Sorry about the length here, but this is a little complicated.
Our house has just been rebuilt after a fire. The original frame including an addition that existed before the fire has mostly been rebuilt as it was. The wall between the original house frame and the addition was so charred it had to be removed and replaced, so we had a laminated beam put in and re-positioned the wall when it was rebuilt for a better kitchen.
The first floor addition area has R30 insulation and bubble foil radiant barrier in the ceiling below the attic space. The room is toasty warm, and the attic space is really freezing cold, as it should be.
The laminated beam creates a wall between the attic where first floor extends further out than the second floor in the back of the house and the space between floors. There is also an area where a concrete block wall also cuts off the addition attic space from between the floors. However, there is a space of four joist bays between the end of the laminated beam and the end of a second floor laundry room where it appears that the joists in the first floor ceiling extend beyond the end of the second floor space into the attic space. There is nothing but the batt insulation to block air from the attic space from entering between floors.
The picture below was taken above this open space. The OSB sheeting is the exterior wall of the second floor laundry room. The joist bay with the insulation sticking up the tallest is the bay where about a foot back further back just under the laundry room are the pipes for the washing machine supply. You can see the top of the laminated beam in the background just left of the center of the frame. There is a tiny insulated gap for wires to run with continued sheeting above it.
The laundry room has waterproof vinyl floor tile. Water spills in the laundry room flow into the space between the wall and floor and then over the edge into the pictured open joist bays and through the insulation and/or run along the ledge to pour down into the kitchen. The contractor is supposed to put vinyl base edging into the laundry room and seal it to the floor, so spills are trapped to mop up. I did not want a drain in the laundry room floor as the trap would probably go dry from evaporation under normal use.
The darling wife has managed to come up with some creative ways to spill loads of water in the laundry room. She would connect the washing machine to use it for a load when it was disconnected for work on the pipes, such as when they froze. Let's just say that it was trial and error for her to figure out how to connect those hoses.
There also was a screw-up where the either the one of the construction crew or the siding contractor installed a dryer vent on the outside where it was cut through the siding but not into the laundry room. We had a proper vent installed in the correct location. However, a set of comical errors on the part of the contractor's employees unloading and loading his truck when he tried to bring over the replacement siding piece ended up leaving the hole in the siding with just tape covering it during some heavy rains. As a result, there is some water damage in the ceiling below those open bays. The siding has been repaired since then.
The batt insulation in those open stud bays is now quite wet and cold.
Last week the laundry washer supply pipes froze. We cut the wall to check the pipes in the laundry room, and there was a freezing wind shooting out of the holes in the floor where the pipes came out. They were frozen by the elbows in the second floor space just about a foot inside from the exterior sheeting seen in the picture. I let the pipes conduct heat from the room with the valves open and supply hoses in the sink until they unfroze. Fortunately, the pipes did not burst. It turns out the wet R30 batt insulation is not enough to stop the airflow from the super cold attic space into the second floor space. The extreme temperature difference increases the airflow.
Just to add to it, when water spills into the open joist bays, it pours down into a smoke detector in the first floor ceiling. Smoke detectors in every room of the house are hardwired into AC power with a battery backup and are networked. If one goes off, they all go off. The water triggers the detector which sets them all off. The children wake up in a panic. This is not good for them when we have just moved back into our house after they had to previously jump out the window of a house in flames in the middle of the night.
The frozen pipes adventure led us down the detective path to access the attic space, find this all out and put it together.
The contractor is committed to making this right. The only question is how to make sure it is done properly.
The easy solution would be to replace the wet insulation from the top, and then cap the open bays up to the ledger/header cross beam (on the right side of the picture) to block the airflow from the attic space to the second floor space. The wires moving though those bays in the picture make that impractical. Can wood sheeting go in between the joists to make a wall extending into the floor space from where the exterior wall of the second floor is? That may not be possible to reach into or secure properly.
Is there some type of waterproof insulation dense enough to stop airflow that will give at least an R30 rating to use instead of fiberglass batt in those stud bays that would solve the problem? Would the Roxul rock wool batt insulation do the trick there?