Per the Pittsburg Corning website:

"The insulation or R value of the Premiere Series (3 7/8 inch thick) glass block when installed with mortar is 1.96. The reciprocal U value is .51. The R value of our Thinline series (3 1/8 inch thick) glass block when installed with mortar is 1.75, the reciprocal U value is .57"

Not a heck of a lot! Two layers of block, with an airspace, would still be less than R-5.

A little better than the average widow though.

Posted By ICFconstruction on 16 Dec 2010 07:25 PM
A little better than the average widow though.

I'd say a glass block wall made with a single layer of blocks would be on par with the typical CHEAP window, not "a little better than average window."    An R-2 window is considered pretty shoddy these days.   

Mine are about an R-7.2 and I've seen them all the way up to R-9.6.

Alternatively, the glass block wall with 2-layers of block and a space between would be quite an expense for the fixed thermal sinkhole you'd be creating.

The double glass block window won't be too expensive. But what do you mean "fixed thermal sinkhole"?

Posted By ICFconstruction on 16 Dec 2010 08:43 PM
The double glass block window won't be too expensive. But what do you mean "fixed thermal sinkhole"?

A glass block wall will likely be the point of greatest thermal loss in the whole structure.

That may be, but at 48" by 32" windows, I can live with it.

Posted By ICFconstruction on 16 Dec 2010 09:32 PM
That may be, but at 48" by 32" windows, I can live with it.

Just as long as you don't need to look out. . .

No I have other widows and doors for that, these will be high on 12' and 17' walls, for light.

have you looked at acrylic block windows? they have fixed, casement, and awning if you need some ventilation. Hylite is the brand I am aware of. they claim a better performance than glass block, and equivalent to a double pane window.

http://www.hy-lite.com/homeowners/products/block_windows/

or double/triple wall polycarbonate panels

Assuming you really want the glass block for the outside portion of the window, I would place a fixed energy-efficient window on the inside. Something that can be removed if you need to get into the space between windows. I have yet to see glass block windows that would not let some moisture diffuse through the many mortar joints. In fact, if you look at the Pittsburg Corning website for installation betails they use "expansion strips" on some of the sides to allow expansion/contraction of the glass block. This will certainly prevent any assembly from being vaportight.

I am concerned about moisture in-between the the two glass block windows. I was thinking when it is warmer in-between, the moisture would be forced out. Not sure how concrete would affect that.

I had thought about one window being a fixed pane, but I would not want a functional pane, and could not take out a nailer flange type.

maybe throw a dehumidifier 'packet' in there before you close it? The kind they include in electronics shipments.

I've been thinking about doing something similar (dual rows of block-windows) but the still-relatively-poor insulation plus the condensation issue has dissuaded me. The newest acrylic or glass block-windows can be purchased with low-e coating and rate an R-3. I think the small 'window' size (hence, lots more framing material) hurts the performance of block-windows, in general.

P.S. Off-topic, but now I'm thinking about making a mini 'sunroom' instead, with regular high-performance windows separated by a foot or two. If I build it on the south-facing wall and include operable openings at top and bottom, then I might be able to use it as a combination trombe wall (winter) and solar-chimney (summer).

Just use a fixed window and stick on vinyl film if you really don't like the transparency.

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?

It might be close except that many windows don't use air and that 5" causes more convection and performs worse than 3/4".

The frame is an even worse insulator than the glass/air/glass combo and a block-window has lots of framing.

should a put a weep rope to the outside? A small hole to the inside?

I like glass block primarily for aesthetic reasons. I have always wanted a curved semicircular glass block shower that extends beyond the envelope of the home for natural light to wake up in the morning! The more I look into it, the more impractical it seems. I know there are all kinds of ways to bring "natural" type light into an indoor space, but I was looking for something unique, not a shower with a bunch of lights or a shower with a bunch of windows.

While I was looking at the Vistabrik that lambabbey mentioned I found these — they are called Vistabrik solar wall tubes and Focus solar wall tubes. I'm new so the forum may not let me post the link, but FWIW here's the current link to that PDF — if the link goes dead it's under Resources/Product Literature.

Would definitely not work for a curved wall at all, and I suppose a stacked wall made of them would be prohibitively expensive, but would work pretty good for what you guys are talking about, except they are not thick enough for most ICF structures. They come in singles and doubles, solid and hollow. For some reason the double hollows have a much better R-value than the single hollows. Maybe I could build a curved solid concrete thermal mass wall and put enough of these in there to give me the effect I am seeking.

Dayum. I guess that idea is out. I just found the Price List — would cost a thousand for a couple of them. It says they are made to order and they do come in different depths up to 16".

Too bad. They look nice. On my budget I'll have to start collecting old pop bottles. If they can be found.