Larson’s insider Storm Windows cut energy loss by up to 50% and drafts by 48% — numbers that matter when Montana winter heating bills arrive. These interior-mount storm windows install from inside, preserving historic exteriors while delivering measurable thermal improvements. The windows mount inside your home, making them the solution for buildings where exterior modifications aren’t allowed.
The I900 series fills a specific gap: retrofit energy performance without touching the building’s facade. For historic homes where exterior preservation is required, these windows deliver thermal upgrades while maintaining architectural integrity. The Low-E I900E model achieves 62% U-factor improvement versus the standard I900’s 50% — a significant jump in heating efficiency.
Energy Performance and Glass Configuration
The performance numbers justify the retrofit investment. Draft reduction reaches 48% while U-factor improves up to 50% with the standard clear glass option. The Low-E upgrade pushes harder against heat loss.
Glass thickness depends on window size, calculated by United-Opening Inches (UI = width + height):
| UI Range | Glass Thickness | Application |
|---|---|---|
| ≤ 95” | 1/8” clear float | Standard residential windows |
| 95-110” | 3/16” glass | Larger openings |
| > 95” | 3/16” tempered (optional) | Maximum strength for oversize units |
Maximum area reaches approximately 23 sq. ft. with dimensions ranging 10” to 80” in both width and height. The clear or Low-E glass options let contractors match performance to budget.
Installation Specifications and Mounting Requirements
Precision matters with interior storm windows. The recommended mounting depth is 1 inch, though actual window depth measures 5/8 inch. Maximum out-of-square tolerance is 1/8 inch — anything beyond that requires shimming or frame adjustment.
Installation requires one tool (likely a screwdriver) and follows this sequence:
- Measure opening width at top, center, and bottom (use smallest)
- Measure height at left, center, and right (use smallest)
- Install top mounting channel
- Install side channels
- Secure with provided screws
- Insert thermal panel
Compatible window types include single or double hung windows, slider and basement windows, and fixed windows. The system works with wood double-hung, single-hung, slider, basement, fixed/picture, and historic window openings.
Historic Building Applications
Interior mounting solves the preservation challenge. The product prevents weathering and allows for easy removal during warmer months or for cleaning. The custom-fit design ensures a tight seal while reducing noise and dust infiltration.
Four color options — White, Antique White, Brown, and Champagne — match most interior trim without drawing attention. Custom-built for a precise fit, suitable for older or historic structures without altering their appearance. The window is easy to install, remove, and clean.
The system fits large commercial, historical, residential or municipal windows, with select styles up to 24-feet in width or height — covering everything from residential double-hungs to institutional oversized units.
The interior mount approach means no ladder work, no exterior scaffolding, and no weather delays. Spring and fall removal takes minutes for buildings that want summer ventilation. For Montana’s temperature swings and historic preservation requirements, the I900 series delivers thermal performance without compromising architectural character.
Frequently Asked Questions
What’s the actual energy savings with Larson insider storm windows?
Energy loss reduces by up to 50%, drafts decrease by 48%, and outside noise drops noticeably. The Low-E version (I900E) achieves 62% U-factor improvement compared to 50% for standard clear glass.
How much mounting depth do these interior storm windows need?
Recommended mounting depth is 1 inch, though the actual window depth is 5/8 inch. The opening can be up to 1/8 inch out of square — beyond that requires adjustment.
What’s included with the I900 storm window system?
The kit includes top mounting channel, two side channels, installation screws, and the thermal window panel (storm glass pane). Installation needs one tool — likely just a screwdriver.
Can these handle Montana’s temperature extremes?
The heavy-duty aluminum frame handles thermal cycling. Interior mounting protects the system from weather exposure. The design allows easy removal during warmer months if seasonal use is preferred.
What window types work with the I900 system?
Compatible types include single or double hung windows, slider and basement windows, and fixed windows. The system specifically works with wood double-hung, single-hung, slider, basement, fixed/picture, and historic window openings.
Is the I900 series still available?
The product line was discontinued as of December 2022. Discontinued status may affect warranty claims; consumers are advised to contact Larson support with proof of purchase. Existing installations remain serviceable, but new orders aren’t available.
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