English Blog Version (SEO Optimized) Foam Sealant or Waterproof Mortar for Window Frame Gap Filling? Here Is the Final, Code-Based Answer

Water leakage around exterior windows is one of the most common and persistent quality issues in residential construction. The debate over whether to use foam sealant or waterproof mortar for gap filling has lasted for years. However, based on national codes, regional guidelines, and extensive project experience, the conclusion is now clear:

Different locations require different materials — no single solution fits all.

1. Source of the Debate: Early Codes Were Misinterpreted

The earlier code GB50327-2001 stated:

“Cement mortar shall not be used for filling gaps between the window frame and wall; elastic materials shall be used.”

Because this clause has never been updated, many mistakenly believed that mortar is prohibited for all window gap filling.

However, as waterproofing requirements increased, this clause became insufficient for modern construction environments.

2. Updated Codes Clarify That Waterproof Mortar Is Allowed

The current code JGJ214-2010 clearly specifies:

• Gaps may be filled with waterproof mortar

• Or foam sealant

• Sealant finishing is required after filling

This means both materials are compliant and can be used simultaneously.

3. Regional Practices Differ: Climate, Wind Pressure & Rainfall Matter

In typhoon-prone areas such as Guangdong, guidelines commonly require:

“All sides of the window frame should be densely filled with polymer-modified cement mortar or waterproof mortar.”

Reason:

• High wind pressure → foam becomes unstable

• Heavy rainfall → mortar performs better in waterproofing

Thus, regional climate conditions directly influence the recommended materials.

4. Fujian Housing Department Offers the “Best Combination Solution”

Based on years of real project experience, the Fujian Technical Guideline for Residential Waterproofing proposes the most reliable practice:

✔ Bottom + 200 mm up on both sides: Waterproof mortar
✔ Other three sides: Foam + sealant finishing

Material characteristics explained:

• Foam: compresses under pressure; unsuitable for water-collecting areas

• Waterproof mortar: may sag vertically; harder to compact

Therefore, combining both materials is essential for achieving long-term waterproofing performance.

5. With Sub-Frames (Auxiliary Frames): Mortar on All Sides Is Mandatory

Because sub-frames are installed earlier, subsequent construction movement can easily disturb foam filling.
Damaged foam structure → potential leakage points.

Thus:

All four sides must be filled with waterproof mortar when sub-frames are used.

6. Recommended Best Practice for Exterior Window Gap Filling

✔ Bottom: Waterproof mortar only
✔ 200 mm up both sides: Waterproof mortar
✔ Other three sides: Foam sealant + surface sealant
✔ With auxiliary frames: Waterproof mortar on all sides

This is currently the most accepted and lowest-risk solution in the industry.