This article was originally published in the March/April 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1999
|Figure 1. A typical concrete block wall system has no interior insulation, and is simpler to build.|
|Figure 2. A semifinished concrete block wall system includes an extra false wall and fiberglass insulation, making the space potentially habitable.|
|Figure 3. This encapsulated form foundation wall has a high R-value and performs extremely well with regard to moisture, air, and heat control.|
This system consists of a concrete strip footing on which a wall is constructed. The wall may be either concrete block (see Figure 1) or CIP concrete. A parget coat made up of Portland cement and sand is put on the exterior of the wall. A dampproof coating is applied above and below grade, along with a drain around the perimeter of the wall, embedded in graded gravel. Anchor bolts are attached to the wall to secure for the sill plate. The foundation wall can be reinforced with rebar cast into the concrete, especially where seismic load is a concern.
The R-value for a concrete block system ranges from R-1 to R-4, depending on the size of the block, whether the block is filled, and the density of the concrete. The cost of materials and labor runs anywhere from about $5.90/ft2 when using 8-inch block, to about $7.50/ft2 when using 12-inch block, based on national average costs.
An 8-inch uninsulated concrete block wall has a thermal material property of about 8 Btu/ft2. This means that when the temperature in the wall falls by 1°F, each square foot will have given off 8 Btu of stored heat. Because the wall is uninsulated, the thermal mass of the blockwork contributes little to the wall's R-value. Soil provides limited insulation and thermal benefits.
An eight-inch block has a two-hour fire rating with no flame spread. It is environmentally safe and inert with respect to off-gassing, volatile organic compounds (VOCs), and so forth. It requires little maintenance, if any. If painted, it may blister or peel as a result of moisture movement.
Semifinished Concrete Block
Better insulation is provided by the semifinished concrete wall system (see Figure 2). It uses the same techniques as the uninsulated concrete wall system, but insulation and drywall (without tape or paint) are added to the interior of the wall, making the basement potentially habitable. The construction procedure is the same as that used for the uninsulated system, except that a 2 x 4 woodframe false wall is built onto the inside of the block wall. Fiberglass batt insulation is put between the block wall and the false wall, which is then covered with 6-mil polyethylene vapor retarder and an insulation option such as 1/2-inch, untaped, unpainted, gypsum wallboard. The wallboard provides the requisite fire protection.
The R-value for this wall system is R-14. The average cost for a 10-inch block semifinished wall system (including materials and labor) is about $8.40/ft2. With the insulation on the interior side of the wall, the surface temperature of the drywall is relatively warm. This makes the occupants feel warmer and lessens the potential for surface condensation.
The advantage of this foundation system is that it makes for a cleaner, brighter space--one that is easily finished. The construction of a false wall also makes it easier to install services. This system would be a good choice for home buyers who want the potential extra living space.
Pouring concrete into walls that have been formed with plywood or steel is one of the most economical ways to construct a basement wall. The costs range from about $6.50/ft2 for a full-height 8-inch-thick wall to about $7.60/ft2 for a 12-inch-wall. The walls are relatively easy to erect, cast, and strip, though to go above an 8-foot height, additional forms must be scabbed on. This type of system tends to be the preferred choice of builders who have access to delivered, ready-mixed concrete, chutes, slings, conveyors, or pumps.
Because of the winter climate in the mid-Atlantic states, poured concrete can cause problems with proper curing and often requires some type of heated housing for the form work, which adds additional expense to the project. The additional time to form in windows, entrances, corners, partitions, doors, and built-in channels for electrical and plumbing is another reason many builders in this area do not use poured concrete. Future renovations are also difficult, requiring specialized tools and labor.
A solid concrete wall is less permeable to air, water, and water vapor than concrete block, but solid concrete also shrinks, and the walls often crack due to lateral loads and settlement. The walls need to be damp-proofed or water proofed as needed. The R-value of an 8-inch poured concrete wall is R-1, with two or more inches of insulation needed to increase the value. An 8-inch solid wall has a thermal mass of about 21 Btu/ft2/°F. It has a four-hour fire rating with zero flame and smoke spread.Insulated Concrete Forms Insulated concrete forms (ICFs) are made of expanded polystyrene (EPS) or extruded polystyrene (EXPS). ICFs are attractive to builders because the light-weight blocks or panels used to make them are easy to assemble, and they reduce construction time and transportation costs. The forms are left in place after casting, for both below-grade and above-grade walls. Currently, more than 23 companies in North America manufacture and distribute these forms.
The lightweight, modular blocks or panels provide the formwork for construction, the thermal insulation, and the air and vapor barriers. Webs made from galvanized steel, plastic, light-gauge steel wire, EPS, or EXPS hold together the insulating layers of the forms. The forms are stacked, braced, and filled with concrete (see Foam Forms Bring Concrete Results, HE July/Aug '98, p. 27).
Gypsum wallboard or other sheathing must be applied using conventional fastening techniques (nails or screws) that can hold onto both the forms and the concrete. If using adhesive, it must be compatible with the EPS and EXPS foam. It should be noted however, that the Council for American Building Officials (CABO) code does not permit adhesives.
The R-value of an ICF system runs anywhere from R-18 to R-35. The average cost, including labor and materials, is about $5.60/ft2 with the inside wall left unfinished. It is unfinished on the outside and does cost additional for outside finishing. The thermal mass of the wall is 16 Btu/ft2/°F.
The ICF wall system has the potential to have the highest overall R-value of all the foundation systems and can be constructed as high as R-35. Because the forms are designed to resist the load of wet concrete, they must be relatively thick to accommodate that load. The resulting two layers of insulation provide the high R-value.
Foam insulations such as EPS, EXPS, and urethanes tend to be vulnerable to termites, and for this reason, ICFs should be used with caution, unless preventive steps are taken to eliminate the termite threat. Most systems will require some type of fire-resistant wallboard finish on the interior.
ICF systems are gaining in popularity. Homeowners like them because they conserve energy, create a habitable space in the basement, and provide the thermal continuity for the home. Builders like them because they are easy to construct, reducing construction time, and because they have integral fastening strips to facilitate finishing.
Modern technology has reduced the probability of toxicity, VOCs, flame spread, and smoke spread with the latest ICF products, making them a healthier alternative for building than they have been in the past.Encapsulated Forms Encapsulated forms are many-celled, interlocking, PVC extrusions that are left in place and filled with concrete, providing a monochrome plastic finish on the interior wall and an exterior encapsulated in plastic. No dampproofing is required. The resulting structure is a two-way, vertically continuous slab that provides both the exterior and the interior wall finishes. The Royal Building System encapsulated form (see Figure 3) is currently being introduced in the United States. The R-value for a standard wall is R-16. The average cost of materials and labor, excluding footings and services, is $8.00/ft2.
The finished wall performs extremely well with regard to moisture, air, and heat control. The system enables the thermal mass to be used to reduce basement space heating. The wall requires virtually no maintenance. Structurally, it performs as well as any concrete wall. It has a two-hour fire rating and a low flame spread. Off-gassing is significantly less than the off-gassing associated with conventional building products, such as natural wood, laminated wood, particle board, foams, and wall or floor coverings, although poured concrete walls have the best performance for lack of toxic off-gassing.
This system would be a good choice for any basement, although the homeowner needs to get used to the plastic interior walls, and approval and acceptance at the code and municipal levels are not yet available in all areas.Preserved Wood Preserved wood with batt insulation can also be used to construct foundation walls. Preserved wood is soaked in a salt solution and pressure treated to make it less vulnerable to water and more adaptable to outdoor usage, such as docks, decks and foundations. Because of wood's tendency to absorb water and its susceptibility to mold and insect invasion, a vapor-and-water barrier must be carefully installed at the interface between the soil and the wood. The R-value for a 4-ft preserved-wood wall is around R-19 if 2 x 4 construction is used with a full-depth fiberglass batt. The average cost is about $8.00/ft2.
Termites cannot work their way into preserved wood unless they have access to a cut end or to a damaged section. In the event of a hurricane, a tornado, or flooding, a wood basement is unlikely to perform as well as a CIP or concrete block system. The walls have little thermal mass, and since the exterior soil is often moist, the relative humidity near the wall will often be 100% even if water is not present. This is why the vapor barrier is so important. A preserved wood system would be a good choice for a house in a rural area because wood is lightweight and easier to transport, store, and work with than ready mix concrete.Making the Choice Builders need to act as the gatekeepers, since they are the ones who typically choose which foundation systems are used in homes. They must be aware of how the different systems perform in homes, so that they can make an informed decision for each specific home in terms of the site, the climate, and how the space will be used.
|For more information, consult Foundation Wall Systems for Houses, by Eric Burnett and John DeGraauw; December 1998. It is available from the Pennsylvania Housing Research Center, 219 Sackett Building, University Park, PA 16802. Tel:(814)863-9788; Fax(814)863-7304; E-mail: email@example.com.|
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