Energy Efficiency and Environmental Separation in a Small Addition Over a Crawl Space
A homeowner is adding a 40 m² one-storey sunroom to a small house. The sunroom will be heated and will have large glazing on three sides, a slab-on-grade floor over a crawl space, and a wood-frame wall assembly. The existing house has a forced-air furnace, and the contractor proposes to extend a duct into the sunroom without a return air path. The local climate is cold in winter, and the homeowner reports condensation on the existing windows. The designer must address energy efficiency, thermal separation, and moisture control, and determine whether the sunroom can be treated as part of the conditioned space or must be separated.
1. Should the sunroom be treated as conditioned space or separated from the house, and what energy and moisture issues follow?
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The decision depends on how the sunroom is built and used. If it is heated and intended for year-round use, it should be treated as conditioned space, which means the envelope must meet the applicable energy efficiency requirements for walls, glazing, and slab or crawl-space insulation. The large glazing area on three sides will drive significant heat loss and can cause condensation on cold surfaces, which is consistent with the homeowner's existing window condensation complaint. If the sunroom is separated from the house by an insulated, airtight wall and exterior door, it can be treated as unconditioned space, which reduces energy requirements for its envelope but means it will be cold in winter. The designer should not allow a hybrid where the sunroom is heated by a duct but lacks a return path and a proper thermal envelope, because that creates pressure imbalances, moisture migration, and comfort problems. A clear decision on conditioned versus separated space must be made before construction.
2. Evaluate the contractor's proposal to extend a supply duct without a return air path.
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Extending only a supply duct into the sunroom without a return path will pressurize the sunroom relative to the house. In a cold climate, positive pressure pushes moist indoor air through envelope gaps, where it can condense and cause concealed moisture damage. It also starves the rest of the system of return air and can cause the furnace to operate with unbalanced airflow. A proper design provides both supply and return, or a transfer path sized for the airflow, and ensures the sunroom's envelope is complete and airtight. The designer should require a return or transfer grille, verify the furnace can handle the additional load, and confirm that the ductwork is insulated where it passes through unconditioned space. If the sunroom is to be separated rather than conditioned, the duct should not be extended at all; a separate heating source should be used instead.