The basement looks dry in August and finished in your head. Then the first damp winter shows you what was behind the drywall. A finished basement fails from water, not from bad framing, so the first job is to find out what your basement is doing, and only then to build.
Check this first: is it dry?
Two free tests, from the planning video by Home RenoVision DIY:
- Garbage bag test. Tape a garbage bag to the floor. If it collects moisture overnight, there is no vapour barrier under the concrete.
- Humidity. He says relative humidity of 60% or lower is fine, that at 65% water builds up on condensing surfaces, and that at 70% everything goes to mould. A hygrometer reads it.
Look, too, for white mineral crust on the walls, a musty smell, staining at the base of walls, and where the water would come from: gutters that dump next to the wall, sloping ground, a missing downspout extension. He recommends sloping the ground away and carrying downspouts at least four feet from the house.
Fix water first
He says sealing concrete walls only postpones a moisture problem. The two real options he names are exterior trenching and waterproofing, or an interior French drain, with a sump pit where water has to be pumped out. For a house with a roughly 1,500 sq ft footprint he estimates 20 to 25,000 for water mitigation, currency not stated in the video, so treat it as a rough order of magnitude and get local quotes.
A bowing block wall is not a DIY job. He calls half an inch a bow, and says a structural engineer should design the fix.
Safety and permits
- Permit. He says everyone building a basement has to get one, including electrical even if you wire it yourself. Adding a bedroom opens up the code. Rules differ by place, so ask your building office first. See Do You Need a Permit?.
- Bedrooms need egress and a closet, he says, and a second living space needs fire separation with at least one layer of 5/8 in. drywall. Ask your building office for your local rules.
- Electrical. Turn off the breaker and test the wires before you touch them. The wiring is the part to leave to a licensed electrician if you are not sure; see When to Call a Licensed Electrician, Plumber or Gas Fitter. Basement outlets and GFCI protection: see GFCI Outlets: Where They’re Required.
- Older houses. The EPA says homes built before 1978 probably contain lead-based paint. Test old paint before you sand or scrape it.
- Foam. Wear safety glasses and gloves with spray foam, and follow the layer limit below.
Step 1: Draw it and get the permit
Plain grid paper is enough, he says: size, posts, furnace as a square, water heater as a circle, sump pit, stairs and any walls. He says the furnace and hot water tank can stay in an unfinished mechanical room, with clearance needed generally where the furnace is serviced.
Step 2: Seal and insulate the rim joist
The rim joist, the band at the top of the foundation wall, leaks air and heat. The rim joist video compares three ways in a 1974 house with an 800 sq ft basement. He says two things matter, R value and air sealing.
| Method | What it is | Cost he gives (800 sq ft) |
|---|---|---|
| Batts alone | Old method, no air seal | About $200 to $300 |
| Rigid foam, spray foam cans and batts | Foam board cut slightly small, edges sealed, batt over it | About $800 |
| Two-part spray foam kit | Seals and insulates in one | $1,200 |
Spray foam rules from the label, as he reads them: never exceed 2 in. of cured foam per layer, aim for about 1 in., and let it cool between layers, because foam applied too thick can overheat. He says to keep the product warm, about 70°F, and to replace the mixing tip if you stop for more than 30 seconds. Currency is not stated in the video.
Step 3: Insulate the walls and frame in front
His simple process for living space:
- Glue rigid foam to the concrete in dabs, so air can travel behind it.
- Frame a wall in front of it. See Frame a wall.
- Run wiring, with a permit.
- Add drywall, then flooring and a ceiling.
For the stairs against a concrete wall, he leaves the stairs, glues 1 in. rigid foam to the wall and glues drywall to it. For soundproofing he suggests two layers of insulation batts in the joists, two layers of 5/8 in. drywall in the ceiling, and resilient channel.
Step 4: Plumbing on a slab
If you add a bathroom, you cannot run drains under a slab without cutting it. In the basement shower rebuild the drain pipe area was dug out, a collar went around the pipe, and quick-set concrete was filled in so the shower system could sit on top. Water lines used PEX with crimp rings. Check with your permit office before you open a slab.
Step 5: Wiring
For living space only, he says code is a plug every 10 ft, with up to 10 plugs on one breaker. That is his statement of one local rule. Your electrical code and inspector decide.
Step 6: Floor and finish
Vinyl plank is common on concrete; see Installing Click-Lock Vinyl Plank and every video for Install vinyl plank flooring. Paint comes last; see Paint a room.
Tools and materials
- Level, tape measure, utility knife, cordless drill, hammer drill for masonry screws
- Rigid foam, construction adhesive, expansion foam, batts, framing lumber, 5/8 in. drywall, resilient channel
- Safety glasses and gloves
Mistakes to avoid
- Finishing over a damp wall, and trusting a sealer to hold the water back.
- Gluing the foam solid so air has no path behind it.
- Skipping the permit because a wall seems cosmetic.
- Spraying foam in thick layers.
See it in the topic pages
- The Basement room, Finish a basement and Insulate a rim joist
- The Drafty basement problem, and the Insulation and Framing systems
- The Basement Shower Makeover project
- Furnace filter changes, since the furnace stays in this room
If that didn’t fix it
If the garbage bag is wet again, or the humidity will not drop below 60%, you have a water problem the finishing work will not solve. Stop, and talk to a waterproofing contractor. If a block wall bows, call a structural engineer. Both come before the drywall.
Sources
- Home RenoVision DIY on YouTube, shown on this site, source of the moisture, permit and framing advice. Planning a Basement Renovation: Water, Permits and Layout (video, Home RenoVision DIY) Source
- Shown on this site, source of the rim joist methods, foam layer limits and costs. Rim Joist Insulation: Batts, Foam Board or Two-Part Spray Foam (video, Home RenoVision DIY) Source
- Shown on this site, source of the slab drain work. Rebuilding a Tiny Basement Shower on a Concrete Slab With Tile (video, Mr. Build It) Source
- US Environmental Protection Agency. Protect your family from sources of lead Source