There are two main approaches to preventing window condensation and frosty windows: reducing an indoor humidity and increasing insulating value of a window by insulating window frame, adding a layer of glass to a window, applying Low-E (Low-Emissivity) coating to all glasses and adding inert argon (krypton) gas, that can be used between panes of glass to offer additional layers of insulation and help reduce thermal transfer.

The National Fenestration Rating Council (NFRC) is a nonprofit organization created by those in the window, door, and skylight industry. This organization developed the energy rating system that building codes, tax credits, ENERGY STAR, and others base their energy efficiency criteria on. The NFRC label provides two key energy performance ratings for windows, the U-Factor and Solar Heat Gain Coefficient (SHGC). It also provides ratings for Visible Transmittance, Air Leakage, and Condensation resistance. U-Factor measures how well a product can keep heat from escaping from the inside of a room. The lower the number, the better a product is at keeping heat in.

Many window manufacturers recommend to maintain an indoor humidity at the very low points especially during the winter months, in order to avoid condensation. However, this should not be encouraged, as it increases the number of bacteria and other unhealthy particles in the air. It is important that comfortable relative humidity be maintained between 40% and 50%.

Therefore I decided to use a second approach - to install new windows with additional layer of glass and with insulated window frame. Triple glazing windows help to reduce cold drafts and therefore will reduce condensation, which will allow me to maintain a higher indoor relative humidity in cold weather. Window manufacturers claim that triple-pane windows can be up to 79 percent more energy efficient than a single pane window and up to 25 percent more efficient than a double pane.

When we were ordering our windows, we explained to Lowe's sales person that our goal was to get windows that are better insulated than our existing dual-pane windows, so we could minimize condensation on the inside window surfaces during cold weather.

Unfortunately, when our windows arrived I discovered some undisclosed issues:

1. The glazing thickness (insulated glass unit - IGU) of the delivered windows is just about 1", which is not the most effective triple glass configuration of 1” 1/4 or 1” 3/8. More importantly, glazing thickness of 1” 1/4 is actually available for Pella 350 series windows which was also not disclosed to us.

2. A very important for us performance characteristic - Condensation Resistance (CR) was also not disclosed and was not printed on the window stickers.

3. The main parts of the window frame, such as large vertical sides (and other hollow areas), are not insulated. Although we paid for the extra option of having the window frame insulated, the fact that the frame and sashes would only be partially insulated was not disclosed to us.

4. Argon issue: "For Pella products labeled as having Low-E insulating glass with argon, Pella injects argon at the time of manufacture. No warranty is made as to the amount or percentage of argon present in the insulating glass. It is known that argon within insulating glass dissipates over time. The manner of use and conditions of installation of the product will affect the rate of dissipation of argon out of the insulating glass. Pella makes no warranty regarding the rate of dissipation of argon or the amount of argon remaining in the window at any time after manufacture." In fact, we did not get warranty terms during our order and this document was not presented, when we picked up our windows from your store.

What Causes Condensation on Windows in your Home?

How to Deal with Window Condensation