Does "Low-E" allow sun to enter house in winter?

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Jay
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Joined: Sun Oct 09, 2005 3:23 pm

Does "Low-E" allow sun to enter house in winter?

#1 Post by Jay » Sun Oct 09, 2005 3:38 pm

We live in Chicago and our house has many south facing windows. Currently, these windows are single pane and made of clear glass. They allow plenty of sunlight to come in during the winter and thus help the house heat up naturally. Similarly, because the Sun is higher in the sky during the summer, these windows do not get too much direct sunlight and thus do not heat up the house during summer. This seems to be an ideal condition. Of course, they are not good from conduction and air infiltration point of view, but that's a different story.

If we decide to replace the windows using "Low-E" windows, will we loose this natural heating effect during the winter? We are afraid that the windows will keep the Sun out during the summer as well as winter. Is that true? A window sales rep told me that Low-E works on the basis of angle of incident rays and does allow for winter sunrays to come in. Is that true? Can we go for only Argon filled windows without Low-E, provided such an option is available?

Thanx!
Jay

FenEx
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Joined: Fri Mar 11, 2005 3:43 pm
Location: Illinois

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#2 Post by FenEx » Mon Oct 10, 2005 1:11 pm

Jay

Yes it is true that the low-e coatings will allow more solar heat gain when the sunlight is more direct during the winter months as the sun is lower in the sky. I'm not sure if you really want to cut the low-e though. Keep in mind that the low-e coatings help reflect heat back towards it's sources night or day. This would include the heat radiating outwards during the heating seasons through windows on the other sides of your home as well as the Southern side for the other 12-16 hours per day/night.

Jay
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Joined: Sun Oct 09, 2005 3:23 pm

#3 Post by Jay » Mon Oct 10, 2005 9:41 pm

Hi FenEx:

Thanks for your thorough reply. Based upon low-E's clear-cut advantage during the night and also the fact that it will allow the Sun to come in during winter, we have decided to get windows with low-E.

I was reading some reports on the web and they seemed to indicate that windows with high SHGC (> 0.55) , high VT (> 60 %) and with UV protection of > 75 % are recommended for colder climates. High SHGC and VT will allow more heat and light, respectively, to come in and the low-E will then trap the heat inside. Would you comment on this? What are the windows that fall in this category?

Thanx,
Jaydeep

FenEx
Posts: 353
Joined: Fri Mar 11, 2005 3:43 pm
Location: Illinois

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#4 Post by FenEx » Tue Oct 11, 2005 4:39 pm

The VT or visible transmittance of light is a desired comfort preference, not an energy issue although it is tied to the SHGC. The SHGC of .55 you mentioned means that it will allow 55% of the solar radiation into the house and reject 45%. Keep in mind that that the high SHGC will not allow you to have a window with a low U-factor which insulates night an day. Selecting the right windows can be complicated. East, West and North facing windows should concentrate on a low SHGC and a low U-factor. South facing windows should only have a high SHGC if the house has a a proper overhang to protect against the summer gain or you will lose any savings with interest. Yes... once the radiant solar heat is absorbed by materials in your house, the low-e will help reflect and keep the infrared heat that is trying to escape in the home. The right choice for those windows would be dependant upon climate and conditions. Sorry, only so much I can do in text.

Fenex
Last edited by FenEx on Wed Oct 12, 2005 8:23 am, edited 1 time in total.

Jay
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Joined: Sun Oct 09, 2005 3:23 pm

#5 Post by Jay » Tue Oct 11, 2005 8:47 pm

Hi FenEx:

Thanks again. One clarification - did you mean "South facing windows should only have a [color=green]high[/color] SHGC if the house..."

One follow up question - Is it even possible to get windows of the same model (e.g. Alside Shefield) with different SHGCs or VTs? It appears that all the windows manufacturers have set SHGCs and VTs for their models. The end user cannot really custom make the windows to that detail. Is that true?

Thanx,
Jaydeep

FenEx
Posts: 353
Joined: Fri Mar 11, 2005 3:43 pm
Location: Illinois

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#6 Post by FenEx » Wed Oct 12, 2005 8:30 am

Yes I meant high... good catch.

Actually you can get different glass options for different windows from most window companies. To get your SHGC higher you will more than likely have to omit the Low-E on those units. The question is, will it help more than hurt. Read up on Passive Solar Energy a bit, it will help you understand the pros and cons. Good Luck.

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