Replies

1. GBA Editor
   Martin Holladay | Oct 29, 2014 10:50am | #1

   B.L.,
   Q. "Is Low-E a good choice for all windows, or should it be selected for certain windows according to their location?"

   A. Low-e storm windows make the most sense for cold climates. Hot climates need storms that are designed to reduce solar gain.

   Q. "Is there any unbiased performance data available for storm windows?"

   A. Yes. Here are two unbiased sources of information:

   "Measured Winter Performance of Storm Windows," J. H. Klems, Lawrence Berkeley National Laboratory, 2002. Klems concluded, “The addition of low-e storm windows to the prime [single-glazed] window provided performance very similar to that of the replacement window [with low-e argon-filled sealed double glazing].”

   "Field Evaluation of Low-E Storm Windows," S. Craig Drumheller, Christian Köhler, Stefanie Minen, 2007. Summarizing the findings of this Chicago research project in an article published in the February 2008 issue of Energy Design Update, I wrote, "Installing storm windows improved the homes’ airtightness between 5.7% and 8.6%. At 50 pascals, air leakage was reduced between 231 and 335 cfm. Energy used for heating was reduced by 8% to 18% in the homes that received clear-glass storm windows, and by 19% to 23% in the homes that received low-e storm windows. Although low-e storms cost more than clear-glass storms, they were shown to be far more cost-effective. The simple payback period for the two houses with clear-glass storms was 8.4 and 12.1 years (average, 10.3 years); at the two houses with low-e storms, the payback periods were 3.5 and 5.1 years (average, 4.3 years)."

   Q. "Is there any unbiased performance data available for Larson windows?"

   A. I don't have any information on Larson windows. Perhaps some other GBA readers will chime in.

   Q. "Is there any data available to help with this decision that many people face with old homes?"

   A. I advise you to start with this article: What Should I Do With My Old Windows?

1. 
   DCContrarian | Aug 30, 2021 01:08pm | #7

   Both of those links are dead.

1. 
   howth | Nov 16, 2021 01:12pm | #8

   Archived links:

   "Measured Winter Performance of Storm Windows" - https://web.archive.org/web/20141205102229/https://ohp.parks.ca.gov/pages/1054/files/berkeley%20storm%20window%20research.pdf

   "Field Evaluation of Low-E Storm Windows" - https://web.archive.org/web/20140311235911/http://web.ornl.gov/sci/buildings/2010/Session%20PDFs/22_New.pdf

2. 
   charlie_sullivan | Oct 29, 2014 10:52am | #2

   1. Low e is good for all window locations. However there are a variety of different low-e coatings. Many of them, in addition to providing low thermal emissivity (which is what low-e stands for), also block IR radiation to reduce solar gain, for the purpose of reducing A/C costs. Whether you want maximum or minimum solar gain depends on your climate, which side of the building the windows are on, and what kind of shading they get.

   2. I can't point you to a resource for data on storm window performance, but I think the performance would be quite consistent across different brands, for each of these categories:
   a. Exterior plain single pane storms.
   b. Interior plain single pane storms.
   c. Exterior low-e single-pane storms.
   d. Interior double-pane storms.

   If you are in a cold climate, for old leaky windows I'd strongly recommend interior storms over exterior storms. Interior storms are made to seal as well as possible, whereas exterior storms have a deliberate air leak included in the design. That's because all windows leak some air, and with a fully sealed exterior storm, you would then have humid interior air leaking into the cavity between the window and the storm. You'd then get condensation on the storm window.

   Interior storms are good for the longevity of old windows, because they keep the humid interior air from getting to the window, as well as keeping the cold air out of the house.

   So if you are convinced that interior storms are the way to go, then you can consider single or double layer versions--the ones you pointed to are double layer, and that is better than single layer, for sure. The U = 0.29 is a little suspect, but it's the right ballpark for a "center-of-glass" value for three panes, air filled. I'd expect more like 0.35.

   A low-e exterior storm would get you close to the same center-of-glass U-value, but would be able to do less to stop infiltration, because it needs to have a deliberate air leak.

3. Expert Member
   Dana Dorsett | Oct 29, 2014 11:20am | #3

   The indium tin oxide hard-coat low-E coatings used on low-E storm windows is fairly high solar gain/low solar rejection. The low-E coatings with high solar rejection are too soft to use on a single-pane window, since they would scratch & scar up too readily. All low-E coatings cut solar gain some, but indium tin oxide improves the heat loss factor more than it cuts the solar gain, enough to make the combined window a net energy gainer in cold climates, even on the north side of the house.

   I'm going to have to contradict Charlie in one respect:

   Exterior storms are FAR superior to interior storms, since they keep direct rain-wetting away from the existing window, and raise the average temperature of the existing window. The materials in the old window simply stay drier due to both the higher average temperature and the avoidance of direct wetting. If warmer humid interior air gets into the space between the old window and the storm window, the condensation (all of it) will occur on the storm window (which is designed to take it), which would be several degrees cooler in winter.

   Exterior storm windows do NOT have to have a deliberate leak. Most will have an adjustable leak point to the outdoors, if there is too much condensation from air leaking by the old window. It's best practice to re-work the antiques to make them as air-tight as possible to mitigate condensation potential, but there are many existence-proofs of very air-tight exterior storms over 100 year old windows that do not have condensation problems. Only if the windows are so beat up & warped that they can't be tightened up would an interior storm be the preferred method.

   I don't have actual leakage data or specs on Larson's low-E window air leakage, but the Bronze series is fairly leaky compared to their Silver & Gold series- it's worth the up-charge. At a specified 0.05 CFM/ft^2 @ 50 pascals air leakage, Harvey's Tru-Channel storms are probably the tightest in the biz, and they do have a hard-coat low-E option. Harvey is a regional player in the northeastern US, and may not be available to everyone, but they tend to be a pretty good $/performance value on most of their window & door products.

4. 
   charlie_sullivan | Oct 29, 2014 02:37pm | #4

   Thanks Dana--that's the first time I've seen someone make such a good case for exterior storms as being preferred over interior. It seems like there are good arguments either way. Probably the best argument for interior would be that you don't need to work as much work on tightening up the old windows. The choice might also depend on how much overhang you have--without much, protecting the old windows from direct rain wetting would be particularly valuable.

5. Expert Member
   Dana Dorsett | Oct 29, 2014 03:08pm | #5

   Roof overhang matters a lot!

   At my house I have exterior storms on some beat up 91 year old windows on a dormer at my house, and they DO have condensation issues (unlike the tighter windows of the same vintage elsewhere in the house.) They became beat up and warped beyond reworking due to the fact that there is only about 8" of roof rake overhang on that dormer, and the exterior sills are below the accumulating snow line of the intersecting roof at least every other year. The exterior snow keeps the sash a lot drier, but if I close the adjustable vent on the storms the sill would get wetted from condensation. Those windows would do well to have both interior and exterior storms, but I'm planning to junk 'em when I re-work that room and install some decent new windows.

   Those are the only windows in the house that I have slated for replacement. Most other windows are protected by 2 foot overhangs, and only a couple of those had experienced much warpage to the sash. It didn't take a lot of effort to tighten that one up sufficiently to not have condensation issues with the exterior storms, despite about 1/8" of bowing at the bottom of of the bottom sash. (Since standard weatherstripping wasn't going to work on the bottom sash, I put down a heavy bead of acrylic caulk at the mating part of the sill, and stretched some plastic food wrap over it, then carefully lowered the sash onto the caulk to mold it to the shape of the bottom of the sash. After a few days of cure time I raised the sash, and trimmed the caulk-mess & plastic a bit with a sharp blade to make it a bit more cosmetic.)

   When I re-roof the plan is to extend the rake of that dormer with the junk windows to about 15", which should relieve the snowpack issue (as well as direct wetting from rain), no matter what windows I have in there at that time.

   So many projects, so little time... Houses are never really "done" are they? :-)

6. 
   r610 | Oct 29, 2014 07:04pm | #6

   Thank you very much for the responses. I will read over them thoroughly.

   And yes, this is for a cold climate,