A room over a garage, over a crawl space, or at the end of a duct run has more exposed surface and less air delivery than the rest of the house. The fix is usually air sealing plus insulation at that specific assembly, not a bigger furnace.
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One cold room is a localized problem, and treating it as a whole-house heating problem is how people end up with an oversized furnace and a room that is still cold. The three usual assemblies. A room over an unconditioned garage has a floor exposed to outdoor temperatures, often with fiberglass batts that have slumped or fallen out of the joist bays, and a band joist at the garage perimeter that is rarely sealed. A room over a crawl space has the same issue with the added factor of ground-source moisture. A bonus room in a roof, with knee walls and sloped ceilings, has an insulation path most installers get wrong at the knee wall base and in the triangular attic behind it. The second cause is air delivery. A room at the end of a long duct run, or served by a duct that passes through an unconditioned attic, receives air that has already given up part of its heat. Duct leakage in an attic is a large and common loss, and sealing accessible duct joints is inexpensive. Diagnosis before purchase. If the room is cold and the supply register delivers weak or cool air, it is a duct problem. If the register delivers strong warm air and the room still loses it quickly, it is an envelope problem. Both can be true. For the envelope side, air-impermeable foam applied directly to the underside of the floor assembly or the knee wall gets the seal and the R-value in one step, which is why it is the common specification for these assemblies rather than batts. Cold floors above a crawl space are the same fix approached from below. Two assemblies account for most of these calls in Greater Columbus and Central Ohio. the mix of pre-war stock in the older Columbus neighborhoods, mid-century ranches in the inner suburbs, and large 1990s-onward subdivisions in the outer counties Rooms over unconditioned garages in the mid-century and later stock, and knee-wall bonus rooms in the newer subdivisions, are the two that come up repeatedly.