Q: Hola, Would a dual pane insulated glass but with no gas, provide any help against cold air? I am looking at an exterior door to our bedroom here in South Carolina. The door is full glass the manufacturer (echo) claims that their glazing, insulated glass will protect against cold air. Is this true without any gas between the panes. Secondly if the glass is glazed should I pay extra for Low e?
—Asked by Jerry H., Hopkins, SC
A: Yes, generally speaking, argon gas and low e will help provide more protection from the cold as long as: 1) it is located on the north or east or 2) you have radiant heat source inside your home.
This is an interesting question that you raise because few people consider all their glazing options before purchasing doors or windows. Dual pane insulated glass is certainly better than old fashioned single pane glass as the trapped air adds a barrier to heat and cold.
That barrier provides thermal as well as acoustical resistance. How much resistance depends upon: how wide the air space is, what’s in the air space, and how well the edges are protected from air infiltration. Also, a thin coating of low emissive (low-e) paint applied to the glass can reflect infra-red energy and thereby reduce heat gain coming into the home as well as reduce heat loss coming out of the house. Let’s look at each of these elements more closely. (This can get a bit technical, so please bear with me).
First, the glass. There are numerous types of glass used for residential and commercial applications that comes in various thicknesses and types of low-e coatings. Normal residential windows use double or triple pane glass that traps more air and provides additional coats of low e. Some high efficiency windows such as Serious Windows use one, two or three pieces of suspended film between the glass that are low e coated creating additional air chambers and even better reflective surfaces.
While low e coatings cost more, they are most effective at reducing solar heat gain and keeping your home cooler during the summer. During the winter, however, low e coating will keep the sun out as well which is not ideal. You may want more heat in the winter, not less. They will also reflect radiant heat generated by wood stoves or baseboard heat back into the home but not forced air because it is not radiant heat. Clearly there is a trade off with low e and there are additional issues affecting the overall efficiency such as size, location in the home, use of radiant heat inside, use of overhangs, trellises or trees, etc.
The space between the glass
Second is the space between the glass. Most double pane windows have air in between them, but newer windows employ gasses such as argon, krypton and xenon. These different gasses can be very expensive but they block the transfer of heat and cold and provide a much better sound barrier than just plain air. There is a science behind the exact width of the air space as some manufacturers claim much better performance with wider or narrower dimensions.
Spacers or gaskets
Third are the spacers or gaskets. Since air infiltration accounts for 30-35% of all heat loss, it’s imperative that the seals between the glass and around the edge of the glass last a long time. If they break down over time due to UV, rain, temperature or wind, air will leak out no matter what type of glass or space between them. Prior to 2000, many double pane low e windows filled with argon have all but escaped due to poor gaskets. My own home had failures in 90% of our windows due to this with no warranty from the manufacturer for this type of problem.
Nowadays, new warm edge spacers between the glass and better gaskets between the metal, fiberglass or wood tend to keep windows performing much longer.
Exceptions to the rule
If, however, the door is on the south or west, the addition of argon and low e will reflect (reduce) any heat you may want. If you don’t have a radiant heat source indoors, the low e and argon will not add much. With all glazing questions, you have to take into account both heat loss and heat gain from inside and outside the home during all seasons. That’s a tall order but one that can be educational and rewarding once you figure it all out.
There are energy calculators online to help you with these types of questions. Your local utility company probably offers free energy audits that can be helpful as well.