Q. Is it Best to Fill the Wall Cavity Completely, or Leave it Somewhat Shy?
A. In general it is best to completely fill the wall cavity. This full cavity approach does cost a little extra in labor and material, but it provides the best results. When the cavity is only partially filled with foam, like in a flash-and-batt (foam and fiberglass system), the sides of the studs will be left slightly exposed. Since the studs act as a thermal bridge, the exposed sides of the stud will transfer heat into and out of the now only partially filled cavity, thereby bypassing the foam and reducing some of its benefits. Leaving more stud surface exposed is similar to the effects of a radiator, exposing a large amount of surface area in a short space to increase heat transfer. Further, what will result are convection currents inside the wall cavity that will actually accelerate the thermal bridging. Even if the remainder of the cavity is filled with fiberglass, the heat transfer will cause convection currents inside the porous fiberglass. In short, by fully filling the cavities we are reducing the exposed surface area of the stud. This is why people usually find that a fully filled cavity with foam “feels” warmer and better insulated. Moreover, a fully filled cavity of air – impermeable insulation may help to slow the spread of fire by limiting the transport of oxygen necessary to fuel the fire.