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Low E is the key here. Unless you get low e glass, the dual panes have very little value in a cooling environment where heat gain is overwhelmingly radiant. Even then, it is not going to make much difference on the bill. And you can always use blinds or curtains. Anything that blocks radiant heat will help. The best energy upgrade you can do (or look for in a home) in Phoenix area is a high efficiency AC. The rest of it is just nibbling around the edges.
Windows make for the greatest energy loss in a home. A dual-pane window gets the low E and High U rating due to the air gap between the two window panes. A single pane window can never achieve the same energy efficiency because it lacks that thermal break/air space in the glass that a two pane window gives.
The best thing you can do for lower energy bills is get really good windows and insulate the roof area. As windows are the greatest energy loss in a home followed by the attic/roof.
Blinds or curtains are never as good as a quality window because the heat is already inside of the home by the time the curtain comes into play. Exterior shutters would work better because they stop the heat gain prior to entering the home.
Quote:
Originally Posted by Ponderosa
There is no window, single, dual or low-e that is as good energy-wise as no window at all. A modern home with huge windows, window walls, two or more 10 foot patio doors and so on loses and gains a lot of heat compared to an older one without those dramatic features. Another modern design feature that hurts energy efficiency here is the trend toward eliminating roof overhangs/eaves.
The problem is that windows are mandatory per safety codes and nobody wants to live in a dark windowless box. Single pane windows are garbage when it comes to energy value. Dual panes are the way to go. One can get an R-7 with quality dual pane windows. Also, window frames must have a thermal break. This is usually done with vinyl or fiberglass or even wood. Aluminum windows are one of the worst windows in Phoenix because they lack a proper thermal break and when the metal frame is blasted with 130F summer sun, the metal transmits that heat from the outside to the inside of the home.
The reason overhangs are not used a lot is that maintenance issues. Most overhangs are framed out in wood and wood in the AZ sun gets destroyed very quickly out here. They require painting and moisture control every few years which most homeowners don't do. That is why the fascia trim boards need to be replaced quite often in homes. Also roof overhangs greater than 12" create tremendous wind uplift loads on a roof. In high winds a 12"+ overhang acts like a sail in the wind and tries to rip and pull your roof off of the walls of the home. This requires better Simpson tie downs and nailing, which builders don't want to do because it costs $$$.
We installed shade screens(on the outside of the window) and they made a huge difference in terms of keeping the house cooler. Even on the east side of the house in the summer, it dropped that rooms temp 3-5 degrees. We went with the highest grade of shade screen we could find(up to 90% efficiency I believe is the rating) and I made the screens myself with metal frames and feel they look professional. I found they were easy/inexpensive to make. The other option is window film if one doesn't want to deal with shade screens/want a different look which according to APS is even more efficient:
I found this url to be very helpful. From my experience and what I think some others have been mentioning, I'd take several other steps before I ever thought about replacing windows in a home.
]Windows make for the greatest energy loss in a home. A dual-pane window gets the low E and High U rating due to the air gap between the two window panes.[/b] A single pane window can never achieve the same energy efficiency because it lacks that thermal break/air space in the glass that a two pane window gives.
The best thing you can do for lower energy bills is get really good windows and insulate the roof area. As windows are the greatest energy loss in a home followed by the attic/roof.
Blinds or curtains are never as good as a quality window because the heat is already inside of the home by the time the curtain comes into play. Exterior shutters would work better because they stop the heat gain prior to entering the home.
The problem is that windows are mandatory per safety codes and nobody wants to live in a dark windowless box. Single pane windows are garbage when it comes to energy value. Dual panes are the way to go. One can get an R-7 with quality dual pane windows. Also, window frames must have a thermal break. This is usually done with vinyl or fiberglass or even wood. Aluminum windows are one of the worst windows in Phoenix because they lack a proper thermal break and when the metal frame is blasted with 130F summer sun, the metal transmits that heat from the outside to the inside of the home.
The reason overhangs are not used a lot is that maintenance issues. Most overhangs are framed out in wood and wood in the AZ sun gets destroyed very quickly out here. They require painting and moisture control every few years which most homeowners don't do. That is why the fascia trim boards need to be replaced quite often in homes. Also roof overhangs greater than 12" create tremendous wind uplift loads on a roof. In high winds a 12"+ overhang acts like a sail in the wind and tries to rip and pull your roof off of the walls of the home. This requires better Simpson tie downs and nailing, which builders don't want to do because it costs $$$.
This is incorrect. The low-e is due to a coating on the glass or ingredients in the glass mix. It has nothing to do with the air gap. The air gap provides insulation against conductive heat loss. It has minimal effect on radiant energy flux. Single panes are used extensively in commercial/industrial construction where a high degree of radiant reflection is desired - the AZ deserts for example. Thicknesses of 1/2 or more are common. They work better but cost more than dual panes found in residential applications.e
As for your R7 windows, there is no such thing. R value is the inverse of U values that are used to rate the conductive transmissivity of window glass. An R value of 7 would correspond to a U value of .14. The very best triple pane, low-e windows you will find for homes with have a U value of about .25 or and R value of 4. Windows, any windows, are lousy insulators.
Typically a single pane, plain glass window might have U factor of 1. A dual pane might be as good as .5 - which would be R2. A low-e could drop that to .35 (low-e is about radiant flux not conduction like R values describe). .35 is roughly R3. Again, windows are not the place to be putting money if you want to reduce energy costs. The benefit cost ratio is much smaller than most other improvements.
Last edited by Ponderosa; 02-05-2012 at 01:05 PM..
This is incorrect. The low-e is due to a coating on the glass or ingredients in the glass mix. It has nothing to do with the air gap. The air gap provides insulation against conductive heat loss. It has minimal effect on radiant energy flux. Single panes are used extensively in commercial/industrial construction where a high degree of radiant reflection is desired - the AZ deserts for example. Thicknesses of 1/2 or more are common. They work better but cost more than dual panes found in residential applications.
This is a quote from the energy council:
"Due to rising energy costs, single-pane windows are just about obsolete these days." Single pane windows are noted as being the worst windows as far as energy loss goes. Yet you recommend single pane windows for residential use?
Quote:
Originally Posted by Ponderosa
As for your R7 windows, there is no such thing. R value is the inverse of U values that are used to rate the conductive transmissivity of window glass. An R value of 7 would correspond to a U value of .14. The very best triple pane, low-e windows you will find for homes with have a U value of about .25 or and R value of 4. Windows, any windows, are lousy insulators.
This is incorrect. A lot of things have changed in the past few years. You are out of the loop but while I am not going to do your homework for you, here is a window that has a verified R-Value of R-10 http://www.seriouswindows.com/reside...ance-data.html
Remember, Lowes and Home Depot are not the source of up to date window technology.
Quote:
Originally Posted by Ponderosa
Again, windows are not the place to be putting money if you want to reduce energy costs. The benefit cost ratio is much smaller than most other improvements.
That above quote would get you laughed out of an green home energy discussion. 30% of a homes energy in lost through the windows. Especially in Phoenix where the solar heat gain is so tremendous and gets amplified with junk windows.
"Due to rising energy costs, single-pane windows are just about obsolete these days." Single pane windows are noted as being the worst windows as far as energy loss goes. Yet you recommend single pane windows for residential use?
Remember, Lowes and Home Depot are not the source of up to date window technology.
That above quote would get you laughed out of an green home energy discussion. 30% of a homes energy in lost through the windows. Especially in Phoenix where the solar heat gain is so tremendous and gets amplified with junk windows.
Well you managed to dig up a manufacturer who makes the claim, so I will give it to you. However, your source will also show that an "energy star window" has, as I said, u values of around .33 or R3. You are not going to find a home, new or used, with better windows here. And that is not very good. And you continue to fail to appreciate the difference of conductive and radiant heat gain, why it matters in our climate and why the number of panes is of less significance than the radiant reflectivity of the glass used. Keep doing your research.
Windows/type of glass/window treatments/energy savings etc, is a really whole separate interesting topic. If you want to discuss that rather than the OP, it's a fine topic for a separate thread.
Windows/type of glass/window treatments/energy savings etc, is a really whole separate interesting topic. If you want to discuss that rather than the OP, it's a fine topic for a separate thread.
A couple of posters have contended that newer is better because a newer home might have dual pane windows where the older would not. My point is that it makes little difference in the annual energy consumption in the Phoenix area. Dual panes typically found in newer construction are only marginally better in cooling climates than single panes. As I said, if one is concerned about energy costs the most important thing, by far and far and far and beyond is the energy efficiency of the AC unit. Newer homes typically have higher efficiency units, but an older home might have an upgraded unit that is better than the typical new home builder puts on as well. If house A has dual panes and a SEER 12 unit and house B has single panes and a SEER 16, house B will cost less to cool than house A.
That above quote would get you laughed out of an green home energy discussion. 30% of a homes energy in lost through the windows. Especially in Phoenix where the solar heat gain is so tremendous and gets amplified with junk windows.
I was being conservative, that is why I stated 30%.
Back to the OP. Older homes lose 50% of their energy thought their windows, the remaining will be the attic and walls. Getting better windows can reduce your energy costs by 30%-50%, then address the attic insulation, then finally getting better A/C units. This can get costly but that is what you get with an older home.
I am not a fan of older wood framed homes in Phoenix or anywhere in Arizona. Termites are a big problem in Phoenix and a lot of people don't do annual termite inspections on their homes. Wood rots, splits, twists, warps, racks etc., while nails pop and pull out. 20+ years of this is not good. Plumbing gets clogged and older homes even had cast iron piping which is a nightmare. Electrical goes bad and wiring problems arise.
The homes in Phoenix are not sealed properly when it comes to water intrusion and mold is prevalent in a lot of the older homes. Builders were lack in their water sealing techniques because, "it never rains in AZ." Also the tar paper is not lifetime and will eventually weather and fail, especially due to the extreme heat, therefore allowing water to coming into the wood framing.
I was being conservative, that is why I stated 30%.
Back to the OP. Older homes lose 50% of their energy thought their windows, the remaining will be the attic and walls. Getting better windows can reduce your energy costs by 30%-50%, then address the attic insulation, then finally getting better A/C units. This can get costly but that is what you get with an older home.
I am not a fan of older wood framed homes in Phoenix or anywhere in Arizona. Termites are a big problem in Phoenix and a lot of people don't do annual termite inspections on their homes. Wood rots, splits, twists, warps, racks etc., while nails pop and pull out. 20+ years of this is not good. Plumbing gets clogged and older homes even had cast iron piping which is a nightmare. Electrical goes bad and wiring problems arise.
The homes in Phoenix are not sealed properly when it comes to water intrusion and mold is prevalent in a lot of the older homes. Builders were lack in their water sealing techniques because, "it never rains in AZ." Also the tar paper is not lifetime and will eventually weather and fail, especially due to the extreme heat, therefore allowing water to coming into the wood framing.
"Getting better windows can reduce your energy costs by 30%-50%"
I think the entire point of the APS publication is the cost savings you are mentioning can be attained much more cost effectively via external window shade screens or reflective coatings, not replacing windows. I think there's a great deal of useful info on those 4 short pages that may give one pause to replace all the windows in a house if the goal is to save $ on cooling costs.
I think the chart on cooling costs savings with just using various types of window treatments on page 4 is very telling.
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