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.
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.
Firstly, do you live in a warm climate, where walls dry to the inside, or a cool climate, where walls dry to the outside? Given your mention of snow and ice, I’d guess you’re in a cool climate and you’re creating a vapour trap with ice/water shield and insulation on the exterior, with typical poly vapour barrier on the inside.
If this is the case, you’ll want a vapour permeable air-weather barrier rather than ice/water shield (or confirm that it’s vapour permeable) and it would be best to use vapour open insulation, like semi-rigid mineral wool, rather than EPS or XPS.
To answer your question, could you not just have L flashing from behind the furring (not furrowing) and bug screen down onto the shingles?
Thanks for the response. The barrier I choose isn’t permeable. I live in a moderate climate. We do get the occasional snow which melts within a few days. My house is pretty leaky at the moment, so the walls will be able to dry from the inside. I would like to use mineral wool in some areas (like near my grill) but it isn’t available in large sheets in my area and its R value per inch isn’t quite as good as the Poly, so I opted to go with Poly in most areas.
Yes, I could place the L flashing behind the furring strips (mentioned in my previous reply), but my hesitation there is I wanted to have a full shield of layered tape and wrap all the way up the wall and I didn’t really want to put wrap on top of the insulation because I didn’t trust that none of the penetrations I would need to make for the furring strips and siding wouldn’t leak. I saw in one video where they were doing 4" of insulation they put the rain barrier 2" back between the two insulation layers. I think this would be a good solution but the install is a bit more complicated when doing multiple layers of insulation. To do this I could have done two 1" layers instead of just 1 2" layer and put the flashing between the two layers.
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).
