Exterior insulation details for a first floor walls in combination with a superior XI foundation and brick ledge
BillEv
| Posted in Plans Review on
Hello,
Putting the final plans together with my builder in SE Michigan, energy zone 5a. Single family residence with one side walkout basement. 3 sides foundation walls in the basement will be below grade. The exterior cladding will be face brick. The exterior walls will be 2×4 with 3″ of closed cell spray foam. I’d like to add 2″ external XPS foam and utilize a drainable housewrap.
A couple challenges with this particular combo of materials.
1. Per my builder and some example manufacturing prints from Superior wall, the brick ledge is 4″ deep. With the 3″ brick, 1″ air gap and 2″ insulation, I’d need 6″. I have a call into superior to see if they can do a custom 6″ deep brick ledge, until then, I assume 4″ is what they provide.
2. Builder said pull in the 2×4 wall towards the interior the required 2″ and he’d use some z-flashing to allow for drainage. To me, this creates perceived air flow and drainage questions for any moisture trapped between the XPS and drainable housewrap since the air flow and drainage route is no longer completely vertical.
Has anyone ran into a similar issue or can anyone talk me off the ledge ? I’m on the verge of giving up my desire to install exterior insulation with this particular set of materials, even though I highly want to externally insulate.
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Replies
Bill,
I'd have no problems with that detail. With your assembly you already have a rain-screen cavity behind the brick. This secondary one is for incidental water that may get behind the foam.
The drainage plane is still continuous. That it moves outward at the bottom of the wall is much the same condition you see over windows and doors with head-flashing.
There is no appreciable air-movement in the small cavity created by drainable house-wrap, whether the cavity is open a the bottom, or diverted by flashing.
Since you have an adequately think layer of continuous exterior foam, is there any advantage to the spray foam in the stud cavities over using batts?
Malcom,
Honestly, it's the preferred technique of my builder and I'm less inclined to fight him over that detail and keep working out other kinks in the design. I wanted to use 2x6 "for strength" in the wall with hd cellulose and zip R6, but he convinced me to go this route. He did say he'd build any wall I wanted, but I decided to leave it at this. I did add the external insulation onto his preferred wall assembly. I was going to do 1-1/2" but figured might as well do round number 2". I had to make the foundation 2" larger here and there to keep all the room sizes the same.
I'm fine with the spray foam and don't want to back track too far on my CAD Work and detail work.
Thanks for your input on the 2" recess.
Bill,
I was just curious. The wall should perform well.
Good luck with your build!
I also don't see any issue with that detail. My own preference though would be to put the housewrap on the exterior of the foam, buck out the windows 2", and order windows with jambs for 2x6 walls.