In my researching I discovered that there seems to be a dispute as to the relative importance of U-factor vs SHGC in this climate. I have ZERO heating days but probably 330 cooling days and a beastly sun beating through my windows much of the year. Is low-E (or low E2) really that important in my climate? Will it keep heat out and cold in? And does anyone know where I can find data for PGT impact glass windows for U factor and SHGC? I need to replace my aged windows irrespective of getting hurricane windows, and I already have hurricane panels which are a PITA to install, but maybe I want to get regular (ie non-IG) windows installed with low-E2 dual panes and just continue to live with my old shutters. Price plays a big role here too as the IG windows are probably 3 times more than ordinary windows.

Also, can anyone give info on the benefits/drawbacks to aluminum vs vinyl windows in south Florida? Unlike the rest of the US, it seems that the vast majority of new and replacement windows here are aluminum. Yet aluminum seems pretty miserable thermally-speaking.

Great question – err – questions. It appears that you have really done your research into this issue so I will try to add a little bit to it.

PGT is the 800lb gorilla in the Florida impact market, but they are not the only option. I say that NOT as a recommendation or not recommendation of any specific window company, but simply as a statement of fact. In the last 10 years or so PGT has gone from a window company not on anyone’s map to one of the 10 largest window companies in North America. Tremendous growth and a very large percentage of that growth has been in the impact market.

Off the top-of-my-head, I can’t think of any window company who meets Energy Star with a typical Dade certified window. There may be some out there, but if so there aren’t very many.

And as a quick aside…IG means Insulating Glass. Technically all dual pane windows are referred to as IG units. I had the impression from your question that you were seeing impact dual pane as "IG" and non impact as "dual pane". Not sure if I was correct in reading it that way, but just in case – a small explanation.

A good part of the reason is that most windows certified to the Dade standard were constructed using monolithic impact glass (the correct term) rather than IG impact glass. Not exclusively of course, but most companies that certified to the Dade standard did so by dropping a single pane of laminated glass into there sash/frame system. This works great for impact protection; but not quite as well in terms of window energy performance.
In the South Florida environment, many folks conclude that SHGC is more important than is U-factor. I totally agree.

In the South Florida environment many folks conclude that SHGC is the holy grail of window energy performance and that U-factor is of no importance. There I don’t agree.

But, the folks who write the codes tend to ignore U-factor in South Florida as having no application – seeing it as a northern climate issue – and so U-factor was never a factor in the older (and some more recent) energy codes in that area. That may change in the future or it may not. We shall see…

Standard LowE has very little application in a severe cooling environment such as South Florida. That is why LowE2 (more correctly as LoE^2 "squared") was developed.

Although LowE2 is used generically for this type of coating, LoE^2 was developed by Cardinal and they "own" the correct designation. Kind of like using "coke" for all cola products even though Coke is certainly a brand identifier.

Standard LowE blocks far or longwave infrared radiation which translates into heat that you feel when exposed to a hot sidewalk (for example) when the sun has been shining on it. In this example you could call it reradiated heat. LowE2 was developed to block shortwave or near infrared energy as well as longwave infrared. Shortwave IR energy translates into direct solar heat gain – which is what you want to block. Does it work? Yep, and very well, too.

However, I would recommend in your situation that you investigate LowE3 which is the newest version of solar control LowE coating on the market. Again, LowE3 is used as a generic term. Cardinal designates there triple solar control as LoE366 (it was the original on the market and it does have the best energy performance numbers of the triple coatings out there). You might also run across someone using one of AFG’s Comfort Ti-AC solar coatings or perhaps PPG’s Solarban 70XL. I mention Cardinal’s coatings for a couple of reasons. First, like PGT in Florida, Cardinal is the 800lb gorilla of the residential coating market, owning something like double the North American share of that market than every other coating company combined – so it is more than twice as likely that you will be offered a Cardinal LoE coating when buying energy performance windows than another coating (and of course, that being said, you will likely come across one of the others in your research…) And second, because Cardinal’s coating designations are really easy to read. For example LoE366 means that the coating has three layers of silver and it transmits 66% of visible light.

Can a LowE coating be placed inside a laminated glass make-up? Yes. And does that make-up have SHGC numbers comparable to an IG with a solar control coating? Again – yes. Does that make-up offer U-factor numbers comparable to using an IG window? In this case – no. A monolithic laminated glass construction with an enclosed LowE coating will offer comparable SHGC performance numbers to a coated IG unit, but the U-factor is a non-factor in that case, being about the same as a clear monolithic laminated glass of the same make-up sans the LowE coating.

Now things become a bit more "interesting".

Impact (or "hurricane") windows are made with laminated glass, upgraded hardware, upgraded frames and other components, and all sealed in place with some of the strongest silicone (or other) adhesives on the market.

Virtually all the major window manufacturers have impact products available. Some perform extremely well, others barely get by.

Laminated glass is simply two (or more) lites of glass bonded to a plastic interlayer for strength. Laminated glass is no stronger against breakage than is the glass it is made of. In other words, laminated glass breaks as easily as the glass it is made of. I mention this because many folks have the mistaken impression that laminated glass / impact windows don’t break very easily. They will break as easily as the glass that they are made of, but the advantage of laminated is that the glass adheres to the plastic interlayer and keeps the envelope of the home closed.

All that said, there is a little difference in breakage because a ½" laminated glass (overall thickness) made up of two ¼" lites bonded to the interlayer, is pressure rated the same as a ½" lite – and not as a ¼" lite. I am not sure if that makes sense, if not I can try to explain it better.

There are a number of options in the impact glass world relating to the interlayers used to manufacture the laminates – and here are some of those options:

First we have PVB or Polyvinyl Butyral. This is the stuff that is in the windshield of your car. It is relatively soft and very flexible, yet it is also tough and doesn’t tear easily. This is probably the most widely used product in the impact glass market since it does great when impacted. It stops whatever hits it and stretches to absorb the impact (such as a persons head in a car accident). PVB is a good product and is the choice for many manufacturers.

A second interlayer type is a hybrid of PVB with a layer of PET film between the PVB layers. This is a very tough product and it performs very nicely.

A third type of interlayer is called SGP or Sentry Glass Plus. This is quite a bit different from PVB in that it is very stiff and very tough. It is becoming something of the product of choice in some of the toughest applications (including some bullet and bomb resistance applications).

It is also can be more than a bit more expensive than PVB and may be overkill for some residential applications – but I think it is ultimately the best product on the market at this date.

SGP has become the product of choice for a number of the wood window manufacturers. Although vinyl folks don’t seem to be using it quite as much yet, SGP does seem to be growing in the vinyl market as well. I suspect that one reason SGP is a little behind in vinyl windows is because SGP’s rigidity tends to transfer the impact force to the frame of the window rather than absorbing the impact in the glass as does PVB. While the best vinyl windows can handle the force of an impact when using SGP as the laminate interlayer, not all vinyl windows are strong enough to take that force.

And a fourth type is the liquid resin laminates. And I will leave that one where it is.

This part becomes a bit sticky…Cardinal, the largest residential coating company in the world and one of the largest, if not the largest, residential laminator in North America, does not offer a monolithic PVB laminated product with an enclosed laminated LowE coating. But, they do offer their coatings in an SGP laminated lite. Beyond that, I have no comment.

Either vinyl or aluminum frames will perform well in your environment…depending of course on the quality of the frame and the materials.

Aluminum is less energy efficient, even with a thermal break, but many folks are convinced that aluminum is more appropriate to the harsh sun and salt air environment in coastal areas.

I would likely consider both and look at the individual performance numbers before making a decision based on material. Also, you might take a look into fiberglass windows. Fiberglass is the newest entry into the sash/frame market and it is a very nice product.

Okay, way long, so stopping now.

I hope this helps…

So it seems the Energy Star designation is somewhat irrelevant to the Florida market if virtually NONE of the impact windows can get that designation. I presume it was mainly developed for cold climates where heat loss is of prime concern. IMHO, it would be helpful if "they" (ie the energy-monitoring powers that be) would give us hot-climate people a useful designation for our needs. I mean, it's not a small market.

In my snooping, er, researching online I came across comments submitted by industry representatives to the Dept of Energy a couple of years ago seemingly attempting to get a variance to permit Energy Star designation in the southern market. It was quite arcane, but it appears that they GOT the modification and yet still can't get the efficiencies good enough to get the designation. (Amazing what you can find online though!)

Unfortunately, I want it all. Impact glass, top notch SHGC and affordable price. Guess something will have to give. Phooey. But again, many thanks for your help!

Energy performance requirements are probably going to get a good bit more stringent in your part of the world in the next few years, but they are still mostly ignored because you live in a warm climate and many (most?) people in your part of the world think that energy performance is a cold-climate thing.

As you said though, you want three things "Impact glass, top notch SHGC and affordable price", and in reality you can get two of them but all three at once is going to be more difficult...

But, in the long run, thermal efficiency and safety count for alot.

And you are very welcome. And if you have additional questions, please feel free to ask. There are a number of pros who visit this site and there are a lot of good answers here.

Have a great evening!

Also, why does the film on the glass have to be green, when most glass has a "dark" or "black" film on it to protect it from the sun. I would appreciate your answering these two questions. Thank you.

The film between the glass is not green, it is clear. Some get green glass tint or darker shaded tint depending on were they are. A few years ago people got green tint on there windows to help with turtles, but it actually doesn't help that much. Now they go more on how dark the window is in turtle areas.

James: 70xl will depend on what type of window. Usually U: will be around .30 - .27 and SHGC around .21. Again it depends on the setup.

Despite my concerns about energy efficiency, my electric costs dropped substantially (about 15%) immediately and have persisted. I used PGT's version of insulating glass and also tinted glass on the sun-drenched windows on 3 sides (not on north), but it's fairly subtle and not too dark and definitely not green. Admittedly, my old windows were really crappy and leaky, but just standing there with the sun shining through them, there was a very big difference in the heat transmitted.

The windows continue to work well and I've had no trouble after 4 years. Of course, we haven't had any hurricane issues like in 2005 (I claim credit for this, as forking out $20K has kept the hurricanes away, IMHO) but it's nice to know I don't need to deal with those awful panels again. Style-wise the PGTs are boring and don't have the appeal of other brands, but they were affordable for me, got the job done and I can recommend them.

We already have storm shutters (yeah, the kind I have to screw in when a severe storm comes), so I'm not concerned with the impact part. (Randomly: Heard one contractor state that only specially licensed contractors can install windows in Broward County and after a storm they're usually booked up so it's better to get regular windows which will ship faster and be installed quicker. Not sure if that's complete BS or not. I have nothing to base it on. Haven't been in South Florida for very long at all.) Our goal was to replace all the windows with new ones (since they're from 1980 and look like they're about 1/16" thick single pane and they're falling apart). But it's more complicated down here than we had anticipated.

The details from this post are helpful, but haven't helped me completely understand exactly what I should be looking for in my specific scenario. Supposedly I want an SHGT lower than the VT... but I'm not sure what the base number should be. Again, supposedly, shaded windows should be 70-80% (how do I know what's best?!) and sunny should be 40-60%. And then I should look for the lowest SHGT I can afford... but I'm still pretty confused about the window glass itself, forget about the framing part! I have no idea if aluminum is better than vinyl for this area or what and there's very little I can find that definitively says one way or the other. :(

Thanks for any guidance!

You really want to look for a double paned IMPACT window. You can get it with LowE/Argon which is recommended. For all of Florida I recommend the lowest SHGC you can get, which should be around .21 for Vinyl windows.

The strength you can get vinyl windows at, I don't push aluminum windows anymore unless it is a commercial job. PGT and most others have windows that meet Dade County requirements for vinyl Single Hung windows.

If your in Broward, and do not have hurricane shutters, then I recommend the impact window. If you have Accordion or panel shutters, then you need to meet a DP50 and can go with a standard window.

Hope this helps.

That said, affordability played a big role. The PGTs were OK price wise and have given us significantly improved electric bills. We installed them in 2008 and now, 3 years later there are newer products available. (eg, when we did this PGT didn't have Dade-approved vinyl windows) But no regrets. I have had 3 years of no worries about shutters (What if I'm out of town when a hurricane is coming?) and the windows are really so much nicer than the old ones the other 364 days per year.