Issues of using a sealed building cavity as duct instead of traditional ductwork
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  1. #1
    Join Date
    Jul 2008
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    Issues of using a sealed building cavity as duct instead of traditional ductwork

    I am a building and solar contractor in Central Florida. Our desire to to build zero energy homes and we've experimented with various ways to improve efficiency.

    We are getting ready to build a duplex for ourselves that will be very tightly sealed between living space and attic. We are even actually considering drywalling the ceiling before building the interior walls so that the drywall envelope is not broken where every wall is located. (The electrical contractor is the main guy affected if we do this but we would leave a sheet or two of drywall off in the middle of the rooms to make it easier to wire. The only thing the plumber has to do in the ceiling is run his stacks through to the roof.) We want to avoid putting ductwork into the attic for the obvious reasons. On an 8' ceiling it gets challenging to put ductwork in conditioned space as you can't drop your ceiling below 7' in those areas where your duct work would go. The floor plan is designed so that we would be able to hit the entire apartment with air by lowering the ceiling in a hallway and one bathroom.

    So the thought is this. . . Since we will have a (virtually) completely thermally sealed separation between attic and living area at the 8' ceiling level, why don't we just hook the A/C supply to this cavity (between the 7' and 8' sheetrock. We would then penetrate the cavity on the adjoining rooms that are 8' tall with an appropriately sized register so that the air is adequately and uniformly circulated (at least that is the goal).

    The reasoning is that it is simpler and cheaper than creating a duct box in the same space and we don't see what benefit the ductboard box would have since the cavity is 100% in conditioned space. Considering that a 7' ceiling is going need to be supported by 2x4s at a minimum, you really only have 8" to work with in the cavity. That's not enough height to do anything with a ductboard system.

    The units are 3 bedroom/ 2 bath, 1,200 Square feet in size and they will have vinyl, double pane, Low-E windows, a galvalume natural metal color metal roof, 2' soffet overhangs, and will be VERY well sealed between living area and R-50 attic. I haven't run the numbers but I suspect that one ton of air would suffice.

    Please advise on the following:

    1. Is there any reason why we should not create an air distribution chase this way? Would there be moisture or condensation issues of some kind? Would be be able to adequately balance rooms by register sizes is this situation?

    2. What is the best high SEER way to deal with such small BTU needs where a ton of air might suffice, considering that most A/C units are much larger than that? We want to avoid short cycling the units and we want to control humidity? Is this where an inverter based variable type of system makes sense? I've thought about using a mini-split and avoiding an air chase all together but it seems like you need a unit in each bedroom and the living area and that would be overkill for a small place such as this.

    Thanks so much for your thoughts and input.

    DavidJ

  2. #2
    Join Date
    Jan 2007
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    Minneapolis, MN
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    Keeping the ducts inside of the thermal envelope is the best way to run them but I would still run metal ducts sealed with mastic to make sure pressure imbalances are not created. Can you raise the ceiling to 9 feet to accommodate a better duct system? If you can't size the AC because of low loads make provisions for whole house dehumidification.

  3. #3
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Without a duct system,you may have moisture condense in the cavity.You'll need a duct system to distribute the correct air flow to each room.

    If the actual btus required is that low,a 6" flex is likely the kargest duct you'll need for any room.If you use the cavity for an an air return,thus getting return from each room,,the supply duct won't need to be insulated,just sealed tightly,so the space will be large enough.

    I don't think you'll find a one ton system,with dehumidification features,but a 1.5 ton may be what you need.

    Be sure to run this by your building dept., as Florida code forbides using cavaties for duct,though yours is not what I think they had in mind.

  4. #4
    Join Date
    May 2004
    Location
    south louisiana
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    I was wondering what your take on this would be, dash.

    I've seen several homes in my area with the exact install of air tight fur down,
    with no ductwork.
    A friend of mine did this 10+ years ago at his home.
    A little smaller than OP's build, but similar layout. Hallway was fir-d down to
    7' and openings cut (..but sealed - between walls..) into eachof the bedrooms, baths & living and kitchen area. Even with a 10 year old 12 SEER and 80% furnace his utility bill
    is less than $70 per month. His utility cost is approx .13 per Kwh.
    Mostly I see this in commercial hvac contractor's personal homes. They 'get it'
    as do you about ductwork in conditioned space.

    I've yet to see the same install with ducts.

    Our summertime temps are 90+ in august & sept, sometimes mid summer.
    RH is usually 80% these times of year. Mild winters.

    We use heat pumps here quite often, and with excellent results.

    What do you think the difference between my climate and yours would be?
    Why would it not be a problem here, and a problem there?


    Lots of questions for a friday!
    have a great weekend everyone.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  5. #5
    Join Date
    Jan 2004
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    Lancaster PA
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    York has 1 ton condensers.

    How would you get return back to the unit.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #6
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    I've seen many here that were retrofit of existing homes,with no A/C,to add A/C.

    Airflow between rooms is difficult to regulate,and the original ceiling often gets condensation,near where the air handler discharges air into the cavity.Most had plaster ceilings,so I'd think they were well sealed from the attic.Maybe hotter attic temps here causes it,or there wasn't enough insulation.

    It might not be an issue here if done correctly,sealed,and enough attic insulation,just not sure exactly how to do it right.

  7. #7
    Join Date
    Mar 2005
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    burlington county n.j.
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    can't see where the little bit of money saved would be worth the risk of condensation and mold.


    would it even pass fire inspection using the chase as a supply duct?

  8. #8
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    May 2004
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    south louisiana
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    You know if I were to do this in my personal home..
    I would make the ..for lack of better description..
    supply plenum (created by fur down) out of ductboard.
    Mastic seal all seams and also seal the opening where the supply grill
    goes into each room through the wall.
    I can't see that the few $$ that ductboard would cost could be anything but
    a good investment.
    I did see a house where they used ductboard to seal between the supply
    and grill to keep air from inside of wall cavity out and to seal this area that
    is open otherwise from top of wall cavity to bottom of wall cavity.
    Several others just left it open, no problems that I knew of...but
    it just seems to me that it would be much better to seal.
    IMO this would be the only area where duct loss would occur and IAQ take a hit.

    I think properly air sealing and insulation in attic would take care of any problems
    in this fir down without ductwork.
    Note that he is planning on a light colored metal roof..


    beenthere..in the homes that I have been in..again see description of my friends home..
    some were a bit larger but none over 2200 sq ft..same general layout.
    R/A located at bottom of heating system closet. it seems that most were either in
    living area or at end of hall that is fur-d down.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  9. #9
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    May 2004
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    south louisiana
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    this is from buildingsciene.com
    hot humid climate..this study is in Orlando Fla.

    Ducts in conditioned space - The preferred method for keeping HVAC ducts and mechanical equipment inside conditioned space is moving them down from the attic. In this building profile, a conditioned attic can be used for HVAC ducts and equipment. In no case should HVAC ducts be placed within exterior wall assemblies-this is not part of what is meant by ducts in conditioned space. A vented attic assembly may be used in this climate as long as the ceiling plane is air tight and no ductwork or air handling equipment is located in the attic.


    here is the link:
    http://www.buildingscience.com/docum...tioned%20space

    There is a Builder's Guide for Hot Humid climates by BS.com.
    it is an excellent resource. I'm on my 2nd copy! it is also online.
    Also the same builders guides for all climates.
    Amazing stuff!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  10. #10
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    If you used 1" ductboard,it could likely be sized correctly for each room and fit within the space.

  11. #11
    Join Date
    Aug 2003
    Location
    Spring Hill, Florida
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    213

    Use Icynene Instead

    Have you considered Icynene. Using the building cavity in Florida is against the Mechanical code to begin with and there are alot of problems that can occur when building cavities are used as duct work. The biggest is MOLD!!! If you want a real tight building you may want to consider using Icynene which is a spray foam insulation that creates such a tight seal in the attic you wouldn't even need to worry about drywall before framing your interior walls. In fact it creates such a tight seal in the buildong envelope you a/c contractor will need to bring fresh air from outside. I can also tell you that you can go into an attic with Icynene on a 95 degree day at 4 o'clock in the afternoon and barely even break a sweat.

  12. #12
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    Aug 2003
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    Fort Worth, TX
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    11,271
    Quote Originally Posted by DavidJ View Post
    I am a building and solar contractor in Central Florida. Our desire to to build zero energy homes and we've experimented with various ways to improve efficiency.

    We are getting ready to build a duplex for ourselves that will be very tightly sealed between living space and attic. We are even actually considering drywalling the ceiling before building the interior walls so that the drywall envelope is not broken where every wall is located. (The electrical contractor is the main guy affected if we do this but we would leave a sheet or two of drywall off in the middle of the rooms to make it easier to wire. The only thing the plumber has to do in the ceiling is run his stacks through to the roof.)
    I realize you were not exactly seeking construction discussion here, other than the furred down duct chase, but a few things about your post raised some thoughts in my mind.

    Curious...perhaps I'm visualizing it incorrectly, but running an unbroken plane of ceiling drywall from exterior wall to exterior wall...are there no load bearing interior walls? You could essentially frame up all the exterior walls, build the ceiling joists and rafters, drywall the joists, and then have one large cavernous space before erecting interior walls?

    More thoughts on this...say you're the electrician up in the attic searching for where to drill through the top plate to send down Romex. He might say, "Hmm...can't see the top plates up here. All I see is drywall. I wonder where that bathroom wall is."

    Concluding thoughts...frame the house as usual. Then, wherever drywall meets an interior or exterior top plate, caulk the joint. If you wanna go crazy, run a bead of caulk along the top plate before slapping up a sheet of drywall. The ceiling is always rocked first, and there's no solid backing behind where the ceiling rock meets the wall top plate (unless you decide to go super crazy and block it), so caulking the ceiling to wall juncture is a good idea, anyway. Before even reaching that point, any penetration through top plates are to be sealed.

    Now, other thoughts about your house thermal design. A few years back I visited a "zero energy" house on display in Frisco, Texas. It had all sorts of cool stuff in it, the coolest of which (besides having a residential chilled water a/c system) was the attic, literally. The entire roof deck had been foamed. It was over 95 degrees outside. The attic had several visitors plus a docent explaining the HVAC. The attic was not hot or uncomfortable. It was completely sealed to the exterior. This was my first experience in a residential attic with a spray foamed roof deck. It's wonderful.

    Have you considered foaming the roof deck vs. a ventilated attic with R-50 with a sealed ceiling plane? With a foamed roof deck you could skip the furred down duct chasing altogether and place the HVAC equipment and ducts in the attic, because it now is conditioned space!

    If I were building a new house, I'd do this without blinking an eye. HVAC equipment and ductwork has NO place in a conventional, ventilated, solar heat collecting attic. It is perfectly fine to have the equipment up there if the roof deck is insulated and the entire attic is sealed to the exterior.

  13. #13
    Join Date
    Jul 2008
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    86
    Quote Originally Posted by mbarson View Post
    Keeping the ducts inside of the thermal envelope is the best way to run them but I would still run metal ducts sealed with mastic to make sure pressure imbalances are not created. Can you raise the ceiling to 9 feet to accommodate a better duct system?
    I could raise the ceiling in that area to 9 feet if necessary since we haven't yet ordered trusses. For reasons of cost, simplicity, and the desire to keep the ceiling plane flat I was hoping to avoid that. I would like to keep the ceiling plane flat for efficiency reasons. Popping up the ceiling increases the surface area between attic and living space and, at least the way they traditionally insulate those attic knee walls, they are usually not insulated very well with fiberglass batting.

    Quote Originally Posted by mbarson View Post
    If you can't size the AC because of low loads make provisions for whole house dehumidification.
    It was my hope that these inverter based units would flex their output enough so that one could size even small efficient envelopes properly. Is that not the case?

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