There can be no question that insulation should be of primary importance when considering new house construction. Slipform stone houses are no exception to this rule. Helen and Scott Nearing, early proponents of this stone building technique, claimed that their New England homes were comfortable year-round without much thought given to insulation – indeed, their first slipform home was built right into a Vermont cliff-face, and made use of bare ledge as one wall. While the thermal mass of the earth itself surely served to maintain house temperatures above freezing, by modern standards, there can be no doubt that this uninsulated stone house would make for a chilly winter. Insulation is key to efficiency, and there are a number of ways to insulate a slipform stone structure.
Stud Wall and fiberglass batting
One common approach to the question of insulation in stone and concrete structures is the construction of a standard framed stud wall against the interior wall. This framed wall is then insulated in the traditional manner, using fiberglass batting insulation between the studs. While this approach works fine, and brings the comfort of familiarity to many builders, the construction of the stud wall is wasteful, and the studs themselves present the opportunity for thermal bridging, significantly reducing the overall insulation of the house.
Rigid insulation offers a number of opportunities when it comes to insulating slipform stone homes. Rigid insulation, correctly installed, allows for no thermal bridging and creates a tight envelope inside the house. There are a number of techniques for building this type of insulation right into the walls themselves, either by laying the insulation inside the forms as you build, or replacing the interior forms entirely with nailbase – rigid insulation mated with OSB, plywood, or even sheetrock on one side.
Adding rigid insulation after the walls are standing can present a challenge to the typical owner / builder. The biggest hurdle here is figuring out how to secure the insulation to the stone / concrete wall. One of the best ways to do this is using furring strips – narrow boards attached to the wall, into which screws can be driven. A powder-actuated nail gun, or ramset can be used to attach the furring strips to the wall. Another approach is to embed the furring strips into the concrete itself as you build the walls. The downside to this approach is that the furring strips will degrade and rot away over the decades, leaving gaps in the wall and weakening the overall structure. Fully removing the furring strips and replacing them could be a challenge.
This nifty product could be used to spray a seamless layer of insulating foam directly onto the interior of the slipform wall. The insulation would then be cut, sanded, or squashed flat before a finish layer of sheetrock is applied. Both of the above approaches present difficulties when it comes to hanging anything on the walls, as there are no interior studs for screwing or nailing into. Strategically placed furring strips could be used for this purpose.
There are any number of other conventional and unconventional approaches that you might pursue in insulating your slipform stone masonry structure, but these three options are both simple to implement and proven in many homes over a number of years. Whatever you decide, it's very important to ensure that your slipform stone house is snug and well insulated.