An Energy-Smart Foundation in Two Days
A compacted gravel footing is the only prep work required, and you don't have to wait for the weather.
Synopsis: Saving time on a building project often means saving money, too. North Carolina builder Tim Robinson saves both when he can by starting a project with insulated, precast concrete foundation walls. The panelized system Robinson uses is insulated with R-12.5 foam and has steel studs with holes punched for wiring and plumbing. Once a gravel footing has been compacted and leveled, the foundation walls can be dropped in place. This type of foundation system often can be set up in only two days: one day for prep work and one day for setting the walls.
Magazine Extra: Associate editor Daniel S. Morrison explains how to level compacted gravel footings for precast foundation walls.
We all know that time is money. That’s why waiting for the footings and foundation to go in can be one of the most frustrating aspects of any home-building project. Trying to get a house out of the ground in the winter is even more challenging.
One time-saving solution I’ve come to like involves the installation of insulated, precast concrete panels to construct a building’s foundation. I use a system from Superior Walls (www.superiorwalls.com), but the same benefits are available from other companies in different parts of the country. In most cases, after a site is excavated, I can install the whole system in two days: one day for prep and one day to set walls. Because these panels sit on a compacted gravel footing, my crew can do the prep work.
Not only are the panels quick to install, but they’re also insulated with 2½ in. of foam (R-12.5). The foam is visible on the inside surface of each panel; so are steel studs punched with holes for wiring and plumbing, making this type of foundation easy to finish inside. The outer concrete face is mixed at 5000 lb. per sq. in. (psi), versus the 2500-psi concrete in standard poured walls, so the prefab walls are practically bulletproof and waterproof. (For the record, the manufacturer says these panels are “damp-proof” and makes no claim about bullet resistance.) On this project, I elected to waterproof the foundation with a sprayed-on sealant and a dimple drain; that cost is minimal for the peace of mind gained.
Drainage matters for all foundations
Because no concrete footing is used, the code governing wood foundations applies (International Residential Code section R402), specifically, ½-in. compacted gravel. The depth of the stone (4 in., in this case) depends on soil type and the combined load per lineal foot (dead load + live load + wind/snow load = combined load).
Drainage is important for all foundations; perimeter drains being the most common. Although code allows exceptions, Superior Walls does not. The perimeter drain’s gravel must extend below the frost line, so if you have a wall with no backfill against it, you need to dig a trench and fill it with gravel to set the wall on. This trench requires its own drain. The width and depth of the trench vary depending on your location.
The overall excavation should be at least 2 ft. larger than the foundation on all sides so that there’s ample room for the perimeter drain and room for the installers to work. If you decide to waterproof (required in some areas), you need this room. It’s also a safety issue (required by OSHA) to have at least 2 ft. of space from a foundation to the edge of the trench. On a simple rectangular foundation, we locate the two back corners first to ensure that we have this 2 ft. of working space behind the wall. Next, we pull the correct measurement perpendicular to this line and make a 3-ft. to 4-ft. arc in the gravel. After calculating the diagonal measurement of the building, I measure from the back pins to where that measurement intersects the arc to find the other two corners.
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