Structure and scope
Basement finishing
A finished basement is a building science problem before it is a finish problem. Water first, then vapour, then insulation, then code-legal egress, then drywall.
Most basements we are asked to finish have at least one of three water problems: surface water arriving from bad grading or a short downspout, hydrostatic pressure pushing through the cold joint where the slab meets the wall, or vapour drive through the concrete itself. Efflorescence on the wall means water has moved through it and left salts behind. A musty smell in August with no visible water usually means vapour, not a leak. The fix is different for each, and drywall over any of them is money set on fire.
Once the water is handled, the code questions are blunt and numeric. A basement bedroom needs an emergency escape and rescue opening: 5.7 square feet of net clear opening, at least 20 inches wide, at least 24 inches high, with a sill no more than 44 inches above the finished floor. A window well serving it needs at least 9 square feet of area with a minimum 36 inch projection, and a permanently affixed ladder if the well is deeper than 44 inches. Ceiling height has to be at least 7 feet, measured to the lowest point of a beam or duct that runs across the room.
Insulation below grade also has its own rules. In Climate Zone 6, which covers Gallatin County, basement walls need roughly R-15 continuous or R-19 in the cavity. Continuous rigid foam against the concrete is the detail that works, because it keeps the concrete face above the dew point and stops the cavity from becoming a condensing surface.
What is included
The scope, item by item
Water assessment
Exterior grading and downspout review, a moisture meter survey of the slab and walls, and a 48-hour plastic-sheet test on the slab to separate vapour drive from a leak.
Water management
Interior perimeter drain to a sealed sump with a battery backup where needed, cold-joint sealing, exterior regrade to a minimum 6 inch fall over 10 feet, and downspout extensions.
Egress
Saw-cut opening in the foundation wall with a properly sized header, an egress window meeting the five numbers, a code-sized well with drainage to the perimeter drain, and a ladder where required.
Thermal and vapour
Continuous rigid foam or closed-cell spray against the concrete, a capillary break under any new framing plate, and no polyethylene sheet trapped on the interior face.
Headroom and systems
Duct re-routing or flattening to protect ceiling height, beam boxing planned to keep 7 feet clear, dedicated circuits, and a radon mitigation rough-in.
Finish
Framing held off the concrete, drywall, flooring chosen for a below-grade slab, trim, paint, lighting and a labelled sub-panel if one is added.
Numbers that govern this work
How it runs
Six phases from first visit to punch list
Water diagnosis
We do not price the finish until we know what the water is doing. This step can take two weeks of observation.
Exterior corrections
Grading, downspouts and window well drainage, done before anything inside is built.
Egress and structure
Saw cutting, header, window and well. Inspected before framing.
Thermal layer
Rigid foam or closed-cell against the concrete, sealed at every seam and at the rim joist.
Rough-in
Framing, electrical, plumbing and mechanical, with a radon rough-in under the slab where practical.
Finish
Drywall, flooring, trim, paint, fixtures and punch.
What moves the price
Five factors, in order of impact
Ceiling height
Anything under 7 feet at the beams needs duct re-routing, beam boxing design or, occasionally, a bench footing and slab lowering, which is a different and much larger project.
Egress cutting
Cutting a poured concrete wall is more work than cutting a block wall, and both need a header. This is the single biggest cost swing in a basement bedroom.
Existing water history
A dry basement with good grading needs far less than one with a visible seepage line and efflorescence at the cold joint.
Bathroom addition
Adding a below-slab bathroom means breaking the slab for drainage or installing a sealed ejector pit. Both are planned early.
Radon
Gallatin County has elevated radon readings. A sub-slab depressurization rough-in during construction costs a fraction of retrofitting one afterwards.
The band of $85 to $185 per sq ft shown on this page is a sample figure for this demo site, not a quote.
This work in practice
Projects that included this scope
Questions
What people ask about basement finishing
You still need the vapour detail and the capillary break, because concrete moves water as vapour even when it never looks wet. You may not need an interior drain or a sump. The 48-hour plastic-sheet test on the slab tells us which category you are in, and it costs nothing but time.
Only with a compliant emergency escape and rescue opening. That is the 5.7 square foot net clear opening, 20 inch minimum width, 24 inch minimum height and 44 inch maximum sill height, with a code-sized well outside it. Without that it is legally a recreation room, not a bedroom, and it will show up on an appraisal that way.
Anything that tolerates vapour and does not rely on a wood subfloor staying dry. Luxury vinyl plank over a proper underlayment, tile over an uncoupling membrane, or engineered wood over a sleeper and foam assembly. Solid hardwood directly on a slab is a call-back waiting to happen.
We install the sub-slab rough-in and the vent stack during construction, and we work with a Montana-certified radon measurement and mitigation contractor for the testing and the fan. We do not issue the test results ourselves.
About 1.5 inches to the thermal layer on the walls, nothing on the ceiling if the ducts can be re-routed, and up to 10 inches under a main beam or trunk duct if they cannot. We measure this on the first visit and tell you before you commit.
More on our questions page, grouped by scope, money, permits, living through the work and building science.
