When performing home inspections and energy audits, I see many well-intended insulation projects that actually end up doing more harm than good.
Here are a few examples of well-intended projects that went awry.
The reasons for insulating the rim joist are important: the R-value of the this framing member is minimal (around R-1 per inch), there is often air movement from outside and between floors, and maintaining a more consistent temperature while reducing relative humidity is beneficial for comfort and building science reasons.
During a recent energy audit, I noticed that the client had installed fiberglass batt insulation. Better than nothing? I suppose. The problem is that batt insulation is not an air barrier and therefore does nothing to stop air movement. And when it is not touching the rim joist, it is allowing pockets of air to change temperature. At right you'll see that this rim joist has a few areas where moisture has appeared. Some of the insulation had become wet (left-most arrow) and was matted.
With a cold rim joist, the warmer air in the area between the insulation and the joist cools as it moves from warm to cold (Second Law of Thermodynamics abbreviated: energy moves from hot to cold and never the reverse unless due to an outside force).
When the warmer air cools and hits the nearest coldest surface - the rim joist - the air becomes more dense, reaches its due point and condenses. Now we have moisture trapped under insulation, which is not a good thing. This is a relatively minor example, but the home is just 2 years old. I'd hate to see it in 10 years.
In this case, one of a few things likely happened with the insulation contractor's installation: (1) The building components were too cold when the insulation was installed, which can prevent proper adhesion, (2) The insulation was applied too thick and layered too soon, which would prevent each layer from curing properly, or (3) Part A and B of the insulation mix was improper.
That's an expensive call-back for the contractor.
Here's a thermal image of what this looks like.
In this case, I crawled into the attic and a wad of old plastic wrapping was keeping the insulation from making contact with the attic floor.
In homes with sloped ceilings such as a cap cod, the area where the ceiling and the knee wall meet is often an area where insulation easily settles or is poorly installed in the first place.
In this case, the batt insulation was not fastened at the top of the knee wall, had fallen backwards and was just hanging there.
You can see the heat from the crawlspace. It doesn't take much for it to conduct through the drywall and cause the room to be cooled even further.
The insulation simply needed to be refastened to the framing members.