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How to Finish a Basement: Framing and Insulating: Building Soffits
Insulating and Framing
Start the job by gluing 3/4-in. extruded foam insulation to rim joists and foundation walls. Extruded polystyrene foam (imprinted on each sheet) can be yellow, pink or blue depending on the manufacturer. Avoid “expanded” foam insulation (the type that has little white beads pressed together) because it isn’t as durable and has a lower R-value. Make cuts by snapping chalk lines to mark and then score it with a utility knife as deep as the blade will penetrate. Then snap the sheet just like you cut drywall. Carefully cut around obstructions and fill spaces with small chunks of foam wherever it’s needed, working for tight fits. Then caulk seams and gaps to seal against air infiltration.You’ll add fiberglass later for a higher R-value. The foam greatly reduces heat transfer through the masonry and framing, and it eliminates the need for a plastic moisture barrier later. Be sure to use adhesive formulated for use with foam (about $3 per tube). Conventional construction adhesive won’t work.
Next frame the stud walls 1/2 in. away from the foam (or more if your foundation’s uneven). We show the “stick framing” method of wall building. That means that you cut, lay out and install the top and bottom 2x4s (plates) first. Then you’ll custom-cut the studs to length and toenail them into place. This method is great for basements because it makes it easy to frame around overhead obstructions and customize stud lengths to handle uneven floors. It can be tough to preframe a wall (as you would normally do with a shed or addition) and raise it in place with a floor overhead. Be sure to use pressure-treated wood for any wood that has contact with concrete surfaces.
Lay out stud locations by laying both plates side by side and then hook your tape measure on one end and mark studs every 16 in. For walls longer than 8 ft., subtract 3/4 in. from each location (e.g., 15-1/4, 31-1/4 in., etc.). That’s so drywall will fall in the center of studs. Otherwise, the sheets will fall just short of a stud at each joint.
To position the top plate, tape your level to a straight 2x4 and mark the blocks or joists at either end of the wall. Then snap a chalk line between them. Top plates that run perpendicular to floor joists can be nailed (with 16d nails) or screwed (with 3-in. screws) to the bottom of every other joist. If walls run parallel to floor joists, you’ll need to nail or screw blocking in between the floor joists about every 3 ft. or so. For most, running in screws is easier than overhead nailing. Likewise, it’s easier to predrill and drive concrete screws rather than pound in concrete nails when fastening bottom plates.
When you’re framing half walls, make all of the studs the same length and cut them so the wall will be even with the top of the masonry. The wall may be uneven because of floor inconsistencies, but you can always sight along the top plate and then shim it until it’s flat before installing the finished top cap. When plumbing the top of the half wall, be sure to sight along its entire length to make sure it’s straight.
Most basements have ductwork and plumbing mounted at the ceiling along an existing wall. Boxing in those pipes and ducts and then drywalling the assembly is the best way to conceal them. The whole structure is called a soffit. Begin by measuring to the floor to find the lowest pipe or duct in the room; that’ll define how low the soffit must be. Mark a point 2 in. lower on the wall to allow space for the framing and drywall and nail on a 2x4 nailing strip using the chalk line to position the bottom of the strip. Then snap another line on the bottom of the joists with a 2-1/2 in. clearance. It’s easiest to preassemble the 8-ft. long soffit side sections and screw them to the bottom of the floor joists. If soffits end at walls, build the walls first.
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