One of the most commonly used insulating materials for the basement is known as extruded polystyrene, which is also commonly referred to as XPS. These are solid sheets of synthetic foam. While it has been around for some time, the way in which it has been used has gone through quite a few changes.
The material was first invented by Dow Chemical. It is made from a compact polymer that water can not pass through. This makes it ideal for dealing with the moisture that commonly accumulates in the basement of residential homes. It works well in concrete flooring, which also makes it beneficial for use in the garage. The material is so resistant to water that it is also used in the construction of docks.
Despite its usefulness, there have been significant changes in the way that it is used as an insulator. In the past, it was sandwiched between two pieces of plywood, and either screwed or nailed to the boards. This method was effective, but not ideal, considering the fact that the nails put holes directly through the insulation. It also took up quite a bit of space, reducing headroom in the basement. There are some contractors who still use this method today, unfortunately.
One of the first changes that was made was to start adhering the polystyrene sheets directly to the concrete floor. The plywood was then installed on top using adhesives, rather than nails. While this system was an improvement, it required concrete screws to prevent the plywood from warping out of shape.
Today, subfloors have been improved upon to include air gap insulation. This system makes it possible to install the insulation without needing to secure it to the concrete. This offers several advantages because there are fewer materials, installation is easier, and there is no wood to break down.
These new panels are held in place using airtight gaskets, so that the insulation is literally floating on air. A secondary advantage of this is the fact that it protects families from radon gas.
XPS is also highly effective as a wall insulator. Fiberglass insulation is not as effective at preventing moisture from creeping into the basement, and it can only be installed between the wall studs. XPS can cover the entire wall, and is much better at resisting moisture.
Originally, the insulation was installed on the inside of the basement. Newer homes install it either on the outside, or even directly inside of the concrete. There are, of course, practical concerns that make it difficult to install the insulation on the exterior of older homes. It is still considered to be more effective to install it in the interior than not at all.
While the methods have changed over time, the effectiveness of the material has only improved. While it is not the only insulator available, polystyrene is one of the best.