Glass and Windows

You know, sometimes you have a piece of fact stored in your head but it doesn’t mean anything until you ask why.  For example, I knew that “Glass used in windows contained lead a long time ago but today most don’t,” and “When you look at a sheet of glass from the side it looks green, like the ones at the display shelves at retail stores.”  Okay, this is basic stuff that you’ve known and observed, right?  Well, I got curious and started a dialog with my husband, Kurt, who is a photographer with physics background and has a gift of explaining science in a simple way.  Our conversation today as we were driving went something like this:

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Chie:  So, hon, I’m curious.

Kurt:  Curious about what?

Chie:  Well, I know that lead was taken out of glass because it’s dangerous.  But why did they have it in the first place?

Kurt:  You know that glass is made out of sand, right?

Chie:  Yeah…

Kurt:  Those tiny quartz pieces, sand, melt at a very high temperature.  By introducing another material into the sand it can melt at a lower temperature.

Chie:  Oh, OK, so they used lead in glass for manufacturing efficiency.

Kurt:  Right.  Not only was it efficient it was also good for light.  Lead will let light through the glass without affecting the colors.

Chie:  Really?

Kurt:  Yep.  When you look through the windows at our craftsman home you’ll see the colors in a natural state.  Lead is pretty nice for letting true colors through.

Chie:  Hmm… So what do they use in glass now?

Kurt:  Mostly soda-lime.  It has a greenish tint.  It looks green because the materials block red from coming through.  You may notice when you look through windows that people look a bit greener and less bright. 

Chie:  Why less bright?

Kurt:  Because with every air-to-glass contact you lose approximately 5% of light.

Chie:  Really?  How do you know that?

Kurt: I know it from studying physics.

Chie:  So, if there is 5% degradation of light for each air-to-glass contact then for double-paned windows it loses 20% of brightness and for triple-paned windows you lose 30%?

Kurt:  Yes, approximately.

Chie:  Wow.  You know what? I noticed those demo windows we saw had a gray tinge to them.  When we get triple paned windows with additional coating to them the house will look darker, right? I wonder if part of the “happy feeling” we get at the house is from the single pane, lead windows letting in bright natural light?

Kurt:  Hmmm…..

Later at home I brought up this topic of light going through glass losing brightness and Kurt decided to do a demonstration for me.  He grabbed one of his light meters he uses for photography and measured the brightness of the kitchen.  It was about 64.  Then he took 3 sheets of thin glass used for his 6×7 slide mounts and stacked them above the light meter so that there is air-glass-air-glass-air-glass-air, which would yield 30% less brightness.  The light reading dipped to about 45, which is 30% less than 64.  What a fun science experiment in the kitchen!

   

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Now, switching gears to windows.  As you know, we are striving for passive house certification and windows are very important for the energy performance of the house as well as the thermal comfort for occupants.  Have you noticed how much colder it feels to stand next to a window than standing next to an insulated wall on a cold night?  This is because there is a heat transfer from inside of the house where it’s warm to outside of the house where it’s cold.  With passive house windows there will be less heat escaping to the outside. The passive house windows specification is quite detailed and only few windows manufacturers in the US provide passive house windows.  In Europe today there are over 50 manufacturers, mostly small to medium businesses, offer passive house windows.  To find out more about passive house window requirements please visit the window requirement page on passipedia. 

From aesthetics standpoint we want our windows to have the arts and crafts look and feel with muntins.  So what are muntins?  They are those strips, wood or metal, that separates panes of glass.  With the windows requirements for passive house it’s not feasible to have windows with true muntins where there are separate glass pieces fitted into the muntins.  The look can be achieved by pasting a strip (wood or other material) on the outside pane and on the inside pane.  We wanted to see what that looks like so we visited the corporate office of Serious Materials in Sunnyvale to see a demo window unit with muntins.  When we looked closely it appears to have a “shadow” around the muntin but we were told that this is not noticeable if you simply look from a drive-by distance from the house.  (I forgot to take a picture of this today.) 

Here are some examples of craftsman style muntins –

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Since windows are the most expensive line item in the bill of material and have a significant impact on the performance of the house we are paying special attention.

Bottom line, we want windows that

Oh, we want to do all of the above without breaking the bank!