Low-e storm window over low-e house window?
gocdunn
| Posted in Energy Efficiency and Durability on
Hello,
I was wondering if anyone knows the answer to this: Is there an advantage putting low e storm windows over low e windows? Low e raises the cost of the storm window and if the house already has permanent low e windows then would clear glass storm windows be the way to go?
Thanks for your time,
Chris.
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Replies
Putting storm windows (low-E or not) over sealed glass windows (low-E or not) can sometimes cause the seal to fail on the glazing, and should only be done with careful consideration of just how much direct sun there is. With heat-rejecting low SGHC low-E windows the risk of creating a heat trap and causing the exterior side of the sealed glass to overheat, but most higher SGHC type low E windows can take it. This is with or without a low-E coating on the storm window.
Building Science Corp has successfully put low-E storms over ~U0.32 low-E windows as part of deep energy retrofits in a Massachusetts climate, bringing the total performance down to U0.25-ish at an SGHC of about 0.5:
https://buildingscience.com/file/3981/download?token=KzlKzjmp
On page 3 under Window Specifications:
"New Harvey Tru Channel storm windows
over existing wood-framed, doubleglazed,
low-E windows; approx.
composite performance: U=0.25,
SHGC=0.52"
Chris,
For more information on this issue, see Insulated Storm Windows? I urge you to read the posted comments as well as the article.
In Comment #31, GBA reader David Jones noted, "Modeling performed for LBNL by sustainability consultant Thomas Culp, Ph.D. has uncovered the potential for serious overheating problems when low-e storms are added to low-e windows: in hot weather, in direct sunlight, temperatures up to 185° F (85° C) may be reached. That kind of heat can cause premature aging or failure of the insulated glazing unit’s seals. Further testing will yield a better understanding of the exact conditions under which this can occur and possible solutions, but it would be wise to avoid using low-e storms in combination with low-e double-pane windows until more is known."