One problem we've discovered in the SE when it comes to tightening up the envelope is how to deal with a basement or crawlspace. The Atlanta area is very hilly, so slab on grade is often excluded as an option. Basements are best designed as a "walk out" down here, but many sites are small and don't have enough elevation change. Crawlspace construction is often the only choice.
Building scientists recommend building crawlspaces as short insulted basements (if you have to build them), which works great in less humid climates with lower water tables and less monsoon like rain seasons. When we first began sealing the crawlspaces, we found that no vapor barrier or waterproofing membrane was good enough to keep vapor from making it's way from the soil into the crawl. The first sign of too much vapor is your hardwoods cupping. Sealing the crawl, of course traps this vapor and too much ventilation defeats the purpose of air sealing. Dehumidifiers can use $50 of electricity per month and some our houses only consume this much without the dehumidifier which sort of hurts the green pride. Small mini split units can dehumidify for less cost and are slightly more expensive than the dehumidifier. It seems that every time you find a new remedy, you find another problem to address. Hot, humid climates are some of the most difficult to build in and then you have the termites that like to eat slab insulation...
Most of our green building science comes from our friends in Canada, but you have to understand your locale before you start implementing the latest technique you hear about because dry freezing winters are one extreme to address, but a wet, hot summer climate is another animal all together. With residential work, you have to be very careful not to get sucked into overdoing it. Appraisals limit construction cost for the majority of people, so bang for the buck solutions are a must if we expect to make these practices mainstream. Enthusiastic clients will often insist on spending a ton of money on something that gives you a marginal return on the investment, so be careful and steer them in an appropriate direction. In Canada, a ton of insulation in the walls and roof is necessary which causes wet wall syndrome if you don't use enough foam sheathing to resist vapor drive. In our area, lots of wall insulation is nice, but not necessary. Foam wall sheathing is better, but has less impact in the SE because we don't have -30F outside and 70F inside causing the vapor drive. In order to direct money to other green features, we will often spray foam a roof and deck bands, batts in the wall (no foam sheathing), house wrap and still get our air exchanges well below Energy Star standards and a respectable performing envelope for the cost.
A more Laconic way of saying all of that is...Be practical or green will remain an exclusive club.
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Eric Rawlings AIA
Owner
Rawlings Design, Inc.
Decatur GA
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Original Message:
Sent: 09-10-2013 11:39
From: Jonathan Herz
Subject: Climate Shift and Vapor Condensation
This is a great observation that should have broad impacts over a wide range of building materials now being used. It would be nice to have some data to go with some of these assertions about mold and rot. Are any available or under study?
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Jonathan Herz AIA
DHHS Office for Facilities Management & Policy
Washington DC
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