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Sandwiching XPS Between 2 Rainscreens

Samuel Burton | Posted in Energy Efficiency and Durability on

I’ve read several articles and have seen several drawings that show the general guidance for installing exterior rigid insulation. Out of an abundance of caution, I worry about the drying potential of an XPS rigid foam. 

My current home has the following specs (inside to out) planned for construction: 
Layer 1: OSB sheathing
Layer 2: Prosoco WRB
Layer 3: 1″ XPS rigid foam (perm <=1.5)
Layer 4: 1×4′ furring strips
Layer 5: Hardie cement siding

My issue is that water will eventually get behind the XPS. Would it be reasonable or overkill to create another air gap between the WRB protected sheathing and rigid foam? 

Alternative construction:
Layer 1: OSB sheathing
Layer 2: Prosoco WRB
Layer 3: Air gap by furring strips or a rainscreen product such as Benjamin Obdyke SlickerMAX or similar product
Layer 4: 1″ XPS rigid foam (perm <= 1.5)
Layer 5: 1×4′ furring strips
Layer 6: Hardie cement siding

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