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.
The construction of the wall assembly has a much larger impact on the wall's ability to dry rather than the airtightness of your home. The concern is that moisture from higher humidity air within the wall cavity can condense as the exterior temperature drops and cools the exterior portion of wall. Standard practice is to have one side of the wall assembly be vapour permeable, typically the outside in cool climates. Typically, vapour impermeable poly is on the inside of the walls, just behied the drywall so the moisture of the more humid, warm air inside the home can't get into the wall. When the outside of the wall is vapour permeable, the wall is free to dry to meet the moisture conditions of the outside air, rather than condensing. But, if you install a system like you're proposing, you're trapping moisture and the likelihood of it condensing within the wall is greatly increased, potentially leading to rot issues over the long term.
I'd recommend using a high performance air-weather barrier (AWB) such as tyvek drainwrap, siga majvest, or pro-clima intello with EPS (not XPS) insulation, as its slightly vapour permeable, and don't tape the seams. Those AWBs are vapour permeable, allowing the wall to dry, but won't allow any water to penetrate from the outside.
For the bottom joint, properly installed siding will prevent nearly all water from getting behind it. The air gap behind it created by the furring will allow any moisture that does happen to get behind it to dry. The most secure solution would probably be to tape the bottom of the new AWB to the top of the shingle or whatever's the top layer, then put L flashing over the AWB, under the insulation with the top of the flashing taped to the AWB (with manufacturer recommended tape, Siga Wigluv is a good choice).