Alright, I found the leak and performed a temporary repair until I can get a roofing contractor out to torch down some mod. bit.
Now, back to the insulation sandwiched between two roofs. First of all, it should be noted that the membrane on the concrete will act like a vapor barrier. Many years ago vapor barriers were all the rage, and people were installing them without needing a vapor barrier until it was determined that un-needed vapor barriers could be detrimental to the roof system. In general, you don’t need a vapor barrier unless you are installing a roof over a cooler, freezer, swimming pool, ice rink, etc. So, I have to ask you why you would want to install a vapor barrier on a residence? I mean I guess it would be alright if the homeowner kept the thermostat at 32 degrees, but I doubt that is the case. For the sake of argument (since I’m not a weatherman), lets say the homeowner keeps his themostat at 72 degrees and it is 98 degrees outside. If the dew point is 74 degrees (I don’t know if that is possible with my example, because I’m not a weatherman), then water vapor in the air will turn to moisture (condensation) at that temperature. So, somewhere between the 98+ degrees of the roof surface and the 72 degrees of the interior temperature condensation will form, although humidity also plays a factor in this. If the 74 degree temp is attained within the concrete, no problem. If it is attained at the underside of the first layer of membrane, you wil have moisture between the concrete and membrane. If the 74 degree dew point is attained within the insulation, then that is where the condensation will form. Naturally, this moisture can also revert back into a gaseous form, and that is why some people install moisture relief vents. My one concern in the example we are talking about though, is that trapped between two roof membranes the moisture likely would contaminate the insulation. That is why when a roofer installs a roof system for me, and then wants to install a cricket/saddle overtop the roof membrane I make them perforate the membrane that will be beneath the cricket/saddle material. That way the insulation and the air space have a way to breathe.
With all of that said, dew point and moisture contamination is not the primary reason I would cut the membrane from around penetrations and along walls. The main reason is that I would want to know when the roof has begun to leak. If the top roof membrane leaks, and it eventually will leak, the underlying membrane will likely not leak. So, you will have a roof system that takes in water and releases very little of the moisture back out except as water vapor on sunny days. You can eventually end up with a “water bed” for a roof system. When you take into consideration that water weighs approximately 62 pounds per cubic foot, you are talking a lot of weight on the roof. In addition, the water saturated insulation (whether it be closed-cell or open-celled, unless you are using insulations like extruded polystyrene or foamglass) will lose its R-value. Why do you think people like me can find leaks in a roof by scanning them with an infrared camera? We are looking for hot spots in the roof where insulation has become wet and lost its R-value. Not to go too far off on a tangent, but the way an IR scan works is ideally you want a day with temps that vary at least 20 degrees from day to night. The roof system heats up in the day time to lets say 90 degrees for this example. At night the insulation and roof system are on their way back down to lets say 65 degrees; however, the wet insulation that has lost its R-value will retain the heat longer than the uncontaminated insulation. So, when you start a scan you are looking for hot spots, or areas that are retaining heat.
Anyway, not to get too far off the subject and to wrap this whole thing up, I just would not want a roof system that can trap water. Furthermore, the water trapped within the roof will attack anything that is exposed. So, if there is exposed metal or perimeter walls, these will be subject to attack from the water. Now, I will say this, if the concrete is solidly poured up to all pipe penetrations and walls, then there may not be any advantage to cutting the first roof. However, if there are openings, I would want them to be a conduit for water so I would know if the roof were leaking.
Once again, I’d want to see pictures of the roof and know more about it before I could absolutely say whether I’d want the first membrane cut or left intact. That is not to say you are right or wrong about my advice Aaron, give certain circumstances you may not want to cut the membrane in which case you would be right, but that doesn’t mean my advice was “whack.”