I have an existing beam ceiling with a thin layer of 3/4 inch foam sheet insulation on top of the roof sheathing.I would like to insulate between the beams with some kind of insulation. I had an estimate for spray in foam but the company said they didn't recommend this because it would change the dew point because of the existing sheet insulation. I do not have the ability to vent this area this is why I chose spray on foam. Can some one give me ideas I have a lot of heat loss through the ceiling.