Photo 9: Trenching For Perimeter Drain. Most rubble foundations have very thin and weak ground slabs—if they have anything at all besides dirt—that are easy to break through with simple tools such as a pick, a pike and sweat. Figure 2: Interior Perimeter Drain. The trench is lined with a geotextile filter fabric and contains a perforated drain and gravel with no fines. The drain tile is connected to a sump crock or to daylight. And then the interior membrane lining and spray insulation thing happens. Epoxy coating over the top surface of the slab reduces vapor transmission into the space. Photo 10: Capillary Break Critical. Many rubble foundations do not project much above grade. The installation of a robust capillary break is a necessity along with lowering the immediate adjacent grade. Photo 11: Rotted Bearing Timbers. Many of the bearing timbers typically have to be replaced due to serious rot. Photo 12: Rebuilding Sill Assembly. Bolts and hangers and more bolts and hangers and still more bolts and hangers. bearing timbers typically have to be replaced due to serious rot (Photo 11) and the sills have to be rebuilt (Photo 12). These are not options, especially when insulation is added from the interior. Energy and airflow is no longer available to assist in drying. Wetting from underneath needs to be eliminated, and wetting from the outside needs to be controlled (exterior drainage planes 76 ASHRAE Journal and back-ventilated claddings, or at minimum a back-ventilated “water table” trim detail). Tell the Historical Commission that grandma heard her grandma say that the house used to have one. Where headroom allows insulating, the top of the existing slab works well. Note the foam bathtub in Photo 13 and Figure 3, but also note the “twist” in the approach. The cleverness of this ashrae.org March 2010