The Larson I900 and I900E are interior-mount storm windows that install inside your home, delivering up to 50% U-factor improvement and 48% draft reduction without touching the exterior. The I900E’s Low-E coating pushes U-factor improvement to 62% — significant numbers for Montana’s heating season. Designed for historic homes where exterior preservation is required, these aluminum-framed units preserve original window aesthetics while cutting energy bills.
The catch? Larson discontinued the line in December 2022. Existing installations still perform, but replacement parts and new orders aren’t happening. For contractors dealing with historic districts or homeowners wanting interior solutions, understanding these specs helps identify comparable alternatives.
Energy Performance and Installation Specs
The 50% energy loss reduction and 48% draft decrease aren’t marketing fluff — they’re measurable improvements in occupied spaces. Standard clear glass delivers 50% U-factor improvement, while the Low-E option (I900E) hits 62%. That difference matters when outdoor temps drop below zero and heating bills spike.
Installation demands precision. Windows need 1-inch minimum mounting depth, though actual window depth is only 5/8-inch. Openings can’t exceed 1/8-inch out-of-square — tighter tolerance than many retrofit products allow. The mounting procedure uses one tool and follows a straightforward sequence: measure opening at three points for width and height (using smallest measurements), install top channel, add side channels, secure with screws, insert thermal panel.
| Specification | Value | Source |
|---|---|---|
| Mounting depth required | 1 inch minimum | manufacturer |
| Actual window depth | 5/8 inch | manufacturer |
| Out-of-square tolerance | 1/8 inch maximum | manufacturer |
| Draft reduction | Up to 48% | manufacturer |
| U-factor improvement (clear) | Up to 50% | manufacturer |
| U-factor improvement (Low-E) | 62% | manufacturer |
| Installation tools | One tool required | manufacturer |
Size Range and Glass Options
The sizing range runs 10 to 80 inches for both width and height, with approximately 23 square feet maximum area. Larson uses United-Opening Inches (UI) sizing — width plus height — to determine glass thickness requirements.
Glass thickness depends on UI: 1/8-inch glass for UI up to 95, 3/16-inch glass for UI 95-110, with 3/16-inch tempered glass optional for larger sizes or UI over 95. The commercial versions could handle up to 24 feet in width or height, though residential models topped out smaller.
Color options included White, Antique White, Brown, and Champagne — standard finishes that blend with most interior trim. The units fit single or double hung windows, slider and basement windows, and fixed windows.
Applications and Limitations
These interior storms targeted large commercial, historical, residential or municipal windows. Historic preservation projects particularly benefited since nothing changed on the exterior. Second-story rooms and urban areas with noise concerns saw additional value beyond energy savings.
Compared to external storm windows, the Insider series offered cleaner interior appearance and easier removal but potentially lower wind resistance than external models. That trade-off worked fine for protected locations but mattered in exposed sites.
Customer feedback praised effective draft blocking, noticeable noise reduction, and simple interior installation. The downside beyond discontinuation? Custom ordering was slow even when available.
The Bottom Line
Interior storm windows solve real problems — preserving historic exteriors while cutting energy costs. The I900/I900E’s specs remain relevant for contractors evaluating modern alternatives. That 62% U-factor improvement with Low-E glass sets a performance benchmark. The 1-inch mounting depth and 1/8-inch tolerance requirements define installation parameters.
For Montana’s temperature swings and historic districts, interior storms make sense. The technology works. Warranties ran 10 years to lifetime on parts with up to 20 years on finishes, though discontinued status affects current claims. Understanding what made these units tick helps identify suitable replacements for energy retrofits that can’t touch the exterior.
Frequently Asked Questions
What’s the actual energy savings from interior storm windows? Tests show up to 50% U-factor improvement with clear glass. Low-E glass pushes that to 62%. Draft reduction hits 48%. Those percentages translate to real heating bill reductions in Montana winters.
Can these handle Montana’s temperature extremes? The heavy-duty aluminum frame handles thermal cycling fine. Being installed inside prevents weathering and allows easy removal — no exterior exposure to temperature swings.
How precise does the installation need to be? Very precise. Openings must be within 1/8-inch of square. You need 1-inch minimum mounting depth. Measure at three points and use the smallest measurement to ensure proper fit.
What window types work with interior storms? Single or double hung windows, slider and basement windows, and fixed windows all work. Wood double-hung, single-hung, slider, basement, fixed/picture, and historic window openings are compatible.
Are replacement parts still available? The product line was discontinued in December 2022. Limited availability for replacement parts now. Consumers should contact Larson support with proof of purchase for warranty claims.
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