Central Air: Attic Ductwork Layout for Single Zone.

Discussion in 'HVAC Heating & Cooling' started by bjferri, Jul 3, 2009.

  1. bjferri

    bjferri DoD Army

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    My other thread was: Central Air Questions for Old Home.

    I am going Single Zone with unit placed in attic.

    New Question:
    What is the best way to run the ductwork from within the attic?
    I’ve heard several ways. One said ductwork around perimeter of attic then tap off that for different rooms. Another said a strait branch with shoots for different rooms off that. Are those the same?

    I’d like to keep some storage in the attic if I can.

    I asked if they can run from the blower outside, whatever they normally up the side of the house, through the house instead. Is this okay? I’m also asking that they put registers in the walls from closets rather than the ceiling, if possible. This all makes my head dizzy. I don't want to lose the character of the home.

    I just don’t want to say to myself…I should have done it this way instead.

    BTW – I’m going 16 SEER Rheem
  2. Dana

    Dana In the trades

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    A decent manual-D software package would probably tell you the answer. Doing the perimeter approach is probably less work, but may require the perimeter duct to be larger. The trunk & branch approach is what I see most often. (And mastic-sealed seams & joints make an efficiency difference, in either approach.)

    But whatever you do, if the mechanicals & ducts are all in the attic, insulating & sealing the attic at the roof deck turning it into semi-conditioned space makes a large difference in operating efficiency (a bigger difference than 14 vs 16 SEER). In a vented attic, any duct leakage results in air infiltration of the worst-sort: Solar-heated attic air than's several 10s of degrees hotter than outdoor air. A sealed & insulated attic is by far the preferable approach, since all potential leakage then occurs inside the insulation & pressure envelope, and the air handler & ducts are then surrounded by room-temperature air, not solar-heat-attic temperature air, so even the conducted losses to the surrounding air go toward cooling the conditioned space, not the overheated attic. See:

    http://www.buildingscience.com/docu...-vented-and-sealed-attics-in-las-vegas-nevada

    For other considerations/issues around sealed attics see:

    http://www.buildingscience.com/docu...tilation.pdf/attachment_download/attachedFile

    If it's just the ducts in the attic (and not the air-handler), sealing them with mastic and burying them in a foot of cellulose works too. (Cheaper than foam-sealing/insulating the roof deck.)
  3. bjferri

    bjferri DoD Army

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    Thanks for the technical information. I actually read most of it - understood some. I have a vented attic with an attic fan so maybe my system won't be as efficient as I thought. Something else besides the ductwork is going in the attic, the blower or condenser - I forget. I try to learn DIY when I can but HVAC is not my forte...

    Nice to know the trunk/branch approach is fine.
  4. Dana

    Dana In the trades

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    This definitely calls for a sealed-insulated unvented attic then. See what it takes to spray ~R19 worth of half-pound foam (Icynene, Sealection 500, Demilec, etc ) between the rafters to seal it all up. It won't be cheap, but it'll :

    A: reduce the cooling load- you may be able to drop a ton (or even two) on the compre$$or sizing and...

    B: It'll allow the system to run at it's true optimal efficiency in an ~80F rather than degraded in a 120F+ environment.

    There may be tax credits & other local subsidies for adding insulation as well.

    The roof probably accounts for ~30%+ of your peak cooling load- cutting that 30% in half reduces the whole-house load by ~15%. Most Manual-J estimates are 25% oversized to the true load anyway, so reducing a 4 ton (derived by Manual J) to a 3 ton is usually just fine, and if you use the difference in money to reduce the load ~10-15%, you've bought some margin. And if you've boosted the operating efficiency at peak loads 25% by keeping it all inside of the thermal-insulation & pressure envelope of the house... (getting the picture?) Something like 1.5 tons less compressor + insulated/sealed attic maybe doesn't look so expensive up front after all, and the operating costs will surely be lower. Your AC contractor isn't in the biz of telling you where the cost/benefit breakdowns are- you're kinda on your own figuring it all out. Their job is to find the answer to, "can you cool this, as-is where-is?", so that's what they do.

    The difference in compressor costs may be only $500-1000, but $1000 buys you ~ 250 square feet of half-pound foam installed @ R20 thickness (before tax credits or other subsidy), and typically 2-4x that where subsidized. But insulation is a lot cheaper to run than compressors, with a much longer life cycle to boot, eh? ;-) But estimating on the low side for compressor sizing ensures that it really runs at full efficiency, and it'll run longer cycles, keeping the air drier & more comfortable. Oversizing tends to make for short cycles & clammy feeling air, and higher power bills. When it doubt, drop a ton. If it doesn't keep up, lower the load by better insulating/shading/air-sealing- it'll be worth it in the long run. With heating & cooling equipment, larger than you actually need is never better- undersizing slightly is usually more efficient, and the equipment even lasts longer since it cycles less frequently.

    BTW: Both the FSEC and Texas A & M did a lot of attic fan (& other cooling strategy studies) during the '80s & '90s. In most cases the power used by the fan equalled or exceeded the power seen by a central AC system to achieve the same cooling target temp without the attic fan. In many instances the attic was cooler because the attic fan was drawing cool conditioned-air up from the conditioned space (!), adding to the overall cooling load. Self-powered solar versions may buy you a handful of percent on the cooling bill if you have ultra-low infiltration between the conditioned space & attic though:

    http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

    But if you're committed to AC ducts & air handler in the attic, make it a conditioned attic- forget the whole attic fan concept entirely- insulate & seal, insulate & seal...
  5. bjferri

    bjferri DoD Army

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    Okay - that all makes sense but I have an attic fan and in the summer when it's not hot enough to run air, it's nice having the fan pull fresh air through the house acting like a whole house fan. Besides, I'm a fan of sprayed insulation (was actually watching a program today on it and all I thought was...What happens if you get a roof leak? How can you trace where the leak is coming from? How will you know you have one? Will the roof sheathing be rotted before you find out? All these things go through my head. I know I'm drifting from my main question but if I did proceed with spray insulation, I'd need answers to these questions... You have any smarty pants? lol
  6. Dana

    Dana In the trades

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    Sure (I just make it up as I go along, can't you tell! :eek: ;) ) I've got answers...

    With half-pound foam, leaks are dead-easy to spot. It's open-cell, and you'll get a drip-drip-drip directly below the leak. The foam isn't hydrophillic- it won't soak up water like a sponge, but it lets gravity do it's thing with the water, and will dry all by itself after the leak is fixed, self-restoring to it's original insulative value.

    The more rigid closed-cell 2lb foams are higher R-value per inch of depth, but it's waterproof and forms a vapor-barrier which can block both detection and drying of roof leaks. It's generally "safer" (not to mention quite a bit cheaper) to use the open cell half-pound stuff (unless you spend a lot of time thinking & designing the roof structure layup to guarantee drying paths calculating where the heating season dew-point lies, etc. etc. there's a whole boatload of discussion on this thread on another forum:

    http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/14/postid/55637/view/topic/Default.aspx

    With all of this stuff, on some aspects it makes a difference what climate zone you're in- what's your zip code?
  7. bjferri

    bjferri DoD Army

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    Zip is 07701
  8. Dana

    Dana In the trades

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    07701 is DOE climate-zone 4, so according to the Building Science Corp data you won't need to use a vapor retarder with open cell foam the way you might in a colder climate. (See page 16 & figure 10 of the "Understanding Attic Ventilation" document.)

    According to the Oak Ridge Nat'l Labs estimators come up with a minimum attic insulation of R38 as cost-effective retrofit for any 077xx zip codes, even with high efficiency heating & cooling equipment. (Check it yourself here: http://www.ornl.gov/~roofs/Zip/ZipHome.html ) I'm assuming you have at least 6" of something (batting or blown) on the attic floor already(?) If you go to an insulated rafter configuration DO NOT REMOVE the existing stuff (even though some foam installers will tell you to "...because it's no longer necessary....".) The total R-value (rafters + attic floor) still counts- particularly during the heating season.

    With the ducts & air handler in the space between insulating layers (roof deck/ducts&AH/attic floor) it's then only semi-conditioned space from an insulation point of view, but with a sealed attic it's fully within the pressure envelope, with at least R19 between the ducts/air-handler and the 100F+ roof deck, which is way better than having it outside the insulation & pressure boundary. The difference in cooling efficiency fully vs. partially within the insulation boundary might be measurable, but would be mostly an academic exercise, and reducing the full R-value doesn't gain you any cooling efficiency even if it reduces the ambient temperature surrounding the ducts, since that lower ambient would only be lower because it's now full-conditioned space- an extra direct-load on the AC. But difference in heating performance would be dead-obvious on the bill. Only remove insulation where it would interfere with installing the foam (like pulling it back from soffits 2-3' so they can get a spray gun in there.)

    You probably arleady know that NJ gives a kickback for high-efficiency cooling equipment like yours (not much, but it's probably worth filling out the forms.)

    http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=NJ10F&re=1&ee=1

    They also subsidize better-than-average energy-audits (which might point you to the most cost-effective paths for reducing the overall heating/cooling loads.)

    http://www.njcleanenergy.com/residential/home/home

    But NJ doesn't subsidize insulation as extravagently as some states, but there's something:

    http://www.energyfinancesolutions.c...rmance_jersey/eligible_measures/building.html

    NJ does subsidize air-sealing fairly heavily if the heating-season performance exceeds 25% reduction (which it just might!):

    http://www.njcleanenergy.com/reside...rformance-energy-star/benefits-and-incentives

    Odds are pretty good that your attic floor/conditioned space boundary is leaky enough that you can hit that 25% estimated heating savings on the air-sealing & insulating aspects of going with a foam-insulated & sealed unvented attic. You may need to foam-seal the foundation sill as well to hit the magic 25% savings mark for the since it's usually one of the largest air-leaks in homes, even new ones. But that's typically only ~5-10% more foam, and at a favorable price if done on the same day as the attic. (It's cost-negative, if that's what gets you over the 25%-heat-savings hurdle for the 50% of up to $5K cash rebate.) It's probably well-worth the cost of an energy audit that utilizes a pressure-door test to find out!

    Based on what I've seen on other retrofits, sealing up the attic with more insulation will reduce your cooling load enough that decommissioning (& sealy up ) the whole-house/attic fan is the right thing to do, especially in higher humidity climates like the Jersey shore, where latent-loads (humidity) accounds for more than half the load, even in the cooler shoulder-seasons. Running a slightly under-sized to right-sized high efficiency AC system will use less total electricity, and provide more comfort (by drying the place out) than sucking in higher-humidity night air with an attic fan. Sure, the compressor & air-handler may use 10x the power of a whole-house fan while running, but it'll run a far lower duty cycle to achieve the same amount of sensible (temperature only) cooling as the attic fan, but will be lowering rather than raising the indoor relative humidity in the process.
  9. bjferri

    bjferri DoD Army

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    Thank you for all this information. I will have to sit down and read/digest it. This is some good material. I placed a call into the NJ Clean Energy Program to learn more about it. See what happens...
  10. Zenman

    Zenman New Member

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    Location:
    New Jersey
    unconditioned attic space with HVAC in NJ

    Wow a lot of information here, I have a somewhat similar situation... Thanks to the original poster and all the contributions.

    Here is my situation;
    I have a gas furnace (~80%) forced hot air and AC in the attic. It is an unconditioned space and in New Jersey (zip 08534). Also I have ridge and soffit vents. The ductwork is all insulated - main branch is ductboard and the individual supplies are round flexible ductwork.

    Correct me if I'm wrong, but It appears from what I read I should seal off the attic or at least build a room around the furnace - correct? My next question would be where would the combustion air come from? (the furnace is not a direct vent.) Any leads on contractors who specialize in this type of work on central NJ? or DIY?
  11. Dana

    Dana In the trades

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    You may have sufficient infiltration from the conditioned space below to provide combustion air, but you may have to duct/louver it (most easily done in a gable-end) if you insulate the roof deck & seal it off. I've seen (haven't designed) a system where a large automatic vent damper was used for combustion air to a boiler room, cut into the same ignition interlock as the exhaust vent damper so that ignition was inhibited until both were proven open. It's likely that a similar system could be retrofitted into your situation.

    If it's easier to build an insulated air-tight chamber around the furnace with enough soffit-venting to the chamber to meet code for combustion air that's another alternative. (Don't rely on ridge-venting for combustion air, since it likely to be temporarily blocked by snow sometimes.) Read the manual for the furnace- it'll have guidelines on the cross-section you'll need for combustion air.
  12. Zenman

    Zenman New Member

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    Location:
    New Jersey
    Thanks for the reply... looks like I have some research to do. :)
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