Insulated Concrete Forms

April 2, 2007

Photo Credit: InsulockInsulated concrete forms (ICFs) are an alternative method for building concrete walls. They are most typically used for foundation (basement) walls, but can be used in some other applications as well. Of course, they offer green benefits.

The most obvious improvement offered by using ICFs is the addition of insulation. Concrete has a very low R-value (an 8" thick concrete basement wall would typically have an R-value of approximately 0.75; even less than a single-glazed window with an average R-value of 1.0). So concrete walls offer very poor thermal performance. Even in the summertime, a concrete basement wall will be cool to the touch, because of this. Adding even a small amount of insulation to the concrete wall makes it better, and ICFs provide a good way of getting an insulated concrete wall.

This article is going to be somewhat less useful to owners of existing homes in general, since installing foundation walls is something that is usually only done in new construction. But understanding a bit more about how foundations work can help when it comes to improving the performance of an existing basement space, as well.

Contractors like them because they reduce labor costs. In typical construction, concrete walls are built by setting up reusable forms which contain the concrete and mold it into its finished shape. The forms are heavy and hard to install because they have to be strong enough to support the weight of the wet concrete and hold it in place for a period of several days to a couple of weeks, until the concrete has cured. Then, workers must be sent back to remove the forms and carry them to the next jobsite. With ICFs, the blocks that are used are much lighter and are easily carried by one person, making the preparation of the forms easier and less labor intensive. ICFs also save after the concrete is poured, because the additional step of removing the forms is omitted. The ICFs are left in place as part of the building.

Early ICFs were basically two pieces of rigid insulation foam held together with plastic ties. This would result in a full thickness concrete wall with rigid insulation on both the outside and the inside. They were originally developed for the speed of assembly and the labor savings, and the insulation was largely an added benefit.

Eco-block is an example of this kind of ICF, although they also offer configurations with thinner concrete cores (as little as 4", which is still structurally sound). More recent versions, such as Insulock, use a block that looks like an oversized Lego made of polyurethane foam. These closed cell foam blocks are lightweight, and can be easily carried and quickly installed. The open cores of these blocks are then filled with a steel reinforcing rod and poured concrete to create a structural wall with properties much stronger than a wood stud wall, much less concrete needed than a traditional poured concrete wall, and much better insulation than either one. Depending on the required wall strength, it may not be struturally necessary to fill all of the cores with concrete, leading to even more savings in materials.

Having insulated foundation walls makes for a more comfortable and more energy-efficient basement. Even for concrete slab-on-grade construction (where the concrete slab floor sits directly on the ground), having insulated foundation walls helps keep the concrete floor at a more comfortable temperature, and keeps the cold from conducting through the concrete as readily.

ICFs are not only for basements. They can be used for walls above ground level as well. The Portland Cement Association has a website with further information about homes built with ICFs, and showing examples of very conventional looking homes that are built with walls constructed in this fashion.