Basement finishing in Colorado, starting with a dry foundation

Solve water problems before you build walls

Concrete is porous, and soil moisture moves through it. Before finishing, look for white mineral deposits on the walls, damp spots on the slab, water marks at the base of the walls, rust on old nails or a musty smell. Tape a square of plastic sheet to the wall and slab for a couple of days; moisture under the plastic means water vapor is coming through.

Fix the outside first. Soil should slope away from the foundation, downspouts should discharge well away from the house, and window wells should drain. If water still gets in, the answer may be interior drain tile and a sump pump. Structural cracks or bowing walls need an engineer's eye, and that work falls under repairs and renovations, not finishing.

Wall assemblies that tolerate a damp foundation

Never put fiberglass batts and a plastic vapor barrier directly against a concrete wall. Moisture condenses on the cold concrete and gets trapped against the insulation. Rigid foam board or closed-cell spray foam applied directly to the concrete keeps the first surface warm and dry, and the framed wall goes inside it. Foam plastic in a living space has to be covered by a fire-rated thermal barrier such as drywall.

Test for radon while the basement is still open

Radon is a colorless, odorless radioactive gas released from soil and rock, and it is common in Colorado. It enters through slab cracks, floor drains, sump pits and gaps around pipes, and basements usually read highest. Test before you finish, using a test kit or a certified radon measurement professional.

If the result calls for mitigation, the most common system is sub-slab depressurization: a pipe through the slab with a fan that draws soil gas from under the house and vents it above the roofline. Routing that pipe is far easier before walls and ceilings close in. Some newer homes already have a passive pipe in place that can be fitted with a fan. Seal the sump lid, and retest after the basement is finished, since sealing and new use can change the reading.

A basement bedroom needs a way out

Any room used for sleeping needs an emergency escape and rescue opening: a window or exterior door large enough to climb through, with a sill low enough to reach. A room without one is not a legal bedroom, whatever the listing calls it. Most existing basement windows are too small, so adding a bedroom usually means cutting a larger opening in the foundation wall.

That cut is structural work. It needs a header or engineering review, a properly sized window well with drainage at the bottom, and a ladder or steps when the well is deep. In Colorado, a well cover keeps snow and debris out, but it must open easily from inside. For help choosing the window unit itself, see roofing, siding and windows.

Headroom, ductwork and the mechanical room

Basements are full of things hanging below the floor joists: steel beams, the main supply and return trunks, drain lines and gas pipe. Codes set a minimum ceiling height for habitable rooms, with allowances for beams and ducts. Measure from the slab to the lowest obstruction early, because headroom often decides the layout.

Options include boxing ducts into soffits along walls, moving branch runs into joist bays or rerouting a trunk. Any ceiling needs access panels at valves, cleanouts and junction boxes. A suspended ceiling keeps everything reachable; drywall looks cleaner and controls sound better.

New rooms need heat and return air. Adding supply registers to an existing system can starve upstairs rooms, so an HVAC contractor should check capacity. Enclosing the furnace and water heater needs service clearances and, for gas appliances that draw room air, enough combustion air. Closing them into a tight closet can cause backdrafting.

• Measure headroom at the beam and trunk lines, not in the open bays.
• Mark every shutoff, cleanout and junction box before framing starts.
• Ask whether the furnace can serve the new space or needs help.
• Insulate the ceiling joists if you want quiet between floors.

Permits and inspections protect the next owner too

Finishing a basement creates habitable space, so it needs a permit from the local building department, and requirements vary by jurisdiction. Expect inspections for framing, rough electrical, plumbing and mechanical, insulation and a final. Inspectors look at egress, smoke and carbon monoxide alarms, fireblocking, stair guards and handrails, and covered foam insulation.

Unpermitted finished basements can cause trouble at resale or with an insurance claim. A permit also documents the work for whoever owns the house next. Before you sign, understand how contracts are priced, and ask whether a bathroom fits your plans; bathroom remodels covers the waterproofing side of that.