High Performing Buildings - Spring 2009 - (Page 58) covered on the inside with 4 in. of closed cell spray foam. The exposed sills at the basement were sprayed from the inside as well. Then the exterior above-grade walls were filled with cellulose, using the traditional method of making small holes in the exterior sheathing. The two 2-in. layers of polyisocyanurate were installed over the whole exterior wall. The roof vents were sealed, and the wall-roof intersections were lined from the inside with building paper to provide a temporary dam for the application of spray foam from the interior. Two additional 2-in. layers of polyisocyanurate were then installed on top of the roof deck. The transfer grilles above the hallway doors provide a return path to the air handler. Vapor Diffusion Profile The roof assembly with its layers of insulation, plywood roof deck, and self-adhering membrane is impermeable. The exterior walls of the house were rendered vapor impermeable by the addition of the exterior foil-faced polyisocyanurate insulating layers. However, the thermal resistance of a i r f l ow D e S i g n the insulating sheathing is sufficient to stop any condensation on interstitial surfaces that might otherwise occur. A small area of the basement wall was left exposed above grade, allowing the concrete and stone structure to dry to the outside. Mechanical System Upgrades The draft hood water heater was inefficient and vented through the masonry chimney, so it also was removed. A low-mass condensing boiler was chosen to provide space heat and hot water, and connected to the air handlers described below. A well-insulated storage tank provides domestic hot water. Three zones are tapped from the boiler, one for each air handler and one for the hot water storage tank. One air handler located in the basement serves the basement and first floor. One central return for that air handler is located on the floor of the first floor central hallway. The other air handler, located on the conditioned attic level, serves the second floor and the attic. One return is on the ceiling of the second 58 HigH Performing Buildings spring 2009
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