How best to insulate in DFW area (zone 3) – new construction
Hey guys,
Brand new here & first post. We lost a house to the Christmas 2015 tornado and are preparing to build again. I have several questions but here is my biggest one today… My previous house was 2×4 with open cell in the walls and ceiling. Now, I’m considering 2×6 & full encapsulation with foam but I’m seeing a lot of information about breaking the thermal bridging. So, my question: do I need to install a minimal 3/4 to 1″ of rigid foam? If yes, does the recommend order look like this: inside>sheetrock|2×6 w/foam|osb|wrinkle house wrap|foam|rain gap|hardie? And finally, does everything pass the OSB hang beyond the concrete slab? Just wondering how you protect the bottom edge from bugs and varmits.
Thanks for all the help/suggestions,
Shawn
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Replies
Shawn,
Q. "Do I need to install a minimal 3/4 to 1 inch of rigid foam?"
A. In your climate zone, there is no minimum foam thickness. But if you care about the thermal performance of your wall, you will install at least 1 inch of rigid foam (R-4 or R-5). Thin foam doesn't do as good a job of interrupting thermal bridging through the studs as thick foam.
Q. "Does the recommend order look like this: inside>sheetrock| 2x6 w/foam| OSB| wrinkle housewrap| foam| rain gap| Hardie?"
A. If you are installing exterior rigid foam -- and it's a good idea to do that -- I recommend that you install cellulose, blown-in fiberglass, or mineral wool batts between your studs rather than spray foam. To learn why, read this article: How to Design a Wall.
Q. "Does everything past the OSB hang beyond the concrete slab? Just wondering how you protect the bottom edge from bugs and varmints."
A. I suggest that you watch this GBA video -- it should answer your questions: How to Install Rigid Foam Insulation Outside a House.
In tornado alley it's worth considering building with concrete. A minimalist 2" + 2" insulated concrete form beats code minimums (by a lot), but will still be there after the next tornado hits, even if the roof and windows disappear. That's only R 18-R19 whole-wall, but with the mass benefit that's plenty sufficient wall performance for building a Net Zero Energy house with a solar array that fits on the roof. (It'll be the quietest house that you ever lived in too!)
In a zone 3 climate it's possible to hit Net Zero Energy type performance can also be had with a 2x6/R20 (open cell foam) + R6 (1" of foil faced polyiso) type construction when done air-tight. It's not nearly as tornado resilient as concrete, but it costs less than concrete too. Going a whole lot higher than that for wall performance doesn't buy much in your climate the way it would in much colder climates.
The other factors that would take get you to Net Zero type performance are:
~R50 attic instead of a code-min R38. Doing it with 14-14.5" of blown cellulose in a 15" deep raised heel truss is pretty cheap and effective. Cellulose is somewhat more effective in the cooling season than fiberglass. You might consider CRRC rated cool-roof materials for the roofing too.
R8 (2" EPS) at the slab-edge (or R10 continuous insulation on a crawlspace or basement wall)
R4 (1" EPS ) under the full slab
U0.30 or lower windows, with an SGHC 0.3 or greater (may need to be adjusted downward for SGHC on west facing windows).
Orient the roof pitches for favorable sun, with few or no stack or flue penetrations or dormers etc to interrupt the plane of the south facing pitch.
Now that Austin's building codes require Net Zero Ready for all new residential construction, there are probably designers & builders reasonably close to you who know how to get there most cost effectively at local material & labor pricing. Doing some online or phone research on how new houses are meeting code in Austin will probably pull up some insights.
Thanks Martin & Dana for the responses... here are a few more follow-up question for clarification.
1. Definitely wanted to use R-4 to R6 rigid foam - any suggestion on what is the best bang for buck? And should it be foil backed?
2. OK, I'm obviously trying to do this as cost effectively as possible (staying in budget). So, maybe blow-in dense cellulose would be cheaper than open-cell foam. I guess my concern is will the house be well sealed compared to the foam?
3. Is there a good source for this sill plate flashing at different widths? And is there an article that better describes it w/photos? Should I use perforated or solid flashing? Here is what I've seen or understand: flashing is nailed to osb, then the tyvek-d is put over the OSB with it ending up ending in the flashing channel, 3/4-1" foam board on next, rain-screen & finally plank goes on last but beyond the flashing. Is that correct?
4. Do I really need a rain-screen gap in DFW area? This really increased the width beyond the slab...
So far, I'm getting a lot of crazy looks from general house contractors.
Thanks,
Shawn
Shawn,
Q. "Definitely wanted to use R-4 to R6 rigid foam - any suggestion on what is the best bang for buck?"
A. The best bang-for-your-buck rigid foam is used (reclaimed or recycled) rigid foam. Check Craig's List, or local companies that offer used foam.
Q. "And should it be foil backed?"
A. Foil facing is OK but not required.
Q. "Maybe blow-in dense cellulose would be cheaper than open-cell foam. I guess my concern is will the house be well sealed compared to the foam?"
A. No matter what type of insulation you install between your studs, you (or your builder) needs to care about air sealing details if you want to get a good blower door test result. Pay attention to seams, intersections, and penetrations -- are read up on air sealing here on GBA.
Q. "Is there a good source for this sill plate flashing at different widths? And is there an article that better describes it with photos? Should I use perforated or solid flashing? Here is what I've seen or understand: flashing is nailed to OSB, then the Tyvek-d is put over the OSB with it ending up ending in the flashing channel, 3/4-1" foam board on next, rain-screen & finally plank goes on last but beyond the flashing. Is that correct?"
A. There are lots of ways to screw up flashing details, so you (or your builder) needs to have a good understanding of flashing basics to get these details right. If you feel nervous about this, start reading GBA articles. I'll try to provide links in a future response.
Q. "Do I really need a rainscreen gap in DFW area?"
A. It's a judgment call. Here is a link to an article you should read: All About Rainscreens.
If local contractors aren't used to exterior foam it's probably better to go with 1.5" ZIP-R, which goes up pretty much like standard OSB sheathing. With ZIP-R the 1" polyiso is between the OSB and the framing and it's a one-step process, not sheathing + housewrap + exterior foam in separate steps, and it simplifies the flashing, since ZIP-R has a factory applied spray on weather resistant barrier. DO pay attention to the seam taping details to get it right, but this isn't rocket science. This has fewer ways for less experienced crews to screw it up than the 3-step approach.
With ZIP-R you also don't have the inherent 3/4" rainscreen gap you get with 1x furring. You still want some amount of vented space between the siding and sheathing, but in your area 1/4" is more than enough. Stapling 2" wide strips of 1/4" fan-fold XPS over every stud location would provide that important capillary break & drain space between the fiber cement siding and the sheathing.
In areas where blown cellulose or blown fiberglass in wall cavities isn't common it will often be cheaper to go with open cell foam. My impression is (rightly or ronglee ) that open cell foam is more common than cellulose in Texas. If there is a damp-spray cellulose contractor available that may come in cheaper than o.c. foam, but if it has to be blown in netting it'll probably cost a bit more. Get quotes.
Even though ZIP sheathing is pretty air tight due to the exterior tapes seams, you still have to caulk the perimeter of every stud bay to the sheathing when using fiber insulation. Whether fiber or o.c. foam, doubled up framing such as top plates need to be caulked. Sealing the stud plate to the foundation can be done using EPDM sill gaskets, but the foamy types don't usually cut it without a bead of expanding can-foam.
Does anyone have any thoughts on ISO Red or Styrofoam SIS compared to Zip R insulated sheathing? The one thing that seems odd about the Zip R insulated is the foam is on the inside and the OSB is on the outside... is that OK? Thanks,Shawn
ZIP-R has been engineered to be structural sheathing despite the fact that the OSB is on the outside of the foam. The fastener specs and spacing need to be followed though- it needs longer nails than standard OSB with no interior side foam.
With SIS the structural part is next to the studs, which means longer fasteners for hanging the siding. With the 1" foam SIS the siding nails might have too much moment arm to support fiber cement siding, in which case you'd need to hang 1x4 furring to hang the siding on, which adds to the wall thickness.
ISO Red is not structural, comparable to other 2lb density sheet polyiso.