Hi. I am currently restoring windows and doors in an old country club that is being converted to a personal residence. I am working on a set of patio doors, 2 inches thick with 10 true divided lites in each. The glass panes are clear single strenth glass and my customer would like me to replace them with double-insulated panes.
If I do this, about a quarter inch of the silver edging of the new pane will be visible around the perimeter of each lite. I feel a better way to go would be to use a single, heavier-gauge pane of low-e glass.
Anyone out there with knowledge regarding the differences in the insulating value of the two types of glass mentioned? Or perhaps any other thoughts on it?
Thank you.
Replies
I have some knowledge of R-values both due to education (engineer) and limited experience in contracting.
There's little doubt that a correctly constructed double-pane window will have a far greater insulating value than almost any single-sheet glass, regardless of the thickness or e-value.
That said, thermal conductivity is rarely where most of the heat is lost (winter) or gained (summer) in a home. With older doors and windows, the largest heat loss is primarily through convection (i.e., drafts). It makes sense to install double or triple glazing with an argon flush between the panes in a modern, very well sealed door with interior conductivity barriers.
However, with an old door, the interior generally is solid wood (whether arranged as solid planks or plywood) without a thermal conductivity barrier. Typically, it's just not worth the expense of installing true divided light double panes in such a door. It makes far more sense to examine the jamb and the molding around the door and install a modern draft seal, replace the floor jamb if its heavily worn, and repair any holes in the caulking around the molding.
I would do those things before replacing the panes with double-glazed panels, because if ultimate insulation is the goal, the money would be far better spent on a new custom door with double or triple glazing, thermal conductivity barriers and draft seals.
Thank you for your thoughts. The doors have the interlocking metal weather stripping cut into them and the frame and sill. I guess the main answer I'm trying to find is this--"what is the difference in insulating value between the insulated glass and a single pane of low-e, say, 3/16 thick in terms the customer could (and myself) can understand. Because I am sure their going to ask. Sad thing that my local glass supply company couldn't give me that information, isn't it?
Thanks.
Well, if the doors are modern enough to have interlocking metal weather stripping, I'm a bit surprised your client wants to renovate them to the extent you're asking about. I wouldn't think the preservation of door made in the 60's or 70's would be a priority, and buying new doors would be a heck of a lot cheaper than ordering and installing separate double-panes for each divided light.
I did a quick internet search on this topic. At the bottom of this article, there's a table of R-values with uninsulated single pane glass as the baseline. If you read the article, though, you find that low-e glass is not typically done in a single-pane format. The table's comparitors are all low-e glass that is double pane:
http://www.askthebuilder.com/B97_Low_E_Glass_-_Types_and_Benefits.shtml
Here is another, more graphic discussion of the differences:
http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/constr/f/f.htm
One thing for your client to remember is that while a pane of glass with an R-value of 2 will have one half the heat loss/gain of a pane with an R-value of 1 (that's a rough approximation based on laboratory condtions, by the way - differences in radiation and convection conditions can alter the values significantly), that will not be the actual energy efficiency gain for the renovated door, primarily because a lot of heat loss goes through the wood and sills of the door. That's why I'd suggest replacing the whole door with a new one. It'll be considerably cheaper (and a lot more energy efficient) to get a door with a single light of double-pane, argon-filled, low-E glass with simulated muntins than to replace the glass in the existing door with divided-light double panes.
dkellernc,
Just to pick up on your thoughts a bit, back in the early 80's I replaced all the windows in a hospital building. My heat loss calculations said it made sense but my engineers were very skeptical. We did 450 Peachtree windows in mid-Michigan...the engineers tracked the heating bills....payback was about 1.5 years. ( we are talking engineers here so you know I did not convince all of them..:))
As much as windows are a big heatloss, the volume of the space is multiplied by the number of air turns per hour and added to the heat needs too. Therefore leaky windows can increase the burden significantly very fast.
I agree - I've never sat done and really done the calcualtions (I don't have a copy of ProE on my home computer - that's a bit expensive for "hobby" use!), but I've seen some example calcualtions that suggest that small air gaps (and the resultant leaks) roughly quadruple the heat lost from thermal conductivity and radiation from the surfaces.
Just a couple of points you might like to consider .The low e coating is designed to be on an interior surface and as such may (and I should talk to the manufacturer) not be durable.Secondly we are very careful to ensure that the silver spacer bar is shielded from sun when we make windows as it shortens the life of the seal and may well invalidate your warranty
According to this site: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html, acrylic has a lower thermal conductivity than glass. Maybe 1/2" panes of acrylic would suit your purpose. I've cut 1/4" on the table saw without problems, but not the thicker stuff. I don't know about the transmission values. OnlineMetals.com sells it.
calmnsense,
Back in the day when I did heat take-offs from blueprints we calculated the heat loss from windows at 80 btu's/sq. ft./ per hour. This was back in the late 60's with an R value of .75/sq.ft. Walls at that time were 9 btu's per hour with a R value of 15/sq. ft. I believe double windows with E stuff runs at an R value of 1.5 I'm not sure how that translates in btu loss per hour in the windows...but I don't think it all that much.
One other important point. The 80 btu/sq. ft./hour I reference was given to me and relates to southern Connecticut. That is, BTU's is determined by multiplying the mass by the coefficient by the change of temp. My point is it never gets that cold in So. CT...maybe 10 degrees faren. That number goes up if your in a colder climate. You could propably check with your local wholesale Heating and Plumbing supplier.
Edited 8/18/2008 7:52 pm ET by BG
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