Choosing double pane vs triple pane windows

Question by email:

Hi Mr. Green,

We want to get triple pane, as you advise. I am looking at Pella and Harvey to keep costs down). However, I am confused as to whether I should trade the R rating advantage and drop to a double pane to improve the SHGC and VT ratings for the Southern facing windows. (I live in Boston Metro-West). 

Right now, all the triple panes are coming in at:

 U of .21-.23, SHGC .23-.24, and VLT .40-.43 

 If I go double pane I get:

 U .28, SHGC .28 and VLT .53. 

So, is the loss in U a good trade off for an increase in solar passivity and light for South facing windows? Your book spoke to this, but I seem stuck with the numbers I am getting, which differ from your options.

Also, Harvey had a triple pane casement half price from Pella, but its VLT  of .33 instead of .43. —  Is .33 noticeably darker?

Thanks for the advise. 

My answer:

I think the difference in SHGC is modest and not a big deal in this decision. Installing an arbor with deciduous plants over it on the south side will provide much more difference than the SHGC difference. The arbor/plants will provide shade in summer and not shade in the winter so the winter sun warms your house. An overhand of 3’ at approx 9’ high does the same thing. Hence arbor plus triple glazed is a better combination. Alternatively a semi-transparent curtain will have a similar effect in rejecting some of the summer sun. This will be far more effective than the small difference in SHGC.

VLT is 0.33 is definitely noticeable from 0.43. See the pictures of windows with different VLTs in the book on page 116. The VT is written on the glass and varies from 0.57 (top left) to 0.53 bottom right. These are all considerably brighter than 0.43. I estimate that 0.43 will look quite dark and noticeably green and the 0.33 will look something like a car window. I suggest getting samples from the manufacturer before deciding.

I hope this helps!

David Green – the “Green Guru”

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