Will application of certain window films help eliminate condensation on old windows?
Our old casement windows are sweating (to the inside right now, in winter) so much that paint on the window is peeling off and we have green mold around some of the panes. We are in Climate Zone 3. Will installing 3M Thinsulate CC75 film help?
We have just completed a year in our 1920 one-story Colonial Revival since doing a major to-the-studs remodel, which included adding central air and heat, tightening the envelope, insulating the exterior walls with NuWool, converting the attic to unvented with closed cell foam, encapsulating the crawl space, and replacing the windows along the back and side of the house with Low-E double pane Marvin windows.
We live in a historic district, and our front casement windows are actually featured in the “windows you should never change” section within the district’s published guide on allowable modifications. Needless to say (but I’ll say it anyway), we kept our wonderful and beautiful original old windows. We actually spent significant time and money rehabbing the windows, including a lot of patching and also equipping them with weather stripping to cut out drafts from gaps when they closed. They now seem to close tightly, and we don’t feel air or see light around them anywhere
My understanding is that condensation occurs when the interior surface temperature of the glass drops below either the dewpoint or frost point and that the differential between humidity on either side of the glass is a part of this because it affects the dewpoint.
The Internet, including my friends here at GBA, tells me that storm windows are the clearest answer (https://www.finehomebuilding.com/2015/10/16/low-e-storm-windows-are-big-energy-savers and https://www.thisoldhouse.com/ideas/weathering-storm). I may end up going that route, but they would have to be interior storm windows because of the historic regulations, and I have read some about the benefits of those here (https://www.greenbuildingadvisor.com/question/interior-storm-window-benefit). My wife finds them unsightly, and I certainly don’t think they are a net positive to our decorating scheme, so I am looking at alternatives.
If the U factor of the window is a good indicator of its resistance to condensation (https://www.nfrccommunity.org/resource/resmgr/fact_sheets/factsheetcondres_oct12.pdf), then it sounds like the application of a film that will lower the U factor will also help with condensation. The NFRC rates the 3M Thinsulate CC75 as reducing solar heat gain from .71 to .46 on 1/8″ clear glass and giving a u factor of .76 (http://search.nfrc.org/search/apd_film/film_search_detail.aspx?cpdnum=MMM-M-043). 3M claims that Thinsulate 75 applied to 1/4″ clear glass gives a U value of .63, down from 1.05.
My questions are (1) is the glass my main problem, or, in other words, would finding a solution to the single-pane glass as a cause of condensation fix the problem? Also, (2) does a U Factor of between .63 and .76 get me to a point where I will significantly reduce condensation vis-à-vis these windows in their current state. I am particularly skeptical of the window film route because I do not see window film companies advertising that their applied films reduce or eliminate condensation, and I assume that companies that can advertise for something like that would do so. But if the U Factor is a relative proxy for condensation potential, and the films reduce the U Factor, well, you see my point.
I have read Martin’s “What to Do with Old Windows,” which I found helpful but did not answer my question. Also, the windows face east and north. Thanks in advance.
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Replies
Sadly, no, it won't help. That's a low-e film and while it will reduce heat transfer to the window surface, the window surface will actually end up slightly colder, and if anything make the condensation worse.
There are a wide variety of interior storms available, some of which are nearly invisible. I'd encourage you to keep searching in that category.
Joel,
The way I've seen this dealt with in historic homes is to replace the existing glass with a thin dual pane IGU. These can be as thin as 3/8" and should fit within the existing wood casing. Looking at it up close you can barely tell that the glass is different.
You are correct that you’ll get condensation if the interior surface of the glass drops below the dew point. The dew point is the temperature at or below which water will condense out of the air. If your surface temperature drops below the frost point, then the condensation will freeze and you’ll have frost on the inside.
Unfortunately there isn’t much you can do to magically improve the performance of a single pane of glass. LowE films work by reflecting a portion of the light spectrum and limiting heat transfer that way. The result is a colder pane of glass in a single pane window.
If you need a quick fix, try the heat shrink window film. This type of film works to create an air space between the film and the pane of glass and basically fakes a double pane window. This type of film is temporary though, maybe one winter season if you’re lucky. For a more permanent fix, try to find a glass company that can retrofit a double pane IGU into your existing window frames if you can’t go the storm window route.
Bill
I have used heat shrink film successfully on my builder-grade 2004 vinyl windows to limit interior condensation in CZ 6A. I managed to install it very cleanly and is not noticeable unless you look close; I installed it on each individual IGU so the windows still accept interior bug screens and remain operable.
Leaving the bug screens on results in overheating on both east and west facing windows, resulting in the adhesive letting go, so I've started leaving the bug screens off. This approach has seen the film last for two years now, still in good shape and doing its job.
Depending on the style of the frame on the interior side, Joel may or may not be able to adopt this approach on his older windows. Peter's comment below also rings true; unless you're sure the film will be airtight to the interior, condensation will form between the film and the glass.
Hi Joel -
No easy answer here; but one rule in managing condensation on windows: in adding anything to existing windows, the most airtight "layer" must be to the interior. Otherwise, any layer that makes the original glass colder without managing vapor's access to the glass will increase condensation. Air molecules are bigger than water molecules so, some materials manage air flow but not vapor.
The best resource for comparing various performance attributes across different window coverings was a website I helped develop called efficientwindowcoverings.org. I just checked and that website does not seem to be up and working, so your best bet is to go here: https://aercnet.org/resources/window-attachments/.
Peter