- Window SLIP™ Slim Line Insulating Pane
Upgrade your original single-pane window to double-pane-level performance.
A permanent, low-profile insulating pane that mounts directly to your existing sash developed over 20 years of hands-on historic window restoration.
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How It Works
The slim line insulating pane installs directly onto your existing sash no removal, no reglazing, no disruption to the original frame. In this short video, you can see the full process on an active historic building project, from measuring through final installation.
The result is a window that looks identical to the original from the street, performs like a modern double-pane unit, and meets the reversibility requirements that preservation boards and landmark registries require.
before
the SLIP is a low profile addition to existing glass
after
the SLIP is a low profile addition to existing glass
What Is the Window SLIP™?
SLIP stands for Slim Line Insulating Pane. It is a sash-mounted secondary glazing system that converts single-pane windows into simulated double-pane windows without removing, altering, or replacing the original sash or glass.
A tempered glass pane is added directly to each sash, overlaying the current glass configuration. The frame is a low-profile pressed metal extrusion, nearly invisible once installed. The result is a permanent insulating air space that modernises thermal performance while leaving the original window completely intact and fully operable.
Unlike traditional storm windows, SLIPs do not require seasonal removal. Unlike insulated glass conversions, they do not require the original glazing to be removed or the window to be reglazed. It is a simpler, less invasive, and more cost-effective path to modern energy performance without sacrificing historic integrity.
- WHY IT WORKS FOR HISTORIC PROJECTS
Designed With Preservation in Mind
Heritage Compliance
SLIPs preserve original sash, muntins, glass, and profiles meeting the visual integrity and reversibility requirements of historic preservation boards, landmark registries, and Secretary of the Interior Standards compliance projects. The product can be installed on the interior or exterior of the sash and can be easily removed if required.
Energy Performance
University of Oregon Energy Studies in Buildings Laboratory testing demonstrated that SLIPs can reduce energy costs by up to 72.9% when installed on single-pane windows. Performance is comparable to insulated glass unit conversions at a fraction of the cost and disruption.
Cost-Effective Upgrade
For multi-lite configurations, insulating glass conversions require removing original glazing, routing new channels, and reglazing every lite individually. SLIPs provide equivalent insulation with a single overlying glass layer applied directly over the existing configuration no alteration to the original window required.
- PERFORMANCE DATA
Tested and Verified Performance
SLIP performance has been evaluated through both independent laboratory simulation (Intertek) and real-world energy testing (University of Oregon).
Thermal Performance (Intertek Simulation):
- Whole-window performance improved from ~0.31 → 0.26 U-value
- Achieves performance levels comparable to modern insulated glass units
Real-World Energy Impact (University of Oregon Study):
- Heating energy reduced from 454 → 123 therms/year
- Cooling energy reduced from 474 → 129 kWh/year
- Up to 72.9% reduction in heating energy use
- Significant reduction in air leakage (~111 → 68 CFM @ 50 Pa)
What This Means:
SLIP delivers a true whole-window performance upgrade—improving thermal resistance, reducing air infiltration, and lowering energy use—while preserving original historic windows and operability.
“The SLIPs were very close to the energy performance of the IGU option… We were able to save hundreds of historic windows and still met our stringent standards of energy performance that exceed Oregon state code.”
— George Bleekman, Owner’s Representative, Capital Design & Facilities Management, University of Oregon
- SPECIFICATIONS
Product Specifications
Window Compatibility
- Single-hung, double-hung, casement, and picture windows
- Interior or exterior installation
Glass Options
- 1/8" or 1/4" tempered glass
- Clear, Low-E, or obscure finish options
Maintenance
- No seasonal removal required
- Eliminates seal failure risk associated with IGUs
- Extremely low long-term maintenance costs
- Easily removable if required
Frame
- Low-profile pressed metal extrusion
- Virtually invisible once installed — no loss in outside viewing space
- PROJECT REFERENCES
Installed on Major Institutional and Historic Projects
SLIPs have been specified and installed on buildings including:
Fort Vancouver National
Historic Site
University of Oregon
Gerlinger Hall and Straub Hall
Oregon State University
Bexell Hall
Redmond City Hall (DeMuro Award for Excellence in Preservation)
Peninsula Park Community Center
- HOW TO ORDER
Fabricated to Your Measurements. Ready to Install.
SLIPs are custom-assembled to the exact dimensions of your windows. We assemble, sell, and ship SLIP Kits containing all materials required for a successful installation. Kits can be installed by contractors, carpenters, and other construction trades. Full installation instruction videos are available.
For projects in the Pacific Northwest, direct installation may be available for select commercial projects. Contact us to discuss your scope.
Ready to
Specify or Source?
Our team can work with your project requirements, architectural specifications, or preservation plan. Storm windows are fabricated to order contact us to discuss your project.