[rwkruger]

Hello.

I recently purchased a house in sunny Dallas, TX and am looking for ways to increase its energy efficiency. I had looked previously at purchasing 3M window film that claims to reduce about 50% of the solar energy. I learned, however, that the previous seller replaced the windows recently with double-hung low-e windows that have a solar heat gain coefficient of .36. Is there value in putting an energy-reducing film on windows that are this efficient? Does the math work that film + the windows would result in .18 transmission (50% of .36)?

Thanks for your help

 

[jerryh3]

I think you're confusing U-factor and visible light transmission. You may want to contact the manufacturer of the window to see if they recommend using film tint. My patio door specifically states not to add any additional tinting to the glass. I believe it's due to differing thermal expansion rates of the film and the glass which can lead to stress cracks.

 

[rwkruger]

Thanks for the suggestion, Jerry. I hadn't considered the different expansion rates. I assume that the window manufacturer will tell me "don't do it" if I call them, so I'll wait to see if I hear any other real-world opinions before calling them or making a decision.

I'm straight on the numbers, however. The windows have a separate U factor (0.33) that measures heat loss via the windows. The solar heat gain coefficient (apparently this replaced the shade factor) measures the heat gain via sunlight passing through the windows.

Thanks again
ryan

 

[Ron6519]

Thanks for the suggestion, Jerry. I hadn't considered the different expansion rates. I assume that the window manufacturer will tell me "don't do it" if I call them, so I'll wait to see if I hear any other real-world opinions before calling them or making a decision.

I'm straight on the numbers, however. The windows have a separate U factor (0.33) that measures heat loss via the windows. The solar heat gain coefficient (apparently this replaced the shade factor) measures the heat gain via sunlight passing through the windows.

Thanks again
ryan

The only, "real world opinion", would be the manufacturer. They made the window, so they should know what works and what doesn't. They also know what voids the warranty.
If you want to reduce solar gain, you can install something to shade the window or replace the unit with the proper glazing for your house.
Ron

 

[Just Bill]

I can't say what the manufacturer will recommend, but installing a film on the inside will definitely cause problems. It reflects the heat back into the insulated glass and will quickly result is seal failure, and as suggested above, probably no warranty. Not sure what installing a film on the outside would do.

The glass in those windows probably blocks 90%+ of UV rays. What are you hoping to accomplish??

 

[rwkruger]

I appreciate your continued help, guys.

What I'm trying to accomplish: I'm trying to reduce the transmission of heat into my house via the windows in the summer. I am aware that I can install awnings, but like the thought of the window film approach for two reasons. first, the window film is "invisible" and does not change the appearance of the house. second, the film is also break/impact resistant and limits the risks of someone breaking into the house by smashing a window, blowing debris breaking a window during a storm, etc.

What I meant by "real world" vs talking to the manufacturer: in many cases, manufacturers will be overly-cautious when making recommendations. For example, if you call a car manufacturer and ask them how often to change your air filter, they likely will tell you every 12 months/12K miles. If you ask an experienced mechanic, however, he/she will probably suggest that you check it regularly but only replace when it's dirty, which may not be for 20-30K miles. I was hoping that I'd get some similar input here.

Window warranty: I'm still sorting through whether the warranty will even transfer to me, since I'm not purchasing a new construction house. In the meantime, I'm trying to get input without regard to the effect on the warranty, since I'm not sure it will apply to me anyway.

Installation: The film installs on the outside of the windows, not the inside. I don't know if this solves all the issues (I still can see the point of the first reply that I could have problems if the film and the glass expand/contract at different rates), but this type of install should guard against the "heat reflected back into the window" issue.

I hope this helps to clarify what i'm trying to do and what I'm asking. Please keep the questions coming.

 

[jerryh3]

Which film are you looking at?

 

[Ron6519]

You will not know the outcome untill you apply the coating. You might not know the outcome for a while. The manufacturer will tell you not to apply anything to their windows. It limits their liability. It doesn't really matter if the warranty is tranferrable or not, once you put a coating on the glass, you have no warranty.
You can experiment if you want or you can look into swapping out the sashes for more appropriate glazing option.
Ron

 

[wrangler]

IInstallation: The film installs on the outside of the windows, not the inside. I don't know if this solves all the issues (I still can see the point of the first reply that I could have problems if the film and the glass expand/contract at different rates), but this type of install should guard against the "heat reflected back into the window" issue.

I hope this helps to clarify what i'm trying to do and what I'm asking. Please keep the questions coming.

This actually does make a difference. Film on the inside of a window can create problems othe than already mentioned. It is like when light is reflected back on itself over and over... it can create heat. But on the outside it will actually reduce the amount of energy transferred into the window/house and will add to your protection, reduction. Many windows with low-e coating is put on one side of one pane of the double sided glass for a reason... if heat reduction (in Dallas) is what you need, I see no reason to not try this.
Before spending too much, you might try it on one of your sun facing windows and learn for yourself if it seems worth the time/cost and report back with your own findings!

 

[rwkruger]

Jerryh3 - I'm currently considering the 3M Prestige 70 and Ultra Prestige 70 (the difference between the regular and the "ultra" is the resistance to breakage).

Ron6519 - I actually was trying to make the reverse point... if the warranty is not transferable to me anyway, then installing a film that voids the warranty will not matter, since I wouldn't have a warranty regardless. I understand your point though: if the warranty is transferable to me *and* I choose to install a film, then I will void the warranty.

Wrangler - I like your suggestion of trying the film on 1 sun-facing window to see what the results are. I have a room with 3 windows on the south-west corner that would be a prime candidate to experiment... If that room feels cooler after installing the film and the windows don't get damaged after some period of time, then I would have reason to go ahead and install on other windows in the house. If there is no difference or the windows fail as a result of the install, then I'm only dealing with the issue on 3 windows instead of the whole house.

Thanks again for the help, everyone. I think I'll investigate the "install on a few windows and see what happens" option.