Efficiency of glass blocks
Last Post 05 Dec 2012 04:33 PM by Dana1. 55 Replies.
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ICFconstructionUser is Offline
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16 Jan 2012 08:14 PM
Can those prices be right?

For my windows, does anyone have an opinion, should a put a weep rope to the outside? A small hole to the inside?
Brad Kvanbek - ICFconstruction.net
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11 Feb 2012 01:14 PM
Brad, good question. I don't know, but would depend on humidity. I don't see how it would hurt though.

I'm also thinking of doing double glass blocks to let light into the top gable ends of my great room/kitchen. Trapizoid & triangle windows are $$$

Rear of house will be built into a hill & I don't want anyone walking on the hill behind my property looking down into my house. I also want the greater strength there, blowing gravel etc.

Front of great room will allign with a drive through roofed portico, so the upper view would only be of the ceiling of portico with regular windows.

R value could be increased by installing 2 layers of clear UV resistant corrigated plastic in the air space between glass blocks, from big box store
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07 Sep 2012 09:10 AM
Maybe you could use a thermopane, storm window for the outside 'layer' instead of another glass block window. Many window companies make what they call a 'dead lite' window for store fronts etc. The ones I had were from Wellington and were like vinyl storm windows but had thermopanes for the glazing. I used them as storm windows over leaded glass windows in a 100 yr old house. Worked great to keep the weather and sound at bay. They could be removed to clean inside the 'well'area. I sealed them in with caulk and needed to clean the insides only every 4-5 years. Worked great.
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07 Sep 2012 03:34 PM
I have put in three double glass block windows so far 32"x48". Hardly cheaper than regular windows, but they look good and since both sides are flush there is no where for dust, water or snow to sit.
Brad Kvanbek - ICFconstruction.net
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13 Sep 2012 01:51 AM
Posted By EnergyWiseBuilding on 29 Apr 2011 07:09 AM
I guess I don't understand why a double glass block window wouldn't be more energy efficient than a insulated plate glass window. Since glass is not a good insulator it must be up to the air in-between. A one inch insulated plate glass window would have 3/4" of air? And the double glass block window 5".

What am I missing?


There are several problems with blocks. The internal gap of the block is too large and promotes convection loops. There is also a lot of surface area (top, bottom, sides) that would contribute to thermal bridging. There will also convection loops in the air space between the 2 sets of blocks. We know cold pulls in moisture so you would be pulling moisture in between the windows which would cause rot and or mold. The mortar will be stressed by expansion/contraction, I wouldnt count on it staying air tight. Plus it would be vapor permeable with moisture being pulled to the cold center cavity. All of the thermal bridging makes this window much different than a dual/triple pane window having the same area. I also think there would be large reduction in light transmission with the second layer of blocks. If you are looking for natural light a smaller standard window would let in a similar amount of light.
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13 Sep 2012 11:18 AM
I will see this winter the temperature of the inside. The light appears the same. No condensation yet.
Brad Kvanbek - ICFconstruction.net
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03 Dec 2012 08:32 PM
When My ICF walls were 70 degrees the double glass blocks were 68 and the Marvin Integrity window glass was 65. It would appear that in reality the double glass block windows are more efficient than thought.
Brad Kvanbek - ICFconstruction.net
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04 Dec 2012 09:43 AM
These look better than typical but a single block still can't outperform a good window. Surface temp is not an accurate test, I'd stick with manufacturer U values.

http://pittsburghcorning.com/products/energy-efficient-panels.aspx
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04 Dec 2012 07:13 PM
Why wouldn't surface temp not be an accurate gauge of energy efficiency?
Brad Kvanbek - ICFconstruction.net
jonrUser is Offline
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04 Dec 2012 07:57 PM
You would have to at least make sure that 1) air flow via convection is exactly the same for all samples (both sides), 2) you don't disturb what you are testing - if you use an IR thermometer, the surfaces have to have the same emissivity and 3) how do you account for the frame? (ie, you have some averaging to do)

This is why ASTM has standards for testing.

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04 Dec 2012 09:00 PM
I don't believe ASTM testing is all that. The two windows are at the same height near each other on adjoining walls, at night, so no sun. I have done it a few times and shoot the glass. It is apparent that the double glass block is more energy efficient than the Marvin Integrity low-e and all.
Brad Kvanbek - ICFconstruction.net
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04 Dec 2012 10:11 PM
Do you mean two layers of glass blocks or a single block (that has two glass layers)? The former (with 4 layers of glass) would do OK (according to ASTM measured U values).

When using an IR thermometer, stick some masking tape on the surfaces to get accurate comparisons.
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05 Dec 2012 07:44 AM
I put in two glass block windows in each opening of the ICF wall. One flush on the inside and one flush on the outside. We have done this before on commercial buildings and I wondered how energy efficient it was. The consensus here was that it was not as a decent low-e.

I am convinced it is better.
Brad Kvanbek - ICFconstruction.net
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05 Dec 2012 12:49 PM
The pittsburghcorning ones are around U .19 when doubled. That is better than most windows.

On the other hand, if you have test results that don't correspond with U values, I'm skeptical.
Lee DodgeUser is Offline
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05 Dec 2012 01:14 PM
Posted By ICFconstruction on 03 Dec 2012 08:32 PM
When My ICF walls were 70 degrees the double glass blocks were 68 and the Marvin Integrity window glass was 65. It would appear that in reality the double glass block windows are more efficient than thought.

How were these temperatures measured?
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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05 Dec 2012 04:33 PM
Posted By jonr on 05 Dec 2012 12:49 PM
The pittsburghcorning ones are around U .19 when doubled. That is better than most windows.

On the other hand, if you have test results that don't correspond with U values, I'm skeptical.

The difference is that manufactured window U-factor values are the average for the whole assembly, not just center glass.

To get to the true equivalency you'd have to factor in the losses through the mortar or silicone, etc. and the thermal bridging would still be pretty severe with some materials.

But I agree that doubled-up, with even a 1/2" of air gap between them would deliver very decent performance. (It passed muster for basement windows on a recent deep energy retrofit I was involved with, but I didn't see what they assigned for a U-factor.)
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