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Community and Q&A

Low-e glass on storm windows

arkitkt | Posted in Green Products and Materials on

We live in a 1920 farmhouse in climate zone 4 and we’re in the process of having our original double hung wood windows reworked – spring bronze and weather stripping added, new window weight ropes, etc. Next summer we plan to reside our house and in the process install new storm windows. Right now we have old triple tracks with broken parts and we’re looking up to upgrade to a product like Allied Windows.

My question is regarding Low E glass – the storm windows are available with Low E on the interior of the storms (second surface). I understand the benefits of Low E glass on new thermal windows, but will Low E glass be of much benefit on a storm window, particular in the winter when the space between the wood windows and storm windows is not completely sealed to the exterior? In the summer it would help reduce heat gain but then would we lose heat gain in the winter? Or would heat reflecting back in from the Low E make up for for the heat gain loss?

Also, on some of the existing storms we have condensation build-up on the interior of the storm glass. We’ve tried to seal up a couple of the interior wood windows but we’re still getting some condensation on the storms. Would Low E glass help because the glass would actually be warmer from reflecting the interior heat? Thanks for any input, we’re just trying to do this right first time.

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  1. GBA Editor
    Martin Holladay | | #1

    Low-e storm windows are a good investment. For more information, see these two articles:

    "Low-e Storm Windows Are Big Energy Savers"

    "What Should I Do With My Old Windows?"

  2. Expert Member
    Peter Engle | | #2


    At the risk of thread hijacking, I've got a question about those two studies. The storm window study says that people can save 4%-27% in heating and cooling costs with low-e storm windows. But the old window article says that people can only save 1%-4% of their heating and cooling costs by replacing all of their windows. This doesn't make sense. The replacement windows must have higher performance than adding a storm, but adding storms saves 4x the energy of replacements?

    Manual J calculations usually show that windows lose 5%-10% of the total energy in a house. So cutting that loss in half by installing better windows does get you into the 3%-5% range of the old window study. But how can storm windows save more energy than the original windows are losing?

    1. Expert Member
      Dana Dorsett | | #4

      >The replacement windows must have higher performance than adding a storm,

      Many code-min replacement windows will underperform a tight low-E storm over a tight wood-sashed single pane. The overall air tightness can be quite a bit higher.

  3. GBA Editor
    Martin Holladay | | #3

    Many homes that pay for new windows already have triple-track storms -- so the thermal performance of the new windows isn't much better in cloudy weather than the old windows. In sunny winter weather, the new windows usually perform worse than the old windows -- because the clear glazing (which has a high SHGC) has been replaced by low-SHGC double glazing.

    Low-e storm windows have a high SHGC, so you don't lose the beneficial winter heat gain.

    I'm more familiar with the Craig Drumheller study in Chicago than the PNNL study, but it appears that the PNNL study documented a reduction in air leakage in addition to reduced heat loss.

  4. Expert Member
    Dana Dorsett | | #5

    >"We’ve tried to seal up a couple of the interior wood windows but we’re still getting some condensation on the storms. Would Low E glass help because the glass would actually be warmer from reflecting the interior heat? "

    It works the other way- the low-E storm reflects the heat rather than absorbing it, and runs colder than a clear-glass storm. The condensation issue could increase with low-E glass.

    If the wood sashes are on pulleys & weights the pulley and weight pockets are substantial air leaks. There are some pretty good retrofit pulley seals available for cheap, but to seal the sash weight pockets requires pulling the trim and spending some quality time with a caulking gun, and sealing the trim will when putting it back up .

  5. Expert Member
    Peter Engle | | #6

    Consider me dubious. I'll give you and Dana the argument if you're talking about cheap replacement windows vs. decent wood sash windows with triple tracks. There won't be much savings there. And I can see how a low-e storm provides better air sealing, decent heat gain, and improved heat reflection. I'm not questioning that the low-e storms are a good thing, and I have recommended them to my clients in the past. But I'm still curious as to how any storm window can save as much as 27% of the total heating/cooling costs, even at a U value of 0 and SHGC of 1. Maybe if the baseline house has burlap windows, but not anything else.

  6. arkitkt | | #7

    We do have significant air flow through the window weight pockets/pulleys but we plan to address that issue when we get new siding and storms. We plan to remove the existing wood clapboards, insulate from the exterior and re-sheath the house. The window weight pockets will be air sealed from the exterior but not from the interior - that is, interior air may make it through the pockets/pulleys - but that shouldn't really affect the storm window cavity.

    As I mentioned we are in the process of the sealing up the interior wood sashes but so far it hasn't helped much. It's only been a few days so I guess time will tell but so far we've had a spell of cold weather and a little snow and the stormed windows are still condensating.

    Interesting that the condensation issue could get worse with low E - I hadn't thought about the glass being colder since it reflects the heat. Right now we have icicles running down the vinyl siding (which is over wood clapboards) from the storms on really cold days and I had to drill additional weep holes in the existing storms to keep the water from running behind the vinyl, now at least it runs on the outside of the vinyl. And for some reason this only happens to the windows on the second level, or at least it's worse on those windows - but the wood sashes have always seemed a little looser compared to those on the main level.

    So, any ideas on how to reduce the condensation if low E glass could potentially make it worse - other than what we've done so far in adding spring bronze to the jamb to tighten the sashes and weatherstripping at the sill?

  7. Expert Member
    Dana Dorsett | | #8

    >"So, any ideas on how to reduce the condensation if low E glass could potentially make it worse - other than what we've done so far in adding spring bronze to the jamb to tighten the sashes and weatherstripping at the sill?"

    Fixing the air leakage from the indoors is the first and most important order of business. Beyond that, venting the space between the storm and original window with outdoor air works. In addition to weep holes at the bottom (check that they aren't caulked shut) many storm windows come with small adjustable vents for reducing condensation. But until the air leakage from the indoors into the space between storm & window is solved the tiny storm window vents won't do a whole lot.

  8. arkitkt | | #9

    One more question - after reading the article "what should I do with my old windows?" I'm still unsure about the low E option. It states in cold weather to install low E (which may unfortunately make the condensation issue we have worse). In hot climates it mentions using a solar control heat film. We live it a mixed-humid climate, the best of both worlds - so what's the best option when you have both hot and cold, and pretty much the same of both.

    1. Expert Member
      Dana Dorsett | | #11

      The difference in condensation with low-E won't be enough to be a real concern- it won't be twice the condensation, probably not even 1.25x.

      The difference in thermal performance is pretty significant though. A clear glass storm over wood sashed single pane has a slightly higher SHGC, but it's still a net energy loser, the higher solar gain not making up for the higher heat loss. A low-E storm over a wood sash single pane is a net energy gainer.

  9. Expert Member
    Peter Engle | | #10

    The low-e storms are probably the better option. They do provide some heat rejection in summer, and the dead air space they create provides substantial R-value in summer and winter, at least if the storms are closed. If they're open, then there's no benefit. One additional benefit that is rarely discussed is that the storms help to protect the old wood windows from weather damage. The wood windows last longer and require less maintenance and restoration when covered by storms. Window film won't help there.

    If you have some windows that take on a lot of solar gain in summer, consider awnings or other outside screening.

    The condensation issues are definitely from interior air leaking into the space between the sash and the storm, probably through the sash pockets. if you are in the process of restoring and upgrading the wood sashes, you could consider getting rid of the sash weights and installing friction strips on the sides of the sashes instead. Then you can fill the sash pockets with spray foam, and the friction strips act as spring-loaded weather seals. This combination reduces air leakage dramatically.

  10. arkitkt | | #12

    So a follow up question to the condensation issue on our storm windows (we have a lot now because it's really cold). As I mentioned before we plan to insulate and reside the house this summer. Right now a lot of cold air comes through the window weight pockets by coming through the exterior wall. And warm air can also enter the window weight pockets. So right now we have a mix of warm inside and cold outside air in the window weight pockets as well in the space between the interior wood windows and exterior storms (this varies depending on wind direction). When we reside and insulate the house the pockets will be sealed from the outside air but not from inside air. Could this potentially make the condensation worse because there's less dry/fresh outside air coming in? Or if I attempt to seal any gaps in the window weight pockets with caulk so the air can't get from the pocket to the space between the windows, will that possibly reduce condensation? Thanks

    1. Expert Member
      Dana Dorsett | | #14

      Air sealing the space between the windows from the humid interior always reduces condensation potential on the storm window.

      Air sealing that space from the exterior can go either way- when air is being pulled out it will draw more humid interior air in, increasing the condensation potential. When exterior air is being drawn in the condensation potential falls. But either way it won't be a lot worse.

      As the house gets ever tighter the condensation potential on the INTERIOR surface (#4, the surface in contact with the room air) goes up as the average indoor humidity rises, which can be mitigated with an HRV.

      1. arkitkt | | #15

        Thanks for the comments Dana - the space between the windows would not be sealed from the exterior and there would be weeps to hopefully help condensation (although the weeps aren't helping now, even after I drilled a few more). My main question is about the window weight pockets, will sealing them from the exterior make matters worse?

        1. Expert Member
          Dana Dorsett | | #16

          Seal the weight pockets, and everything but the weeps (that you can control). Counting on random air leakage from the outdoors into the sash pockets for moisture control is a dubious strategy at best, and one that is likely to partially defeat the potential thermal performance gains.

  11. arkitkt | | #13

    And one more question - several years ago we converted our attic to an unvented attic (I've discussed this topic on separate threads). And we're going to spray foam the walls so the house will be much more air tight (except the the windows which will hopefully be a little tight when we're done). I've read that this will allow less moisture to escape the house - so could this be making the the condensation worse on the second level (closer to the unvented attic) - and could it make storm window condensation worse if the house is more sealed up?

    I know we could install an ERV to bring in more fresh air but I was curious if this could potentially make matters worse?

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