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  1. #1

    Ducting on a roof. Smart?

    We're designing a second story master suite addition in Los Angeles. It's about 500 square feet for the bedroom, bathroom and walk-in closet.

    This will have a flat roof, and routing the HVAC ducting is turning out to be a problem. In a nutshell, we can route ducting through the ceiling in the East-West direction, but not the North-South direction.

    The architect is proposing two fake "roof ridges" that would be built on top of the flat roof that run in the north-south direction. Basically two triangles, each one foot high, that run 22 feet from the north end to the south end.

    He wants to line these with drywall (or something else that would give a smooth surface) and run air through them. Each of these rooftop "ducts" would have an area of roughly 144 square inches BEFORE insulation.

    That's my concern. This is Los Angeles, which gets hot. Running an important part (these would be the main upstairs supply lines) of my HVAC ducting on TOP of the roof seems like it's asking for trouble, since it's directly exposed to the sun (or cold,) and even with insulation, I'm going to lose efficiency compared to running all my ducting through much cooler walls or a ceiling or under the floor.

    Or am I wrong? Is it actually only a very slight loss when running ducting on top of a roof?

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Sounds liek a bad idea. Why not just use a minisplit which doesn't need ducts? Or look at a high velocity system that uses smaller diameter ducts.

    Adding edges, seams, valleys and pretty much anything on a flat roof is a bad idea. Its' a good place for leaks... and yes it will likely warm the ducts.

    Why not run the ducts inside at the ceiling and enclose them in drywall creating a bulkhead. You can make it a little oversized and throw in some recessed can lights and make it a interesting design element.

    Besure to spec out spray fomm under the flat roof deck for insulation. It will really help on cooling bills.

    Make sure you get a laod calculation done... and again, in thsi case, it might be better ot use a minisplit system. Soem system can have up to 8 indoor units that you mount on the wall. Some units can be built into the floor or ceiling and use short duct runs. The pro's on here seem to love the Mitsubishi's and I've heard great thigns about them from the pros I work with.

  3. #3
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,485
    Around here they run metal ductwork on the roof and build a little wall in front of it to hide it.

  4. #4
    Join Date
    Jun 2011
    Location
    sfv california
    Posts
    110
    Quote Originally Posted by motoguy128 View Post
    Sounds liek a bad idea. Why not just use a minisplit which doesn't need ducts? Or look at a high velocity system that uses smaller diameter ducts.

    Adding edges, seams, valleys and pretty much anything on a flat roof is a bad idea. Its' a good place for leaks... and yes it will likely warm the ducts.

    Why not run the ducts inside at the ceiling and enclose them in drywall creating a bulkhead. You can make it a little oversized and throw in some recessed can lights and make it a interesting design element.

    Besure to spec out spray fomm under the flat roof deck for insulation. It will really help on cooling bills.

    Make sure you get a laod calculation done... and again, in thsi case, it might be better ot use a minisplit system. Soem system can have up to 8 indoor units that you mount on the wall. Some units can be built into the floor or ceiling and use short duct runs. The pro's on here seem to love the Mitsubishi's and I've heard great thigns about them from the pros I work with.
    i like the mini spit idea fujitsu halycon can have multiple fan coils, or lg u can have a picture frame fan coil

  5. #5
    I hadn't considered a mini-split idea. (I had to Google it to even know what it is.) I'm hesitant to go this route partially because I'm not sure my wife would go for a ductless look, and also because we're the types who keep the central air fans constantly on (not on auto) because we like air being constantly circulated and filtered through the whole house. Still, I'm going to look into mini-splits further, because it does have a lot of upside. Thanks for suggesting this.

    Friends of ours had a high velocity system and it was really noisy. The whine of the air would drive me nuts. But maybe that was just their system was not being set up right or something.

    Bulkheads and soffits are indeed another option we're considering, but we'd like to avoid them for interior design reasons.

    I'm noticing that a lot of commercial buildings have a lot of metal ducting on their roofs. I wonder if they lose a lot of efficiency this way? Or maybe it turns out to be fairly insignificant?

    Thoughts and advice are much appreciated. Thank you.

  6. #6
    Join Date
    May 2007
    Location
    Dry as a bone Tucson
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    4,544
    Loses thru exposed roof top duct can be very substantial. Considering that your supply air temp is around 60 degrees and the outdoor temp around a 100. That is a large differential and the cold duct would absorb the heat like a kid and a popsicle. Even if the duct has the proper minimum insulation of 2" there still is a tremendous amount of heat gain or another way to say it would be cooling capacity loss. I have had customers that have built "dog houses" over properly insulated and sized duct work. This helps tremendously.
    I usually have some pictures to go along with my advise. But they will not be posted till next thursday...sorry.
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  7. #7
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    All this for 500 square feet of space??? Look at a PTAC unit. PTACs what most hotel rooms use without ducting to a bathroom or a closet. I'll bet you never thought about how well that works.

    The other options are big bucks.
    • Don't use a building cavity if you do a duct above the roof.
    • Do use sealed metal ductwork above the roof.
    • Do get a duct pro: Any grilles delivering air from the trunk duct inside will have much more air velocity and noise than those branches going up, over and down into the living space. Let the pro prove his stuff by letting him tell you about the pitfalls of doing it wrong.
    • Do inuslate the duct enclosure space on all sides to R40 or so using polyurethane board or spray foam to cacoon the duct before the roof goes on.
    • Do talk to a roofer BEFORE you make any changes.
    Or do a mini split or high velocity system.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  8. #8
    Join Date
    Apr 2010
    Location
    southern california
    Posts
    535
    Being from southern calif. , My first question is with the additional sq. footage are you trying to tie this ductwork into existing system. How do you know the existing system is sized correctly. Because you are wanting to add more than 25 feet of ductwork this will trigger all title 24 requirements. You will also need to add an exhaust fan , carbon monoxide detector, smoke detectors, earthquake seismic shut-off valve

  9. #9
    Thanks for the replies, guys. This is really helpful.

    I should probably have made more clear that this is a complete remodel of the house. I only described the 500 feet of the upstairs section because that's the only problematic part. The rest of the house is all easy access for ducting.

    The entire HVAC system will be new. We will indeed be hiring pros for this. I'm voicing my concerns here because I don't want the architect to box me into a corner that will make things impossible for the HVAC guy (who we haven't hired yet.)

    A PTAC unit does look efficient and simple, but it wouldn't be a look my wife would ever go for. We really do want this to be a straight central air type system through the whole house. It's just a matter of figuring out the ducting. If the roof ridge duct is a bad idea (and it sounds like many here think it is,) then I need the architect to come up with some other design solution now, before we sign off on the plans. I just need to feel pretty certain before I tell him, "No, we're not going to do the roof ridge idea."

  10. #10
    Join Date
    May 2004
    Posts
    600
    Quote Originally Posted by acwizard View Post
    Being from southern calif. , My first question is with the additional sq. footage are you trying to tie this ductwork into existing system. How do you know the existing system is sized correctly. Because you are wanting to add more than 25 feet of ductwork this will trigger all title 24 requirements. You will also need to add an exhaust fan , carbon monoxide detector, smoke detectors, earthquake seismic shut-off valve
    A room addition will need a title 24 CF-1R-ADD form to cover the windows, insulation, HVAC, etc.

    If all your new duct work is in the interior space, you can skip the duct testing requirement of title 24.

    Read up here, this is what I would install. these are really cool units.
    http://www.mitsubishipro.com/en/prof...n-and-addition

  11. #11
    Join Date
    Jun 2011
    Location
    Houston area
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    1,493
    Quote Originally Posted by MikeInLosAngeles View Post
    ....The architect is proposing two fake "roof ridges" that would be built on top of the flat roof that run in the north-south direction. Basically two triangles, each one foot high, that run 22 feet from the north end to the south end.

    He wants to line these with drywall (or something else that would give a smooth surface) and run air through them.....
    Your architect is a HVAC idiot. Is it too late to get a new one? I'd strongly suggest Listening to Mr. Comstock's advice.
    The picture in my avatar is of the Houston Ship Channel and was taken from my backyard. I like to sit outside and slap mosquitos while watching countless supertankers, barges and cargo ships of every shape and size carry all sorts of deadly toxins to and fro. It's really beautiful at times.....just don't eat the three eyed fish....

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    LMAOSHMSFOAIDMT

  12. #12
    Join Date
    May 2004
    Posts
    600
    look at the inverter technology

  13. #13
    Join Date
    Jul 2002
    Location
    Yuma, AZ
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    2,361

    Aha!

    Quote Originally Posted by MikeInLosAngeles View Post
    ...I should probably have made more clear that this is a complete remodel of the house. I only described the 500 feet of the upstairs section because that's the only problematic part. The rest of the house is all easy access for ducting.

    The entire HVAC system will be new. We will indeed be hiring pros for this...
    Well that information makes me suggest, "Put the ducting for the bedroom in the bedroom floor and above the ceiling of the first floor." This might be easy to do and to make it work well. You only need one or two supply vents at most.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

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