The thing most window companies will not tell you
Low-e storm windows pay back in about nine years in this climate zone. Replacement windows take decades. Here is the whole calculation.
We sell replacement windows. This page argues that a lot of people should buy something else first, and we would rather tell you that now than have you find it out afterwards.
The comparison, using the government's own numbers
The Department of Energy's national laboratory modelled low-e storm windows city by city. The nearest modelled city to St. Louis in the same climate zone is Kansas City, and for a Zone 4A house:
| Low-e storm windows, Climate Zone 4A | Modelled result |
|---|---|
| Simple payback | 8.9 years |
| Site energy savings | 18 percent |
| Savings-to-investment ratio | 1.5 over single-pane; 1.4 to 1.7 over double-pane metal frames |
| U-factor improvement | 0.88 down to 0.36 over a single-pane wood window; 0.51 down to 0.28 over a double-pane wood window |
Against that, replacement windows: EPA models about $493 a year for going from single-pane to ENERGY STAR in this zone, and about $253 a year for going from older double-pane. DOE puts installed replacement windows at about $29 per square foot for double low-e in a non-metal frame and $34 for triple, and its own payback tables run from ten years to more than 240 depending on the numbers you put in.
So the honest comparison is roughly a nine-year payback against several decades. That is not a close call, and it does not depend on our opinion of anything.
The bit that is genuinely surprising
Interior storm inserts measured an 18.6 percent cut in whole-house air leakage in one 3,792 square foot home, comparable to a professional wall or attic insulation retrofit. That is not a comfort measure, it is an envelope measure, and it takes ten to thirty minutes a window.
Where storm windows are the right answer
- Historic houses, and especially St. Louis historic districts. Where the ordinance limits replacement windows to wood or colour-finished aluminium — as Shaw's does — or requires public-facing windows to be wood, as Lafayette Square's does, a storm window is often the only thermal upgrade you can make without a fight. The National Park Service's own recommendation for these openings is a single casement wooden storm, rabbeted along the inside edge to keep the original moulding visible, with weep holes in the bottom.
- Original wood double-hungs that are structurally sound. These were built to be repaired. Sash cords, glazing putty and a storm on the outside will get another few decades out of a window that a salesman will tell you is beyond saving.
- Steel casements. Documented repair economics put a sixty-five-window job at around $360 a window, comparable to replacing them — and the replacements could not match the narrow original sightlines and cut the glass area by a fifth.
- When money is the constraint. Exterior storms run roughly $90 to $400 a window depending on frame and style. That is a fraction of a replacement and it buys most of the comfort.
Where they are not
- When the frames are actually rotten. A storm over a failing window is putting a lid on a problem.
- When the windows do not operate, or fail egress in a bedroom. Those are safety issues and a storm does not touch them.
- When you want the house to look different, or you are tired of painting. Perfectly good reasons to replace, and no amount of payback arithmetic changes them.
- Interior storms in a cold snap, if they are not airtight: Condensation can form between an interior storm and the original window if the storm is not airtight, worst in the shoulder seasons. Exterior storms on older houses need weep holes cut in the bottom so the water can get out.
One buying note
Low-e storm glass performs roughly fifty per cent better than plain storm glass and costs little or nothing more; where there is a premium, the Department of Energy puts its payback at two to five years in every climate zone. Either way the answer is the same: if you buy storms, buy low-e ones.
Why we are telling you this
Because you will find it out eventually, and because the version of this trade that oversells energy payback is the version that gives it a bad name. If storms are right for your house we will say so. If you then want replacement windows anyway — for the look, for the operation, for the maintenance — we will quote them and neither of us will pretend the reason was the utility bill.
Sources: pnnl.gov · basc.pnnl.gov · mncee.org · nps.gov · energystar.gov · efficientwindows.org
Common questions
Do storm windows look bad?
Cheap triple-track aluminium ones do. A properly proportioned storm, sized to the lines of the primary sash and finished rather than raw, is close to invisible from the street — which is exactly why preservation offices accept them where they will not accept replacement windows.
Interior or exterior?
Exterior protects the original window from weather as well as adding the thermal layer. Interior is easier to install and remove, is invisible from outside, and measured better on air leakage in the study we cite — but needs to be genuinely airtight or you get condensation between the layers.
Will a storm window stop condensation on my old windows?
Usually it moves it. The interior surface gets warmer so it sweats less; the water can end up between the two layers instead. Exterior storms on older houses need weep holes at the bottom so it can drain out.
Do you install storm windows?
Ask us. If it is the right call for your house we would rather do the smaller job than lose the customer by pretending it is not.
Not sure which your house needs?
We will look at the existing windows and tell you honestly whether they are worth keeping.
Tell us about the house
We answer the phone. If we miss you, we call back.