The Larson Insider series delivered measurable energy performance improvements for historic homes before its discontinuation in 2022. With 50% energy loss reduction and 48% draft reduction, these interior-mount storm windows solved the classic problem of improving efficiency without touching the exterior of historic buildings.
Performance Specifications That Mattered
The Low-E glass option (I900E) achieved 62% U-factor improvement versus 50% for standard clear glass. That 12% difference translates to real heating savings in Montana winters. The standard I900 with clear glass worked fine for most applications, but the I900E earned its premium in bedrooms and living spaces where comfort matters most.
Interior mounting required only 1-inch depth — shallow enough for most historic window casings. The actual window measured 5/8-inch deep, leaving clearance for window locks and hardware. Maximum coverage topped out at 23 square feet per window, which handled most residential openings except large picture windows.
| Specification | I900 (Clear Glass) | I900E (Low-E Glass) |
|---|---|---|
| Energy Loss Reduction | 50% | 62% (U-factor improvement) |
| Draft Reduction | 48% | 48% |
| Glass Thickness | 1/8” (UI ≤ 95) | 1/8” (UI ≤ 95) |
| 3/16” (UI 95-110) | 3/16” (UI 95-110) | |
| Maximum Size | 80” × 80” | 80” × 80” |
| Maximum Area | 23 sq. ft. | 23 sq. ft. |
| Mounting Depth Required | 1” | 1” |
| Out-of-Square Tolerance | 1/8” | 1/8” |
The sizing system used United-Opening Inches (UI) — width plus height. Simple math that contractors could calculate on site without conversion charts.
Why Historic Homeowners Chose Interior Mount
Historic homes where exterior preservation is required represented the core market. You couldn’t touch the outside of a registered historic property, but you could improve efficiency from the inside. The Insider series offered a cleaner interior look and easier removal but may have lower wind resistance than external models.
Color options included White, Antique White, Brown, and Champagne — period-appropriate finishes that didn’t scream “modern retrofit” in a Victorian interior. The Antique White matched most historic trim without requiring custom paint.
Customer feedback praised effective draft blocking, noticeable noise reduction, and simple interior installation. That noise reduction mattered for homes on busy streets or near highways — double-pane performance without window replacement costs.
Installation Reality Check
One tool installation (likely a screwdriver) wasn’t marketing fluff. The mounting system used channels that screwed into the window frame — top channel first, then sides, then slide in the thermal panel. Pre-installation required ensuring minimum 1-inch mounting depth and that opening was within 1/8-inch of square.
Out-of-square tolerance mattered. Historic buildings settle. That 1/8-inch forgiveness meant you didn’t need to rebuild window frames to make these work. Measure top, center, and bottom for width. Left, center, and right for height. Use the smallest measurements and order accordingly.
The included hardware covered standard installation: top mounting channel, two side channels, installation screws, and the thermal window panel itself. No trips to the hardware store for special fasteners.
The Discontinuation Problem
Discontinued as of December 2022 created immediate problems for owners counting on these for ongoing renovations. Custom ordering can be slow and limited availability for replacement parts now means damaged units might not be repairable.
General Larson storm windows often featured a 10-Year or Limited Lifetime Warranty on finish and parts, but specific model warranty details are now limited due to discontinuation. Discontinued status may affect warranty claims; consumers are advised to contact Larson support with proof of purchase. Keep your receipts if you have existing units installed.
Who Should Still Consider Used or Remaining Stock
Historic building owners still need these capabilities. The 48% draft reduction and interior-only installation remain unique advantages. If you find old stock or used units in good condition, they’re worth considering for:
- Second-floor bedrooms where ladder access makes exterior storms impractical
- Buildings with historic district restrictions on exterior modifications
- Rooms where seasonal removal matters (spring cleaning access)
- Noise reduction needs without full window replacement budgets
The Low-E version justified its premium through measurable energy savings. In Montana’s temperature extremes, that 12% additional U-factor improvement paid for itself. But even the standard clear glass version delivered real draft reduction — half the air infiltration gone with one afternoon’s installation.
Compatibility included wood double-hung windows, single-hung windows, slider windows, basement windows, fixed/picture windows, and historic window openings. That flexibility made them work for mixed window types throughout older homes.
FAQs
Q: Can damaged Insider units still be repaired?
A: Limited availability for replacement parts now makes repairs difficult. Contact Larson directly with model numbers and proof of purchase to check parts availability.
Q: What’s the actual energy savings difference between I900 and I900E?
A: Low-E glass reflects summer heat and retains winter warmth better than clear glass, with higher U-factor improvement (62% vs 50%). The 12% difference matters most in extreme climates.
Q: How much out-of-square can these handle?
A: Maximum 1/8-inch out-of-square tolerance. Measure carefully — openings beyond this tolerance won’t seal properly.
Q: What tools are actually needed for installation?
A: One tool (likely a screwdriver) handles the entire installation. The mounting channels use standard screws into wood frames.
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