Innovative Structural  

Foundation walls are no longer just poured concrete. Today you'll find them made of concrete blocks, concrete forms, and even preserved wood. Builders today have many choices to make.

The Durisol wall system has been used in Europe for about 50 years and is just coming onto the U.S. market. It goes one step beyond an ICF system in that it incorporates both an interior and an exterior finish (other finishes can be added). Durisol is a wood-and-cement composite that is insulating, lightweight, capillary inactive, fire-resistant, self-draining, and highly sound absorbent.
During construction, the large, self-aligning wooden blocks are dry stacked, reinforced if necessary, and filled with concrete. A layer of inert, hydrophobic mineral wool can be built into the exterior side of the blocks to enhance thermal insulation.

The insulated block has an R-value of R-12 to R-23, depending on the thickness of the block and the amount of insulation used. As with any other type of insulation, if the water seal fails, it would get wet and be damaged and have to be replaced. The wall has a four-hour-plus fire rating and is highly earthquake and hurricane resistant. It is completely resistant to rot, freeze/thaw, rodents, and termites.

The Durisol wall system is currently being improved so that it can be marketed directly for basement walls. Improvements include better drainage, incorporating a dampproof layer, and using various interior and exterior finishes. The system should work well in any wall, once it is available.

Twenty years ago, standard concrete masonry block and cast-in-place (CIP) concrete basement foundation wall systems were the norm for single-family, low-rise housing. Today, new products are becoming more available. Examples include insulated concrete forms and encapsulated forms. This article discusses several foundation systems that are available today, how they perform in the climate of the mid-Atlantic region, and the energy efficiency, safety, and comfort attributions of each type

Standard Uninsulated Concrete

Uninsulated concrete foundation is the most basic foundation system. It is helpful to use it as a basis for comparison with other systems.
This system consists of a concrete strip footing on which a wall is constructed. The wall may be either concrete block 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.

Concrete Block

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. 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.

CIP Concrete

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

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 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.

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