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Mixed-humid climate construction details

Rob R | Posted in General Questions on

Hello,
I am in the planning phase of building a home in Knoxville, TN (Zone 4). I have spent a ton of time reading material on this site and have come up with a preliminary plan that I have discussed with several custom home builders.

Wall:
Brick cladding (areas with stone gables)
Air gap
1 1/2″ Polyiso (foil out)
Housewrap
Taped and sealed OSB sheathing
2X4 Wall cavities filled with damp spray cellulose
Gypsum board

Roof:
ADA ceiling with 5/8″ drywall (minimize or eliminate can lights)
Blown cellulose insulation to depth to provide 2015 IRC Code R-value (flat ceilings)
Vented attic with no HVAC ductwork

My plan is to also have a sealed, unvented crawlspace with all HVAC equipment in the crawlspace. I plan to insulate the interior of the crawlspace with 2″ EPS foam along with the vapor barrier.

Based on early feedback from builders, I have the following questions:

1) Does it make “cents” to eliminate the housewrap and taped OSB with ZIP sheathing? I think it is an installation/labor cost vs. material cost dilemma, right?

2) If builders cannot find a cellulose insulation contractor in our area, is the best option to go open cell spray foam in the wall cavity and blown fiberglass on the flat ceiling areas? No one uses cellulose here and I’d love pros and cons of other options.

3) I have two rooms with vaulted ceilings but the ceiling pitch is much shallower than the roof pitch in these areas. What’s the best insulation plan here? High density batts? Install truss/rafter baffles and blow and fill insulation?

4) For the bonus room above the garage, I should just follow Martin’s recommendations in his article on insulating a cathedral ceiling, correct? Most builders want to fill the rafters in the area of the home with closed cell foam. I will make sure to select one of the options he presents.

5) Any recommendations for fabricating a termite barrier/drip edge for the exterior rigid foam?

Sadly, all of these GBA standard recommended practices are never used in this area (moderate climate and cheap energy), and I am teaching every builder what I have learned here with the goal of building in the spirit of the Pretty Good House; R-values at ~2015 IRC code levels and 2.5ACH. I received bids for windows from Accurate Dorwin and Fibertec and both are viable options at this time.

Any help or advice is greatly appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rob R,
    I noticed that your planned R-value for your crawl space wall (about R-8) is below the minimum code requirement (per the 2012 IRC) for your climate zone. In Zone 4, the 2012 IRC requires crawl space walls to be insulated to a minimum of R-10 (assuming continuous insulation). That means at least 2.5 inches of EPS.

    Q. "Does it make 'cents' to eliminate the housewrap and taped OSB with ZIP sheathing? I think it is an installation/labor cost vs. material cost dilemma, right?"

    Either housewrap over conventional OSB or taped Zip sheathing can work as your water-resistive barrier (WRB). The most important thing is to be sure that this WRB is properly integrated with window and door flashing. The integration details are more important than your choice of WRB. For more information on WRBs, see All About Water-Resistive Barriers.

    Q. "If builders cannot find a cellulose insulation contractor in our area, is the best option to go open cell spray foam in the wall cavity and blown fiberglass on the flat ceiling areas? No one uses cellulose here and I'd love pros and cons of other options."

    A. I would probably choose to install blown-in fiberglass (for example, Spider) in the walls if cellulose were unavailable. If you are forced to install blown-in fiberglass above your ceilings, make it a little thicker than you otherwise would -- fiberglass is so air-permeable that you have to add extra if you expect it to perform well in cold weather.

    Q. "I have two rooms with vaulted ceilings but the ceiling pitch is much shallower than the roof pitch in these areas. What's the best insulation plan here? High density batts? Install truss/rafter baffles and blow and fill insulation?"

    A. The answer depends on your R-value goal and the depth of the trusses. If you end up with recessed lamps, it may be better to just install SIPs or nailbase, to get the insulation above the roof sheathing. There are lots of ways to detail these areas.

    Q. "For the bonus room above the garage, I should just follow Martin's recommendations in his article on insulating a cathedral ceiling, correct? Most builders want to fill the rafters in the area of the home with closed cell foam."

    A. I don't think I have ever presented just a single way to insulate a cathedral ceiling. There are lots of ways to do this. Closed-cell spray foam works -- but it is expensive, environmentally nasty, and doesn't address thermal bridging through the rafters. Usually, a combination of rigid foam insulation above the roof sheathing and fluffy insulation between the rafters is the best way to go.

    Q. "Any recommendations for fabricating a termite barrier/drip edge for the exterior rigid foam?"

    A. Details to limit termite infestation are intensely local. I urge you to follow local advice.

  2. Expert Member
    Dana Dorsett | | #2

    Copper flashing is a better termite barrier than most other physical barriers, because the copper oxides leaching off the surface are toxic to the gut flora of termites needed to digest wood fibers. They don't have to injest very much to end up dying of starvation.

    Fiberglass blowing wools designed for open blown attics are much improved over prior decades' products, but don't use products designed for walls in open-blown attics.

    If you're dense-packing a cathedralized ceiling a wall-cavity wool such as Optima or Spider works, but it then needs sufficient insulation above the roof deck for your zone 4A climate, which would be 30% of the total-R. At R49 that's the IRC prescribed R15 above the roof deck, with R34 cavity fill. For every R of cavity fill, you need at least R0.43 above the roof deck. With dense-packed cellulose in a 2x10 rafter bay you have you'd be at about 9.25" x R3.7/inch= R34 in the cavity, and would need a minimum of R0.43 x R34= R15 above the roof deck, exactly meeting the R49 code min. But if you installed dense packed Spider you'd have 9.25" x R4.3/inch= R40 in the cavity, and would need to put a minimum of R0.43 x R40= R17 above the roof deck for dew point control at the roof deck. The ratio matters, since that's what determines the average temperature at the roof deck, which mustn't dwell below the average dew point of the conditioned space air in winter or it'll load up with moisture.

  3. Rob R | | #3

    Martin and Dana,
    Thank you for the responses. The frustration I'm experiencing with my project is a Supply and Demand issue. My area has no demand for tighter, better insulated building envelopes, so there is no supply of products or contractors with the expertise to install these products. As a homeowner, I'm left trying to come up with a practical, available option to achieve what I believe to be a worthwhile objective for my home.

    After striking out with local contractors for damp spray cellulose and now JM Spider insulation, and the fact that the installation of exterior rigid foam is basically unheard of in Knoxville, I'm left with builders and an energy consultant recommending Zip-R sheathing with R-13 fiberglass batts in the wall cavity. Based on my location, I know that I will compromise by preferred plan (derived from reading GBA).

    My question to the experts is about the significance of the compromises. How should I prioritize some of the building strategies that I have read about on GBA (based on being in Zone 4A)? For example, from most important to less important- 1) Air sealing to a specific ACH level; 2) Continuous exterior insulation to reduce thermal bridging; 3) All HVAC ducts in conditioned space; 4) Type of wall cavity insulation 5)Eliminate use of recessed lights....etc.

    I am struggling to create a practical, realistic plan prior to construction, and the availability of expertise in Knoxville seems to be the biggest constraint. Thanks again for all the advice and information.

  4. GBA Editor
    Martin Holladay | | #4

    Rob,
    You wrote that "the installation of exterior rigid foam is basically unheard of in Knoxville." You might want to contact roofers who do commercial work -- all of them should be familiar with the installation of rigid foam on the exterior of the roof sheathing.

    I can't think of any reason why builders in Knoxville wouldn't be willing to frame walls with 2x6s. After all, the methods are identical to those used when framing with 2x4s. A wall with 2x6 studs and R-6.6 Zip-R sheathing will work in your climate zone. That gives your wall a nominal R-value of about R-26. Not great, but not bad for Zone 4.

    When it comes to prioritizing different energy-efficiency goals, by far the most important goal is to lower the air leakage rate of your building envelope. If you have to educate your builder and subcontractors, these are the three most important rules:
    • Pay attention to airtightness.
    • Pay attention to airtightness.
    • Pay attention to airtightness.

    Here's a question for GBA readers: Does anyone know of a builder in Knoxville who understands the "house as a system" approach?

  5. Coop Mag | | #5

    I'm in Nashville and just started my build. I'm running into the same issues. Nobody needs work around here. They are not going to slow their crews down to train them up on a one off technique. I opted to go the Zip route that Martin mentioned. Though none of the framing crews had installed Zip-R (the salesman at Builders Source in Nashville says they've never sold it!), they had all installed regular Zip panel before and it seems to install about the same. I'm going behind it with a insulated batt or spray cellulose.

  6. D Dorsett | | #6

    What he said: Framing & sheathing 2x6 + 1.5" ZIP-R isn't much different from 2x4 + OSB. R23 rock wool can be found in most box stores in my area, and would be cheaper through distributors. At the very least local contractors & distributors should be able to come up with R21 fiberglass rather than low-density R19s (which really only perform at R18 at 5.5" thickness in a 2x6 wall.)

    At a 25% framing fraction, allowing a total of R1 for the interior wallboard & siding, the whole-wall-R of a 1.5" ZIP-R 2x6/R23 wall comes in at about R22. With R19s it's only a bit over R20.

    A 2x4 / R13 wall with 1.5" ZIP-R only comes in at about R16 whole-wall, bumping to R17 if using R15 fiberglass or rock wool.

    An IRC code-min 2x6 / R20 wall comes in at about R15 whole wall.

    If you go with 2x6 and go to 24" o.c. stud spacing to hit the same structural capacity as a 2x4 16" o.c. wall, and your framing fraction drops to about 20%, adding another ~R1+ to the whole-wall numbers, so a 2x6 wall with 1.5" ZIP-R + rock wool you'd be at ~R23 whole wall a bit more than 50% above code min, and in a range where it's at least possible to think about Net Zero Energy in a zone 4A climate.

    If the contractor pleads ignorance on air sealing, buy yourself a powered caulking gun and caulk the sheathing to the framing on every edge in every stud bay yourself, as well as the bottom plate to the subfloor, and between the seams of the top plates etc. It takes remarkably little time when you have the right tools.

  7. GBA Editor
    Martin Holladay | | #7

    Dana,
    I'm not sure that it makes sense to caulk the sheathing to the studs if the builder uses Zip sheathing. Hopefully, the builder will also use Zip System tape. If that's done, the air leakage problem areas are windows, penetrations and transitions (foundation to wall, wall to ceiling) -- as well as the ceiling, of course.

  8. Rob R | | #8

    All: Thank you again for the information, and I'm glad that you understand the difficulty that I'm facing in my area.

    And C. Maglio (Post #5), I appreciate the empathy and you're exactly right. Knoxville housing is booming and no subs need work; especially work that involves doing something WAY different than anything outside of their normal. Building supply houses have rarely purchased or had requests for polyiso sheets or even ZIP-R 1.5".

    Martin and Dana: There is very limited use of 2x6 Advanced Framing here in Knoxville so deepening the cavity to 2x6 with traditional framing comes with a heavier cost penalty; I do not know the exact additional cost. I had hoped to achieve > or ~R20 walls with 2x4 cavities by using exterior rigid foam and better cavity insulation, but I'm struggling finding anything other than batts for the walls (or OCSF $$$). With everything that I've read on this site, are batts in the walls a poor choice? I will also begin discussions with the builders about the use of 2x6 walls to increase the R-value of the wall. Sadly my desire for >R20 wall or 2015 IRC code is challenged due to the perception of little to no ROI in this area.

    Martin, thanks for the additional thoughts on air sealing when using taped and sealed ZIP sheathing. I questioned where to tape and apply caulk/sealant when dealing with multiple layers of the wall system (rigid foam, ZIP, wall insulation, drywall, etc.).

    The challenge for C. Maglio and I is building with techniques (cellulose, Spider, rigid foam, etc.) that will most likely be common or at least available in our area in 3-5 years based on demand for energy efficiency. Down here, it's done "'cause that's the way we've always done it".

  9. GBA Editor
    Martin Holladay | | #9

    Rob,
    Q. "Are batts in the walls a poor choice?"

    A. Fiberglass batts can work, especially if (a) you do the work yourself, or (b) the work is closely supervised and your contract with the subcontractor includes a requirement that the job meet Grade 1 specifications. As Dana said, specify the densest fiberglass batts you can find.

    For more information, see Installing Fiberglass Right.

  10. Bill Daugherty | | #10

    Re termite shields:

    I always use G90 galvanized. Be sure to isolate the PT sill plates from contacting the termite shields.

    Look into using BoraCare (and/or TimBor) in combination with the termite shields.

    Incorporate an inspection strip for termites as recommended at crawlspaces.org.

    Bill

  11. Expert Member
    Dana Dorsett | | #11

    Martin- believe it or not I've seen ZIP installed without the tape (balked at the price, couldn't read the instructions?), so don't count on anything if it hasn't become a common product in the local market. And even if they do tape the seams, don't count on them to seal the ZIP-R properly to the top plate or at the bottom. Take allegations of local contractor ignorance seriously- I've seen it in action even in regions where the average builder knows better.

    Case in point: I was looking at a home addition currently under construction on Martha's Vineyard a few weeks ago, where they had stopped the R10 foundation insulation from the footing on up at grade on the exterior, with no insulation on either side for the above-grade portion of the foundation, for what was intended to be a finished walk-out basement. (I advised the owners of the property of the error, but apparently the inspector had seen it and hadn't flagged it as an issue, the contractor apparently thought it was OK, so... At least they hadn't started finishing that level yet.)

    Rob: They don't have to take a course on Advanced Framing to simply use 24" o.c. spacing with 2x6 studs, keeping their more familiar doubled up top plates, etc. At 24" o.c. spacing wall flatness is sometimes an issue. Most 2x6 homes in New England are still built 16" o.c., but 2x4 R13 hasn't been code min in this area in decades, whereas it still met code min in zone 4A under IRC 2009. While a 2x4 R13 +5 wall meets code here in MA and it is more resilient than 2x6/R20, most opt for the latter, and most are 16" o.c..

    It looks like TN is still operating under IRC 2009, in which case any type of 2x6 framing is probably rare:

    https://www.iccsafe.org/about-icc/government-relations/map/tennessee/

    With 1.5" ZIP-R and 2x4/R15 the center cavity R is about R21 (not counting the OSB of the ZIP-R), and the whole-wall R is going to come in around R17, or U-0.059.

    An IRC 2009 code-min R13 wall comes in at about R10 whole-wall, so an R17 wall is still WAY better than your local code-min.

    Pouring huge money into closed cell foam for cavity fill would only add about R2 to the whole-wall figure (at most!), and would be a crime against the planet. Air-tight sheathing and high-density batts are the better investment.

    At R17 whole-wall you can think about going to higher performance windows to lower the energy use profile, since IRC 2009 U0.35 code-min windows (with no maximum SHGC!) would start to be as large a fraction of the heating & cooling loads as the walls. IRC 2012 specifies the same U0.35, and a max SGHC of 0.40, which is still a pretty low hurdle. That would be fine for south facing windows with appropriate overhangs to limit summertime gains, but the rest of the house it's worth looking at sub-U0.30 / sub-U0.20 windows. (Andersen 400 series has plenty of standard offerings that hit those marks, but there are many other vendors that would too.)

    In any new construction it's worth taking the site factors & house orientation into consideration with an eye toward future potential rooftop solar.

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