Soundman2020 wrote:Sorry to be rather late with this!
As Glenn pointed out, there are things you can do to help deal with it. Which it looks like you already did!
It will create a 3-leaf system, yes... but the real questions are then: How bad that would be? and Can you compensate?
There are equations for calculating resonant frequencies of 3-leaf systems, but they are a little complicated ( ), so maybe not really useful.
If you have a large air path above that "middle-leaf" and below the roof deck itself, up through the eaves then out through a ridge vent (possibly required by code, but a good idea anyway), that would help to mitigate the situation, a little. If the roof deck itself "leaks" air, such as through an old-fashioned tiled, shingled, or slate roof, that helps too. Insulation in that gap (but not filling it, due to the need for the air path) also helps a little. Making that gap as large as possible (oversize rafters, or spacers under the rafter for the middle-leaf) also helps. A large air gap between the middle and inner leaf, completely filled with porous insulation (eg, "pink fluffy" fiberglass insulation, or mineral wool, in both cases of suitable density) will also make a difference. Adding extra mass to the middle leaf and/or inner leaf also helps.
So there's several things you can do to improve isolation if you are forced to use a 3-leaf system. You can do the math if you want to figure out f it is enough, but be warned: it's complicated!
Reasonable rules of thumb: increase your air total gaps by at least 50% above what it would have been for the 2-leaf, and double the total mass on the leaves if possible (or at least increase it as much as the structure can safely handle).
Basic question underlying all this: How much isolation do you need (in decibels), and what's the lowest frequency you need to isolate? Knowing those two can help you make better decisions.
- Stuart -
Going back to this topic as I am growing concerned. Rather than packing the roof rafters with rigid foam, drywall, and rockwool, and potentially creating that 3rd leaf, would it be easier to simply add some rockwool up there, then add a third layer of drywall on the iso clips? Im worried that only 1" of space isnt going to create enough air flow to vent the soffit to the ridge vent. If I just put rockwool into the rafters, that will give me several inches of space for air to flow up and out. Then I add some additional mass to the inner leaf to compensate....