About the most frequent complaint we deal with is a problem in a room over a garage. Heating or air conditioning this type of room takes special consideration, and there are four different causes of comfort problems that need to be looked at:
First is the increased exterior wall surface ratio – your dining room may have one exterior wall as the only exposure to the outside, but your room over the garage can have three walls, a floor and a ceiling with outside temperatures on the other side – this means very high conductive losses. R-Value is the only way to stop conducted heat loss, so it is important that full insulation thickness be installed in walls and ceilings. Floors (garage ceilings) are typically insulated to R-19or less, but we recommend filling the cavity even if it takes R-38 or more to do so.
Improper installation of insulation is common and results in getting only a fraction of the stated R-Value in this area. Proper installation means that the insulation must touch the surface it is insulating: in the walls and ceilings, the insulation must touch the back of the drywall, however in the garage ceiling, it must touch the bottom of the plywood floor deck of the room above, the floor you stand on. Remember that no amount of insulation will bring the floor of these rooms up to the same temperature as the air in the room. Tile floors can be bad because your feet are wet and the cool floor is much more apparent even though it is the same temperature the floor would be if it were carpeted – the difference is the carpet will hold the heat as it insulated your foot, while the tile will pull the heat off your foot..
The second problem is air leakage. There is more potential for leakage in a room over a garage than in other rooms in the house because of the way it is built. Proper tightening would include checking and sealing band joists, knee walls, duct soffits in garage ceilings, block/drywall joints in garages, lolly columns and other penetrations of the garage ceiling, as well as general tightening.
The third problem is HVAC supply. We frequently find these rooms under-supplied because the increased heat loss has not been properly accounted for. We have never seen one of these room with “too much” heat!
Your HVAC installer should also take into account that these are usually the longest ducts in the house, and that they pass through the colder space between garage ceiling and room floor. This means that these ducts deliver a lower flow with cooler air, reducing the amount of heat delivered to the room.
The last problem is what we call a “sensing” problem. This room has increased heat loss, more infiltration, the air supply is slower due to duct length, and that air supply is also cooler due to the colder space that the duct must travel through. So we have a room that cools off quicker and also does not heat up enough. The thermostat is located inside the main body of the house and is “sensing” the temperature changes in, say, the dining room. The dining room, however, has an average amount of heat supply and an average amount of heat loss. If the thermostat is calling for the furnace for ten minutes on and ten minutes off, that is enough for the dining room, but the room over garage is cooling off too fast to respond to the heating cycle. The thermostat is not “sensing” the temperature in the cold room. In extreme cases, the solution is to supply a separate system that can “sense” the temperature swings in the cold room. This could be as simple as a small strip heater with its own thermostat or as complex as a motorized damper on a thermostat in the cold room.
Every room over a garage should be carefully insulated and sealed, and we recommend looking at the amount of heat delivered to be sure it is adequate.