Insulating a basement is not the same problem as insulating the rest of the house. A below-grade wall is cold, in contact with soil, and passes water vapour through concrete even when it looks and feels completely dry. Build a standard above-grade wall assembly against it and you create a cold surface inside the assembly where moisture condenses — inside the wall, where you cannot see it.
In a climate with Cheyenne's heating demand, insulating the basement is worth doing. It just has to be done in the right order.
The principle that governs everything
Keep the concrete's moisture out of the framing, and keep warm indoor air away from cold concrete. In practice that means continuous rigid insulation against the foundation wall, with framing inboard of it, rather than fibreglass batts stuffed into a stud wall built directly against bare concrete.
The rigid layer keeps the concrete surface warmer than the dew point of the indoor air that reaches it. That single detail is what prevents condensation inside the wall, and it is the thing most commonly got wrong in a DIY or bargain-bid basement finish.
What not to do: batts against bare foundation wall, and a full interior polyethylene vapour barrier on the warm side of a below-grade wall. The second traps drying in both directions and is a well-documented way to build a mould sandwich.
The rim joist is where the heat actually goes
The rim joist, the band of framing sitting on top of the foundation wall around the perimeter — is typically the single largest air leak in an older house, and it is right there in the basement, usually uninsulated or stuffed with a fibreglass batt doing very little.
Air-sealing and insulating the rim joist is one of the highest-return improvements available during a Cheyenne basement finish. The area is small, it is accessible while the ceiling is open, and the effect on comfort and heating cost is disproportionate. If budget is tight and you have to choose, do the rim joist.
What about the slab?
Insulating under a slab is only realistic if you are pouring one. On an existing basement, the practical options are a floating subfloor system that creates a thermal and moisture break under the finished flooring, or accepting a cold slab and dealing with it through floor covering choice and, in bathrooms, electric floor heat.
In this climate the slab stays cold most of the year against ground with a deep frost line. Tile directly on bare slab is unpleasant enough that people avoid the room, which is why heated floor under tile is the upgrade Cheyenne homeowners most often say they are glad they took. See basement bathrooms.
The Cheyenne-specific trap
This is a dry climate — around 16 inches of precipitation a year and low relative humidity for much of it. That leads people to conclude that vapour management in a basement wall is a concern for humid places and can be skipped here.
It cannot, and the reason is that the moisture source is not the air. It is the concrete, which is in contact with soil that holds meltwater from 58 to 63 inches of annual snow, and it passes vapour inward regardless of how dry the air in the room is. A dry climate outside does not mean a dry foundation wall. The assembly has to handle vapour arriving from the soil side no matter what the weather is doing.
Fix any liquid water problem before insulating anything — moisture control comes first in the sequence, always.
Common questions
Can I put fibreglass batts against my basement wall?
Not directly against bare concrete. That places a cold, vapour-passing surface inside the assembly where moisture condenses. Use continuous rigid insulation against the foundation with framing inboard of it.
Do I need a vapour barrier in a basement?
Not a full interior poly sheet on the warm side of a below-grade wall, that traps drying in both directions. Vapour is managed by the rigid insulation layer and the assembly design rather than by a plastic sheet.
What is the highest-value insulation improvement?
The rim joist. It is typically the largest air leak in an older house, it is accessible while the basement ceiling is open, and the effect on comfort and heating cost is out of proportion to the area involved.
Should I insulate the basement ceiling?
Not for heat, if the basement is conditioned space, that would separate it from the rest of the house. Insulate the ceiling for sound instead, which matters a great deal under a media room.
Is the concrete floor worth insulating?
Under-slab insulation is only realistic on a new pour. On an existing slab, a floating subfloor system or, in bathrooms, electric floor heat under tile are the practical answers.