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  1. #508
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    The following link is to an article about attic ventilation written by Dr. Joe Lstiburek, one the worlds leading Building Science experts.
    http://www.greenbuildingadvisor.com/...-venting-roofs
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  2. #509
    Join Date
    Feb 2010
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    In a boiler room
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    7,159

    Thumbs up

    Quote Originally Posted by tipsrfine View Post
    The following link is to an article about attic ventilation written by Dr. Joe Lstiburek, one the worlds leading Building Science experts.
    http://www.greenbuildingadvisor.com/...-venting-roofs
    Very good article. Thanks Tips.

  3. #510
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,358
    Quote Originally Posted by tipsrfine View Post
    Ducts are enclosed areas with conditioned air in them, same as the home. If you have air leaks, and a pressure difference on either side of the opening, you're gonna have air flow. If the attic is at a lower pressure than what is in the ducts, due to the pav, then yes I believe air flow would occur. If you only have r-4 or 6 flex ducts, your gonna have some good heat transfer via conduction also. Then convective currents through the ducts and into the home.
    Just add up how much surface area the conditioned air in those ducts come into contact with up in the attic with only r-4 or 6 insulating them. On average it's easily the equivalent of the surface area of a large living room, with only r-4 insulation and the other side of that area is 125 F, or more?
    I agree with what you said. When you state a 30% loss of a/c or heating capacity via having ducts in a ventilated attic, do you also factor leakage, such as we've discussed above?

    Thanks for the Lstiburek link. That was good stuff. Seems he is not outright opposed to ventilated attics. He just wants to see them done right, as would many of us about many things in residential construction.

    I must admit I had not given much thought toward what Lstiburek said regarding provision of more ventilation area down low vs. up high, and to ensure that each rafter bay is ventilated. Also good stuff about ensuring minimal R value of ceiling insulation over top plate matchs R value of wall cavity below it. I have known about providing wind deflectors over insulation where roof and wall meet to prevent wind washing of exposed insulation, but I can see based on what he said it would also facilitate natural convection of air from the soffit vents to the exit ventilation areas located high on the roof, without those same convective currents coming in direct contact with the insulation.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #511
    Join Date
    Sep 2009
    Location
    Arnold mo
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    3,969
    Quote Originally Posted by Shophound View Post
    I agree with what you said. When you state a 30% loss of a/c or heating capacity via having ducts in a ventilated attic, do you also factor leakage, such as we've discussed above?
    Thanks for the Lstiburek link. That was good stuff. Seems he is not outright opposed to ventilated attics. He just wants to see them done right, as would many of us about many things in residential construction.

    I must admit I had not given much thought toward what Lstiburek said regarding provision of more ventilation area down low vs. up high, and to ensure that each rafter bay is ventilated. Also good stuff about ensuring minimal R value of ceiling insulation over top plate matchs R value of wall cavity below it. I have known about providing wind deflectors over insulation where roof and wall meet to prevent wind washing of exposed insulation, but I can see based on what he said it would also facilitate natural convection of air from the soffit vents to the exit ventilation areas located high on the roof, without those same convective currents coming in direct contact with the insulation.
    I havn't researched what you've asked (above in red). That 30% loss in efficiency is just a number I've seen accepted & used throughout the community of HVAC & Building Science. Whether or not it takes into account duct leakage, I have no idea. Obviously HUGE leaks in the ducts would raise that number proportionately to the size of the leak.
    Not getting too caught up in the science is one of the things I like about Dr. Joe's lectures; while knowing all of the miniscule details of what is going on in buildings, he nevertheless has that attitude of keeping it basic & simple. It seems the more he knows about the science of building science, the more he demonstrates the philosophy of keeping it simple.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  5. #512
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,358
    Quote Originally Posted by tipsrfine View Post
    I havn't researched what you've asked (above in red). That 30% loss in efficiency is just a number I've seen accepted & used throughout the community of HVAC & Building Science. Whether or not it takes into account duct leakage, I have no idea. Obviously HUGE leaks in the ducts would raise that number proportionately to the size of the leak.
    Not getting too caught up in the science is one of the things I like about Dr. Joe's lectures; while knowing all of the miniscule details of what is going on in buildings, he nevertheless has that attitude of keeping it basic & simple. It seems the more he knows about the science of building science, the more he demonstrates the philosophy of keeping it simple.
    Hmm...are we looking at the Steve Jobs of building science? One who does not ignore nuance, but when presenting it to the public, keeps it simple, makes it appealing, makes it reliable.

    What you said above (what I placed in bold italics) is what I've always liked about Lstiburek. One can tell reading his works and listening to him speak that he knows his field very well. Unlike many engineers I've encountered, he's driven by not only keeping his findings relevant, but ensuring those he engages can understand where he's coming from. To me it's one thing to deeply grasp the nuances of one's particular discipline, be it HVAC or building science or aeronautics. It's another to effectively communicate to anyone interested in those topics in a way that does not alienate them or make them feel inferior for asking.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #513
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,969
    Quote Originally Posted by Brian GC View Post
    I was incorrectly under the impression this was a discussion board where people are allowed to express their experiences, views, and opinions on HVAC matters. There is also a clear delineation from administrators between giving advice, giving technical advice and giving advice that may be harmful. It is very confusing to the point that maybe I should keep the knowledge I’ve accrued from my twenty-five years of professionally working on building envelopes to myself.

    My apologies to anyone who was mislead by my post.
    It is very confusing to me also. Based on a post by Dad in http://hvac-talk.com/vbb/showthread....1#post11484662, it would seem my interpretation of the rules has been wrong.
    I believe you are entitled to post your past experiences, and to also disagree with what a pro with an * after their name says in regards to a topic. My apologies for reporting your earlier posting, which I had believed was in violation of the rules.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

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