On Tuesday my friend Grant came by to say hello and check in on the progress at The Casa. He made a comment along the lines of ‘that doesn’t look like any framed house I’ve ever seen…’ Since I have heard that several times since starting the project I thought I would take a minute to explain what Insulating Concrete Forms (ICF) are and how they work.
Although it is not yet widespread in our area, ICF represents a whole new set of options for new buildings, including residential construction. ‘Stick built’ or wood framed construction is still the prevailing choice, and it seems that most of the subcontractors are so accustomed to working with wood framed walls (read: they can do their work very quickly without a steep learning curve) that they don’t want to do anything but that. There have been some very welcome exceptions to that to be sure, and I’m sure in time more of the trades will continue to focus on this new building trend.
Even construction with brick or stone, like my father’s house, is essentially a facade applied over walls constructed out of (mostly new growth) timber. This is necessitated by the relatively low insulating value of brick or stone on their own. They are all fairly good thermal conductors, meaning that temperature changes on one side of the wall will be relatively quickly transmitted to the other side. The use of fiberglass or foam in the stud cavities of wood construction allowed houses with masonry construction to improve their insulation factor.
Decades ago a primary limiting factor for the use of concrete was aesthetics. While construction with concrete reinforced with steel was fairly cost effective and enjoyed more longevity than the average wood framed house, overcoming the ‘ugly’ factor was an issue. More common in commercial construction, penetrating the residential market was difficult when the primary option was to paint the concrete walls (or leave them bare). Couple this with the fact that concrete alone is also a very good thermal conductor, and it is no surprise that there remained relatively few concrete houses.
These insulation and aesthetic issues are both soundly addressed through the use of ICF construction. There are a number of different manufacturers of the blocks (Logix, Fox, Nudura) but all employ an elegantly simple approach. Instead of a single-layer concrete wall, or a layer of wood frame covered with a brick or stone veneer, Insulating Concrete Forms consist of three layers. A standard steel-reinforced concrete wall is sandwiched in between two layers of rigid foam insulation. The materials themselves are cost competitive with standard construction, and the systems are arguably more simple to construct than standard stick-built homes.
The standard building block (in our house at least, built with Nudura) is shown above. It is 8′ long and 18″ high. The inside and outside is made of rigid foam between 1 1/2″ to 2″ thick. The top and bottom of each form has teeth (like Legos) that allow them to easily fit together to build up the wall. In the space between the two sheets of foam are rigid plastic webs that space the forms apart in addition to providing an easy place to snap in the steel reinforcement (rebar). At the top and bottom of each plastic web are small plastic teeth that click together when you assemble the forms. These teeth keep the forms from ‘floating’ apart when they are filled with liquid concrete. These straight pieces are combined with corners and t’s (all with the same interlocking components) to essentially form the negative wall space. Once the forms are assembled (an incredible variety of shapes and configurations are possible by combining different forms) you literally pour in concrete from the top to form the core of the wall. The foam remains in place and becomes a permanent component of the structure of the building, providing a huge insulation boost among other benefits.
Concrete construction that doesn’t use ICF involves building the forms, pouring the concrete, and removing the forms once the concrete has cured - which adds labor to the process and takes away from the r-value of the finished product. So regardless of the system you choose, you wind up with a simple mechanism to build the forms (anyone who can assemble legos can put blocks together), great insulation for the walls, a good vapor barrier to prevent moisture from coming into the house, as well as ideal surfaces on the interior and exterior to attach siding or drywall. I can’t speak for the other systems, but Nudura has a nailing strip that runs vertically every few inches, and acts just like a stud in a wood framed house - allows for trades to drive nails or screws.
There is a great deal of information about and discussion on the benefits of ICF construction just a Google search away, but there are a few that helped us make the decision, including the low cost of heating and cooling, sound insulation provided by the thick walls, and best of all no wood to decay and need replacing down the road. Additionally, while the expansive pale green walls may look a bit odd to folks like Grant right now, by the time the construction is complete and the siding is applied the house will be virtually indistinguishable from a traditionally constructed home.
Hopefully that is helpful in understanding ICF construction, please don’t hesitate to ask if you have any questions or I can clarify anything.