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  1. #196
    Join Date
    Jun 2009
    Location
    Texas
    Posts
    13
    Please see my post below. I do have another question though. What happens when you add insulation - say you put R49 on top of R19 - do you add the two together and now you have R68 ???

    Also, is blown insulation as effective as the rolled stuff??

  2. #197
    Join Date
    Apr 2002
    Posts
    11,808
    they add up as long as you do not squish it

    blown vs rolled is like asking what is heavier a ton of rocks or a ton of feathers

    you can roll out R40 or blow in R40
    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/

  3. #198
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Quote Originally Posted by Carnak View Post
    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
    Yep. After this realization sunk in I found no reason to move my attic temperature sensor from on top of the insulation to higher in the attic. I also realize the temperature may not be the same throughout the entire attic. Nevertheless, it serves as a good illustration to crunch the numbers as above.

    I'm all for going with the approach that yields the best bang for the buck. We have our staunch defenders of powered attic ventilation here, but the more I study the matter, the more it makes sense, short of insulating the roof deck itself (which is not affordable to many), increasing the attic floor insulation thickness and creating an airtight seal between the attic floor and the ceiling below it seems more and more an effective approach, combined with improving the windows. The radiant barrier on the roof deck can't hurt anything, and with ducts in the attic it is a boon, along with reducing heating of the insulation surface.
    • 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. #199
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    So...If I take a temp reading in the hallway at 4ft level and it's 75*...then I take a temp at the ceiling(8ft) and it's 80*...is that the hot air rising or the heat transfer coming down from the attic?

  5. #200
    Join Date
    Apr 2002
    Posts
    11,808
    it's both

    The air stratifies a bit, measure the ceiling temp should be warmer than the 'high 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/

  6. #201
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by shophound View Post
    We have our staunch defenders of powered attic ventilation here
    There are still people in the world who think the earth is flat, and people who think the moon landings were faked...
    If more government is the answer, then it's a really stupid question.

  7. #202
    Join Date
    Jun 2009
    Location
    Texas
    Posts
    13
    Thanks for the comments gents. Shophound mentioned improving the windows. Does anyone know a cheap(est) way to reduce heat transfer through a window short of replacing w/ double or triple pane?? I have single pane..... I have seen exterior framed glass covers (kinda like a storm door) that simply fit over the existing window - curious if that would work...

    I am looking for economical installation to reduce the heat transfer.... Any ideas or suggestions???

  8. #203
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Quote Originally Posted by AlphaRat View Post
    Thanks for the comments gents. Shophound mentioned improving the windows. Does anyone know a cheap(est) way to reduce heat transfer through a window short of replacing w/ double or triple pane?? I have single pane..... I have seen exterior framed glass covers (kinda like a storm door) that simply fit over the existing window - curious if that would work...

    I am looking for economical installation to reduce the heat transfer.... Any ideas or suggestions???
    Storm windows help. I've measured their performance with an IR thermometer. Not nearly as impressive as double pane low e argon filled vinyl frame windows, but better than single pane aluminum frame windows. For existing windows on tight budgets short of storm window additions...shade the windows! Use insulating window covers on the interior and leave them shut during the heat of the day.

    Your R19 attic insulation is doing you no favor.
    • 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.

  9. #204
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Good shading can help a lot with single pane windows. Think about window film, or awnings, or a trellis with grapevine or some other kind of foliage. That last idea has the advantage of being summer-only shading. I have seen some advertising for a 2nd pane installed on the indoors side, it looked very nice but the price quote knocked my socks off.

    My house is built with lots of glass area and it's all single pane. Great daylighting, but those windows count for almost half the summer AC load on a Manual J. However it's mitigated by lots of tree shading and plantation shutters inside.

    Hope this helps -- Pstu

  10. #205
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,296
    Quote Originally Posted by pstu View Post

    My house is built with lots of glass area and it's all single pane. Great daylighting, but those windows count for almost half the summer AC load on a Manual J.
    Many times, when studying heat gain for a particular room, the heat gain from a single pane window can be more than the ceiling and exterior walls combined. And this is with a window that is shaded all day.
    • 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.

  11. #206
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by pstu View Post
    Good shading can help a lot with single pane windows. Think about window film, or awnings, or a trellis with grapevine or some other kind of foliage. That last idea has the advantage of being summer-only shading. I have seen some advertising for a 2nd pane installed on the indoors side, it looked very nice but the price quote knocked my socks off.

    My house is built with lots of glass area and it's all single pane. Great daylighting, but those windows count for almost half the summer AC load on a Manual J. However it's mitigated by lots of tree shading and plantation shutters inside.

    Hope this helps -- Pstu
    Besides when it is an absolute pyschrometric requirement, the only other time I would design with a dehumidifer is for projects with excessive glass (usually west to face the caribbean)

    The glass elevates the cooling load aritificially high per square foot.

    When we get out worst case scenario here which is almost 80F fog, suddenly there is no more solar heat coming through all the glass. If there is a lot of infiltration, the system sized for a lot of glass will not run that long during overcast periods. So buildings with infiltration problems can get overwhelmed under these situations so I go with a dehumidifer.

    These places also utilize a lot of sliding glass doors which are notrious for air sealing, these doors allow the multi-million dollar down wind view of the sea.

    Up wind on the east is typically all the exhaust vents. And all the back draft dampers on bathroom fans , combination microwave range hoods still let in air.

    When you have multi-story condos, the wind is stronger the higher you go and the east west alignment of vents and leaky doors creates a wind tunnel effect as the prevailing winds here are from east to west, with a northern component. Same winds that pushed Columbus to the new world. Then they zig-zagged north to catch the west to east winds to get back to Europe



    No humidity problems when it is sunny, but overcast and high dewpoints and they are doomed.
    Last edited by Carnak; 06-23-2009 at 11:39 AM.
    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/

  12. #207
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,139
    alpha rat,
    there is a good, better and best way to reduce heat gain in windows.

    best is to stop the heat before it gains entry to the conditioned space. plantings, (I have banana trees that block my west windows by mid May, die back in winter, have view back) awnings, shutters
    like bahama or plantation shutters work well.

    better is solar screens. not stopping the heat before it gets in as it will conduct thru metal frames, but you are addressing heat gain through the glass.

    good is window tint. while it won't achieve low e numbers of IGY"S but it will help. a long time ago these tints were dark and reduced what you could see thru the window. now we have improved products.

    double windows are called insulated glass units. low e is effective in igu's because the location of the low e can be applied to specific glass surfaces. for hot climates it is the inside of the outer pane, for cold climates inside of inner pane.
    the other thing is the distance from the inner glass from the outer glass. I'm not sure of the exact number 5/8" is close, so we'll go with that for now. if the glass is further than this specified distance
    convective currents set up between panes of glass. argon gasses are added to IGU's to mininize this air movement. also there is a seal and window frames are thermally broken to not conduct heat in IGU's.
    you just can't achieve this by adding storm windows. instead you end up trapping hot air between the glass surfaces and increasing the heat gain.
    if you have a storm door that is kept closed and the interior door is also closed, you can feel the heat build up between these two doors when you open one. this is what is going on between existing window and storm window. and it's not good.

    there are companies that custom make solar screens, as there are several mfgs of window films, I'm sure there are even companies that install tints on windows.

    and as always there are choices about most everything.

    btw, if you add insulation to existing insulation, use unfaced batts
    so as not to have two vapor barriers in the insulation. if you blow insulation this doesn't apply.

    good discussion folks!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  13. #208
    Join Date
    May 2005
    Location
    Pomona, California
    Posts
    3
    Howdy!
    I have been reading alot about the radiant barrier on the roof. Wanted to know what a radiant barrier is like. Is it a cloth with foil on one side? and does the foil face the roof surface inbetween the rafters?
    Hope this question doesnt' disqualify me as a DIY question, just wanted clarification on roof barrier... if it does, I am sorry.

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