What Type of Eco-Home Is Best For You?
(Page 2 of 5)
July/August 2008
By Carol Venolia
COB is a mixture of clay, sand, water and straw hand-sculpted into walls without formwork or machinery. It allows for highly organic forms and lends itself to construction parties. There is no building code for cob in the United States, but many cob homes have been built in areas with owner-builder codes. Cob can be relatively inexpensive for DIYers.
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Straw building methods
Straw is the stalk that’s left after the seed heads are removed from hay. As an agricultural waste product that also makes good insulation, it’s a natural for an eco-home. But, as with any fibrous material, moisture can be problematic in straw building.
STRAW BALE construction has captured hearts worldwide. Relative amateurs can stack the highly insulating walls in one community work day, and the addition of thick earthen plaster adds thermal mass. The system can be inexpensive if you provide your own labor, but poorly shaped bales may require subcontractor work-arounds, increasing costs.
“I love straw,” says architect Kelly Lerner (www.One-World-Design.com), “but if you’re in a very wet climate, you will need to design carefully. Provide a deep roof overhang and keep the walls well above the ground. If you have an exposed site with wind-driven rain, straw bale walls may not be the best option.” Bale walls might not be best for hot, humid environments, where insulation is less valuable than open structures that admit breezes.
CLAY-STRAW involves packing loose, clay-coated straw into structural forms that are later stripped. As with bale walls, clay-straw needs protection and isn’t appropriate in tropical or driving-rain environments. Long used in Europe, clay-straw techniques have been developed extensively in this country by Robert Laporte and Paula Baker-Laporte (www.EcoNest.com).
“Clay-straw balances insulation and thermal mass,” Baker-Laporte notes. “We often put more clay for thermal mass in the south walls of a house and more straw for insulation in the north walls.”
Building with composite materials
These materials require little or no wood; can be assembled by most construction professionals; encounter little resistance from building code officials; create airtight, energy-efficient buildings; and provide flat, planar surfaces. The materials are high in embodied energy (the total energy that goes into creating and delivering a product), but the energy savings over the lifetime of a composite building is even greater. One thing to keep in mind: Concrete composites inhibit moisture passage, so they require careful ventilation strategies to inhibit mold growth.
STRUCTURAL INSULATED PANELS (SIPs) consist of a core of rigid foam insulation surrounded by a composite board (usually oriented-strand board, or OSB). Precut in the factory, they’re delivered to the jobsite by truck and lifted into place by a crane. SIPs provide insulation but no thermal mass.
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