My mom has glass bocks in several basement windows. Her basement was always cold prior to the glass block windows, so I suspect the real problem is due to a lack of heat, rather than heat loss at the windows.

I'm no expert, but 3 heat registers doesn't sound like enough for 1200 sq ft. Also, the heat registers do better job close to the windows.
Do you have any returns? My mom's basement doesn't have any returns and probably could use a few more registers, especially over the windows.
Hope this helps.
-jasper

We also added glass block to a couple of basements, which improved their warmth. If you simply have cinderblock walls with no insulation, that's also going to be a source of heat loss. Cool air is going to settle, which makes your basement floor potentially the coolest spot in the house. If your basement registers were like mine, they were just cut into the main trunk that went down the center of the house. Since heat rises, and since the heat registers were at the ceiling, the warm air never reached the floor. I'm guessing the cheapest thing to do is circulate the air in the basement, which is probably best done by adding a return air duct.

I'm just guessing in all of this, though.

You have the best hope of achieving a warm basement by making changes on the furnace side. My guess you'll need to add some more registers and returns. You will still need to run the furnace with the fan on continously to keep the cold air from settling in the basement. I have a well insulated basement with returns and plenty of ducts. I use a programable thermostat that allows you to also program the fan setting. When we are home and active, I run the fan all the time and the entire house is the same temperature. The thermostat also has a nice feature, in addition to "fan" and "auto" fan settings it has a "circ" setting that runs the fan with heat demand and 33 % of the time randomly to circulate the air. The thermostat is well worth the money.

good luck

Some insulating window treatment ideas:
http://www.builditsolar.com/Projects/Conservation/conservation.htm#WindowTreatments

Here is a link that might be useful: builditsolar.com

I also am in Michigan and my experience is the same. The glass blocks are tranferring cold into the house for sure. I always thought that glass block was a good insulator but my experiece says no to that. My home was built in 1938, The block was installed later but they are pretty old. However for security reasons Blocks were added to the big kitchen window and several lower level windows on the farside of the home in the crack 90's I guess, and they all let in a lot of cold weather. Now I am not certain if it is the mortar surrounding or the window glass but these blocks are very energy inefficient. What I have done is added double pane windows infront of the blocks which has beautifed and insulated the kitchen window andthe windows in the living spaces. In the basement I also thought about putting plexiglass over them but now I am thinking about doing something creative with epoxy to make them look interesting but I don't know if it will seal them up .

While building our home I researched the insulating value of glass block windows and finally found that (around ~2007) they were being built with an R-value of 2. Nobody was advertising an inert gas filling, as stated above, which would only add about 1 to the R-value anyway. Modern low-E windows have an R-value of 3 and higher, depending on number of glass panes, multiple low-E coatings, etc. Being a 1950' era house, the glass block windows you speak of likely had a steel frame (?) which would only lower the composite R-value from the above value, not help it. Yes, a plexiglass covering could help as each additional trapped air layer adds a nominal R-value of 1 to the system. But, if the plexiglass is on the inside, warm moist air from the interior will eventually get behind and condense on the cold glass blocks, causing cosmetic, rust, and or mold issues. The plexiglass placed on the outside to add another air layer there, as storm windows do, would be a better (and safer from condensation, mold issues, etc.) stop-gap measure, IMO. Ideally, I'd suggest changing the glass block windows to a modern vinyl or fiberglass framed low-E glass window.

Oh, gees! Just saw that the original post was from 2007! Maybe the older posts should be color-coded for those of us denser individuals????