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  1. #1
    Join Date
    Feb 2005
    Posts
    39

    Question

    Hello,
    Can anyone tell me what the U value on a new construction, double pane, working vinal frame window would be?

    Will the value change depending on which direction the window faces?

    What else should be considered when figuring out the U value of window.

    Thanks

  2. #2
    Join Date
    Oct 2001
    Location
    Waterford Michigan
    Posts
    2,668
    It's in the book!

  3. #3
    Join Date
    Feb 2005
    Posts
    39
    I don't have a book, I'm just a new home owner who wants to find out why we are hot and cold. We got a Manual J and they have a u value of .55 for all windows. Most of them face south and west. Why is the heat gain from the north windows the same as the west windows?

  4. #4
    Join Date
    Dec 2003
    Posts
    184
    U value represents resistance to heat flow/transfer through a given material or composite. It is a sum of the insulating factors around a body. It does not look at the actual heat load placed on the material (ie how much sun shines through window or cold air blows up against it).

    It would be like comparing hp to speed. 350 hp in a dodge truck is differnt than 350 HP in the hyundai or element of your choice.

    U is part of a heat transfer equation.
    What is there to be unhappy about?

  5. #5
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by help3
    and they have a u value of .55 for all windows. Most of them face south and west. Why is the heat gain from the north windows the same as the west windows?
    They should be given a differnt values...how the are factored in,as part of the west wall or north wall,or indivdually?

    I would have to get up from my chair to find the book and look it up.

    Good question.

  6. #6
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by help3
    Hello,
    Can anyone tell me what the U value on a new construction, double pane, working vinal frame window would be?

    Will the value change depending on which direction the window faces?

    What else should be considered when figuring out the U value of window.

    Thanks
    U value is inverse of R value,right?

    Each rooms windows are given a value based on infiltration and heat loss.The u value will be the same for all the windows at a given constant temperture.The u value is important,but there is more to it...The work sheet will differentiate each window based on size and a multiplier...

    I always get a real bad headache when thinking about this.
    Trying to explain it is even worse.

    Sometimes there are compounding complexities of multiple variables that are not intuitively obvious

  7. #7
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,233

    U value

    U x TD = HTM .55 x 50 = 27.5 Where TD = temperature difference across material or indoor verses outdoor temp. HTM = Heat transfer multiplier in BTU per sqft. of material or unit. This is used for heat loss calc. in Manual J. The closest window I find in J is .551 double pane clear glass wood frame. A single pane & storm with clear glass wood frame is .475 U. For heat gain of double pane east or west facing window with a design TD of 20° with no external shade screen is 72 btu per sqft. North facing is 23. For all directions is 23. I'm stopping there but there is more to it.
    Be safe not fast. body parts don't grow back

  8. #8
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    a north facing window has little heat gain, at least not from sunlight. --

    doesn't ManJ list vinyl & alum frame as the same? seems like the older edition did -- [my copy downstairs]

  9. #9
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,233

    Manual J

    Yes mine is 1986.
    Be safe not fast. body parts don't grow back

  10. #10
    Join Date
    Nov 2004
    Posts
    2,927
    Originally posted by cem-bsee
    a north facing window has little heat gain, at least not from sunlight. --

    [my copy downstairs]
    I am going to continue to blab on without looking it up.
    I think the poster was asking about heat loss.Dont know how I got that idea...maybe cause it is frezzen cold out.. Anyway,heat loss calcs do not consider direction windows facing the same as heat gain calc do.Heat loss calculates temperture difference(cant remember the actual terminology)
    and infiltration only.

    I could be wrong...

    I also use an old copy.Older than 86.

  11. #11
    Join Date
    Feb 2002
    Posts
    516
    To know the specific u value of your vinyl windows, you would have to contact the manufacturer or their sales rep (supply house). Vinyl window u values range from .31 to .40, depending on the manufacturer, the type of glazing (low E or clear glass) and the type of spacer between the glass. The u value doesn't change for the different orientations (north, south, etc). The thermal heat gains of any window is affected by the direction it faces and the amount of shading (outside and inside) that the window has. Even the new Manual J-8 doesn't list these lower u values.

  12. #12
    Join Date
    Feb 2005
    Posts
    58

    If nothing else, I have learned to ask questions

    We are in the process of building a new house, I have asked alot of questions and one of them is on windows.., so, here is my Pella Wood/Clad info..
    .34 U Vllue
    .33 solar heat gain?
    55 % visual light trnsfer
    Air Infiltration = .3 CFm.

    Not a bad set of specs..

    Most of the Aluminum insulated windows used on the spec homes around this area average .55/57 on UV and Solar heat gain... Quite a difference..

  13. #13
    Join Date
    Jun 2000
    Location
    WYO
    Posts
    2,019
    U and R values are reciprocal. There is also an outside air film factor involved regardless of direction. This is an R value,
    never say never

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