There are several factors that can cause the room over the garage to be cold in winter or warm in summer.  Wall Insulation – Some rooms over garages have “knee walls” (i.e., walls with open attic space behind), the insulation in a knee wall is often left exposed on the attic side, which degrades the effectiveness of the insulation.  Floor Insulation – Houses in New Jersey prior to 2007 were built to comply with the 1995 CABO MEC Energy Code, which required insulation to be installed under rooms over a garage but with no requirement on the quality of its installation, the insulation was poorly installed down on the garage ceiling.  Airflow – in houses with only one furnace in the basement, this room is the furthest from the furnace, resulting in longer duct runs and more bends than the ducts that serve the other rooms, thus reducing the amount of heating airflow that reaches this room. Oversized Equipment –  In many old houses, the furnaces typically have 50% or more heating capacity that the home requires. The room where the thermostat is located reaches the set temperature causing the system to shut down long before enough heat reaches the furthest rooms from the furnace. Houses with forced air heating systems are comfortable when the system is running, then start to cool down when the system shuts off, oversized systems result in this heating / cooling cycle to happen more frequently than a proper sized system. (Read full article)