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Foundations are typically one of three types: full basement, crawlspace, or slab-on-grade. Basements extend the farthest underground, and are typically used in areas where there are deep frost lines and low water tables. A home addition will often have a craw/space — essentially, a basement with shorter walls one to three feet deep. Slab-on-grade is just that: a cement slab on top of the earth. Most foundations are made of cement, although there are various mixtures you can add to the cement (such as fly ash) to reduce the overall cement content (and the impact on our natural resources), and to strengthen the cement.
Insulating foundations is important — an uninsulated foundation may account for up to 50 percent of the heat lost from an otherwise tightly sealed, well-insulated house in a cold climate. Proper installation of foundation insulation material is necessary to avoid the condensation, material damage, and structural decay caused by the difference in temperature between the house interior and the adjacent earth. Poor design may also aggravate radon infiltration and insect infestation.
Slabs or crawlspaces can be insulated on the interior or exterior, and there are various advantages and disadvantages to each option (see Table below). Whereas other rigid foam materials absorb water, extrudable polystyrene (XPS) is ideal for exterior insulation because water can't penetrate the material. If a foundation is insulated on the interior, batt or wet-spray is traditionally used, and this is then covered with drywall. Rigid foam insulation boards such as extruded polystyrene (XPS), expanded polystyrene (EPS) or poly-isocyanurate insulation boards might also be used on the interior. (See Section 14 to compare the environmental impacts of these insulations.) Most insulation boards are then covered with drywall.
Exterior foundation insulation uses only XPS, also known as closed cell insulation, directly on the outside of the basement walls. The insulation is then covered with a material like a cementitious board or stucco finish. You might also use a foam-form foundation system in which polystyrene foundation forms are set on conventional footings, much like building a child’s Lego wall. Concrete is poured into the core of the forms, where it cures to form the structure and thermal components of the basement wall.
We will describe the various types of insulation and their respective environmental and health risks more completely in Section 14.
Table of Interior Versus Exterior Insulation
The Foundation as a System
The following green alternatives deal primarily with the foundation as a whole system, and with its structural materials. They are intended to help you choose among the many systems available, and to ensure that you understand the impact of your choice.
$ = cost effectiveness; EE = energy efficiency; RC = resource conservation; HB = health benefit
Excavate Frost Protected Shallow Foundations (FPSF) - $, RC, EE
• The U.S. consumes over 120 million metric tons of cement every year. However, you can minimize the amount of cement used for your addition when you reduce your excavation, foundation wall depths, and slab width. Choosing a FPSF typically results in a reduction in concrete use of up to 50 percent. In 1999, the National Association of Home Builders (NAHB) estimated there were 3,000 FPSFs in the U.S. with builders reporting about a 40 percent savings in foundation costs. Moreover, excavating to 16 inches rather than 48 inches will result in significantly less soil removal and piling on the site, thereby reducing soil compaction and vehicle disturbance. Finally, the NAHB Research Center claims that the required insulation levels for a frost protected shallow foundation make your home more energy efficient, exceeding existing code requirements for foundation insulation. By placing insulation horizontally two to four feet from the foundation, heat loss can help keep the ground from freezing around the foundation, reducing freeze-thaw cycle damage.
• Recommendation: Excavate the foundation to i6 inches rather than the 36 - 48 inches typical for a cold climate application. Ask your building code official about appropriate depth of the foundation for your specific project.
Use Concrete Containing Recycled Waste (i.e., slag, bottom ash, fly ash) - $, HC, RB
• The manufacture of cement is contributing more to global warming than most other building materials: The CO2 emissions produced for US cement consumption is equivalent to the CO2 produced by 22 million passenger cars. Recycled fly ash, a coal-fired power plant waste product, can replace up to o percent of Portland cement used in conventional concrete — thereby decreasing the overall environmental impact of cement production — and will also increase the strength, water resistance, and durability of the concrete. Other industrial and agricultural waste products, including ground blast furnace slag and rice hull ash, can also be used to replace some of the Portland cement in concrete.
• Recommendation: Typically, 15 to 50 percent of cement can be replaced with fly ash or other industrial waste products in concrete mixes. Note that it must be cured longer than standard concrete.
Use Autoclaved Cellular Concrete (ACC) – RC, EE
• ACC is a lightweight precast concrete product, usually manufactured as blocks or panels, which can reduce the total amount of concrete used and the negative environmental impacts associated with concrete. The ACC process uses aluminum to aerate and foam the concrete, which is then steam-cured in an autoclave to create a high strength-to-weight ratio material. ACC insulates much better than concrete and has very good sound absorbing characteristics.
• Recommendation: Check with local codes for structural applicability, and if possible, choose ACC in place of concrete. (It can be assembled with regular masonry methods and tools.) ACC is typically more available in the southern US and California.
Use Insulated Concrete Forms (ICFs) - $, RC, EE
• Lightweight interlocking rigid foam blocks or panels hold concrete in place during curing and remain in place afterwards to serve as thermal insulation for concrete foundations and /or above-grade walls. Foam form systems have higher thermal efficiencies than solid concrete foundations or walls, and can reduce the total amount of concrete used, yielding material cost savings. Unlike untreated lumber, ICFs are not subject to rot and result in a higher strength wall than standard cast-in-place concrete. Most ICFs have metal or plastic ties that will hold a screw so that you don’t have to frame a wall to install the drywall, thereby saving resources and money.
• Recommendation: Use rigid foam forming systems wherever an insulated foundation is desirable. Sizes and styles vary by manufacturer, but generally, rebar is placed in the hollow foam cores and concrete (preferably containing slag or fly ash) is poured into the cores to create a load-bearing structure.
FYI: Insulation R values -- R-values, used to rate insulation, are a measurement of the insulation’s resistance to heat flow. The higher the R-value, the better the insulation.
Reuse Form Boards - $, RC, EE
Forms are used whenever a slab is poured, and form boards are used to mold the foundation. The boards are often two-by-tens or larger solid- lumber made from old-growth trees. Reuse of forms saves money and conserves resources, as large dimension, solid-sawn lumber is becoming increasingly expensive and scarce.
• Recommendation: By carefully removing and separating the forms, they can be reused several times. Use non-toxic form release treatment to make it easier to remove the form, minimizing the risk of splitting the form during removal.
Use Metal Forms - RC
• Metal forms, like wood forms, are used to mold the foundation. They come in all sizes and shapes, and produce a smooth finished surface on the concrete. Metal forms can be reused almost indefinitely, thereby reducing wood use and the cost of buying new forms.
• Recommendations: Metal forms can be used in most applications where wood forms are used.
Use Biodegradable, Non-petroleum Form-release Agents – RC, HB
• Concrete form-release agents are products used to remove the cement structure from the form used to mold it. They are typically made from petroleum byproducts, and can be a major source of VOCs and soil contamination. Biodegradable or rubber-based, non-petroleum form- release agents meet or exceed the EPA’s VOC and toxicity regulations. They are better for your health and provide a smoother finished surface, with fewer “bug” holes than conventional form release agents.
• Recommendation: Use biodegradable, non-petroleum form-release agents wherever concrete form agents are specified. Water-based products must be protected from freezing temperatures during storage and application.
Use Non-asphalt-based Biodegradable Damp-Proofing – HB, RC
• Damp proofing materials protect your foundation from water intrusion and decay, but the petroleum in asphalt damp proofing can leak into the soil and ground water. Synthetic rubber and cement-based damp proofing products are available that don't contaminate soil and ground water.
Recommendations: Use synthetic rubber and cement-based damp proofing instead of traditional damp proofing products.
Install Non-vented Crawlspaces – EE, HC, $
Non-vented crawlspaces require less insulation than vented crawlspaces because there is less air infiltration and energy loss. In the summer, the non-vented crawlspace stays cooler, thereby helping to keep the adjacent living space cooler. In winter, the non-vented crawlspace will be drier, reducing the potential for mold. Since the crawlspace is not a living space, ventilation is an unnecessary and expensive feature.
Recommendation: Check with your building department to find out it they will approve non-vented crawlspaces. Many codes require low-volume mechanical ventilation. Install polyethylene sheeting with overlapping seams on the crawlspace floor and up the foundation wall to keep moisture out of the crawlspace.
Insulate Foundation Before Backfilling - EE
• All foundations, including slab floors, can be insulated to minimize heat loss from the floors and basement, a major source of energy loss and higher utility bills.
• Recommendation: Insulate the foundation with at least R- insulation.
Use Recycled Content Rubble for Backfill Drainage - RC
• Concrete and rubble can be crushed and used for backfill and drainage purposes at the base of foundations, saving money and natural resources.
• Recommendation: Use recycled materials for backfill. Make sure the foundation drain is on top of the footing, underneath the siding, and at the outside perimeter edge of the flashing.
Incorporate Radon Mitigation - HB
• Radon gas is one of the most dangerous contaminants you may find in the air in your home. The EPA has estimated that exposure to radon may be the second leading cause of lung cancer, after cigarette smoking. In the US, the average indoor radon level is 1.3 picocuries per liter (pCi/L), while average outdoor level is only 0.4 pCi/L. It is estimated that in the Us, 6 percent of all houses are above the threshold value of 4pCi/L; reducing levels in these homes would reduce the incidence of radon-related lung cancer by one third. The EPA and the Surgeon General recommend that all home owners test their radon levels and take action to increase ventilation if tests result in a radon level above 4pCi/L. (See “Radioactive Contaminants” in Section 5).
Recommendation: To reduce concentrations, place an airtight membrane under carpets or provide some form of sub-slab ventilation. Cover any exposed earth with a polyethylene air barrier and seal all cracks and joints in the foundation wall and floor slab with caulking or foam. Install a self- priming drain or gas trap in the floor drains leading to a sump or to drainage tiles. Remove radon from well water using activated charcoal filters or aeration units. If radon is suspected, lay perforated PVC pipe under the foundation floor for future retrofit of an above-ground radon mitigation system. For further information about radon, go to the EPA’s radon webpage.
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