Ventiliation

• Glass block windows cannot be opened. Replacing your basement windows with glass blocks is the ventilation equivalent of bricking them over with cinder blocks. If your basement is unfinished, and subject to flooding or general dampness, glass blocks may not be the right choice for you. If you are attached to the aesthetics of the block window, consider replacing only the windows in the front of your house with glass blocks, and keeping the existing basement windows on the sides and back.
  
  

Emergency Exit

• You may not consider your basement windows to be viable emergency exits, but your local fire code may say otherwise. Consult your local ordinances to see if replacing your basement windows with glass blocks will infringe upon any fire or safety codes. If you live in an area subject to extreme weather, realize that blocking over your basement windows will render your basement a poor storm shelter. This is doubly true for basements that do not contain doors to the outside.
  
  

Security

• Glass blocks are, when installed properly, as secure or more secure than traditional basement windows. Blocks must be broken apart once they're cemented in place, and they cannot be pried open or pushed in with the same ease as a traditional swinging basement window. If security is an issue, a glass block basement window installation, along with window grates, is your best bet to keep out intruders.
  
  

Natural Light

• Glass blocks transfer significantly less natural light when compared to traditional basement windows. This is simply due to the relative thickness of the blocks. Keep this in mind when deciding upon traditional basement windows or glass blocks, as you will be cutting down on the amount of light in your basement.
  
  

Waterproof

• If you have window wells in your basement windows that are subject to flooding, glass blocks can more easily be sealed to be watertight. Glass blocks themselves are waterproof, as is the cement or mortar that holds them together. This leaves only the area surrounding the glass blocks to be caulked and sealed. This is a substantially easier process than waterproofing traditional basement windows.