While you can skip around the page and read what is only relevant to you, there are a few terms I want to define so there is no confusion:

Subfloor: 

The floor of the first floor and the ceiling of the basement. This includes the floor joists (wood beams) and the plywood/planks ontop of the wood beams.

Slab: 

The concrete floor of the basement. This can sometimes be dirt or stone. For simplicity on this page, I will refer to all basement floors as the slab.

Grade:

Grade the the level of the ground outside. Above grade is above ground. Below grade is below ground.

Foundation Walls:
The walls of the basement. Usually poured concrete, block, or stone.

Conditioned Space:

Area of the house that is within the thermal envelope that is heated and cooled. All areas in the conditioned space shares air and heat.

Rim Joists:

The floor joist (floor beams) that connects all of the other floor joists at the end. The other side of the rim joists is the exterior of the house.

    There are less options for types of insulation in the subfloor, usually either batt/rolled insulation or spray foam, but it opens other factors that need to be considered. When you install insulation in the subfloor you are separating the basement from the conditioned space of the house, meaning that the air, humidity level, heating, and cooling that was shared with the basement before will now be cut off.  

    This means that now in the winter the basement will become much colder and in the summer the basement will become warmer and more humid. This is a problem when you have pipes, you heater, or your washer and dryer in the basement.

    Since basements are mostly below grade and unvented, the temperatures usually won't be as extreme as the outside or something like an attic. Usually the foundation walls will stay relatively mild throughout the seasons. That being said, it is still possible for it to get cold enough for pipes to freeze so that has to be something that is carefully considered.

Basement Sub Floor Insulation Pros:

    There is a long list of methods and materials that can be used to insulate basement walls. All of the methods have their pros and cons, which we will get into later.

Some examples of types of basement wall insulation:

    • Solid foam board directly against the foundation walls or drainage mat

    • Spray foam directly against the foundation walls or drainage mat

    • Batt/Rolled insulation in framed 2x4 walls

    • Spray foam in framed 2x4 walls

    • Vinyl or Foil faced basement insulation rolls

    • Rolled radiant barrier

    While insulation in the subfloor separates the basement from the conditioned space, insulation on the foundation walls establishes the basement as conditioned space. That means the basement will be the same temperature as the rest of the house, as long as the HVAC is configured correctly.

Foundation Wall Insulation Pros:

    Those are the places where it's possible to retrofit insulation. Below are some more places that are impractical or impossible to add insulation to but can be done during construction. Most of these are required on new construction but are rare to find on home built before the 2010s. You can skip to this section and go right to the section that compares the different methods and materials for basement subfloor insulation and basement wall insulation with the button below.

    That blue material is solid foam board applied to the outside of the foundation wall. When they are finished work on the foundation everything shown will be below grade. You can see the ground level on the top right of the image.

    That white material is solid foam board installed on the ground before the concrete slab is poured. The grid in the picture is the rebar support for the concrete. This insulation is unfortunately very to find completed.

    There are a number of insulated basement systems that integrate the insulation into the structure of the foundation. The example in the above picture is ICF. It is lego style blocks made out of insulation that are then filled with concrete.

    Fiberglass is a good option for basement ceilings (subfloors) because they have a very high Rvalue per dollar. You can get the best of all worlds by installing fiberglass insulation and then installing solid foam board insulation on top of it.

    IMW Specialists does not install spray foam. It is a dangerous and toxic process that is unhealthy for the technicians and for the clients. When doing spray foam homeowners need to leave for 3 days while the foam off gasses dangerous chemicals.

    Mineral wool, often referred to as the brand name Rock Wool, is a great alternative to fiberglass batt insulation. Rock Wool 1ups fiberglass batts in every aspect other than cost. And Rock Wool comes in less sizes than fiberglass, so the picture shown above is two layers totaling 11 inches.

    Foam board shouldn't be installed by its self on the ceiling/subfloor. It should be installed after another type of insulation fills the joist space like fiberglass or rock wool. Foam board is great because it takes away almost every con of the batt insulation options.