheathing_systems.pdf).

Lets now talk about durability of shingles and shingle temperature. Venting or non-venting a roof has about a 5 percent impact on shingle temperature and roof sheathing temperature and even less on shingle durability. The color of the shingle is more important than venting or non-venting. And temperature is less important than the shingle getting a sunburn. The biggest impact on shingle durability is ultra-violet light. UV is more critical than temperature. The best roof for hot, humid climates for all applications (including unvented attics) is a concrete or clay tile roof. Period.

Crawl spaces are real simple to understand and deal with. When you vent crawl spaces you bring in hot, humid air and cause moisture and mold problems. The ground surface is typically colder than the dew point temperature of the exterior air. The underside of crawl space floor insulation is radiation coupled to the ground surface and is very close to the same temperature of the ground. Moisture droplets can be seen all over the top surface of typical polyethylene ground covers as well as hanging from the bottom surface of the crawl space floor insulation. Gee, I wonder how all the water got through the poly ground cover? It must have leaked through the walls. Give me another break. Now, when the moisture is in the insulation where do you think it wants to go? Where is the air conditioning? Moisture moves to the cold surface. Venting crawl spaces made sense only when you had no air conditioning and no insulation and no crawl space walls.

Interested in unvented design strategies? It will take a while to change common practice as builders and contractors learn to adopt new approaches. Be sure to consult local experts and code officials before attempting unvented attics or crawl spaces on your own. Many design details that cannot be covered here are important to achieve best performance. See Houses That Work (www.housesthatwork.com) for more information."

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House Design Recommendations by Climate Region

In 2001, the Building America Building Science Consortium developed the Web-based Houses That Work (HTW) as a climate-specific technical resource for designing and building homes that are 30% or more energy efficient than the 1993 Model Energy Code (MEC). The resource reflected the experience gained from five years of Building America teamwork, including insights gained during the construction of more than 8,000 production homes from across the country. Houses That Work was and is a freely-accessible learning resource and reference for builders, building product manufacturers, building researchers, and the general public.

We call this Houses That Work II. HTWII is more comprehensive and more detailed and represents the latest Building Science Consortium experience and results under the Building America program. Specifically HTWII includes:

• Updated North American hygro-thermal regions map that is aligned with the Department of Energy Model Energy Code proposed climate zones and map.

• Climate-specific Best Practices with performance criteria for high performance home design and construction.

• Three Building Profiles per climate.

• A Building Materials Property Table¹ (populated with technical and performance specifications from product manufacturers and building research members of the Building Science Consortium).

HTW II has five sections - an overall introduction and a section for each of five hygro-thermal regions: Hot-Humid, Mixed-Humid, Cold, Very Cold and Hot-Dry/Mixed-Dry. The introduction contains the hygro-thermal map and each climate section contains Best Practices and Building Profiles. See the table below for a quick reference for the components in each hygro-thermal region's building profiles.