Insulating walkout basement wall
I’d like to correctly insulate an exterior wall of our walkout basement. The wall is 2×4 wood framed, sitting on a single course of concrete block, and sheathed with 1×12 boards. Home was built in the mid 50’s and we are in the twin cities, climate zone 6.
Having read various helpful articles on this site and Taunton literature, I have decided on XPS foam board over the existing sheathing to form a thermal break so as to avoid condensation in the wall cavity (with housewrap underneath the foam board for air barrier). The recommendation for a minimum R value for that thermal break has been stated at 7.5, so 1-1/2” board ought to do the trick. However, I’ve also read about thermal loss over time for XPS and since I’d be right on the line of that minimum, I’m concerned that over time I’d no longer have the fuller value of that original 7.5, this exposing us to condensation potential in the cavity.
My plan for in-cavity insulation is rockwool R15, which likewise creates a slightly greater than recommended ratio between the outside and inside cavity insulation if only using 1.5” thick foam board (R7.5). The difference in percentage is small but it’s there.
The simple solution would be to simply up-size to a 2” foam board externally as recommended, however, this poses other dimensional/space challenges with existing structural elements so not as easy a solution.
my question: could I add a thin layer of closed-cell spray foam insulation on the inside of the cavity against the backside of that sheathing in an effort to extend the thermal break insulation segment slightly and thus successfully avoid the condensation potential? Or is this quasi flash and batt solution really not a solution at all?
I will not be using an internal vapor retarder either, beyond painting the gypsum board that will finish the internal wall. On the outside, siding will be fiber cement on 1×4 rain screen over the foam board.
thanks for any insights you might share.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part