high shgc for north-facing window in mn?

konoctiApril 8, 2013

I'd like to know if anyone has an opinion about my choice for a north-facing bathroom window in Minnesota.

I have the option of installing Cardinal loE-270 or loE-180 glass in the window.

The two types have very similar insulating properties. The winter u-factor for the 270 glass is .25, and the 180 glass is .26. So, very close!

However, while the 270 has a solar heat gain coefficient (SHGC) of .37, the 180 glass is much higher at .69.

This results in the 180 glass having a light transmittal value that is considerably higher (nearly 10%).

Our issue is that this north-facing window gets no direct sunlight whatsoever, and in fact is partially blocked by the neighbor's house and even a large tree.

So, light is at a premium, and as we're also having patterned glass (privacy glass) installed owing to the proximity of the neighbor's windows, we really want to maximize the light.

Because the u-factor is nearly identical for the 270 and the 180, I'm not worried about losing more interior heat in the winter with the 180. Should I be?

But because the SHGC is so much higher, I'm hoping for more light to come into our already light-compromised space.

Am I making the right decision?


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Windows on Washington

I think you have noodled it out pretty good.

Given your location, would triple pane not be worthy of consideration?

    Bookmark   April 8, 2013 at 1:44AM
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Thanks for the input, it's appreciated.

A triple pane window would bump the light transmittal right back down to the level that we're trying to move away from by switching from the 270 to the 180. So, probably not ideal. And while the insulating factor is greater with the triple pane, this is a pair of inswing French windows, and the sashes would be a fair bit thicker with the triple pane installation, which seems not so desirable either.


    Bookmark   April 8, 2013 at 1:59AM
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Windows on Washington

You are correct. It does bump it down to a VT (on the glass) of 0.70 but you get an R-5+ glass package if you go with the 1"+ size glass packages.

Here is a link that might be useful: Cardinal Glass stats

    Bookmark   April 8, 2013 at 9:17AM
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A followup:

I called the manufacturer to ask about the 270 vs. 180 and they advised sticking with the 270 on a north-facing window. According to them there would be additional winter heat loss at night with the 180, even though (as I noted above) the u-factor is shown as being just slightly different, .25 vs. .26. I don't understand this much, but I respect their judgement.

I still feel the 10% gain in light transmittal during the day in the bathroom would be worth it, especially as there isn't a single modern insulated window anywhere else in our house anyway. They're all hundred-year-old double hungs with glass storm windows.


This post was edited by konocti on Mon, Apr 8, 13 at 12:19

    Bookmark   April 8, 2013 at 12:17PM
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+1. Sounds like you are on the right track... I would give some consideration to the triple pane however. If you don't get much sunlight there anyway, the added insulating value could outweigh the shgc difference.

    Bookmark   April 8, 2013 at 12:22PM
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Will do. Thanks a lot for the suggestions.


    Bookmark   April 8, 2013 at 5:33PM
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