Whether or not you need permission to change the windows in your house depends on your local laws. If you’re enlarging an opening or installing a new window and need to make structural changes to the house, you’ll need a permit. In most locations, you won’t need permission if you’re just replacing a window with another window of the same size. The window installation company you work with will be able to help you navigate any planning permits you may need.

Energy-efficient windows are judged on their performance according to a few different factors, including heat transfer, air leakage, light transference, and condensation resistance. If you're unsure whether a window is energy-efficient, look for an ENERGY STAR® label from the U.S. Environmental Protection Agency (EPA). If various products, including windows, meet energy-efficiency standards set by the federal government, the EPA issues this label to help consumers identify energy-efficient products.

The gas between window panes, typically argon or krypton, acts as a barrier to heat transfer. This means it helps keep your home warmer in the winter and cooler in the summer by preventing heat from passing through the glass. This gas layer also reduces condensation by stabilizing the temperature across the window and helps minimize noise. Essentially, it’s all about improving energy efficiency and comfort while lowering those utility bills.

Mulled windows are often more expensive than standalone windows. However, this is mostly because mulled windows tend to be larger than standard windows, meaning they require more materials and labor to manufacture and install, both of which drive up the cost. If mulled windows take up the same square footage as separate windows, they should cost about the same unless you need to reinforce your exterior wall to support the larger wall opening.