Planning tool 2 of 4
The five numbers that make a basement bedroom legal
An emergency escape and rescue opening is not a design preference. It is a set of dimensions, and a window that misses one of them turns a bedroom back into a rec room.
All six dimensions are met and the well is shallow enough that no ladder is required. Drain the bottom of the well to the perimeter drain regardless.
The six numbers
Water reading 0 of 4
Nothing checked. Even a basement with no visible water needs a capillary break under new framing and continuous insulation against the concrete, because concrete moves moisture as vapour whether or not it ever looks wet.
Based on the 2021 IRC as commonly adopted in Montana. Confirm current requirements with the City of Bozeman or Gallatin County before you order a window.
The most common disappointment in basement finishing is not cost. It is discovering, after the drywall is up, that the room the family has been calling a guest bedroom cannot legally be called a bedroom, because the window does not meet the requirements for an emergency escape and rescue opening.
The second most common is finishing over a water problem. Drywall over efflorescence is money set on fire, and the fix depends on whether you have liquid water arriving through the wall, vapour drive through the slab, seasonal surface water, or simply humid air condensing on cool surfaces. Those are four different problems with four different answers.
The trap in the arithmetic
A window that is exactly 20 inches wide and exactly 24 inches high gives you 3.3 square feet, not 5.7. Meeting the two minimum dimensions does not get you the area. In practice one of the two has to be considerably larger, which is why a wide casement or a large slider is the usual answer in a basement.
The sill height catches people out for a different reason. It is measured from the finished floor, so a thicker floor assembly helps. But if the existing sill sits at 52 inches you cannot solve a shortfall of eight inches with flooring. The opening has to be cut lower, which means more concrete cut and a deeper well outside.
Requirement by requirement
| Requirement | Minimum or maximum | What we most often find |
|---|---|---|
| Net clear opening area | 5.7 sq ft minimum | Existing slider gives 3.1 sq ft |
| Net clear opening width | 20 in minimum | Usually met |
| Net clear opening height | 24 in minimum | Hopper windows give 14 to 18 in |
| Sill above finished floor | 44 in maximum | Typical existing sill is 48 to 56 in |
| Window well area | 9 sq ft minimum | Stock semicircular well is under 5 sq ft |
| Window well projection | 36 in minimum | Stock wells project 16 to 24 in |
| Ladder in the well | Required over 44 in deep | Frequently omitted entirely |
Scroll the table sideways to see every column.
Based on the 2021 IRC as commonly adopted in Montana. Confirm current requirements locally before ordering a window.
Reading the water before you frame anything
- Efflorescence, the white crust at the base of a wall, means liquid water has moved through the concrete and left salts behind. Start outside with grading and downspouts.
- A two-foot square of plastic taped to the slab for 48 hours tells you about vapour. Condensation on the underside means vapour drive, which drainage will not fix.
- Water that appears only with snowmelt is almost always surface water, not a rising water table, and is usually solved outside.
- A musty smell in August with no visible water is humid air condensing on cool surfaces. Continuous insulation against the concrete keeps the wall face above the dew point.
The order the work has to happen in
Exterior corrections first: grading to a minimum 6 inch fall over 10 feet, downspouts extended well clear of the foundation, window well drainage. Then interior drainage if the diagnosis calls for it. Then the egress opening and its header, inspected. Then the thermal layer, continuous rigid foam or closed-cell against the concrete with a capillary break under any new framing plate. Only then does anything get framed.
Gallatin County also sits in a high radon zone, so we rough in sub-slab depressurization during construction on every basement finish. Retrofitting one afterwards costs several times as much.
Questions
About this tool
In a poured or block foundation wall, enlarging an opening is structural work. The wall above needs a properly sized header and the cut has to be made wet with a track or wire saw. It is a day of noisy, dusty work and it is the single biggest cost swing in a basement bedroom.
Yes, and the reason is that a firefighter in full gear has to be able to get into it. Nine square feet with a 36 inch projection is the minimum, and the window has to be able to open fully within it.
Ducts can often be re-routed or transitioned to flat oval to recover headroom, and beams can be boxed to keep the clear height where it is needed. Below about 6 feet 6 inches you are into bench footings and slab lowering, which is a different and much larger project.
Usually yes, sized to the space and plumbed to a drain rather than emptied by hand. A finished basement with correct insulation still benefits from active humidity control in a Montana summer.
The other three
Load path and footings
Trace an addition's weight from the roof to the soil, and size the pad that catches it.
OpenFloor height and transitions
Stack up the subfloor, underlayment and finish before you order, not after.
OpenElectrical service capacity
Run the NEC optional calculation and find out whether 100 amps still carries the house.
Open