Full Basements Should Be Insulated and Protected From Water
Bird's eye view
Green basements are comfortable and healthy
Most basements have dampproofing and a footing drain, but they can still be damp, clammy, and musty. Green basements go farther with a full waterproofing system, better footing drains, and foam insulation. With warm walls, moisture doesn't condense so basements don't get moldy.
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Waterproofing is better than damp-proofing
Basement walls need to stop groundwater from getting in. There are a number of products that can be applied to foundation exteriors, including asphalt-based damp-proofing, spray-on waterproofing coatings, dimple mats, and plastic membranes that shed water. Water-resistant coatings (damp-proofing coatings) by themselves do not make a wall waterproof. Bentonite, a type of clay, is sometimes used as one component in a basement waterproofing system.
Is building a basement worth the effort?
Basements take a lot of energy and resources to build. There are several situations that may justify the extra expenditures. For example, if a home needs storage space, basement space may be cheaper to build than above-ground space.
Sometimes it's more comfortable underground. No matter where you live, there will be some part of the year when the temperature deep underground is more comfortable than the air temperature. Creating conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. underground may take some of the load off of your heating or cooling system, depending on where you are and what season it is.
Frost lines require you to dig deep anyway. In a cold climate, you have to protect the foundation from damage caused by frost heaving. If your footings need to be several feet below grade, it's probably worth it to create a full floor with usable living space, instead of a less functional crawl space.
A hillside home has a few advantages. A hillside can shelter a home on some sides while offering sunlight and accessibility on others. Depending on how steep the site is, there could be two or more ground levels instead of just one. In a cold climate, an earth sheltered home should face south to take advantage of passive solar heating and to shelter the north side from winter winds.
Keep water at bay and encourage drainage
The International Residential Code, in section N1101.6.1, includes requirements for the protection of above-grade exterior basement insulation.
Foundation drainage requirements are covered in section R405.1, which states, "Drains shall be provided around all concrete or masonry foundations that retain earth and enclose habitable or usable spaces located below grade. Drainage tiles, gravel or crushed stone drains, perforated pipe or other approved systems or materials shall be installed at or below the area to be protected and shall discharge by gravity or mechanical means into an approved drainage system."
Invest in a waterproofing system
Basement foundations should be insulated to minimize energy losses and should be protected from water infiltration. Even if a basement is not intended to be used as a finished space, it's best to assume that it may be finished in the future. That's why it's a good investment to upgrade from damp-proofing to a true waterproofing system when the basement is built. It's always cheaper to perform this work at the time of construction than years later, when landscaping is in place and excavation is difficult.
For information on ways to improve a damp basement, see Fixing a Wet Basement.
Types of basement foundations
Full foundations that create usable basements can be built in a number of ways: formed-in-place concrete walls or concrete block on footings are the most common. Insulated concrete forms (ICFs) and precast concrete panels are two newer options. Although it's possible to build basement walls with treated wood lumber on footings of compacted gravel, all-wood foundations are rarely installed.
Insulated concrete forms
The most common type of ICF is made from inner and outer layers of rigid foam insulation with internal cavities that are reinforced with steel and filled with concrete. The forms are light, easy to handle, and quick to assemble. The foam provides an efficient thermal barrier. ICFs can be used to build above-grade walls as well as basements. ICFs can also be made from recycled polystyrene or wood chips combined with cement.
Treated Wood. Foundations made from treated lumber can be less expensive than building with concrete or concrete block. Despite their novelty among many builders, these foundations have a long history of reliability. Regular carpentry crews can erect them, and wood foundation walls can be set on a gravel base rather than a concrete footing. All of that helps make construction simpler, and sometimes faster, than a concrete foundation. Also, framed walls are easier to insulate and wire than concrete. Structural insulated panels with a treated-wood exterior are a similar option.
Like wood foundations, precast panels can be erected quickly without the need for concrete footings. They consist of an outer panel of concrete and top and bottom plates connected by concrete studs. One advantage of these systems is that most include integral insulation.
MORE ABOUT BASEMENTS
When the basement will be used as a home office, bedroom, home theater, or other living space, it has to have windows that are big enough to be used as emergency exits. Local codes may vary, but the requirements listed in the International Residential Code include these provisions:
Windows must measure at least 24 in. high and 20 in. wide and have a minimum net clear opening (not including the frame) of 5.7 sq. ft.
Windows must be operable from the inside, without the use of keys or tools.
The sill must be no more than 44 in. from the basement floor.
If windows open into a well, the floor must be at least 9 sq. ft. with a minimum dimension of 36 in.
A permanent ladder or steps must be provided if the well is more than 44 in. deep.
A metal or plastic well can be installed during construction or as a retrofit to meet code requirements for egress basement windows. In addition to providing emergency access or escape from below-ground living areas, the wells provide a lot more natural light than standard basement windows.
Many concrete basement foundations are probably stronger than they need to be. A typical foundation wall 8 in. thick rests on a 12-in.-wide footing that distributes the load evenly. Because of the enormous compressive strength of concrete, it’s very unlikely the house will fall down. But it takes a great deal of energy (and greenhouse gas emissions) to manufacture cement, so it pays to reduce the amount of concrete in a foundation wall if possible.
Building codes are based on assumptions about the bearing capacity of the soil beneath the footings as well as the weight of the structure. Calculating the loads more precisely, along with testing soils on the site, may allow thinner foundation walls, lowering costs and consuming less material.
This is the approach advocated by Fernando Pagés-Ruiz, a builder in Nebraska and author of Building an Affordable House (The Taunton Press, 2005). Among his many suggestions are using fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info (a by-product of coal-burning power plants) to replace some of the more expensive cement in the concrete; specifying concrete with lower load-bearing capacity where appropriate; and replacing wire mesh and steel reinforcing bar with synthetic and steel-fiber additives.
The point is to design foundations for the load and soil conditions on the site, not a conventional standard that may not apply. Although Pagés-Ruiz doesn’t always bill his suggestions as "green," his cost-saving measures are all about resource conservation, which is very green.
Where radon is a hazard, the crawl space can be safely vented by installing perforated plastic pipe in gravel beneath the polyethylene ground cover and running the stack up through the roof.
For more information, see All About Radon.
BuildingScience.com: Renovating your Basement
- Charles Lockhart
- Charles Bickford/Fine Homebuilding #190
- Paddy Morrissey, Code Check Building 2nd Edition
Jul 15, 2010 3:10 PM ET
Jul 5, 2010 8:03 PM ET