I am replacing the siding on one wall of my house. I am going all out with it and including a rain screen in the design. It will consist of the sheathing, ice and water shield on sheathing, 2" Polystyrene Board Insulation, furring strips, and finally Hardie Panel siding. There will be a top and bottom bug shield (Cor-A-Vent). The Polystyrene boards will sit on top of a 2"x1.5" board screwed into the sheathing and taped to prevent water intrusion (tape layered under ice and water shield).

This wall is above another roof, meaning there are shingles that butt up against the wall with flashing. My assumption is that the flashing is done between the shingles and the wall sheathing and the 2"x1.5" board floats just a little above the shingles. This allows for the air gap for the rain shield and keeps the water barrier at the roof line along the flashing and wall sheathing (not the 2"x1.5" board or Polystyrene surface).

My confusion though is that the bottom of the 2"x1.5" board will be exposed underneath (I wasn’t planning on taping the bottom of the board since the tape might come loose and drop down or hold moisture). This might only be a problem in snowy and icy conditions, in most weather, in my area, it would dry fairly quickly I think. I was planning on a 7/16" gap between the 2"x1.5" board/siding and the shingles (same as the gap for my rain screen), should it be larger, should it be a 2" gap? Somewhere in between?

Here is a side view:

  • |#^)
  • |#} )
  • |#} )
  • |%^)
  • L--------

| = Sheathing, L = Flashing, # = Insulation Board, % = 2"x1.5" board, } = furring strips, ^ = Bug screen, ) = Hardie board, - = Shingles.

  • @pdavisOP
    1•1 year ago

    On further research it seems that in this case I could have brought the polystyrene all the way down to the roof deck and flashed it with the shingles (cutting them back 2") to make the front of the polystyrene the rain barrier. Assuming I could do a good enough job to guarantee no water penetrated behind it. This would prevent the need for the 2"x1.5" board all together and the gap all the way to the sheathing which would be a big source for thermal bridging and transfer. Not sure I am confident enough to take this approach though and the flashing has already been installed by the roofer.

    So my question remains, how big should the gap be above the shingles.