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Energy review for my new home

user-7710245 | Posted in General Questions on

I am building a small single family home in North Florida. I am building the home to be airtight, energy efficient and net zero.

Here are the main energy savings features I am planning on incorporating

-Silver metal roofing
-zip sheathing + highly detailed sealant for air sealing
-encapsulated attic with spray foam
-24k btu single zone ducted heat pump
-heat pump water heater
-all energy star appliances
-led lighting
-rooftop solar
-low e double glaze windows

I am attaching a screenshot showing the homes layout. It will be positioned with the front porch facing due north, on a lot with little shade.

Any ideas on how much insulation I should use? Would filling up the 2×6 wall cavity be overkill?
Any thoughts, suggestions or changes on the project?

Thanks,

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi user-7710245 (would be great to get your real name).

    That's a nice efficient floor plan and it's great that you are thinking through the details now. Too often people are asking these questions about houses that are under construction when it is too late and/or expensive to make changes. With all of the effort you are planning to put into this project, it would probably be worth getting a local expert to help you with some energy modeling.

    For example, if you haven't decided on insulation values, how can you know what size heat pump you need (A Beginner’s Guide to HVAC Design)? In Climate Zone 2, the minimum R-value for cavity insulation according to the current IRC is R-13, which can be installed in a 2x4 wall, but often it is advantageous to insulate beyond code minimums or to use some continuous insulation on the outside of the wall to minimize thermal bridging. An energy rater can answer this question, not in isolation, but in the context of your particular project.

    What level of air tightness are you shooting for? It's great to say that the sheathing will be detailed as an air barrier, but air sealing is not something you have to guess at. It can be tested with a blower door (Blower Door Testing). And the testing process can help you identify areas that need attention (Blower-Door-Directed Air Sealing).

    Also, an energy rater can tell you how much money to put into insulation and when to stop insulating and start adding money to the PV system (Revisiting Net Zero Energy). Net-zero can also be calculated before construction begins. No need to guess here either.

    1. user-7710245 | | #6

      Hey Brian, it is Jacob Hall. Ishould note that I am an owner builder for this project. I am a solar installer/electrician by trade and have extensive carpentry experience so the plan is for me to handle those scopes and sub out everything else. I am building this out on a tight budget so havent paid to consult with a local energy rater... as of yet. The cost of adding additional solar after the fact is much lower (for me given my profession) then it likely would be to hire consultants and run additional energy modeling based on preliminary quotes I had received. I have an hvac contractor friend who sized the hvac system for me. He ran the manual j calcs and came up with the 24k btu unit based on the floor plan and a "moderately airtight" envelope. I am not sure what all info he used to come up with the 24k btu system but this is something i will double check on

  2. walta100 | | #2

    When I read your list the thing that jumps out at me is “encapsulated attic with spray foam”

    This line pushes my buttons.

    1 The word encapsulated implies the attic is separated from the rest of the world and it is just not possible. If you insulate the roof the attic will be more connected to the conditioned space than the unconditioned space. To avoid a moldy mess the attic needs to be well connected to the inside or the outside. Any space in between is likely to be below the dew point at times and get wet and grow mold.

    2 Why would anyone choose to use the most expensive and least green form of insulation Spray Foam? Have you read the stories foam installations gone wrong with the fume driving people from their homes?

    3 Why would anyone choose to insulate the roof instead of the attic floor? All roofs have more surface area than attic floors many are double.

    This decision is almost always an attempt to mitigate the very lazy and stupid choice to put the ductwork and HVAC equipment in the attic. Do the hard work now with your pencil and find the room inside the conditioned space for your HVAC equipment and ductwork.

    Rant over sorry if I hurt anyone’s feelings.

    Walta

    1. user-7710245 | | #7

      I may have misspoke here.. the plan is to have the attic encapsulated at the roof line and not the ceiling, so really the living space wouuld be "encapsulated " to the roof line.

      I am hoping that the heat pump + water heater will help with humidity. I will also have an ERV tied into the hvac system. Would you think humidity is still likely to be an issue? I had considered adding an air exchange between the main living space and attic but not sure if this would be beneficial or not as far as removing humidity

      2. THis is what I am most unsure about.. I planned on hiring a reputable company to do the spray foam but have pondered using foam board + canned foam at the board seams.. this is something I could DIY.. but to be completely honest this portion of the project is one of the things I am the most ignorant about. Any insight on spray foam vs foam board or other insulation strategies for the build are greatly appreciated.

      3. I will be using attic trusses and would like the attic to stay at a comfortable temperature as I would like the "attic truss room" to offer more utility and usuable space than a traditional attic

  3. Jon_R | | #3

    Put more thought into interior humidity and dehumidification. The heat pump alone won't suffice.

    1. user-7710245 | | #8

      Forgot to mention but will also be using an ERV. Am hoping that ERV + ducted mini split + heat pump water heater together would be sufficient to handle any humidity issues. Am is missing anything here or is there anything else to incorporate to better plan for humidity?

      1. Jon_R | | #12

        Yes, you will need a dehumidifier and a way to distribute the dry air.

        1. user-7710245 | | #13

          So dehumidifier in the attic and perhaps a couple of living space to attic air exchangers?

          1. Jon_R | | #14

            Maybe. How much attic->room airflow you need depends on how much you over-dry the attic and how much moisture needs to be removed.

  4. walta100 | | #4

    Will you please tell us your name.

    I do like your floor plan. You have packed a lot of house in a small footprint.

    The only thing that could be better is that the plumbing was all together. Every design is a comprise and I made a similar choices.

    Consider adding a suntunnel in the master closet.

    Have you posted this here before as it looks very familiar?

    Walta

    1. user-7710245 | | #9

      Yes sir it is Jacob Hall. The plumbing on the drawing is not up to date, the plan would be to use a heat pump water heater in the attic as opposed to the 2x exterior water heaters shown in the drawing. I wanted to try and group the bathrooms/kitchen as close as possible but there were only so many designs that would allow for that, and the ones I found with everything so close had other pitfalls

  5. walta100 | | #5

    You may want to invest to 20 + hours to model the house in BEopt

    This will tell you if the low E windows will ever pay for themselves in dollars or how much insulation is worth buying, but some things are worth doing form a comfort point of view.

    https://beopt.nrel.gov/home

    Walta

    1. user-7710245 | | #10

      What would that usually run? As mentioned I am an owner builder and trying to keep this project on a tight budget. As much as I would like to preplan with energy modeling and incorporate as many green features as possible.. I am a solar contractor and the home will be in an area with net metering. with an additional $1000 in panels I am able to add up to 2kw of solar capacity that would deliver 2800kwh of usable energy in year 1.. So there is the trade off, is it better invest more and to design for hyper efficiency, or just add more solar at my cost.

      1. walta100 | | #11

        The computer software is free to use it was paid for with tax dollars. You will need to spend your time watching the training videos learning to use and understand the program.

        BEopt is a very powerful program that can show whether it is better to spend your money on solar or insulation. Part of the answer has to do with your interest rate current electric rate and your guess at electric rate inflation.

        Walta

  6. AlanB4 | | #15

    You keep going on about reducing cost but are looking at making monumental mistakes.
    Its false economy to save money by cutting corners then those corners cost you even more in the end.
    You need to plan the structure and map out the energy costs. Not knowing if you should even fill stud bays and your attic spray foam insulation plan shows you don't know how this works. There is no shame in that but look at building a house as a system, not a place to cut costs that costs you more after nickel and diming and picking bad products. And building a house will always cost more then you expect. If you cannot actually afford to build it properly then don't build, get a bigger loan or save up more cash.
    And your throwing away money, not insulating properly will add a lifetime heating and cooling penalty, and spray foam is overpriced, you can save cash and get more insulation with better alternatives.
    So spend some money to hire a proper expert to plan this and you will save money in building and save money for the rest of the build's existence.
    So you need to decide a few things, how much money do you have available, how many sq ft are you looking for, is your aim for net zero or better, when there are cost overruns will it sink you, are you willing to pay a professional to model your house properly?

  7. harrison55 | | #16

    Hi Jacob

    Last year I built an owner designed house in Virginia, zone 4. It worked out fine and I have a net zero certificate now.

    But there are two things I would do differently.

    1. I would do the energy model sooner rather than later. I needed to do it in the end anyway, and it would have saved money to do it earlier

    2. Install a fully ducted dehumidifier. Do not count on a portable dehumidifier. Portable dehumidifiers are apparently adequate up in New England. They are not adequate down south. I will be retrofitting a ducted dehumidifier soon.

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