Gas furnace in SPF sealed attic
The purpose of applying spf (spray polyurethane foam) to the roof deck is to seal the attic area in. It is sprayed on the roof deck and down over the soffit area to effectively seal in the attic. The temperature in the attic is then only 5 - 10 degrees differential from the living area below. The advantage of the foam over traditional insulation like fiberglass or cellulose is that it insulates and seals out all infiltrating air. It is usually not completely sealed because there is still air infiltration from the living area below. The result is less wear and tear on the hvac equipment, less loss/gain in ducting, and so on. In some new construction where the walls, floors and attic are sealed, an erv or hrv vent system is required. To spray foam the attic deck is a common practice.
My question of course is whether there is enough oxygen to fuel the furnace. If not, is there some type of air exchange system that can be installed that will feed the furnace but not compromise the seal.