Modeling Attics with Insulation Extending Over the Framing Cavity

It is somewhat common in attics insulated with fiberglass batts (or other types of batts) for the batts to be deeper than the attic framing cavities, and therefore to extend several inches above the top of the attic floor.

In these cases, depending on the width of the batts, the portion of the batts that extends above the attic floor may expand outward to varying degrees such that the batts might touch or become fairly continuous.  The thermal characteristics of this situation are tricky, and highly dependent on the specifics of how the batts are installed.

When modeling this type of situation in Ekotrope, you will not be allowed to specify insulation that extends beyond the stud cavity layer.

There are several possible approaches that you can take instead:

1. (Most conservative) You can simply pretend that the portions of the batts that extend above the top of the attic floor do not exist.  In other words, reduce the R value of the cavity insulation to only reflect the R value that is nestled snugly between the studs.  So if 2” of a 10” thick R19 batt are exposed, devalue the R value to 0.8 * R19 = R15.2.  Set the depth equal to 8”.  This is most appropriate if the batts do not extend very far and do not touch each other.

2. (Moderately conservative) You can model the entire batt as if it fits entirely within the cavity.  So if 2” of a 10” thick R19 batt are exposed, simply model the batt as R19, 8” deep.  This is most appropriate if the extended batts fill some of the gaps above the studs (potentially touching each other) and restrict some of the free convective airflow.

3. (Least conservative) You can model a portion (or all) of the extension of the batts as continuous insulation.  So if 2” of a 10” thick R19 batt are exposed, model R15.2 cavity insulation 8” thick, and somewhere between R0 and R3.8 continuous insulation as a separate layer.  This is only appropriate if there is significant expansion at the tops of the batts such that the gaps above the studs are essentially filled with insulation.  This scenario might also result in compressed batts in cavities, which should be de-rated appropriately.