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As discussed in the previous installment, a building foundation performs a surprising number of tasks. The four critical tasks are to:
Bear the weight of the building loads,
Anchor against the forces of wind and earthquake,
Prevent vertical motion due to frost, and
Isolate the building from ground moisture. In general, there are nine different foundation types. Each is capable of satisfying these four basic requirements if built properly. Poured-Concrete Full Basement This is such a common foundation in the northern states that most northerners don't distinguish between the words basement and foundation. It provides convenient storage space (when dry), protection against frozen pipes, space for a furnace or boiler, thermal mass for temperature stability in both winter and summer, and potential living space. It is also the most expensive, most likely to fail and most difficult kind of basement to construct. Full foundations are problematic around bedrock. Blasting is extremely expensive, and allowing bedrock into a basement almost guarantees a water problem. As shown in Illustration 1, the foundation wall is insulated on the outside with rigid extruded polystyrene. There are great thermal benefits to be gained from insulating outside a concrete wall.
First, counter to intuition, concrete is a poor insulator. A single inch of extruded polystyrene has the thermal resistance of 5 feet of concrete! As a result, 15 to 25 percent of the heat loss of a typical northern home occurs through its uninsulated concrete foundation. Second, large quantities of heat energy can be stored in a concrete wall. The thermal mass of the wall acts like a massive flywheel, holding the building at a constant temperature.
As a result, the interior living space of the building becomes less responsive to daily swings in ambient temperature. Unfortunately, exterior foam insulation provides ideal cover for termites making the trip from ground to wood. Exterior foundation insulation (exsulation) should be used only where there are no termites (ask your building inspector). Note all of the steps taken to prevent incursion of groundwater into the basement (grading away from foundation, wall dampproofing, granular (porous) backfill, crushed stone covered with filter fabric, vapor retarder under the slab, and drains placed above the footing but below the floor. The subslab crushed stone also provides for suctioning of radon gas, if it later proves necessary.
All-Weather-Wood Full Basement If you confuse concrete with foundation, you'll love this one. Imagine starting your house by building the first-floor wood walls 6 feet below ground. About 40 years ago, a bunch of back-to-basics Canadians (Canadians are like that; they can't afford not to be) asked what would happen if one were to start building a wood house underground instead of on top of concrete, especially if the wood were immune to moisture and rot. They couldn't think of any problems, and indeed there haven't been any. The all-weather-wood foundation (AWWF) has proven to be quicker, less expensive and simpler to build than its concrete equivalent. It is also warmer and easier to insulate.
Of course the masonry industry and lending institutions have been less than enthusiastic, but I can't see why the AWWF won't eventually replace the concrete basement. Here's how the AWWF works. After the building dimensions are staked out and the batter boards (boards outside each corner of the foundation that serve as a temporary reference) are erected, the basement hole is excavated 12 to 15 inches below the future basement floor. Drainage can be provided either around the perimeter or under the center of the floor.
If installed around the perimeter, as shown in Illustration 2, a perforated 4-inch PVC pipe (perforations facing down) is carefully sloped 1/8 to 1/4 inch per foot on a bed of crushed stone in a slightly deeper perimeter trench. The loop of pipe connects at a single point to an unperforated pipe, which then drains either to daylight, the city sewer or a sump pump. A less expensive alternative is a sump pit 3 feet in diameter and 3 feet deep, which serves to collect all groundwater at a central point before similarly draining away. A 4-inch bed of crushed stone is then carefully leveled over the entire area. Water will never rise above the level of the gravel and, therefore, of the basement floor - provided the drainpipe remains unclogged. The walls are constructed of water- and termite-proof pressure-treated framing lumber and plywood. It is simplest, especially in bad weather, to construct the walls off-site in 8-foot sections and then bolt them together in place.
The wall framing is either 2 by 6 inches or 2 by 8 inches, depending on the depth of the soil backfill. I prefer 2-by-8-inch framing because it accommodates R-30 fiberglass batts. The footing plate is always one size larger; i.e., 2-by-8 or 2-by-10. After all wall sections are level, square and fastened, the interior gravel bed is again leveled, a 6-mil polyethylene sheet spread, and a 3- to 4-inch concrete slab poured to the level of a pressure-treated screed board (guide for screeding - or leveling - the slab) nailed to the bottom plate of the wall. The slab below and the floor above resist the inward pressure of the soil during backfilling. Further protection against incursion of groundwater is provided by a 6-mil polyethylene sheet wrapped around the below-grade area of wall and held in place by a strip of protective board or flashing at ground level. Finally, painting the strip gray makes it look like concrete. The inside of the wall is finished as if it were above ground: wired, plumbed, insulated, vapor-barriered and drywalled. With sufficient window area, this basement comes close to aboveground space in livability. Poured-Concrete Crawlspace This is what results when you delete the space requirement from the list of functions of a concrete foundation. The wall extends only to frost depth, and no floor slab is required. Builders like the concrete crawlspace because it looks like a full foundation but costs a third less.
Although no slab is required, a perimeter drain and a 6-mil polyethylene vapor retarder should be installed to prevent excess ground moisture from rising into the house. Provide a trapdoor and adequate crawling room in case the furnace, plumbing or wiring requires attention. Like the full concrete basement, a concrete crawlspace insulated on the exterior can provide thermal benefits. The termite problem in central and southern areas of the country, however, requires the crawl walls to be insulated on the inside. You might use rigid foam interior insulation, but depending on the accessibility of the space, the fire code may require the foam to be covered with drywall. Fiberglass blanket and batt is another solution. All-Weather-Wood Crawlspace From the systems above, you should be able to imagine how to build a crawlspace foundation using pressure-treated wood: simply a shortened version of the AWWF with the concrete slab replaced by a polyethylene sheet. Recalling the four basic functions of foundations, however, make sure that: 1) the soil is backfilled against the outside to a depth of at least a foot to provide wind resistance, and 2) the bottom of the gravel footing is below the maximum depth of frost for your area.
The wall can be insulated as in the full-basement version, or with exterior foam. Again, exterior foam is recommended only in termite-free areas. Slab on Perimeter Walls There are several versions of the concrete slab, the difference being what supports the building walls and what supports the slab. The version in Illustration 3 is the most conservative, with the walls supported by a perimeter concrete wall extending below frost depth and the floor floating independently on a bed of crushed stone. Rigid foam insulation between the slab and wall retards heat loss and provides a cushion for relative movement. Slab on Grade If your building inspector approves, you can save money by pouring a slab on a bed of gravel or crushed stone at ground level (on grade), as shown in Illustration 4. To prevent heat loss and frost heaves in northern areas, the perimeter of the slab is insulated with extruded polystyrene foam to a depth of 1 foot and out horizontally a distance equal to the frost depth. Since the edge of the slab carries all of the building loads, it should be reinforced in both directions with reinforcing rod (rebar). Builders often recommend the slab as the cheapest way to build a floor.
That's because they think a concrete slab is a floor. It may be a floor in a cow barn or a Wal-Mart, but it will never be one in my house. By the time you have achieved an acceptably attractive floor by laying tile or slate and then adding area rugs for comfort, you've spent any savings. And covering a slab with wall-to-wall carpet ruins its only other advantage - thermal mass storage. Another disadvantage of the slab is the difficulty of running pipes and wires. Rather than simply running them any which way under the floor to pop up wherever convenient, wiring must be fed through plastic or metal conduit. This is not only expensive; it also requires coordination with the plumber and electrician, and makes remodeling nearly impossible. I recommend slabs for just three applications: basement floors, garage floors, and sunspace and greenhouse floors. But keep your eyes on a promising new variation: cast-in-place tile floors. The contractor adds dark brown, red or green dye to the concrete mix, presses a mold into the still-wet concrete, and finally waxes the cured concrete. The result is a "tile" floor at half the cost. This is not a do-it-yourself project, however. Even the professionals get nervous when the concrete starts setting up faster than they can mold it. Rubble Trench Frank Lloyd Wright's rubble, or stone-filled, trench makes a lot of sense. Whether it makes a good foundation depends on what you're looking for. Frost heaving occurs when water-saturated soil beneath a foundation freezes. That being the case, there are three ways to prevent the phenomenon: 1) place the footing below the maximum depth of frost, 2) insulate the ground so that it never freezes, or 3) make sure there is no water in the ground to freeze.
Rubble trenches operate on the third principle. Trenches are dug to below the frost line, and perforated drain tiles are placed at the trench bottoms, sloped at 1/8 to 1/4 inch per foot to daylight. The trenches are then filled with crushed stone or coarse gravel. Water running into the trenches immediately drains away, leaving nothing to freeze. Form and pour a footing or concrete-grade beam with reinforcing bars and anchor bolts on top of the rubble trench. Illustration 5 shows a concrete-block crawlspace wall on top of a reinforced footing. While a pressure-treated wood sill would work as well, the building might not have sufficient weight to resist wind. Concrete Piers If you have no need for space under the house (for a heating system, for example), you can auger holes to below frost, pour 18- to 24-inch-diameter concrete footings and fill cylindrical cardboard (Sonotube) forms to above grade. Reinforcing rods, splaying out into the footings, are required because: 1) Unreinforced concrete is strong in compression, but weak in bending; 2) the piers alone have little sideward stability if placed in creeping soils or if projecting far out of the ground; and 3) frost tends to fasten onto the rough concrete piers and lift them from their footings.
In Illustration 6, a wood box sill is built on the piers. First, a pressure-treated 2x4 is nailed inside the flanges of galvanized tie-down straps cast into the tops of the piers. Next, load-carrying 2x8s or 2x10s are nailed to the 2x4. Finally, a 2x4 is nailed inside the top, creating a box beam. Shingles can be shimmed between the concrete and the bottom 2x4 before fastening in order to level the beam. Due to the potential for frost in poorly drained soils and the lack of protection for pipes under the building, concrete-pier foundations are recommended only for well-drained soils and for structures containing no plumbing, such as ells, porches, decks, outbuildings or summer cottages. Pole Foundation Replacing the concrete piers of Illustration 6 with pressure-treated telephone poles has several advantages: 1) The pole is tapered and slippery so the frost cannot grip it; 2) poles sticking far out of the ground can be braced easily; and 3) mistakes are easily corrected with a saw (making the poles shorter) or a hammer and nails (making it taller). As with concrete piers, the freedom to cantilever floor joists over the sills allows a margin of error of up to a foot in the placement of the piers.
This advantage cannot be overestimated in the case of novice builders. Knowing that concrete piers set up fast and are hard as rock can lead to quickened pulse, weak knees, indecision and finally depression. In my opinion, aside from providing additional space, the pole foundation is the best. Experience has shown that the pressure-treated poles, if protected from sun and rain, will last indefinitely. Unhappily, I speak from experience in regarding the major deficiency of the pole foundation. Although technologically superior in many ways, few of the unenlightened masses believe it to be a real foundation. The assumption that all wood placed in the ground will quickly rot and crumble must be genetically encoded, for of a hundred prospective buyers of my first pole-mounted home, just one failed to object to the poles. He worked for the telephone company. Floors over concrete-pier and pole foundations must be carefully insulated. The method is simple and results in floors as well-insulated and warm to the touch as insulated walls or ceilings. And the removable panels allow the running of pipes and wires when remodeling. Considering resale value, however, the pole foundation is best for outbuildings, porches, decks and seasonal homes.