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  1. #183
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,322
    Quote Originally Posted by Carnak View Post
    The big gains are the glass and the infiltration or ventilation. Elite spits out a pie chart, pretty good visual. Really emphaszied the window issue like you did on another thread.

    But still, when you look at numbers like that, the attic is a big part of the infiltration, and air from a vented attic will have a lot more enthalpy than 'your design dry bulb and wet bulb'
    I agree re: attics being a large factor for infiltration. I attribute the air sealing efforts made to the ceiling plane in my own home as one of several key factors why my a/c system performs well during extremes in outdoor climate. I am now vacationing in Northern California, where the weather is so wonderful we have all the windows in the house open. If I could import this weather to DFW for the summer I'd do it, but then how could I marvel at how well building thermal boundary improvements perform?
    • 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.

  2. #184
    Join Date
    Jun 2009
    Location
    Texas
    Posts
    13
    2:15PM here, it is 94 deg outside, my house is 77 degF and my attic is 135 deg. Am I like not getting any air movement or what??

  3. #185
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    6:30 here...outside 94.1* inside 77.3* Attic 103.6*

    Sunny

    Dewpoint 71*

    Heat Index 101*

    I am taking my attic measurement 8ft about the floor...

  4. #186
    Join Date
    Jan 2009
    Location
    Colorado
    Posts
    88
    Joe,
    In your description you said "I added an attic tent". What is meant by an "Attic tent"?
    Did you add a radiant barrier? Trying to understand the details.
    Thx,
    Gary

  5. #187
    Join Date
    Jun 2009
    Location
    Texas
    Posts
    13
    Gents, so basically if I seal my light fixtures and leaks into the house, that's as good as I'm gonna do????

    What should attic temperatures run to allow for efficient operation of that dang A/C unit ??

  6. #188
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    http://www.attictent.com


    no radient barrier...but I wish I could do it...

    7:52 sun is getting lower in the sky...outside temp 91.5* Attic 99.2*

  7. #189
    Join Date
    Jan 2009
    Location
    Colorado
    Posts
    88
    Thanks Joe. Now I am even more curious. How about some more questions?
    - What is the roofing material?
    - Is the house in the shade most of the afternoon? Surrounded by large trees?
    - Do you have continuous soffit venting, one vent per rafter bay, or occasional soffit vents?
    Thanks,
    Gary

  8. #190
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    Hey Gary...

    Roof is a 3 Tab Art shingle (elk is the manufacturer)(They are thick & heavy)

    No trees at all...full sun

    Occasional soffet venting

  9. #191
    Join Date
    Jan 2009
    Location
    Colorado
    Posts
    88
    Thanks Joe. Very impressive. You must have done a great job with the venting, sealing and insulating!
    Gary

  10. #192
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    Now, it's been a little breezy the past couple of days, I don't know if the readings will change when it's hot and dead calm outside...we shall see...

  11. #193
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,322
    Quote Originally Posted by AlphaRat View Post
    Gents, so basically if I seal my light fixtures and leaks into the house, that's as good as I'm gonna do????

    What should attic temperatures run to allow for efficient operation of that dang A/C unit ??
    Your attic insulation level is R19, correct? Let's do a little object lesson to see where you may wish to focus your attention - insulation vs. more ventilation.

    Assumptions:

    A 12 x 12 room with a flat ceiling that is airtight to the attic.
    R19 on the attic floor.
    135 degrees on the top surface of the attic insulation.
    75 degrees air temperature in the room.

    Formula for estimating rate of heat transfer from attic into room through ceiling surface:

    U factor (inverse of R value) x Area x Temperature Difference (Delta T)

    Scenario 1: .053 x 144 x 60 = 458 btus per hour transmitted into the room through the ceiling surface with R19 attic insulation.

    Scenario 2: .020 x 144 x 60 = 173 btus per hour transmitted into the room through the ceiling surface with R49 attic insulation.

    By increasing your attic insulation from R19 to R49 (increasing R value by 30), you decrease the heat transfer rate by 285 btus per hour. Try to achieve that same reduction with R19 attic insulation and a powered attic ventilator. You might reduce the air temperature in the attic but you won't reduce the amount of radiant heat beating on the attic insulation from the roof deck that is cooking in the sun. It will only allow the surface of the attic insulation exposed to the attic to have a better heat sink to transfer radiant heat to the attic air in motion.
    • 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.

  12. #194
    Join Date
    Apr 2002
    Posts
    11,808
    top of the insulation will be hotter than the 135 degree air when the sun is beating down on the roof

    heat radiates from the under side of the deck to the top of the insulation. Heat trickles down through the insulation and convects up to the air
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  13. #195
    Join Date
    Jun 2009
    Location
    Texas
    Posts
    13
    Quote Originally Posted by shophound View Post
    Your attic insulation level is R19, correct? Let's do a little object lesson to see where you may wish to focus your attention - insulation vs. more ventilation.

    Assumptions:

    A 12 x 12 room with a flat ceiling that is airtight to the attic.
    R19 on the attic floor.
    135 degrees on the top surface of the attic insulation.
    75 degrees air temperature in the room.

    Formula for estimating rate of heat transfer from attic into room through ceiling surface:

    U factor (inverse of R value) x Area x Temperature Difference (Delta T)

    Scenario 1: .053 x 144 x 60 = 458 btus per hour transmitted into the room through the ceiling surface with R19 attic insulation.

    Scenario 2: .020 x 144 x 60 = 173 btus per hour transmitted into the room through the ceiling surface with R49 attic insulation.

    By increasing your attic insulation from R19 to R49 (increasing R value by 30), you decrease the heat transfer rate by 285 btus per hour. Try to achieve that same reduction with R19 attic insulation and a powered attic ventilator. You might reduce the air temperature in the attic but you won't reduce the amount of radiant heat beating on the attic insulation from the roof deck that is cooking in the sun. It will only allow the surface of the attic insulation exposed to the attic to have a better heat sink to transfer radiant heat to the attic air in motion.
    ShopHound - now THAT's something I can use!!!! Thanks for providing this - I can do some sensitivities to see what makes sense. I am a complete novice at the A/C and insulation thing. So this really helps. Thanks - I'm sure I'll have some other questions.

    I do have another thread I would really like some feedback on... Here's the link - anything appreciated. http://hvac-talk.com/vbb/showthread.php?t=316462

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