Rescheck question

[Squirl]

I am purchasing used double paned windows for my cabin.  Rescheck requires me to enter a U-value for windows.  Does anyone know how to find this out on used windows?

[MountainDon]

As far I know the only marking placed on a new window is a paper label that is on the glass surface. They state the U-factor and the solar gain factor. I could not see markings of any kind of code on the new ones we purchased.


If your building will be inspected you may have a problem with used windows. Here at least, the labels must be left on the windows until the inspector checks off on them.

[Don_P]

U is the inverse of R, so going from ancient memory and being brutal call each pane R1 for a total of R2. 1/R...U=.5. I doubt they could be worse than that but that number might blow you out of the water. Is there a manufacturer name anywhere on the units?

[MountainDon]

Depends. I've seen cheapy aluminum frame dual pane windows at the big box stores with u= 0.7   

[Squirl]

They are simple double paned vinyl double hung windows left over from jobs gone wrong or wrong sizes that I find on craigslist.  They look just like the ones at the big box stores with no names on them for 1/4 the cost.  My reasoning in this. Windows seem to be a lot cheaper because they have a limited market because of their size.  Most plans call for a specific size and people usually don't rip down whole walls to change the size of a window in an existing building.  If I buy the windows before I submit the plans, I can contour the plans to the windows that I have and save a bundle.  It seems silly to pay 4-5 times the price for an official U-value. 
I didn't know if there was a standard or average value table that anyone heard of using.

[MountainDon]

I am assuming that you do not have to provide proof of the windows u-factor or SHGC rating?

In that case whether or not it is silly to pay for new windows with a certain rating is a personal choice, between taking a chance on what comes up and wanting to know that the windows are of a certain quality. There are many variables in how the frames are constructed that affect the values as well as whether or not they have low-e glass, argon or other insert gas. An average u-factor might be 0.50.

[MountainDon]

If you want to check a window to see if it is low-e or not here's something I found. This will not give you any values but you can determine if there is low-e glass or not.


Hold a flame to the window in the dark and count the reflections.

The easiest is to hold a lighter (or other open flame) near the glass and look at the reflection in the window. If you have Low-E, and it is on the #2 or the inside of the exterior lite surface, then you will see three flames reflected in the glass. If you are doing this test from the inside of your house, the two "flames" farthest from you will be close together and then a space and then the third flame.
If you perform this test outside, there will be two flames close to you and then a gap and one flame further away.
The center flame will be a slightly different color than the two flames that bracket it.
It is best to do this test in the dark, although you can do it in daylight as well. It is simply easier to see it in the dark.
The middle flame will be the reflection from the Low-E coating.

[Squirl]

True, buying windows new does have good reasons.  I should have written it seems silly for me.

I don't know if the inspector is going to check the U-value of windows.  The building permit just needs a passing rescheck print out prior approval.  I figure I could make a valid argument of averages or lowest rated value if at the time of inspection it matches the figures I put in the rescheck printout.  Example, if the worst averaged double paned vinyl windows have a .7 u-value and my design passes rescheck with a .7, I could make a reasonable argument that I am compliant.