What Size Glazing Packers Do I Need? Professional Selection Guide

Choosing the right glazing packer size is one of the most critical decisions in window and door installation. Get it wrong, and you're faced with misaligned frames, weak seals, and potential warranty issues. Get it right, and your installation sits perfectly, supports the weight of the glazed unit, and maintains a consistent sealant line that will last decades.

This guide walks you through everything you need to know to select the correct packer size for your project — whether you're installing standard UPVC double-glazing, working with aluminium frames, or tackling heritage timber windows. We've broken down the 1mm to 6mm range, explained what each size does, and provided a simple decision tree so you can order with confidence.

Quick answer: Most installations use 3mm or 4mm packers. But read on to find your exact specification.

Understanding Packer Thickness — The 1mm to 6mm Range

Glazing packers come in six standard thicknesses, each designed for specific frame and application types. Understanding what each size does is the first step to choosing correctly.

1mm Packers: The thinnest option, used for shimming and fine-tuning alignment in lightweight frames. Typical application: timber sash windows, secondary glazing, or where minimal spacing is essential.

2mm Packers: Ideal for shallow rebates (15–20mm depth) and lightweight to medium-weight frames. Common use: aluminium frames with narrow rebates, timber frame adjustments, and applications requiring precise alignment with minimal gap.

3mm Packers: The industry standard. Used in the vast majority of UPVC double-glazing installations where rebate depth is 20–25mm and unit weight is up to 350kg. This is your default size for modern residential windows.

4mm Packers: Specified for deeper rebates (25–30mm) and heavier units (350–500kg). Common in premium UPVC systems, commercial installations, and larger frames. If your rebate is visibly deep, 4mm is often the answer.

5mm Packers: Used for wide frame profiles, architectural systems, and high-specification installations where rebate depth exceeds 30mm. Support capacity extends beyond 500kg.

6mm Packers: Maximum thickness, reserved for specialist applications: heritage timber frames with substantial spacing, coastal high-wind installations, or bespoke projects specified by architects. Rarely needed but essential when called for.

Why thickness matters: Thicker packers support heavier units without compression. Thinner packers minimise the sealant bead width (the visible gap around the frame). Too thin, and your frame sags. Too thick, and you've got an ugly wide bead that's harder to seal effectively.

How to Measure Your Frame Rebate

Before you order a single packer, you need one critical measurement: rebate depth. This is the depth of the groove inside your frame where the glazed unit sits.

Step 1: Identify Your Rebate

Look at your frame from the inside. You'll see a horizontal lip or groove — that's the rebate. The depth is measured from the bottom of that rebate to the front face of the frame.

Step 2: Measure at Multiple Points

Rebates aren't always perfectly consistent, especially in older frames. Measure at:

• Top left corner
• Top centre
• Top right corner
• Middle left, centre, and right
• Bottom left, centre, and right

Use a ruler or depth gauge. Record the smallest measurement — that's your limiting factor.

Step 3: Account for Frame Deflection

Here's where installers often slip up: your frame might bow inward under the weight of a glazed unit. If your measurement is 24mm at rest, the frame might deflect to 23mm under load. Reduce your measurement by 1–2mm to account for this.

Step 4: Check for Frame Twist

Place your level across the rebate. If it's not perfectly level, you may need packers of different thicknesses on different sides — but this is advanced. For most installations, use the smallest measurement across all sides.

When to Measure: Measure before removing the old frame. Once the frame is out, the aperture changes shape, and your measurement becomes unreliable.

Common Mistake: Guessing. Professional installers measure every frame. It takes two minutes and saves hours of problems.

Colour-Coded Sizing Reference Chart

GSD packers are colour-coded for on-site identification, following UK and EU professional standards.

Thickness	Colour	Common Use	Unit Weight Range
1mm	Green	Shimming, secondary glazing, heritage timber	Up to 150kg
2mm	Black	Shallow rebates, fine alignment	150–250kg
3mm	White	Standard UPVC double-glazing (industry standard)	Up to 350kg
4mm	Grey	Deeper rebates, premium frames, commercial	350–500kg
5mm	Blue	Wide profiles, architectural systems	500kg+
6mm	Red	Heritage timber, high-wind coastal, specialist	500kg+

Why colour-code? On a busy job site, colour coding prevents mix-ups. Your team can visually confirm they're grabbing the right size without checking labels every time. It's a simple quality-control measure that professional installers rely on.

Packer Selection Decision Tree

Use this decision tree to narrow down your exact requirement.

START: Is your frame UPVC or aluminium?

UPVC: What's your rebate depth?

• 15–20mm: Use 2mm packers
• 20–25mm: Use 3mm packers (most common)
• 25–30mm: Use 4mm packers
• 30mm+: Use 5mm or consult technical specs

Aluminium: Is it a standard commercial profile or bespoke system?

• Standard profile (typical rebate 22–28mm): Use 3mm or 4mm
• Deep rebate (28mm+): Use 4mm or 5mm
• Sloped or non-standard: Consult frame supplier specs

Timber (heritage/sash windows): Is this a restoration or new installation?

• Restoration (working around existing frame): Use 1mm or 2mm
• New installation (new timber frame): Use 2mm or 3mm depending on rebate

Secondary question: How many windows are you installing?

• 1–5 windows: Order 100-pack (usually sufficient for small projects)
• 5–15 windows: Order 500-pack (cost-effective for medium work)
• 15+ windows: Contact GSD for bulk pricing (trade accounts available)

Real-World Installation Scenarios

Scenario 1: Standard UPVC Double-Glazing (New House Build)

You're installing replacement windows in a new build with standard UPVC frames. Rebate depth measures 23mm. Unit weight is approximately 280kg.

Decision: 3mm packers. 23mm rebate falls in the 20–25mm range. Unit weight is well within 3mm capacity. This is the default for 90% of UPVC replacements. Order a 500-pack for four windows.

Scenario 2: Aluminium Frame with Deep Rebate (Commercial Facade)

You're fitting large aluminium-frame units into a commercial office building. Rebate depth measures 31mm. Unit weight is 450kg.

Decision: 4mm or 5mm packers. 31mm rebate exceeds the standard range. Unit weight is significant. Thicker packers spread the load more evenly. Architect specifies 5mm. Order 500-pack minimum, consider 1,000-pack for cost efficiency.

Scenario 3: Timber Sash Window Restoration (Heritage Property)

You're restoring a period sash window in a listed building. The original frame is intact but needs new glazing. Rebate depth is 18mm (narrower than modern frames).

Decision: 1mm or 2mm packers. Original timber frame has shallow rebate. Heritage conservation requires minimal intervention and narrow sealant lines. 100-pack sufficient for small windows.

Scenario 4: Coastal Property in High-Wind Area (Exposed Location)

You're installing UPVC windows in a coastal property subject to exposure conditions. Rebate depth is 26mm. Spec calls for maximum support.

Decision: 4mm or 5mm packers (plus additional packer density). Deep rebate suggests 4mm minimum. Exposed location means maximum support and potentially closer packer spacing (350–400mm instead of standard 400–450mm). 500-pack per frame.

Common Packer Sizing Mistakes (And How to Avoid Them)

Mistake 1: Guessing Size Without Measuring
Problem: "I'll just use 3mm like I always do" — works fine 80% of the time, fails the other 20%.
Fix: Measure every frame. Takes two minutes. Prevents callbacks.

Mistake 2: Using Wrong Packer Width (24mm vs. 28mm vs. 32mm)
Problem: Your packers don't fully support the unit, or they're too wide and squeeze out under pressure.
Fix: Confirm frame dimensions with your supplier. Standard is 28mm wide; specialty frames may vary. Order matching width.

Mistake 3: Over-Packing (Too Many Packers)
Problem: Stuffing packers every 300mm instead of 400mm — seems safer but actually weakens the seal.
Fix: Follow FENSA spacing standard: 400–450mm apart around perimeter.

Mistake 4: Under-Packing (Insufficient Support)
Problem: Skipping packers at corners — frame sags, rattles, or deflects under unit weight.
Fix: Use the formula: (2 × width + 2 × height) ÷ 400 = number of packers needed. Round up.

Mistake 5: Mixing Sizes in Same Installation
Problem: Using 3mm on sides and 4mm on bottom because "I had some left over."
Fix: Use consistent thickness throughout. Mixing creates uneven support and misalignment.

How Many Packers Do You Need?

Professional installers space packers 400–450mm apart around the entire frame perimeter. This ensures even support without over-packing.

Formula: (2 × frame width) + (2 × frame height) ÷ 400 = number of packers

Example Calculations:

• Frame 900 × 1,200mm: Perimeter = 4,200mm → 10.5, round up to 11 packers
• Frame 1,200 × 1,500mm: Perimeter = 5,400mm → 13.5, round up to 14 packers
• Frame 600 × 900mm: Perimeter = 3,000mm → 7.5, round up to 8 packers

Pro Tip: Corners typically get one packer each regardless of spacing rule. This ensures maximum support where stress concentrates.

Pack sizing guide:

• Single windows: 100-pack covers 5–7 installations
• 5–10 windows per project: 500-pack covers one project
• Ongoing work: 1,000-pack or bulk supply

FENSA Installer Best Practice

FENSA compliance requires proper packer selection and installation. Here's what the standard requires:

Packer Spacing: 400–450mm apart, with minimum one packer per corner. All FENSA inspectors check this.

Packer Material: Must be UV-stable plastic or foam — no cardboard, no wood (except heritage timber frames where specified). GSD packers are polypropylene, fully FENSA-compliant.

Documentation: Your installation records should note frame size, packer size and type, number of packers used, and spacing.

Quality Standard: Frame must be level and plumb after packing (tolerance ±2mm over 1 metre). Any deviation means re-packing.

Frequently Asked Questions

Q: Can I use different packer sizes in the same frame?
Not recommended. Mixing sizes creates uneven support and misalignment. Exception: heritage timber windows may use variable thickness for historical accuracy. For standard installations, consistent thickness throughout.

Q: Do I need different sizes for top vs. bottom of the frame?
No. Use the same size all around. The rebate depth is consistent. Consistency is key to a level frame.

Q: What if I can't find the exact size I need?
Standard ranges (1–6mm) cover 99% of UK installations. If your rebate is between standard sizes (e.g., 24mm), round down unless the unit is unusually heavy — then round up. When in doubt, contact GSD technical support.

Q: How do I remove packers if I need to adjust the frame?
Use a thin chisel or scraper. Gently work from one end, sliding the packer out without forcing. If the frame is sealed, carefully cut around the sealant first.

Q: Are recycled packers suitable?
No. Always use new packers. Recycled packers may be compressed, contaminated, or degraded. FENSA requires new material.

Ready to Order?

Choosing the right glazing packer size is straightforward once you understand the system: measure your rebate, reference the size chart, and order accordingly. The vast majority of installations use 3mm or 4mm. Heritage work drops to 1–2mm. Specialist projects go to 5–6mm.

Don't overthink it, but don't guess either. Two minutes of measurement saves hours of problems later.

Browse GSD's complete glazing packer range — all sizes in stock, FENSA-grade, next-day delivery available. We supply professional installers across the UK with competitive bulk pricing.