Ventilation Ducts in Wood Floor Trusses
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  1. #1
    Join Date
    Jan 2005
    Posts
    4
    Hello,

    I am building my own house in SE Texas and wanted to
    ask if anyone has any concerns about the HVAC Ducts I am
    installing:

    House is a two-story on a slab. I am having my home
    insulated with icynene so it will have a very strong
    moisture barrier.

    The 1st floor ceiling/2nd floor is 18" open wood trusses.
    The airhandler will be on the first floor - floor and
    the ducts will be distributed thru the open wood
    trusses.

    The ducts will be made out of 1.5" ductboard on 3 sides
    and use the plywood floor for the 4th side. None of
    the 4 sides will be next to an UNconditioned space.

    Is this OK?

    If not, why & what solution do you recommend?

  2. #2
    Join Date
    Dec 2003
    Location
    Location!, Location!
    Posts
    929
    Both supplies and returns will have one side as plywood? Not good for either- what will happen to air when plywood outgasses in heated and cooled airpath? What will happen to plywood when surface is exposed to changes in temp/humidity?
    Was this recommended by a pro?
    Is this your house?
    Why don't you want to use regular 4-sided ductwork?
    I am partial to steel, but any closed system would be better than what you are planning.
    If you can't afford to do it right, how can you afford to do it over?

  3. #3
    Join Date
    Jan 2005
    Posts
    4
    supply will only have one side plywood.

    WHAT HAPPENS to air when plywood outgasses in heated and cooled airpath? (Only Cooled to 60F & Heated to 85F)

    WHAT HAPPENS to plywood when surface is exposed to changes in temp/humidity? (Only Cooled to 60F & Heated to 85F)


  4. #4
    Join Date
    Aug 2003
    Posts
    626
    All ducts in HVAC need to be hard ducted. Using floor joist cavities in any manner will cause leaks to the outside of the home.

  5. #5
    Join Date
    Jul 2003
    Location
    sacratomato
    Posts
    535
    Running your duct work inside condition space is great! Best thing you could ever do. You’re not breaking any code concerns. The issue would be warm moist air from the outside getting near the cool air condition air, water will condense and mold will grow on wood, as wood is food for mold. Duct board is ok, but there are advantages to steel, like it can be cleaned later on with out a concern for damage. To bad they don’t make duct board with foil on both sides.

  6. #6
    Join Date
    Jan 2005
    Posts
    4
    Under normal conditions there will not be any way for warm moist air to come in contact with these ducts. None of the four sides will be in contact with any exterior wall - All four sides will be surrounded by conditioned space.

    Without warm moist air coming into contact with these ducts, does anyone see negatives with the air coming in contact with the plywood or 2x4's in the wood trusses?

  7. #7
    Join Date
    Jan 2005
    Location
    Seattle
    Posts
    191

    Talking Tree side duct!!!

    Setexas.
    You will probibly sart to get some pretty
    opinionated responses here.But to
    some this is a pretty silly idea.

    First, in my neck of the world this
    plan will not pass code.I can tell you
    the inspectors around here would be
    smirking,and breathing funny through
    their nose.As they wrote me up with
    a really big RED pen in one hand
    and their cell phone in the other.
    Cause he would just have to get ahold
    of the other inspectors for the
    "YOU JUST GOT TO SEE THIS"
    conference call.

    I can appreciate a solid "CAN DO"
    attitude.One of coolist parts of
    thisforum is that homeowners can ask
    questions and learn from people
    who have been at this for a long
    time.WE all like to help the good
    folks out ther in internet land.
    What you need to understand is
    that based on your question.It
    IS fair to say that you have a lot
    to learn before you make the choice
    to install this system your self!

    If you can put your ego in the drawer
    for a while.Think about what things
    were brought up right off the bat
    in response to your question.
    You would not come back with
    WHAT WOULD THAT HAVE TO DO
    WITH IT.You came here to learn.
    Not to badger the people who
    bring a life time of experience
    to this forum for free!!!!!!!!!
    So sit back think and type back
    that you can see why "X" would
    be a problem.But I don't under
    stand how 'Y' would be a problem.
    Remember you only want to pay
    for your system once.Good luck
    to you.And I hope we can help
    steer you to a good contractor.
    41Gasman






  8. #8
    Join Date
    Jan 2005
    Posts
    4
    41Gasman, I did come here to learn, I am looking for facts & logical reasoning, not one-line statements. I think binford is the only one that read ALL of my initial post.

    To help my education, could you answer at least my question number #1?

    1. Why will this not pass code? Where is your neck of the world?

    2. Exactly what statement did I make that caused you to think that I had a LOT to learn? I asked my original question because even thought it is OK according to IRBC 2003, there may be issues the code people may not have considered.

    3. I was NOT asking "what does offgassing got to do with it?" but rather "What is offgasing from plywood, and is it really a concern over the temp range we are talking about?

    4. I apologize to everyone that I unintentionally badgered.

  9. #9
    Join Date
    Oct 2004
    Posts
    202
    Sounds like you know ALOT about this proposed duct system...maybe even more than some of us here...

    From what you have described... it sounds like a wonderful idea to me....

    You came here for opinions from people in the hvac trade...

    You got some...

    Let us know how it all works out...

    Good luck




  10. #10
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    Originally posted by setexas
    41Gasman, I did come here to learn, I am looking for facts & logical reasoning, not one-line statements. I think binford is the only one that read ALL of my initial post.

    To help my education, could you answer at least my question number #1?

    1. Why will this not pass code? Where is your neck of the world?

    2. Exactly what statement did I make that caused you to think that I had a LOT to learn? I asked my original question because even thought it is OK according to IRBC 2003, there may be issues the code people may not have considered.

    3. I was NOT asking "what does offgassing got to do with it?" but rather "What is offgasing from plywood, and is it really a concern over the temp range we are talking about?

    4. I apologize to everyone that I unintentionally badgered.
    1. Because it is in the National Code.

    2. How are you sizing this ductwork? (what method)
    It sounds like you are a DIY'er...
    How tall are you Private???!!!!

  11. #11
    thank you.

    under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

    Does the National code not allow this?

  12. #12
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    if you are using the second story floor as the top of the duct you are gonna have a hot/cold spot on the floor material. i have never heard of an idea such as yours. how are you sizing this? did you contact the building department in your area? they are the ones that have to be satisfied for occupancy. that would be my first call.
    FILL OUT YOUR PROFILE!!

  13. #13
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    My last thought is:

    If you are builing your own house, why do it half assed?

    Have your ductwork sized properly. Also use sheetmetal. (all 4 sides)
    How tall are you Private???!!!!

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