How to Finish a Basement: Framing and Insulating

With special framing and insulating techniques, your basement can be as comfortable as any other room in your home

Planning and getting started

Your basement can be more than a utility and storage area. With some forethought and good techniques, you can make it as warm, comfortable and inviting as any other room in the house. But, make no mistake about it: Finishing a basement is a big job. In this article, we'll focus on the framing and some unique problems, such as:

  1. Finishing against cool masonry walls without creating moisture problems.
  2. Framing around obstructions like posts, heating ducts and pipes.
  3. Keeping access to valves and cleanouts.
  4. Framing and finishing a wall that's half masonry and half wood frame. You only need basic carpentry skills for framing and one special tool—a hammer drill for concrete fasteners.

Get started by making a scale drawing of your plans to submit to your local building inspections department. Your plan should include wall dimensions, window and door sizes, and each room's purpose (e.g., family, bedroom, etc.) along with any special features like fireplaces. Some rooms may require large windows, called “egress” windows, for fire safety. Ask your building inspector if you need them. Also measure the future finished ceiling height and low-hanging pipes or ducts that'll lower headroom. Sketch the details of the exterior wall construction you intend to use as we show in this article. If you’re uncertain about the best use of space, hire an architect to help with the design. The permit itself will outline at what stages inspections are required. If you choose to do your own electrical work, draw up and submit that plan as well. With your plan and permit in hand, clear everything out of the basement and you're ready to go. Walk around the basement with caulk and cans of spray foam and plug every gap you can find between framing and masonry and around pipes or wires that penetrate the rim joist or exterior walls. This is your last chance to seal air leaks from the inside.

If you have a wet or damp basement, you must deal with the problem before you get started. To tell if walls are damp from exterior water or just condensation from humid interior air, tape a 2-ft. square sheet of plastic to the masonry. If moisture collects on the front of the plastic, you have condensation. The method we show for finishing will take care of that problem. If moisture collects on the backside after a few days, then water is wicking through the foundation wall from outside. The basement should be treated the same as if it were leaky. If you have regular seepage or water puddling after storms (even once every few years), you have to fix it permanently before finishing. Remedies for damp or wet basements can be as simple as rerouting downspouts, regrading slopes away from foundation walls, or applying water-resistant paints to interior surfaces. As a last resort, hire a pro to install perimeter drains and a sump pump. The bottom line is that it's senseless to spend time and money finishing a basement if leaks or moisture will ruin your work or cause mold to grow.

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Insulating and framing

Start the job by gluing 3/4-in. extruded foam insulation to rim joists and foundation walls (Photo 1). Extruded polystyrene foam (imprinted on each sheet; see Photo 5) 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 (Photo 2). 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 (Photos 3 – 6). Then you'll custom-cut the studs to length and toenail them into place (Photo 7). 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 (Photo 5). 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 (Photo 8), 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 (Photo 9), be sure to sight along its entire length to make sure it's straight.

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Building soffits

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 (Photo 1) 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 (Photos 2 and 3). If soffits end at walls, build the walls first.

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Framing partition walls

Partition walls are any walls that aren't against exterior foundation walls or walls that support floors above. Lay out partition walls by snapping chalk lines to mark both sides of the bottom plates (Photo 1). That keeps you from building walls on the wrong side of single lines! Mark door openings on the floor (Photo 2) to avoid putting glue under doors. Frame partition walls as you did the outside walls, again installing blocking between joists wherever it's needed. Add 2x6 backers on walls that meet partitions (Photo 1). They provide support and nailers for drywall. Before you tie the partition walls to exterior stud walls (non-masonry, without foam), staple 2-ft. wide strips of polyethylene over the 2x6 backers (Photo 2). That way you'll be able to seal this type of outside wall with a continuous moisture barrier in cooler climate zones.

In a basement, the top and bottom plates are often different lengths. That's because top plates may project past foundation walls and be longer or run into soffits and be shorter. (See both cases in Photo 3.) When you line up the plates to mark stud locations, be sure to account for differences (Photo 2).

Frame the door openings 2-1/2 in. higher and 2-1/2 in. wider than the door you're installing. This “rough opening” allows adequate space for the door plus its frame. Use a regular stud plus a “trimmer” on each side of the door (Photo 3). If you have low headroom, you may need to cut your doors down or special-order shorter ones. Remember to allow overhead space for the door trim. Trim that's either missing or ripped too narrow over doors with inadequate clearance will really detract from the appearance of the room.

TIP: Partially cut through the underside of the bottom plate at the edges of the door rough opening to make removal easier later on.

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Framing around obstructions

Nearly every basement has something that will project past finished surfaces. That can include beams, posts, drain lines, water piping or surface mounted wires. It’s a simple matter to frame or fur out around projections and then drywall and finish them to blend in with surrounding surfaces. You'll have to maintain access to other things like electrical junction boxes and plumbing shutoffs and cleanouts. If you need future access to anything, just frame around it and cut out the opening when you drywall (Photo 1). Then, after taping and painting, screw a “return air” grate over the opening to conceal it but still have access. Return air grates are available in various sizes for about $5 at home centers. Check the sizes of available grates and frame the accesses slightly smaller.

Sometimes furring down part of or the entire ceiling is the best way to bury surface-mounted pipes or wires. Use either 2x4s or 2x2s running perpendicular to the joists to add 1-1/2 in. of dead space so you can drywall over the top of everything (Photo 3). Be sure to run all the wiring and other things you might want before hanging the drywall.

If you have a lot of deep projections from the ceiling or you need a lot of access, consider installing a suspended ceiling rather than drywalling. The downside is that you'll lose at least a few additional inches of ceiling height.

Finish round steel columns by framing around them with 2x4s. You can then face the framing with drywall or decorative wood as shown in the opening photo.

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