# Thread: U value for window on Manual J

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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. It's in the book!

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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?

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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.

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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.

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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.

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## 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.

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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]

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## Manual J

Yes mine is 1986.

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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.

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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.

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## 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. U and R values are reciprocal. There is also an outside air film factor involved regardless of direction. This is an R value,

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