Exterior Insulation And Drainage Plane Woes

The IECC’s requirement for 1” continuous exterior insulation has prompted wall configurations that integrate various types of outbound insulating materials.    With all of these systems, adequate considerations must be made for the amount of moisture penetration from wind driven rain and the presumed location of this moisture load.   It is generally assumed that exterior surfaces of the outbound insulation serve as the drainage plane.  We see many configurations of this approach – the most common being taped or sealed extruded polystyrene boards that are either foil-faced or un-faced.  The XPS may or may not contain an exterior weather-resistive barrier (WRB), but generally the wall sheathing does – intended either as protection during construction or as secondary sheathing protection in the event of water penetration.  The underlying assumption of this approach is that sealed exterior insulation serves as a perfect moisture barrier.  It is further assumed that incidental moisture behind the insulation is adequately managed by the greater assembly.  Unfortunately, these assumptions can lead to very questionable outcomes.

We have previously published Hygrothermal Snapshots showing poor moisture performance when 0.5% or 1% rain penetration is placed behind 1” XPS.   These earlier studies lacked rainscreens, but the potential benefit of a rainscreen in light of the low-permeable XPS would be negligible.   In other words, once water has entered behind the XPS, the potential for damage exists.   These earlier studies are linked below: Hygrothermal Snapshot No. 3 (Minneapolis, Minnesota)Hygrothermal Snapshot No. 4 (Atlanta, Georgia)

But What about Mineral Wool?
Energy upgrade walls for residential and commercial construction have employed exterior mineral wool panels as a ‘greener’ substitute to extruded polystyrene.  Another proposed benefit is mineral wool’s ability to release water that has entered behind the insulation. We examined 3/4” and 3/8” ventilated rainscreens on 2×6 framed walls that included 1.5” exterior mineral wool over housewrap and OSB sheathing.  The selected climate was Minneapolis, Minnesota.  The 3/4” ventilated rainscreen, with an assumed 120 ACH, provided suitable moisture performance when 1% wind-driven rain was placed solely against the exterior face of the mineral wool (Hygrothermal Snapshot No. 7):

Hygrothermal Modeling of Continuous Insulation

Relative humidity is increased by reducing the rainscreen depth to 3/8” and placing the 1% rain penetration load solely on the WRB.  Still, conditions remain slightly below our ASHRAE 160 criterion  (Hygrothermal Snapshot No. 8):

Hygrothermal Modeling of Continuous Insulation

We see similar results for walls configured with a 3/4” rainscreen and 0.5% rain penetration directly against the WRB (0.5% is also placed on the exterior face of the mineral wool) .  For example, Hygrothermal Snapshot No. 9 illustrates conditions for Seattle, Washington:

Hygrothermal Modeling of Continuous Insulation

Conclusions
The use of exterior insulation is highly problematic when considering the potential for moisture entrapment or high humidity at sheathing surfaces.   Low-permeable materials, such as XPS, require a drainage plane at the exterior side of the insulation.  But the plane must be perfect.  Flashings at wall penetrations must also be perfect.  Even small amounts of wind driven rain behind the insulation rain can cause failures.   The use of highly permeable exterior insulation, such as mineral wool, does not necessarily remove the risks associated moisture entrapment or high humidity between outbound insulation and wall sheathing.  Our studies also show that exterior insulation can reduce the benefits of rainscreens.  For example, manufacturers’ requirements for a minimum 3/8” rainscreen behind fiber cement panels may not be adequate for walls containing exterior insulation.