Question

Would a dual pane insulated glass door help against cold air?

Asked by Jerry H.
Hopkins, SC

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?

Answer

Dear Jerry,

This is an interesting question that you raise because few people consider all their glazing options before purchasing doors or windows.

Yes, generally speaking, argon gas and low-e will help provide more protection from the cold as long as:

  1. it does not cost too much more,
  2. it is located on the north or east, or
  3. you have radiant heat source inside your home.

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 in the winter. 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.

Dual pane is better

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 between the panes,
  • 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 coming into the home as well as reduce heat leaving the house. Let’s look at each of these elements more closely. (This can get a bit technical, so please bear with me).

The glass

There are numerous types of glass used for residential and commercial applications that come 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 (www.seriousenergy.com) 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.

Low-e coatings

While low-e coatings cost more, they’re most effective at reducing solar heat gain which keeps 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’s not radiant heat.

Clearly there’s 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.

Filling the space

Second is the space between the sheets of glass.

Most double pane windows have air in between them, but newer windows employ gasses such as argon, krypton and xenon.

  • These 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’s a science behind the exact width of the air space, as some manufacturers claim much better performance with wider or narrower dimensions.

Sealing the edges

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.
  • Our 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.

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.

Good luck.

Joel Hirshberg
[email protected]
641-209-9979

 

For more information:

Read "Where can I find eco-friendly exterior doors that will match my historic home and keep the cold out?" a Q&A answered by Christine and Robert Boles.

Tagged In: replacement door

Do you have a question about greening your home? GreenHomeGuide invites you to Ask A Pro. Let our network of experienced green building professionals – architects, designers, contractors, electricians, energy experts, landscapers, tile & stone specialists, and more – help you find the right solution.