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Community and Q&A

Load bearing attic space between cathedral ceilings

Stephen Watts | Posted in Green Building Techniques on

I planning to build a “pretty good house” in northeastern Oregon (climate zone 5).  The house is a rectangle with a simple gable roof.  There will be a cathedral ceiling over rooms on each end of the house, but not over the rooms in the middle of the house.  The ceiling over the middle of the house needs to be load bearing for storage, solar PV inverters/batteries (~1000 lb), and HRV.  If the weight of batteries becomes a design issue, the batteries could be moved to a closet on the main floor, but I would still want to be able to use the area for storage.

What is the best way of making the ceiling above the middle of the house load bearing?  The architect has suggested an attic truss over the middle of the house; however, I am concerned that this would make insulation install and air sealing difficult.  An alternative may be to utilize a cathedral ceiling over the entire house and make a “floating” load bearing ceiling inside the building envelope.  Are there any other ideas?

I would appreciate any thoughts/experience on the best way to design this.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    DO NOT put your batteries in an attic space!!! Besides the weight, you have the periodic maintenance issues of bringing them up to and down from that space. If the batteries ever have an issue and leak, that leak is going to be much bigger problem in an attic space. You also have the issue of trying to keep the batteries at the correct temperature which means you need to condition that attic space.

    Put a mechanical room on the first floor or off the garage. Ideally on a sealed concrete slab. Keep all of your solar system equipment in there.

    Regarding the storage room, if you’re trying to avoid support columns in the middle of the lower level, you’ll probably need some kind of truss. An example of a suspended structure would be the big displays suspended above the middle of large stadiums. It takes large trusses or beams to support a large load in the middle of the span since the middle of the span is the worst place to put a concentrated load.

    And remember to NOT put your batteries up there!

    Bill

    1. Stephen Watts | | #2

      Bill,

      Thanks for the response. I wasn't clear... regardless of the design, the attic space I am talking about is inside the building envelope, so temperature is not an issue. In addition, the batteries that would be used are not lead acid; they will be Lithium Ion, with no maintenance of any kind required and no fluids that can leak (unless the battery fails catastrophically, which is possible but unlikely, and fire is a bigger issue with failure than fluid leakage). In other words, this is like a cell phone or laptop battery; just larger with a more protective design. These batteries are designed to be installed inside the building envelope. Examples of this type of battery includeTesla PowerWall, LG Chem RESU, or Enphase Encharge.

      Thanks,
      Steve

  2. Expert Member
    Malcolm Taylor | | #3

    Steve,

    How are you planning to frame the two areas of cathedral ceiling? That will inform how best to support the centre section .

    1. Stephen Watts | | #4

      Malcolm,

      Exterior walls will be double stud with the outer wall being load-bearing. The cathedral ceiling trusses would be either raised heel scissors truss or parallel chord.

      One other detail is that these batteries are typically mounted on walls, so even if mounted in the attic, the floor wouldn't have to support the weight if mounted on walls that start from the slab and rise to the top of the attic. However, as stated, I also want the ability to use the floor area for storage and to mount the HRV and ducts.

      1. Expert Member
        Malcolm Taylor | | #5

        Steve,

        If those two areas are going to be trussed, I'd use trusses on the middle part as well. You can either use attic trusses or keep the same scissor (or parallel chord) trusses as the rest.

        The problems with attic trusses occur when there is an unheated garage below. They are fine over living spaces. If the chord is too shallow where the walls and sloped ceiling portions meet, you may have to furr down a bit.

        If you use the same trusses throughout, you may be able to frame a floor clear-spanning across the width using TJs. If the span is too long you will need a load-bearing wall or beam somewhere near the centre.

  3. Joe Norm | | #6

    I work with these types of batteries daily and can see the temptation to putting them up in an attic and out of the way.

    That said, I second the notion of encouraging you NOT to put batteries up there.
    Design in a mechanical room on the first floor.

    1. Stephen Watts | | #9

      Joe,

      Thank you for your response; especially from the point of view of someone who installs battery backup systems. Can you be more specific about why I should not put batteries in the conditioned attic space? Is it installation/maintenance inconvenience and/or safety and/or? Does this recommendation also apply to inverters and other balance of system (BOS) components?

      The house has been very carefully designed (but not accounting for batteries and BOS except in the attic), and adding an electrical room would be very difficult. There is the possibility of placing batteries in the utility room, closets, or converting portions of closets to an electrical room, but that would negatively impact other things planned for those locations.

      1. Joe Norm | | #12

        My concern was primarily ease of install/replacement and secondary fire related.

        We had a Lithium battery get overcharged once and melt the BMS. It plumed toxic smoke. Luckily it was in a a tall, ventilated crawlspace otherwise it would have been a MUCH worse situation.

        Point is these types of systems are new and rapidly changing. Manufacturers claim they are very safe and I mostly believe them. But the one time they go haywire its nice if they're quickly accessible/sealed off. Plus you may be replacing it for something better in the near future.

        That said, make sure the NEC will allow the system up there if you decide to go for it. Not sure what code cycle you're on or the details of your attic and whether it would be permitted to be there. Run it by an installer in your area.

        1. Stephen Watts | | #14

          Joe,

          Thanks for your response. Yes, it is certainly possible for a lithium ion battery to have a catastrophic failure and burn. Such fires, although extremely rare (from what I can see) are also serious because they are difficult to extinguish. However, from a safety standpoint, I don't see any significant difference due to being in the attic vs. somewhere else in the building envelope.

          Of course, if putting batteries or BOS in the attic is a code violation (I'm not aware that it is), then that would preclude this no matter what I think. If an installer refuses to install in the attic, then that probably also ends that option. I'll see what I can find out.

          1. Joe Norm | | #16

            If there was a code violation it would have to do with "accessibility" to the equipment. Not knowing the details of your "attic" its hard to say. Are there stairs leading to it? If so, then you're probably fine.

            But regarding the NEC, battery/solar systems are some of the most updated sections per code cycle because of how rapidly they are changing/being adopted. So, I imagine the 2020 code cycle will have different wording for batteries and their various disconnecting means than the 2017 did. I haven't looked yet.

            Regarding Inverters/BOS-we have put them in attics before, as long as they're reasonably accessible.

  4. Expert Member
    Michael Maines | | #7

    Without knowing more about your project, I'd be inclined to build the entire ceiling with parallel chord trusses, with ventilation baffles and an air barrier on the interior. The attic space can be framed like any floor system. It's hard to air seal attic trusses, but it can be done.

  5. Walter Ahlgrim | | #8

    Consider a tall flat ceiling. If you read a few pages of this Q&A you will find cathedral ceiling question litter this space with failures because they are is just not enough space for decant insulation and ventilation to make matters worse most then insist on dozens of pot lights? My guess is you can build a flat R60 12 foot ceiling for less money than a R30 spray foam cathedral.

    Unless you are off grid batteries do not make economic sense today.

    Understand batteries are fire hazards and can leak fumes you do not want to breathe. Put them someplace with easy access because they will be replaced every 3-10 years.

    There are a bunch of very smart people that make a living designing load bearing ceilings they will consider all the variables and guarantee it will work. Generally they tend to keep their mouths shut unless they are being paid.

    Walta

    1. Stephen Watts | | #10

      Walter,

      Thank you for your practical comments. I prefer a cathedral ceiling for aesthetics. For me, aesthetics trump practicality in this case. I will be the one guiding and approving plans and build of the house and there will be n0 pot lights in the cathedral ceiling. Insulation will be R60 minimum.

      I totally agree that solar, battery backup, and other green building techniques I plan to employ are not economic, in that they will never pay back over my lifetime compared to doing nothing or using alternatives such as a cheap generator. However, this house is not about cost effectiveness (although it obviously plays a significant role in evaluating alternatives). I have planned my entire life to build a house that is as self-sufficient and green as possible, and now that I have the chance, that is what I plan to build. That's what makes me happy. Of course, at some point, potential cost over-runs could force me to change my plans.

      Yes, batteries could be a fire hazard, but that is true regardless of where they are installed and fumes are not an issue with sealed Lithium Ion batteries unless there is a catastrophic failure. I am curious whether you have a cell phone and/or laptop in your house. Do you consider them to be a fire hazard and source of dangerous fumes enough to prevent them from being inside your house? I don't consider having to haul an ~200lb battery into or out of the attic once every 10 years to be an insurmountable problem; certainly more difficult and inconvenient, but not insurmountable. If someone can identify a specific code violation and/or a specific safety issue compared to mounting in a mechanical/electrical room on the ground floor, that would be more convincing to me. There will be no garage for this house, and even if there were, an unheated garage could put these batteries below their design specs during the coldest parts of the winter.

      Yes, I understand that people design houses for a living; I have an architect working with me on this house. However, the whole point of GBA, as I understand it, is to provide input on the best ways of accomplishing building green houses. Therefore, I appreciate the feedback that I receive here, especially from those with personal experience.

  6. Walter Ahlgrim | | #11

    I have a very healthy respect for Lithium Ion batteries most people do not.
    Incorrect charging is very dangerous but so is physical damage and thermal damage all end in similar failure modes.

    From my one experience with exploding Lithium Ion batteries I would say a double A Li battery is every bit as powerful as an M80 firework and it came with more toxic smoke. So yes I so consider them a hazard and No I do not want 200 pounds of M80s in my house.

    You may find this YouTube video interesting and search Lithium Ion battery fire
    https://www.youtube.com/watch?v=XTIiIXGVVIY

    Walta

  7. Expert Member
    Zephyr7 | | #13

    The primary reasons to keep the batteries on the ground floor are ease of access and safety. I design telecommunications facilities for a living (datacenters, switching sites, etc), and some other kinds of facilities that have similar reliability requirements. We use primarily VRLA batteries which are also supposed to be recombinant cells that don’t offgas. Surprise though! Sometimes they DO offgas, usually due to cell defects or over charging or excessive discharging.

    We don’t use lithium batteries in normal facilities. The reason is that they don’t have much use history in this application so they represent TOO MUCH RISK. This is a multi billion dollar industry using dedicated battery rooms and clean agent fire suppression systems too. There is a big difference between the small batteries in your phone and thousand pound BIG batteries. Lithium ion batteries tend to fail catastrophically when they do fail. DO NOT trust marketing information here.

    If you have an attic room that is difficult to access for maintenance, you’ll be less likely to check on things. Less checking means less chance of catching potential problems early. Less chance of catching small problems means more chance of an unexpected BIG problem, that is right above your head! It’s a big tradeoff of saving some ground floor square footage at the potential expense of life safety which is my primary reason for advising you against it. I advise my commercial clients to keep power systems on lower levels for the same reasons.

    At the very least, build the attic room with at least a 1 hour fire rating, use steel trusses sprayed with fire retardant (more time before failure in a fire), and ALARM THE SPACE with combination IR/ionization type smoke detectors with remote annunciators in your living space. I would also put heat detectors. Commercially I use hydrogen sensors interlocked with exhaust fans to keep gas buildup in the safe range.

    It is much easier to effectively limit fire spread in a ground floor room, and much less chance of a catastrophic failure killing someone. I have seen system failures result in deaths before and you DO NOT want to risk something like that. Always prioritize safety above aesthetics.

    Bill

  8. Hugh Weisman | | #15

    what is the span from one side to the other?

    1. Stephen Watts | | #17

      Hugh,

      Span is 26 ft from exterior to exterior.

      1. Hugh Weisman | | #20

        Personally, I'm not into scissor trusses. If it was my job, I would be working with an engineer and do one of two things depending on the look we wanted.

        1_ a deep ridge beam spanning the open area with 2x10's at 16 or 24" o.c. from ridge to wall. insulation would be within the cavity with an interior finish of wood planking or sheetrock. Insulation could also be above the roof deck.

        2. simply heavy timber trusses spaced 6'-8' o.c. with wood decking above and exterior insulation.

  9. Stephen Watts | | #18

    Joe (post #16; I can't seem to respond directly under the post),

    The planned access is a pull-down attic access stair, not a permanent stair.

    In looking at NFPA 7o: National Electrical Code with Oregon Amendments, it doesn't look like there is an issue, but there are several places subject to interpretation by inspectors.

    The only place specifically prohibited for PV components is the bathroom.

    The PV system disconnecting means shall be installed at a "readily accessible location."

    A personnel door(s) intended for entrance to and egress from rooms designated as ESS rooms shall open in the direction of egress and shall be equiped with listed panic hardware.

  10. Joe Norm | | #19

    "readily accessible" is the key here, and you're exactly right, it's often up for interpretation.

    My recommendation is giving your local inspector a call and asking.

    Also note these batteries are heavy. Getting them up (and eventually down) pull down ladder stairs will be tough. As an installer I would push back on the idea. Buts it's your house in the end.

  11. Expert Member
    Zephyr7 | | #21

    Having dealt with battery rooms many times, you’re likely to find more code issues with the mechanical codes (which govern things like makeup air and gas detection and exhaust systems) than you will in the electric code.

    The electric code will pretty much want things sized to be safe with the voltages and currents in use in the system, and will require adequate over current protection and disconnecting means. The mechanical codes may take issue with the physical placement or type of equipment in use. I’ve specifically had issues with mechanical inspectors wanting continuous makeup air instead of interlocked fans with gas detectors.

    You may also want to check with your local fire Marshall. It probably won’t be the electrical people that you’ll have problems with here.

    Bill

  12. Stephen Watts | | #22

    Summary so far:

    Load bearing floor between cathedral ceilings:

    As usual, there are multiple ways of accomplishing this. I will need to consult further with the architect, but I am leaning toward his recommendation, which is ventilated parallel chord trusses for the cathedral ceilings and attic trusses for the middle of the house. For those who may remember my previous posts on cathedral ceilings focusing on fire resiliency I have been convinced that intumescent soffit vents will stop fire intrusion into the vented space.

    Batteries in the attic:

    A number of people are trying to apply the rules for lead acid batteries to engineered package lithium ion batteries. This is not appropriate in my opinion. Code makes a distinction between these types of batteries, and states in several places that installation of a pre-engineered or self-contained energy storage systems (ESS) shall be in accordance with manufacturer's recommendation and listing of the system.

    The drawbacks to attic installation is difficulty in transporting the batteries into the attic and whether inspectors will interpret attic installation as meeting the readily accessible location standard. This could potentially be an issue for attic mounted inverters and BOS components as well.

    In terms of the specific house design, it would not be possible to install two LG Chem RESU batteries on the ground floor without a major redesign due to battery clearance requirements. However, a minor redesign of a storage closet would allow installation of two Tesla Powerwall 2 batteries on the ground floor with minimal impact (loss of that storage space and a little less space in the adjacent utility room). The Tesla Powerwalls are a little more expensive and a little less efficient due to AC connection, but also a little more resilient because they have their own inverters.

    Therefore, I am leaning toward Tesla Powerwalls on the ground floor in a dedicated closet. Note that I am still 2 years away from build, so things could change during that time as more information becomes available, technology improves, and budgetary constraints intrude. I appreciate all the feedback.

  13. Expert Member
    Zephyr7 | | #23

    Chances are you’ll have new options for most everything in two years time. This stuff has been evolving pretty quickly. You’ll also have the benefit of two years of additional field experience that the manufacturers will have gained during that time.

    I’d plan for a little extra space than you expect to need to have some margin in your design. I’d especially leave additional space on the sides, top and bottom of any wall mounted equipment to allow for possible configuration changes of the equipment that may happen in two years for new models.

    I do think you’ll find the ground level installation is going to be a good idea. “Readily accessible” is usually interpreted to mean you can get to the device without tools (no need to cut through a wall for example), or special equipment. Stairs or a ladder is usually sufficient to meet the requirement.

    I would also strongly encourage you to put a heat sensor in that equipment room as a minimum, and wire it into a remote annunciator in the living space to alert the occupants to any issue.

    Bill

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