Aluminum vs Vinyl for Phoenix -- U-factor question

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azbearoh
Posts: 2
Joined: Sat Aug 11, 2007 2:03 pm

Aluminum vs Vinyl for Phoenix -- U-factor question

#1 Post by azbearoh »

I am looking to replace windows here in Phoenix where the temps are above 100 all summer long. I have spoken and looked at vinyl and aluminum windows (with thermal break, of course). I have heard some stories from aluminum reps that talk of expansion/contraction in summer heat with vinyl and the effect on seals/caulking. Also some issues with larger windows (60"X72") and vinyl. The vinyl reps talk of the u-factor issue with aluminum and heat. Looking at equivalent SHGC factors in the glass (using the identical glass), is there really much difference between a u-factor of 0.40 and 0.33 in the summer heat (cooling load)? I think I have used my heater here 4-6 times in the 12 years I have lived here in the winter. So the cold is not a concern of mine. Is the u-factor more for cold than heat?

Oberon
Posts: 280
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

#2 Post by Oberon »

Two considerations you need for your environment.

First is Solar Heat Gain Coefficient or SHGC. This is a measurement of how well your window can control direct solar heat gain. In your environment this measurement is the more significant of the two.

Second is U-factor or U-value. This is a measurement of the ability of your window to control indirect heat gain - indirect meaning not direct solar gain.

In both cases the lower the number the better.

From your question I get the impression that you are thinking of SHGC as the glass and U-factor as the frame material? If so, then consider that the entire window system (including install) is going to affect both SHGC and U-factor to some degree - likely several degrees!

Energy Star and other rating systems do tend to downgrade U-factor in your part of the country and concentrate on SHGC. I would suggest that this is not an accurate accessment of overall performance and I would also suggest that in time folks in your part of the country will be addressing U-factor as well as SHGC. But, keeping in mind that direct solar gain is still significantly more important than U-factor in your environment only because direct solar gain is potentially more of a problem.

The better vinyl windows will perform in your environment. Lower end vinyl windows might be problematical. Aluminum windows, even with a thermal break, are not as efficient as are vinyl, wood, or fiberglass, but they are proven reliable in your environment so they are still relatively common for that reason and also because they are relatively cheap. Windows in the southwest are still something of an afterthought to many folks and energy considerations don't always go in that direction.

If you are concerned about vinyl then I would probably recommend looking at fiberglass as an alternative material worth consideration.

azbearoh
Posts: 2
Joined: Sat Aug 11, 2007 2:03 pm

#3 Post by azbearoh »

Thanks for the reply. Currently looking at IWC Ambassador series for aluminum with thermal break and a low-e cubed glass (Energy Shield Xtreme) vs Simonton vinyl windows with similar glass, Not sure which Simonton series I was quoted a price on. Both quotes were very close. Like the look of the aluminum since less frame-more window. Any knowledge of the windows?
Been playing with Resfen software and comparing cooling costs by changing the u-value (factor) while keeping the SHGC the same -- each change in hundreths results in very little change in costs -- even changes in tenths results in a couple of dollars difference in annual cooling costs.
How do you evaluate the overall rating of the window? The tags I have seen on the windows show separate values for u-value, SHGC and VI -- are these the values for the overall window?

Oberon
Posts: 280
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

#4 Post by Oberon »

Personally, I am not a fan of RESFEN. It has its uses, but remember that it is a computer program that makes a heck of a lot of assumptions - my opinion and I imagine other folks may think differently.

I would not worry about playing with RESFEN (except for fun - and I have done that as well), but would consider the actual performance numbers listed by the manufacturers on the tags (as you mentioned). The labels do list overall window performance. U-factor and SHGC and VT (not VI) list how the entire unit performs as a unit - not as individual parts.

If energy performance is your primary concern then look for the lowest SHCG first and then at the lowest U-factor (again remembering that both numbers reflect overall unit performance). I would go with the best SHGC value and don't worry about the rest - in your environment - provided the window is a quality product.

As to companies, I don't comment on or compare different window manufacturers, but other folks on the site do, so hopefully you will see a few more answers over the next few days concerning the different manufacturers.

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