Overall thickness
How thick the complete sealed unit is from outside face to inside face.
It has to fit the available glazing depth in the sash, casement or metal frame.GLAZIE LTD · TECHNICAL GUIDE
Measure the rebate. Control the sightline. Glaze the unit correctly.
Slim double glazing needs more than a width and height. This guide explains the measurements, edge cover, setting blocks and glazing sequence that matter when fitting reduced-profile sealed units into traditional or shallow rebates.
General technical guidance for competent installers. The sealed-unit manufacturer’s instructions and the project-specific glazing system take priority.Outer pane
Narrow cavity
Low-E inner
Slim edge system
Rebate / putty cover
Check 01Overall thickness
Check 02Edge sightline
Check 03Clearance & support
Do not order slim glass on overall thickness alone.
A unit can physically fit the rebate depth and still look wrong if its spacer or perimeter seal remains visible. Treat overall thickness, sightline, rebate cover and glass size as separate checks before manufacture.
01 · MEASURE FIRST
Four measurements decide whether the detail works.
Width and height manufacture the rectangle. These four checks decide whether the sealed unit belongs in the window.
Cavity
The sealed gap between the two panes.
Its width affects gas performance and limits what can physically fit inside the unit.Sightline
How far the spacer and edge-seal system remains visible in from the edge of the glass.
A heritage unit can be thin overall and still look wrong if its edge system is not hidden by the rebate or putty line.Rebate cover
The amount of glass edge concealed by the frame, bead or traditional putty line.
It determines whether the spacer and seal disappear visually once the unit is installed.SIZING PRINCIPLEFinal glass size = verified frame opening − the edge clearance required by the chosen glazing system.
Do not apply a universal deduction without checking the frame, unit manufacturer and glazing method. Measure the opening at multiple points and size from the verified condition.
02 · CURRENT REFERENCE BUILDS
Know the build before you set the rebate.
The store treats slim units as engineered constructions. The exact live specification remains controlled by the active supplier catalogue.
| Build | Make-up | Cavity | Reference sightline | Rebate guide |
|---|---|---|---|---|
| 11.5mm11.5mm Ultra Slim | 3/5.5/3 | 5.5mm krypton | 3–4mm | Confirm with build |
| 12mm12mm Heritage Slim | 4/4/4 | 4mm krypton | Build-specific | Confirm with build |
| 14mm14mm Slim Performance | 4/6/4 | 6mm krypton | 5.5–6mm | 7mm+ cover |
| 16mm16mm Slim Performance | 4/8/4 | 8mm krypton | 5.5–6mm | 7mm+ cover |
Reference figures help with early detailing; final order data and installation instructions must follow the confirmed production build.
03 · CHOOSE THE GLAZING METHOD
Putty-fronted and beaded details solve the edge differently.
Both methods need continuous support, compatible materials, enough edge cover and a detail that sheds water away from the perimeter seal.
Traditional appearance, controlled edge cover
The unit is bedded continuously in a compatible glazing compound, supported on setting blocks and secured without point-loading the glass. The external front is then tooled to a weather-shedding line that conceals the spacer and perimeter seal.
- Continuous rear bedding
- Correct setting and location blocks
- Compatible compound
- Spacer concealed by the finished putty line
A clean retained-bead or new-joinery detail
The sealed unit is supported and bedded in the rebate before compatible beads are installed. The bead and bedding system must hold the unit securely without forcing the glass or exposing the edge seal to water.
- Dry-fit bead depth first
- Keep edge clearance around the unit
- Pin or fix without glass contact
- Seal the perimeter continuously
04 · SITE SEQUENCE
A practical six-step installation flow.
The detail varies by frame and manufacturer, but the control points remain consistent.
- 01
Inspect and prepare
Confirm the frame is sound, dry and suitably coated. Remove debris, failed compound and anything that would stop the new unit sitting evenly.
- 02
Measure the opening
Record the verified rebate opening, available glazing depth and the cover available to hide the slim edge system. Check width and height in more than one place.
- 03
Dry-fit and block
Test that the unit can be centralised with the required edge clearance. Support its weight on suitable setting blocks and add location blocks where the sash movement requires them.
- 04
Bed continuously
Apply only an IGU-compatible glazing compound, tape or sealant. The rear bedding should support the glass continuously rather than leave water-trapping voids.
- 05
Secure without stress
Fix beads or approved glazing sprigs so the unit is retained but not pinched, twisted or point-loaded. Metal fixings must never touch the glass edge.
- 06
Front, tool and inspect
Finish the external line so water sheds away from the unit, keep drainage paths clear and verify that the spacer and edge seal are concealed as specified.
SETTING & LOCATION BLOCKS
Support the weight. Control movement. Keep the edge clear.
Setting blocks carry the unit. Location blocks help control it in opening sashes. Positions depend on whether the sash is fixed, side hung or top hung; the objective is to support the glass without transferring harmful loads into the edge.
05 · BEFORE PRODUCTION
Keep the PDF on site →Ten checks before a made-to-measure unit is released.
Overall depth
The complete sealed unit fits the available glazing depth.
Sightline
The spacer and edge seal will disappear behind the bead or finished putty line.
Width & height
Final manufacturing sizes allow the clearance required by the chosen glazing system.
Glass type
Safety, Low-E, heritage and decorative requirements are confirmed before manufacture.
Frame condition
The sash, casement or frame is dry, sound and capable of carrying the extra glass weight.
Setting blocks
Block positions suit the way the sash is hung and support the unit correctly.
Compatibility
Compounds, tapes, sealants and packers are compatible with the insulated glass unit edge seal.
Drainage
Water can shed and escape without being trapped against the perimeter seal.
Consent
Any listed-building or conservation requirements have been checked for the individual project.
Final specification
The ordered build, dimensions and edge detail match the verified opening before production begins.
STANDARDS & RESPONSIBILITY
The product guide does not replace the installation specification.
Installation should follow the sealed-unit manufacturer’s current instructions, the selected glazing-system guidance and the applicable requirements of BS 6262, BS 8000-7 and relevant Glass and Glazing Federation guidance. Historic or listed buildings can add project-specific conservation and consent requirements.
READY TO SPECIFY THE GLASS?
Take the checks back to the Slim configurator.
Choose the engineered build only after the rebate depth, sightline cover and final manufacturing sizes have been verified.