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Building a house in Greenville, SC — insulation and air sealing questions

jander5984 | Posted in Green Building Techniques on

Building a house later this year. Plan to live in the home 30+ years. So I’m interested in the “long term.”

The House: 3,350sf above grade. Very very little shade on this house. Kinda like building in the middle of a field. Lots of sunshine bearing down. Galvalume metal roof with 3/4″ strapping between metal and synthetic felt (TBD). Zip System for exterior walls. 2×6 exterior walls. I’m planning on ponying up for the super efficient Trane XV20i heat pump. Cement fiber siding painted white.

The Plan:
Foam attic with open cell to 5.5-6″.
Dense packing exterior walls with fiberglass or cellulose (let’s not debate between the two on this post).
Plan to apply elastomeric sprayable caulk (EccoSeal+) on all joints from the inside.
Zip System sheathing (as mentioned before)
Pella 450 wood Windows

The Question:
I am torn over whether or not to pursue rigid foam on the outside of the zip. I really like the idea of a radiant barrier on the outside the rigid foam since I would plan to have a 3/4″ air gap after the strapping is installed. Remember, this house will get full sun over basically all of the Southern/East/West sides. Seems most people are focused on exterior foam in colder climates and not as much in the south.

Is the foam overkill? After the cost of the rigid foam, labor to install foam, strapping, rain screen, boxing to set out windows, etc…etc….. I know we’re are talking about more than just a few thousand bucks and I’m already spending extra in other places to make the home efficient. Foil faced foam make the most sense? I was only thinking about doing 1″ or 2″ of rigid foam.

Plan to add detached garage later and the roof line faces due south at almost the perfect pitch for a solar array! Net Zero farm house here we come?

Thanks in advance.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    If you dream of a net-zero-energy house, you want your house to have a thermal envelope that is better than average.

    First of all, you are planning to install open-cell spray foam in your attic with an R-value of between R-20 and R-22. That is significantly less than minimum code requirements (R-38). I suspect that you have been bamboozled by a spray foam contractor who is trying to convince you that spray foam is so miraculous that you don't need to meet code. Ignore this sales pitch. Instead, read this article: It’s OK to Skimp On Insulation, Icynene Says.

    Second, you didn't mention where the spray foam will be installed. Do you want to install it on the attic floor? Or do you want to install it on the underside of the roof sheathing? If you were thinking about installing it on the underside of the roof sheathing, you probably want to read these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

    If you are interested in building a net-zero house, install the foil-faced rigid foam on your walls. You won't regret it.

    -- Martin Holladay

  2. brendanalbano | | #2

    If boxing out your windows is causing you to hesitate on the exterior foam, my understanding is that if you limit your exterior foam to 1.5" or less, you typically don't have to box out your windows.

    If you're thinking about net-zero, plan to live there 30 years, etc., it seems like a rain-screen gap and a little exterior foam will be good from both an energy and a durability standpoint. But I'm from climate zone 5, so maybe the standard advice up here doesn't apply down in climate zone 3.

    But at the very least, hitting the IECC 2015 residential minimums is a no-brainer. That's R-20 (or R-13+5) walls and R-38 ceilings. And that's just code-min. You can do better!

    The standard advice is to "pony up" on your envelope before you pony up on your HVAC equipment.

    If you're serious about net-zero, it seems like you're going to want to do some energy modelling. BeOpt seems like perhaps the simplest option, but perhaps other GBA readers have suggestions: https://beopt.nrel.gov/

  3. jander5984 | | #3

    Martin & Brendan- thank you for sharing your thoughts. I will offer more detail on the foam which might make more sense. This house is kind of a 1.5 story. Not a Cape Cod, but there are roof lines that will serve as the ceiling in almost all upstairs rooms. My intention is to spray open cell at the roof. The duct work and the air handler that services the second level will be in the attic, so I would never consider installing insulation anywhere other than the roofline. Likewise.... the insulation has to go in roofline in many places because the roofline and the ceiling are one and the same for about 20% of the upstairs ceiling. I've read articles on high humidity issues with foamed attics in southern climates. The duct design includes a small return in the attic that terminates 5' or so from the top of the roofline (there's also a small return). Plus, I'm not cutting corners on ventilation in the bathrooms, air sealing, etc.... Just to keep a close eye on things, I plan to have a device in place to measure humidity at the top of the roof line. Net Zero isn't really my main goal here, it's just that the house happens to set up well for solar and assuming tax credits/incentives and rebates are in place.... the numbers can make good common sense.

    I promise you, I haven't been bamboozled by spray foam contractors, several of them were removed from consideration the minute they suggested I foam the entire envelope.

    In my area, HVAC contractors & home builders alike have all commented at how amazed they have been by the impact of foaming the roof. I value 3rd party advice that doesn't have a skin in the game. Keep in mind.... we don't have many months of cold weather, but that sun can overheat the attic 8 months out of the year. The attic of this house would easily reach 120-130 degrees on hot summer days. I don't care if you blow enough loose insulation to achieve R-55 on the ceiling floor........... with the duct work up there, I'll bet my Dad's C-1 Corvette the spray foam out performs the loose fill all day long. Unless you live in a warm climate, you're probably not very aware of the value associated with the Trane XV20i because you're interested in completely different equipment.

    Back to my question: is there's anyone out there that can chime in on whether or not exterior foam (with or without a radiant barrier), strapping, rain screen, etc....... is really worth it? I would think thermal bridging is a little bit bigger of an issue on a house that's wide open for sunshine, but would I be hitting a point where it's just silly. i.e.- Lets say spending an extra $8,000 will only reduce my power bill (just because it's easier to talk dollars instead of kilowatt hours) by another $8.00/month.

  4. JC72 | | #4

    Hello form Atlanta,

    I would add exterior rigid foam if it were my build. Exterior foam with your rainscreen will really help on those days when its over 90 degrees , high humidity, and not a breeze to be found. You can also think of it as "future proofing" your home. Especially come resale time.

    Here's a build in Decatur GA (Same CZ as you): ZIP-R was used in this case.

    https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/topping-out

  5. brendanalbano | | #5

    Insulating at the roof line is great, but why aren't you planning to do at least a code-minimum R-38 for your roof?

    Regarding the cost vs payback of exterior foam, I'd imagine that because the costs is dependent on your local market and your energy use is dependent on the complete design as well as your energy use habits, it's going to be hard to make a payback calculations without doing some energy modelling. Is doing an energy model or hiring someone to do an energy model something you are considering? How are you sizing your HVAC system? Would adding exterior foam change your HVAC sizing calculations?

    You might be right that the economics don't work out, but it's hard to know without doing the math! Seems like it'd be worth crunching the numbers if you're planning for 30 years.

  6. walta100 | | #6

    Story and a half is the toughest building to air seal the only reason to build it is to get the most square feet per dollar spent on framing, roofing and siding.

    If you plan on growing old in this house make it a single story.

    Spray foaming a roof is the best way to fix the stupid idea of ducts on an attic. I you have not built it yet just say no to ducts in the attic. If you must insulate the roof put at least R38 ridged foam on top of the roof sheeting and top that with your steel roof.

    Walt

  7. jander5984 | | #7

    Walter, your thoughts/suggestions I feel are a common sentiment in the Green building community that significantly stints green building growth. Some of the most "green" homes built today look like ugly houses that only impress architects, green building scientist and green building enthusiast.

    My opinion, find a way to make homes that people want..... green. Don't design green homes and tell people they should want them.

    By the way, this homes has a full basement. A basement that won't even require the 1 ton of cooling the load calc. came up with. If I build 3,350sf on one level, I've got to heat and cool 3,350 for 30-40 years. With a 1.5 story, I can virtually shut down the heating & cooling for the 1,400 sf upstairs for 300+ days a year once we are empty nesters.

  8. JC72 | | #8

    @Farm

    I think what Walter is getting at is that all else being equal it's much easer to air seal/insulate a full 2-story vs a 1.5 because there are no dormers or cathedral ceiling to work with. The 2-story (colonial, farmhouse), because of its wide open attic space , is usually much easier to insulate with copious amounts of loose fill, has a basic roof design, give 2nd story occupants maximum head room

    It's not a matter of aesthetics. Living and growing up in the South I like the low-country southern style myself (esp with metal roof), and I can see the appeal of having a 1.5 because it allows you and your wife to age in the house without having to climb stairs to a master bedroom. Also, I imagine that it's difficult to build a full 2-story with only 2 bedrooms on the second floor.

  9. STEPHEN SHEEHY | | #9

    If you build a tight, well insulated house, it won't cost much to heat or cool. you'll spend more on plug loads and not water.

    Building a single level house means it's easy, and cheap, to insulate the ceiling. It's easier to air seal. You can shrink the size a bit because you aren't wasting floor space for a stairway to the second floor. You can run all ducts in the basement. Plumbing and wiring can all be run in the basement. Easier to install ventilation.

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