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Flash-and-fill or just seal well and fill?

BuildingEnthusiast | Posted in Energy Efficiency and Durability on

Hello, We are talking about building a pretty good home with a local contractor in a semi custom solar ready community. I am of the mind, we should reduce energy consumption before adding solar, but the builder is trying to create an affordable option for the average person.

I appreciate and respect what he is trying to do. That being said, I have been pushing him to make our potential home more efficient than the average house in our community. I should start with, we are in  climate zone 5. It is a moderate climate generally and very dry here.

Currently his wall assembly  is 2×6 24 on center, sealed with spray foam and netted blown in cellulose with sheet rock on the inside and osb sheething, tyvek, lab board siding. They are carefully sealing headers, footings, cutouts, ect and have blown 3.0 ach/50psc. Not fantastic but much better than the average home being built here.

I have challenged him to add 3 inches of ridged board to the outside of my home 2.5 in. being the minimum according to my calculations of 27% in the outside for our climate zone and minimum precipitation. R23 in walls R9 or better on the outside to avoid dew point problems.

He is also doing a partially conditioned crawl space and has agreed to a fully conditioned crawl for our house, upgrading to spray foam around the perimeter for better insulation and sealing. He also agreed to go from a typical gas hvac to a minisplit system. We are currently debating water heaters but he has also agreed to helps us fit our water heater within our conditioned space.

He will not budge on windows so they are typical windows (hopefully) well sealed and installed. Surprisingly most of our bigger windows will be south facing and I plan to add overhangs at some point to reduce summer sun, helping a bit in the winter passively. The ceiling will be well sealed and have R50 blown in.

Back to walls, I proposed 3 in. ridged foam out board of the osb sheathing with furring strips. My contractor said that option is very expensive in out region due to availability. He is suggestion 2 in. spray, flash and bat. Or a zip sheathing option with insulation (I need to clarify this option). I appreciate any and all suggestions. I would like this pretty good house to be as efficient as possible with out blowing our current budget.

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  1. GBA Editor
    Martin Holladay | | #1

    Are you building in the United States, or some other country? I don't know what you mean by "A5 climate." Does that mean Climate Zone 5 on the U.S. Department of Energy climate zone map? Or does that mean something else?

    Here is a link: Climate zone map.

    Until we have a better idea of where you live, we can't give you advice.

    [Later edit: Thanks for correcting your question.]

    1. BuildingEnthusiast | | #2

      Martin, thank you for pointing out my mistake. We are in climate zone 5. I am in Boise, Id.

  2. spenceday | | #3

    Ask the builder to consider Zip R-9 panels. You dont need to spray foam the wall cavities with that system. Easier detailing than exterior foam and no need for Tyvek.

    1. BuildingEnthusiast | | #4

      Thank you. I believe this is one option he quoted for me, but I need him to clarify that. Waiting for my contractors response.

      Have there been precipitation issues with zip R sheathing?

      1. spenceday | | #5

        Not sure what you mean by precipitation?
        If you mean condensation on the interior side of sheathing then I dont really know but it shouldn’t be an issue if you size the insulation to the climate zone. There should be a table on the Huber site.

        1. BuildingEnthusiast | | #6

          I did mean condensation. Sorry, about mis-spelling and wording. I believe R9 or better will be enough ridged insulation.
          Thank you for your response. I really appreciate all opinions at the moment. I want to make the smartest decision we can.

      2. spenceday | | #7

        Do note that there is a difference between ZIP ad ZIP R panels. The ZIP R panels are an OSB sheathing with foam adhered to the back. The R panels reduce thermal bridging.
        I’m about to do a build in zone 4 with R6 sheathing and rockwool insulation, 2x4 wall construction. R-21 wall that should perform better than a 2x6 R 23 wall.
        If you do 2x6 framing and ZIP R9 panels you could end up with a R32 wall.

        1. BuildingEnthusiast | | #8

          An R32 wall is my goal. Am I over thinking the thermal bridging and wall value?

          I do know the difference between zip and zip R. I introduced zip R to my contractor while trying to find a cost effective way to reduce thermal bridging and raise my walls overall R value. The debate has been what building techniques will give us the most bang for our buck. My concern with thermal bridging is once the wall is up, it is very pricey to take off siding and add ridged insulation later.

          My contractor believes he can achieve net zero status with a well sealed r23 wall and a semi conditioned crawl space. I tend to disagree. Most of the houses he will be building in our community will have gas stoves, gas fireplaces, gas water heaters, and gas heating systems. with all that and a 5kw solar array panel you can achieve a $0 electric bill.

          I want all electric accept the water heater (I really can not find a better alternative currently) and a near $0 electric bill. We have a big family and I am giving up space to afford this build. Let's just say I am nervous. My dream would be a net zero home nearing passivhaus standards. That is not affordable for us so I am trying to find a good alternative. Am I on the right path?

          1. GBA Editor
            Martin Holladay | | #9

            You wrote, "My contractor believes he can achieve net zero status with a well sealed R-23 wall and a semi conditioned crawl space. I tend to disagree."

            In fact, your contractor is right. As long as your projected annual energy bill is matched by the output of your PV array, you'll have a net-zero house. But your contractor's method of getting to net zero may not be the cheapest way. (Then again, it might.) You need to compare the cost of each proposed energy upgrade with the cost of the PV array needed to make your house net-zero. A better envelope costs more money, but allows for a smaller PV array. Someone needs to sharpen their pencil and do the math.

            You wrote, "Most of the houses he will be building in our community will have gas stoves, gas fireplaces, gas water heaters, and gas heating systems. With all that and a 5 kw solar array panel, you can achieve a $0 electric bill."

            But a zero electricity bill doesn't mean you have a net zero house. You'll still have a big gas bill. If you have your heart set on a net zero house, it has to be all-electric. So if your contractor is bragging about getting to net zero, and he is also proposing a lot of gas appliances, his logic is flawed.

            You wrote, "I want all-electric accept the water heater (I really can not find a better alternative currently) and a near $0 electric bill." Why not install an electric water heater -- either an electric-resistance water heater or a heat-pump water heater -- with the idea that your PV array will balance the electricity load of the entire house, including the electricity required for hot water?

  3. Expert Member
    Peter Yost | | #10

    Hi Tiffany -

    This modeling tool is a great way to evaluate different packages that get to net zero, optimizing on the way to PV.

    Free and great online tutorials.


    1. BuildingEnthusiast | | #11

      Thank you Peter, I will check out the optimizer tool you are suggesting.

      Your points are the reason I disagree with my contractor. For one thing having so many gas appliances means we are not net zero we are 'electric zero'. And while a large enough PV array will take care of almost any homes load, at what cost? And in the long term as far as maintenance and replacement is concerned, I think a well sealed home with proper insulation and a smaller PV array makes more financial sense. It all really does come down to numbers. I will use Peters suggested tool to see if I can hone in on the best way to get what we want at the price we can afford.

      Every article on GBA I have read about water heaters seems to conclude that the only decent electric option for water heaters is a heat pump. Our house will have a crawl space that will not be large enough to install a hot water heater in. Arguably a heat pump on the main floor of a house being heated by a mini split will simply raise my electrical use even more as it draws warm air from my home and replaces it with cold air. Is that correct?

      The home will be around 1900 sq ft. An eco27 water heater may be large enough for the whole home. This may be an options that makes sense?

      I just don't know enough to be confident. I am going to work with the tool Peter mentioned and see if that helps me understand the choices we are making.

      1. Expert Member
        BILL WICHERS | | #12

        In a heating climate, heat pump water heaters do essentially rob some heat from the house to heat the water. What they don’t do is waste any heat energy at all out an exhaust like a gas fired water heater will. You could also look into an electric resistance water heater (the “regular” kind of electric water heater), but your average cost to operate that unit over an entire year may well exceed the average cost to run a heat pump water heater.

        If your goal is to keep energy costs down as much as possible, some gas fired devices probably make sense. Gas tends to be cheaper per BTU than electricity. If your goal is to consume as little energy as possible, then using all heat pumps makes sense. Electric devices allow you to offset their energy consumption with a larger solar system.

        I would argue that either approach is green. Myself, I try to maximize efficiency and lower cost as much as possible. You need to decide if your goal is to minimize energy cost, minimize energy use (these aren’t always the same), or minimize installed system cost. Using a gas water heater and furance is probably cheapest to install, and might be cheapest to operate, but won’t be the lowest energy use. Using All heat pumps will be the most expansive to install, should be the lowest total energy use, but may cost more to run.


        1. BuildingEnthusiast | | #13

          Thank you for your input Bill,

          What you are saying totally makes sense and that is why we are considering the gas water heater. Gas is so cheap here, we know that our gas bill will be under $10 a month with a tankless gas water heater for a family 0f 6. That is hard to beat.
          If we have a mini-split for heating and cooling, an induction range for cooking, and an electric fireplace for looks, Then having a gas water heater is our give.

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