The most popular post on this site is, Should I Insulate My Basement Ceiling. Yay, 1st spot on Google searches! The number of reader comments is also a pretty good measure and I get a number of emails. There are tons of questions and some dissenting opinions. My view is that the easiest, most consistent approach in any house is to marry the structural boundary with the thermal, vapor and air controls as closely as possible. Put the insulation etc. where the wall is.
Put The Insulation On The Wall
It is what makes closed cell spray foam great (ignoring the merits of it’s green credentials for a moment) and the foundation wall lousy. Closed cell spray foam is the Swiss Army knife of insulation, combining the thermal, vapor and air control layers of the building enclosure in one application. Concrete foundations do exactly one thing well: sitting there and holding up your house. They have almost no insulating value (about r-1 per 8 inches of concrete), they’re porous to moisture and depending on a bunch of factors can even have some small air permeability (for example, a giant crack).
Add to that the difficulty of truly defining the building enclosure at the basement ceiling. You’ve got radon mitigation pipes, electrical penetrations, plumbing chases, chimneys, stink pipes, thermal bridging across the floor joists and oh, yeah, the stairwell. Did you insulate and seal that giant hole for the stairway? No? Then your basement ceiling isn’t the thermal boundary.
So where should we stick the insulation?
Here are a couple of thermal images of foundations:
These were taken in July with an outdoor temperature around 80 F. You can see the heat radiating through the concrete.
During the winter, these images would be inverted. Outdoor temperatures would’ve plunged to 10 F or lower and the above grade portion of the foundation would be cold. The above grade concrete would show very aggressive heat loss, right next to the hot water pipes strung along the basement ceiling joists. Yech.
Stick your basement insulation on the structural barrier to the exterior and where the most aggressive heat loss is occurring.
Options for Insulating the Basement
I’m copying this over from the other post as it is a good summary.
It is worthwhile to add insulation over your exposed foundation walls. Before insulating, the basement moisture should be well controlled with exterior and interior systems (french drains, gutters and contoured landscaping outside, sub-slab ventilation, perimeter drains and vapor barriers inside).
You should opt for a water resistant insulation such as closed-cell spray foam or XPS foam board. Avoid using fibrous insulations like fiberglass, rock wool or cellulose which could trap moisture, especially if installed with an interior vapor barrier. Be sure you check with your local building department as some insulations that are flammable – such as closed cell foam – must be covered with drywall or other approved material to prevent rapid fire/flame spread.
This article details the questions and concerns with basement insulation in greater depth.
Other Reading
For more details on the exact thermal and hydro dynamics, Joe Lstiburek of Building Science helps with ‘Understanding Basements‘.
A brief how to guide on insulating a basement wall.