Larson I900/I900E Insider Storm Window (Interior Mount)

Larson’s I900 and I900E Insider Storm Windows deliver up to 50% reduction in energy loss and 48% reduction in drafts through an interior-mount design that preserves exterior facades. The heavy-duty aluminum frames fit inside existing window openings from 10 to 80 inches wide and tall, with a maximum area of approximately 23 square feet. These interior storm windows excel in historic homes where exterior modifications are restricted, while still providing measurable energy savings without altering the home’s outside appearance.

Montana contractors deal with -20°F winters and 100°F+ temperature swings. The I900E’s Low-E coating delivers 62% U-factor improvement versus the clear glass I900’s 50% improvement. That 12% difference matters when heating bills arrive.

Installation Specifications and Tolerances

The I900/I900E requires 1-inch minimum mounting depth and tolerates only 1/8-inch out-of-square openings. Those tight tolerances aren’t suggestions. Exceed them and the thermal seal fails.

Larson uses United-Opening Inches (UI) sizing — width plus height. Critical glass thickness requirements:

• UI ≤ 95: 1/8” clear float glass
• UI 95-110: 3/16” glass required
• UI > 95: 3/16” tempered glass optional for larger sizes

Installation requires one tool and follows a straightforward sequence: measure opening at multiple points (use smallest measurement), install top mounting channel, add side channels, secure with provided screws, insert thermal panel.

The system works with wood double-hung, single-hung, slider, basement, fixed/picture windows, and historic window openings. Each kit includes top mounting channel, two side channels, installation screws, and the thermal window panel.

Energy Performance and Applications

The I900/I900E targets bedrooms, living spaces, historic homes requiring exterior preservation, second-story rooms, retrofits for energy improvement without full replacement, and draft/noise control in urban or windy areas.

Compared to standard external storm windows, the Insider series offers a cleaner interior look and easier removal but may have lower wind resistance than external models. Montana’s chinook winds make that trade-off worth considering.

Four color options match interior trim: White, Antique White, Brown, and Champagne. The finish matters less than function, but matching trim prevents the add-on look.

The product line was discontinued in December 2022, which may affect warranty claims. General Larson storm windows featured 10-Year or Limited Lifetime Warranty on finish and parts, with up to 20 years for premium finishes.

Bottom Line

Customers praised effective draft blocking, noticeable noise reduction, and simple interior installation. Key negatives included slow custom ordering and limited availability for replacement parts after discontinuation.

The I900E with Low-E coating makes sense for Montana’s temperature extremes. The 62% U-factor improvement translates to real heating bill savings. Historic home owners get energy efficiency without touching the exterior. The discontinued status means finding inventory, but the performance specs remain solid for contractors with access to remaining stock.

Frequently Asked Questions

What’s the actual energy savings difference between the I900 and I900E?

The I900E with Low-E glass delivers 62% U-factor improvement while the standard I900 achieves 50% improvement. Low-E glass reflects heat in summer and retains warmth in winter. For Montana winters, that 12% difference justifies the upgrade.

Can these handle Montana’s temperature swings without seal failure?

The heavy-duty aluminum frame and 1-inch mounting depth requirement create a robust thermal barrier. The critical factor is maintaining the 1/8-inch out-of-square tolerance during installation. Exceed that and thermal performance degrades.

How do interior storm windows perform versus exterior models in high winds?

Interior models like the I900/I900E may have lower wind resistance than external models. They’re protected from direct wind exposure but can’t reinforce the primary window against pressure. For extreme wind zones, exterior storms provide better structural support.

What happens to warranty coverage now that the line is discontinued?

Discontinued status may affect warranty claims, and consumers are advised to contact Larson support with proof of purchase. Standard coverage included 10-Year or Limited Lifetime Warranty on finish and parts. Document everything.