Gas furnace in SPF sealed attic

I have a gas furnace in an attic that I am getting ready to insulate with spray foam on the roof deck. This will seal the attic area in except for any infiltration from the living area below. I don't think there will be enough oxygen to fuel the furnace and I don't want to open to the outside to ventilate the attic which would defeat the purpose of sealing with the foam, because instead of the attic space being 5-10 degrees difference from the living space, it would then be the outside ambient temperature.

Does anyone know how to solve this problem? Is there some type of air exchanger that would feed the furnace???

I would appreciate some help here.

Thanks, David