Replies

1. Expert Member
   Dana Dorsett | Sep 04, 2015 03:28pm | #1

   Kinda depends on how many square feet of stemwall you have, and what R-value you would otherwise install between the TJIs, doesn't it?

   If your stemwall is 8' from the footing to the foundation sill 240 linear feet of stemwall adds up to about 2000 square feet. If insulated to R30 with sheet EPS at about 10 cents/R-ft^2 that comes to $3 per square foot, or about six grand. But if it's only 4' from footing to sill it's half the area, half the cost.

   Why you'd want R30 stem walls in that temperate climate is beyond me, given that code-min is R15 continuous insulation, and part of the stem-wall is below grade, with much lower heat loss numbers.

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

   The analysis by the Building Science folks also puts R15 c.i. at roughly the financial-rationality balance point, per Table 2, p.10 of BA-1005:

   http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view

   (The link to the pdf is in the lower right corner of that page.)

   If you dialed back to R15 that would be about $3000 for an 8' stemwall, and $1500 for a 4' stemwall.

   If you go that route, it's worth putting at least R6-R8 on the floor under a 1.5-2" rat-slab, so at 60-80 cents per square foot and 2600' of slab that's another $1500-2000 plus 12-16 yards of concrete (any grade). I don't know what the installed cost of a rough unfinished rat slab would be in your neighborhood- it varies (a lot) by local market. The concrete itself runs $90-100/yard (with wide divergence in some markets), so it's going to be on the order of 1500-2000 depending on how much rat-slab you're going for. Call it $3500-4000 for floor foam + rat-slab.

   Blown cellulose is about 3-4 cents/ R-ft^2 so for a code-min R30 that would run about $90-1.20 per square foot. Times 2600' would run you about $2300-3100, plus whatever sheathing you put on the bottom of the I-joists. If you bumped that to 2x code (the way you did with R30 on the stemwalls) it's $4600-6200 plus sheathing. But it also means you can't accommodate the mechanicals in the crawlspace without a performance hit.

   Batts really don't work with TJIs, since they're not designed to fit, and sculpting them to fit isn't worth the labor. Blown or sprayed insulation is really the only reasonable approach to cavity fill with trusses / TJIs.

2. 
   fourforhome | Sep 04, 2015 03:50pm | #2

   Thanks Dana for your reply. I was too abbreviated on my specs. The R-30 was for the framed house walls and the stem walls are minimal at 2 - 3' tall.
   I'll have to digest the rest of your reply at the end of the day.

3. Expert Member
   Dana Dorsett | Sep 04, 2015 04:19pm | #3

   At 2-3' tall from footing to foundation sill you might consider using R20 insulating concrete forms for the stemwalls to save time & labor. It may come in less than if pouring it with rented concrete forms and insulating it with 4" /R15 EPS on the interior afterward, and the bonding of the EPS to the concrete is already a given. The exterior above grade EPS can be finished with a cementicious EIFS material such as Quikrete Foam Coat applied directly to the foam without undue expense.

   How are you getting to R30 for the above-grade wall (and why)?

   That's somewhat on the high side for that climate zone if it's the "whole-wall R", but if it's 2x6/R23 rock wool with a couple inches of exterior EPS it's in the mid-20s, which is still in the long term financially-rational range per that table in the BA-1005 document.

   With an ICF stemwall and 2x6 wall with exterior foam, with exactly 2" of foam sheathing and 1/2" structural sheathing the above-grade foam aligns nearly perfectly with the 2.5" exterior foam of the ICF when the sill plate is at the edge of the concrete, for a perfect continouse thermal break. In termite country you may need to install copper flashing to prevent insects from reaching the foundation sill via tunnels in the EPS, and if you're mounting the windows "innie" you'd need flashing to direct bulk water to the exterior of the foundation EPS, but despite the thermal bridging of the flashing at that point it's still better performance than a stemwall with only interior side insulation.

4. Expert Member
   MALCOLM TAYLOR | Sep 04, 2015 05:29pm | #4

   Mark,
   Two other cost considerations are:

   - Whether your jurisdiction wants exposed foam covered by a fire stop and what material they accept.

   - The mechanical venting requirements for your conditioned crawlspace.