Insulation in a new flat ceiling under an old vaulted ceiling?
We are in the middle of an addition on the back of our house and have run into a question about insulation in the existing portion of the house. We currently have a vaulted ceiling covering the front room and the back kitchen (separated by a wall and an opening with the peak of the vault mostly overtop the separating wall). The new addition has 8' walls with a 12" stepped tray ceiling in the middle. To tie all the rooms together, we have furred down and built a flat/stepped tray ceiling below our vault in both the kitchen and the front room. It honestly looks fantastic We are very happy with this change and the way it makes the front room feel more formal/intimate, and combines the new kitchen with the new living room into an 'open concept' space
The problem comes in what to do about insulation. We left the vaulted ceiling where it was to maintain the insulation value, but after putting up the flat ceiling, I note that there is a lot of heat seemingly trapped between the new flat ceiling and the old vaulted ceiling (a 'dead space'). That dead space is closed in with no venting and I'm wondering if this is going to be a problem?
On one hand, the blown in insulation in the attic should provide the needed R-Value. On the other hand, the 'trapped' heat up there could possibly cause problems with warping the drywall, or even possibly retain moisture that could then become a problem later.
SO - the question is: Can I leave the existing insulation above the vaulted ceiling and close in the new 'dead space' with no worries, or do I need to vent that dead space add insulation? Assuming the first, then we are good to go. Assuming the 2nd, then I'll need to vent that new dead space out into the existing attic by cutting holes through the vault, and call the insulation company to come back and add into that dead space.