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Posts: 243
Reply with quote  #1 
I did some testing for transmittance on some common DIY solar collector glazing materials.

Also included some older Suntuf and twinwall polycarbonate to see how much loss in transmittance there is over time.



Scott Davis

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Reply with quote  #2 

Thanks for adding the link Gary.  Great stuff!

Take care, Scott MD
troy allen

Posts: 12
Reply with quote  #3 
Thxs G
This is aswesome. as i am just designing for my heat collector.

Posts: 135
Reply with quote  #4 
awesome info thanks for testing Gary!

Great news that the older materials seemed to maintain high transmission.

I wonder how the tempered glass used in commercial collectors, and PV panels, would compare to the glass you tested.  There's so much solar in use here in Colorado that it is quite easy and affordable to find used "solar" glass for ~1$/sqf. or less. I snagged a 4x10 sheet for my in progress downspout collector.

If your doing further testing perhaps throw in a couple plastic sheets? I think routine contractor plastic at 6 mil is made from polyethylene. Usually only expect 3-5yrs life on a greenhouse, I don't think its UV protected.

Posts: 243
Reply with quote  #5 
Hi Joe,

I've heard the transmittance for the solar collector glass quoted as 90%.  This Pilkington glass appears to claim up to 92%.

Pretty close tot he single layer polycarbonate glazings.

It would be a good idea to get the transmittance of polyethylene film as its a nice low cost and quick option.


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Reply with quote  #6 
Had a foot of snow overnight. The sun is really melting it fast though. All snow should be gone after the weekend.
I've had my 6 mil "clear" film from the big box fall apart after one winter season. It was out of the sun too. The cold and wind were just too much and by spring it was a shredded mess. I did use it as a temp wall on the sides of my deck, but...
It may do better as a short-term glazing for a collector as the wind won't be whipping it around from all sides, but then the sun, UV and other elements will be playing havoc as well.

Greg in MN

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Posts: 1,845
Reply with quote  #7 
The 6 Mil I used held up very well though I did accidently poke a hole in it trying to get the overhead ice and snow down before it damaged something.  I would think it would easily be good for another year but I am going to rework these collectors.  We only got another 8 inches or so but still have 30 or so inches on the ground.  Warmer weather forecasted so hopefully it will put a good dent in it.


Posts: 303
Reply with quote  #8 
..."only got another 8 inches"...."30 or so on the ground"...

Thank You for reminding me WHY I moved down south 24 years ago...


Posts: 29
Reply with quote  #9 
I found low iron glass available at Menards, special order "interior storm windows" from Larson. I can be ordered in 1/8" increments including a narrow frame around the glass. The 1/8" low iron tempered glass transmission is 90% and the 3/16" low iron tempered glass transmission rate is 89%.  A 40" x 40" section will cost you about $75. I bought several of these for my house and the work very well. I am now considering using the 3/16" glass on my new thermal collector.

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Reply with quote  #10 
Hi pianoman!

Welcome aboard! While you could certainly use storm windows for glazing, the cost of separate window sections might be high compared to other options, depending on the size of a collector. You may want to check the price of glass directly from a glass shop. There won't be a frame around it, but if you can build a collector, you can surely build a frame for your glass. Also consider using twin-wall polycarbonate from Menards. It's often on sale, and a 4'x8' panel can be less than $40. It's also very lightweight and easy to work with. The twin-wall poly also adds a bit of R-value if you live in a cold, northern climate. 


Greg in MN[wave]

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