Best way to insulate attic rooms

[Wiley]

The room over the garage is cold in the winter and hot in the summer. The attic insulation was checked and OK by an insulation company. The roof was replaced last summer, installing ridge vents and removing two turbines.

As the house is reasonably new, 2007, would it profit to spray foam in the garage ceiling to reduce some of the heat leaching in the above room floor? Also, the attic now is Hades hot. I would appreciate any input from a knowledgeable neutral party.

[home improvement guy]

The mistake a lot of contractors make when insulating is that they either fail to insulate the floors over an unheated area such as a garage or the insulation is not properly installed. The conditioned air inside the room travels in all directions meaning through the walls, ceilings and floors. The walls and attic area may be insulated, but many times the floors are not.

Even when the floors are insulated, the insulation is applied from the room below and instead of touching the floor of the room above, they simply hang the insulation between the floor joists. This leaves a cavity where conditioned air can escape and be replaced with unconditioned air. The space between the floor and the ceiling below can be filled and compacted with a loose fill insulation. The most important part of the insulation in both the walls and floors is the air sealing via air barriers at the insulation.

If you have good insulation but incomplete air barriers, the insulation's effectiveness can be greatly diminished. In the attic, the roofers installed a ridge vent which is ideal for venting as long as the building has soffit vents at the overhang of the roof. Soffit vents allow air to enter the attic, travel up through the rafter spaces and exit through the ridge vent. If the attic has gable vents they will need to be covered from inside the attic to be sure the ridge vent is working in combination with the soffit vents as intended.

If you want to use spray foam, it is an ideal product for reducing heat loss or gain and it also stops air flow, something conventional insulations do not do. Spray foam may cost up to 10 times that of conventional insulation, so most contractors use it only when requested by the homeowner.