Options and Analysis
|alternatives||foundation cost||percent of budget||total house cost||total house cost ($/sf floor area)||yearly energy cost||energy cost ($/sf floor area)||IAQ||practice|
|slab with stem wall (864sf floor area)||$18,040||18.1%||$99,780||$115.49||$1,318||$1.53||better||standard|
|crawlspace (864sf floor area)||$24,324||23.0%||$105,880||$122.55||$1,307||$1.51||good||standard|
|garden basement (finished,1728sf floor area)||$73,044||47.2%||$154,600||$89.47||$1,846||$1.07||typical||standard|
|full basement (finished, 1728sf floor area)||$69,795||46.1%||$151,550||$87.70||$1,682||$0.97||typical||standard|
|full basement (unfinished, 1728sf floor area)||$33,421||28.1%||$118,930||$68.83||$1,682||$0.97||typical||standard|
The floor area includes the main floor and the basement. Other than adding the basement, the house dimensions remain the same in all respects.
Construction costs and energy costs are divided by 1728sf for foundations with basements and 864sf for foundations without basements.
Cost information is based on RS Means CostWorks 2007.
The construction of an unfinished basement can increase the total cost of a home significantly - up to 20% - compared to the use of a slab-on-grade foundation. However, if the basement space is built to livable standards, the space can be included in the total square footage of the house and the cost per square foot is reduced dramatically. For example, in the chart above, a slab-on-grade foundation costs $20,000 less than an unfinished full basement foundation, but the overall cost per square foot of the home with the basement is close to half that of the slab-on-grade home. Even with a full, finished basement, home costs measured per square foot are cheaper than slab-on-grade construction. This shows that constructing a livable basement is a much cheaper way to increase the square footage of a house, compared to building more above-grade space. A garden basement is slightly more expensive than a full basement. Although the garden basement offers savings with a half-height foundation wall, these savings are typically spent on more and larger windows. Therefore, he garden basement option is slightly more expensive, but offers more livable, well-lit basement rooms.
In the same way that total construction costs go up with a basement while construction cost per square foot goes down, total energy costs increase with a basement while energy cost measured per square foot goes down. This is because extra energy is needed to condition the increased space, even though basement space is more efficient to heat and cool than above grade space. For example, in the chart above, total energy costs increase by several hundred dollars per year for homes with basements, but the energy cost/sf actually declines by close to 40%. Garden basements are not nearly as energy efficient as full basements because garden basements typically rise 4 feet above grade, exposing the walls to ambient air conditions and reducing the earth-tempered effect. Ideally, basements should be insulated from the exterior. This is why there is no difference in energy use between the unfinished and finished basements in the chart above. Although unadvisable, many builders choose to insulate finished basement walls from the interior, which can increase the energy efficiency of a finished basement at the expense of decreased indoor air quality and a higher chance of mold and moisture problems.
The primary durability concern regarding basements is moisture. Moisture attacks basement walls and floors from all sides. Bulk water draining down through the soil, water rising up from the foundation through capillary action, and condensation from warm, humid indoor air can all add significant amounts of moisture to basement walls and floors. In addition, water vapor can pass though unprotected concrete by diffusion and damage interior finishes and assemblies. Adding insulation and finish assemblies on the inside of basement walls and floors can exacerbate these conditions by reducing the ability of the basement surfaces to dry. Moreover, the extra insulation and finish materials lead to colder concrete walls and floors, increasing the chance of condensation. Eventually, excess moisture leads not only to the growth of mold and mildew (with negative repercussions for indoor air quality and occupant health), but also to damaged basement finishes, such as drywall and flooring.
To ensure good indoor air quality, limit the chance of mold growth, and improve the durability of finish materials, all sources of moisture in a basement must be managed. This begins with managing water above grade. Generous overhangs, gutters, and downspouts should direct water away from the foundation. The top of the foundation wall should be a minimum of 6 inches above grade and the ground should slope away from the house in all directions for a minimum of 10 feet. Dirt backfilled against a basement wall or under a concrete slab should be free-draining sand or other porous material, such as gravel. To prevent vapor diffusion through a concrete wall, the exterior should be damp-proofed, while a vapor retarder like polyethylene sheeting should be placed below a slab to prevent vapor diffusion through the floor. The vapor retarder should be well-sealed to the basement wall to create a continuous barrier. At the base of the footing, a perimeter drain wrapped in fabric should be placed all the way around the foundation. The drain should either be connected to a mechanical pump or drained to daylight. To prevent capillary rise of water up the basement wall, a capillary break should be installed between the footing and the wall. Insulation placed on the exterior of the basement wall and floor slab is important for several reasons. Exterior insulation acts as an additional drainage plane, but more importantly, it warms the concrete surfaces. This reduces the chance that warm, humid indoor air will condense on cold concrete. In addition, the concrete is kept at a more uniform temperature, which reduces vapor diffusion as well as cracking and subsequent bulk water intrusion. Finally, if interior finishes such as carpeting and drywall are installed, it is generally recommended that these continue to allow inward drying. This implies omitting a strong vapor retarder such as polyethylene sheeting. Many studies have shown that a warm side vapor retarder in a basement setting is inconsequential at best and can often lead to trapped moisture and moldy conditions. 3,4
Indoor Air Quality
Wet or damp basements can lead to an increase in mold and mildew within the house. This, in turn, can lead to a decrease in indoor air quality and occupant health. Follow the discussion in the durability section above to properly manage moisture in basements. In addition to moisture, basements can have high levels of gas. There are a number of steps that can be taken to prevent problems with radon gas. These include installing a well-sealed, continuous vapor retarder directly below the concrete floor slab (see the page on vapor retarder placement for page for more information) and installing a passive soil gas ventilation system at the time of construction. This simple system consists of continuous drain tile placed in the layer of gravel below the floor slab and vapor retarder. The loop of drain tile should be connected to a stack that passively drafts soil gas through the loop, up the stack, and out of the house through the roof. If tests show that radon is still an issue, a fan can be added to the stack to actively draw soil gas out from beneath the house. Radon must be addressed with radon-resistant construction techniques before a basement can be considered livable.
Concrete is generally the material of choice for use in basement walls and floor slabs. It has a high compressive strength, enabling it to take the loads of the house above and is relatively impervious to damage from water and moisture. However, concrete does conduct and store large amounts of water, and any material that comes in contact with concrete should be water resistant or treated to handle moisture (e.g. green-treated lumber). Concrete also has very significant environmental impacts, as mentioned in the section on environmental context. Minimizing its use can have environmental benefits. However, the energy-saving benefits of full-height basements quickly outweigh the embodied energy and pollution impacts of greater concrete use. Therefore, garden basements and walk-out basements, which use less concrete, but require more energy to heat and cool, do not usually result in a net environmental benefit. Permanent wood foundations are another option to reduce concrete use. However, the preservatives used to treat wood that is employed for below-grade installations are highly toxic and can eventually leach into the surrounding soil. They may also negatively impact indoor air quality. Concrete is considered safe and inert once cured, with no effect on indoor air quality.
Concrete has the best program for recycling: it is used for fill or as aggregate in lower grade mixes. Wood foundations, because they are chemically-treated, have little use as recycled material. They cannot be used for landscape mulch and must be landfilled or burned, releasing toxins into the air or water. Insulated concrete forms used for basement walls reduce the recyclability of both the concrete and insulation material because the concrete bonds to the insulation.
Although construction of basements is standard practice, they continue to be one of the main causes of poor indoor air quality and durability issues in homes. Certain common practices, such as the installation of interior insulation and warm-side vapor retarders below grade, often lead to conditions that generate mold, decrease indoor air quality, and ultimately damage the assemblies and materials used to create finished basements. Even simple steps like backfilling against basement walls with free-draining fill or providing proper gutters and drainage away from the house are often ignored or short-cut. If a basement is built, careful attention should be paid to ensure that best practices are executed thoroughly.
1 "Cement from CO2: A Concrete Cure for Global Warming?" Scientific American, August 2008.
2 Ernst Worrell, Lynn Price, C. Hendricks, L. Ozawa Meida. "Annual Review of Energy and Environment" Vol 26, 2001. Lawrence Berkeley National Laboratory.
3 Straube, J. 2009. Field Monitoring and Hygrothermal Modeling of Interior Basement Insulation Systems. Research Report 0906. Building Science Press.
4 Robert W. Anderson and Associates. 1989. A Survey of Moisture in Minnesota Home Interior Foundation Wall Insulation. Final Report for Energy Division, MN Department of Public Service. St. Paul, MN.