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Thoughts on PERSIST construction in Chattanooga, zone 4a

[email protected] | Posted in Green Building Techniques on

I’m designing my home using the PERSIST technique and would like input on insulation ratios that are suitable for my climate zone. I’m in Chattanooga, climate zone 4a, with 3587 heating degree days and 1544 cooling degree days.
Practically speaking, we have 3 months of trying to stay warm, and 8 months of desperately trying to stay cool.
My plan is a 1200 square foot, 2 story, flat-roofed house with a simple footprint, oriented for passive heating, windows placed accordingly.
I’m wrapping the OSB sheathing in Ice & Water shield (including the roof), layering to 3″ of EPS for outsulation (r-15), then furring and metal siding. The 2×4 walls will be covered inside with 1×6 t&g boards (no drywall, no cavity insulation, which seems crazy).
The foundation is a raised, floating slab on concrete stem walls, all insulated with 2″ borate-treated EPS, and with serious detailing at the mudsill.
The problem I’m having is an effective “case study” in my region with which to compare this to, and of the many questions I’m having, the issue of whether to insulate the cavities at all (given the R-values and lack of thermal bridging of the wall assembly I’ve described above), and whether my interior finish needs to be replaced with airtight drywall.

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  1. GBA Editor
    Martin Holladay | | #1

    You don't need airtight drywall, because the Ice & Water Shield is your air barrier.

    Three inches of EPS won't give you R-15; it will give you about R-12. (Perhaps you meant to write XPS?)

    Your proposed R-values are too low to meet minimum code requirements. In Climate Zone 4, the prescriptive requirements of the 2009 IRC call for a minimum of R-38 insulation in your ceiling. If you plan to use EPS, you will need about 10 inches of EPS on your roof.

  2. [email protected] | | #2

    Martin- thank you, and yes, that would be a thin ceiling at R-12! I'd planned on thickening it up top.
    My plan was to use the EPS, and my logic, or lack therein, is based on the articles I've read concerning long-term R values of the other foams, as well as photos of EPS being used in REMOTE walls, SIPs, and Passive Houses. I'm open to recommendations- foil faced polyiso is the runner-up,
    I'm still wondering how thick to outsulate before considering filling the cavities. For example, I would have a 50/50 inside/outside ratio with R-13 between the studs and 3" R-12 foam outside. The Alaskan formula for this ratio seems to be 1/3 inside, 2/3 outside, if any cavity insulation at all.
    Since the PERSIST technique seems to be one of the most exportable wall assemblies (mentioned in your article) in terms of climate, my question is what ratios/ thicknesses would work safely in the sweaty South? If I use 4 inch EPS foam outside, I'm insulating at nearly the same levels as parts of Alaska, at least according to some of the REMOTE diagrams. I'm missing something here (besides an engineering degree!).

  3. jinmtvt | | #3

    If you are considering a fully wrapped construction in zone 4, why consider inside insulation at all ?
    I would be pretty confortable with recommending you to use ~ 4" of EPS on exeterior walls
    and up to whatever is required for your flat roof ( stack up of EPS , don't forget to use the dense variety for the last layer )

    What is your coldest night temp during winter time? ( let's say coldest average for a few days , not a single peak )

    I just completed insulating a "re-do" of a small 14X16' roof exit on my house, using mostl reclaimed 5" EPS . Plywood with peel-stick screwed on 2X4 framing onto which i installed from 6.5" to 10" of EPS ( 2 walls accessible on the roof received 2 layers of 5" of the EPS and the 2 exterior walls 1x5" + 1.5" of R4 EPS with glued on tyvek )
    Will be using probably 2 stack of 5"EPS on the roof + 0.5" recover boards of iso before the EPDM .

    As you have more heating than cooling EPS is probably a great choice, if you have a good cheap supply in your region.

    If your temps are not extreme, you could use some thin FG bats inside stud bays maybe ?
    Depending on your dew point calculus .

    I will probably use some reclaimed 1" EPS inside the plywood surface , have some strips laying around that are asking for it . :p

    4" EPS exterior is nothing overkill, unless you plan on using more insulation at the interior.

  4. [email protected] | | #4

    My feelings exactly on thickening the EPS to 4" rather than filling the cavities, with my hesitation listed at the end of this post. I'm looking at the grade 2 EPS (spec sheet claims R4.6 per inch), which is locally available and affordable, and treated with PerformGuard. This is my last line of defense in a multi-pronged approach to keep insects out of the structure. Besides termites (we're at the very edge of the Formosan termite's northward march), I've done battle with flying ants, carpenter bees, wasps, roaches, cave crickets and black widows- several of these have been infestations, so insect management is a driving factor in the shell construction. Keeping the building dry obviously prevents the conditions that attracts them, hence the Ice and Water Shield as cheap insurance at a critical, extremely inaccessible location. Without the foamboard , however, I'd be nervous about using it. Hence the whole PERSIST package, all or none- yet there's no data for adapting it to this area.
    Winter nighttime temps are in the thirties, dipping into the single digits for 4 to 6 weeks, depending (this year was a cold one), but the primary effort here is to keep the damp heat out. The shoulder seasons seem to be disappearing.
    Detailing at the stemwall/ mudsill junction is making it difficult to push exterior foamboard thicknesses past 3 inches, which I'm having a hard time resolving. Getting full plate bearing over the stemwall, providing a ledge for the slab edge, insulating and termite-proofing the slab edge, working a termite shield in there, providing an opening for the rainscreen while denying access to insects, and water/air proofing the whole shooting match is daunting and no diagram online has completely solved it. In addition, without unnecessarily insulating the exterior of my stemwall to the same thickness of the structure above, I'm out of plane and have an aesthetic issue. Thank goodness for Sketchup- I've made a lot of models of this area, but still, no dice.

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