What happens when the space between your floors is connected to an attic space or directly to the outdoors? Condensation and lots of it. In the picture above you see the floor joists being sealed from an attic space behind. The client called because the insulation on the ducts was saturated and dripping down on to the ceiling tiles. The ceiling tiles were falling from the weight.
The problem was warm moist air was allowed to contact the ducts, and even the R-3 installed was not enough insulation to prevent condensation from the metal duct in an attic-like space. The solution was to seal the ducts, apply new insulation on ducts per code and create an air barrier between the indoors and outdoors.
We used a perforated foam board aligned with the exterior of the wall, R-13 fiberglass then solid foam board aligned with the interior of the wall. After this photo we used a foam sealant around the perimeter of each piece of foam board to make it air tight. Within an hour of putting in the first layer of foam board to seal the walls the duct work dried up.
This home had an obvious issue that was bad enough to expose itself to the home owner. But, often these conditions exist and are creating moisture, energy and comfort issues in the walls and floors without being detected. An Energy Auditor can help you find these weaknesses in your home. You can also look for them yourself in areas where the exterior wall does not align with the next level exterior wall or if there is a porch or overhang outside. For this home it had both; on one side the ceiling joists extended outside for a porch and on the other side of the 2nd floor was cantilevered out about three feet.
In this next photo the home has brick on the bottom two levels and siding on the top. The framers cantilevered the floor joists out 5 inches so the brick and siding would align. But this left a 1/4″ to 3/4″ gap all the way around the home. This meant outdoor air was between the floor joists. You can tell air was flowing thru because the spiders had built webs to capture bugs traveling with the air flow. We addressed this one from the outside, pulling the bottom piece of siding and air sealing.
Both of these were older homes, but sadly homes are still being built today with these kinds of issues. If you want to ensure your home is being built to current energy code hire a 3rd party inspector such as a HERS Rater.