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Low-E film retrofit on windows?

Nola_Sweats | Posted in General Questions on

I’m considering getting a low-e film retrofitted to some south- and west-facing glass.  These rooms get very warm in summer afternoons here in the deep south.  Winter cold is not really an issue.

The articles on this (excellent) site say the proper location of a low-e coating is inside of “surface 2” on a double-pane window in hot climates, i.e., between the panes where it has to be done at the factory.  All but one of my windows is double-pane, so the only retrofit option is inside of surface 4, facing the room.  I’ve also got a pair of glass doors that are single-pane.

There’s an encouraging GSA report that says the retrofit films work great, even on the inside (surface 4) of double-pane glass:
Is there a differing school of thought here that would have me proceed cautiously? 

FWIW, the windows are wooden, double-pane, no coatings, about 12 years old.  Doors are single-pane and several decades older.  3M’s product looks pretty impressive, but almost all of the info online is from 3M or its installers, so I’ll take opinions on products, too.  Thanks!!

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  1. Expert Member
    Dana Dorsett | | #1

    What climate zone are you in?

    Most south facing windows can get greater benefit by using awnings to block summer sun. Though that's more expensive than window film, it's more effective, and if done right still allows wintertime sun to warm the place when the mid-day sun angles are low compared to in the summer. That can still be a valuable consideration in climate zone 2, not so much in zone 1.

    Rejecting the heat at the exterior pane is usually more effective than trying to do it a the interior. Since the double panes are clear glass (no low-E coatings) a hardcoat low-E exterior storm window with the coating on surface #1 or #2 will reject heat without over heating the sealed double-pane to the point that it blows the seals, (exterior operable shades would do better). Hard coat low-E coatings don't cut as much visible light spectrum as much as window films or soft coat low-E use in sealed insulated glass units, but also not quite as much heat rejection as soft-coat low-E.

    Living in a heating dominated climate I have limited direct experience with heat rejecting window films, but some (not 3M's though) - they do have a measurable effect on heat gain but also cut a significant amount of visible light. The most effective versions can really darken a room. In my own home I installed a medium grade window film on the bug screen frame for a skylight (rather than the window itself), in a summertime-overheating family room to be able to seasonally swap it in or out. I haven't tried to measure the performance, but it feels like the right compromise.

    1. m854 | | #3

      Dana, wouldn't a Low-E film also help in the winter, reflecting infrared energy back into the house? Or maybe it's not enough to make up for the reduced solar gain?

      1. Expert Member
        Dana Dorsett | | #5

        As-modeled in my skylight case (which is itself glazed with a modestly heat-rejecting film on surface #2), leaving the film there in the winter would increase heating energy use. My confidence level in that result isn't super-high.

  2. Nola_Sweats | | #2

    Thanks for the response. I'm in Zone 2A, near the Gulf Coast. Fixed awnings are most notable around here for being mangled in hurricanes. There are fabric retractable awnings that I'm looking into, but I'm concerned about the durability of the mechanisms.

    I don't have a place to store storm windows -- small house with no garage -- and I'd like to be able to remove them seasonally to open the windows on nice days. That said, storms would work on two small windows that are just a fixed plane of glass, if condensation between them is not a problem. They condense on the outside in summer almost daily, and rarely on the inside in fall or spring when the a/c and heat are not running.

    You mentioned operable exterior shades -- that is my other realistic option. The downside is that they cut off the light and views, making the narrow old house claustrophobic, and they're much more expensive. That's why I'm looking into window films.

    Can you tell me more about seal-busting problems? I've seen concerns expressed online, but it's not mentioned in the GSA study. I can't tell if it's a real problem, or more hypothetical, like the old warnings that unvented attics would broil roofs and destroy shingles.

    1. Expert Member
      Dana Dorsett | | #6

      >"Can you tell me more about seal-busting problems? I've seen concerns expressed online, but it's not mentioned in the GSA study. I can't tell if it's a real problem, or more hypothetical, like the old warnings that unvented attics would broil roofs and destroy shingles."

      I have no direct experience, but it's partly a function of the temperature ratings of the goo the glass manufacturer uses, some of which can be pretty low, like 150F or something. For clear glass windows I suspect it would be pretty hard to blow the seals with an exterior side low-E storm compared to an argon filled soft coat low-E window with the coating on surface #3.

      Exterior storm windows will of course be substantially more expensive than window film, but they last longer, and protect the original window from the elements, and a better quality storm can tighten up the air leakage by quite a bit. The tightest in the biz are made by a regional player in the northeast (Harvey Tru Channel storm windows) but Larson low-E storms sold through box stores aren't bad. In most of Zone 2 they may have to be special ordered, but at least some box stores have some models listed on the websites (but not necessarily currently in stock, in a very small sampling.) There are other sources for low-E storm windows that may be more convenient/better service/ cheaper. In a quick search it seems like the blue box store shows more storm window offerings than the orange box store in the handful that I looked at in zone 2 cities.

  3. Nola_Sweats | | #4

    After reading about storm windows, it appears I am behind the times -- I was thinking of traditional inoperable, seasonally removed storm windows. Still, the film would be thousands of dollars less ... if it works and does no harm.

  4. Nola_Sweats | | #7

    Update: Based on the responses here, I decided to take a different route: Bahama shutters. I'm getting them on the south and west-facing windows, which will sort of function as the awning that Dana recommended, keeping heat out before it reaches the house. Unlike awnings, these are hurricane-rated, so I can close and bolt them when a storm is approaching. The slats are spaced a bit more widely than traditional Bahamas, preserving a good bit of the exterior view.

    I did get the 3M film on a single-pane patio door that faces west, where there wasn't really another good option. It's only been about 10 days, but it's performing well, and the room is noticeably more comfortable in the afternoon. I don't think the film is polarized, but there's a lot less glare, which was an unexpected benefit.

    Thanks again for the advice!

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