💧 Zero Water Absorption
Even when wet, it absorbs no water, so it never loses its thermal properties (unlike rock wool which, if not properly ventilated, can become sodden and fail).
Placing Extruded Polystyrene (XPS) Between the Bricks - The Modern Cavity Wall
If you (or your architect) decide to build a new home using the cavity wall method, things have changed dramatically since the 1980s. The "plain white polystyrene" that masons used to toss loosely between the bricks has been replaced by strict specifications.
The definitive material dominating cavity core insulation today is Extruded Polystyrene (XPS) - the familiar "blue", "green" or "pink" rigid boards. Let's see why this material is the only option for the cavity wall and what the golden rules for correct construction are.
1. Why Exclusively XPS in the Core: Two "Superpowers"
The space between the two brick leaves (the cavity) is a dark and potentially damp environment. The outer brick, when wet, absorbs water. That water can pass into the cavity as vapour or droplets. XPS is chosen because it has a closed-cell structure, giving it two "superpowers" essential for the cavity:
🪨 Enormous Mechanical Strength
It doesn't crumble, doesn't break easily during construction and doesn't "settle" over the years. Ideal for the rough conditions of a building site.
2. The Golden Rule of Construction (Step-by-Step)
Success doesn't lie only in the material but in the sequence of construction. The correct team must follow this process:
1️⃣ Build the Inner Leaf
First, the inner masonry leaf (the bricks facing the room) is raised.
2️⃣ Bond the XPS
The XPS boards are not placed loosely. They must be bonded tightly to the inner leaf. Joints must interlock perfectly (tongue-and-groove) leaving no gaps.
3️⃣ Install Wall Ties
Because both leaves are thin, they must be "tied" together for seismic resistance. Stainless steel wall ties (hooks) are used that pierce through the XPS and connect both leaves.
4️⃣ The Air Gap (Critical!)
Between the XPS and the outer leaf to be built, an air gap of at least 2-3cm must remain.
5️⃣ Build the Outer Leaf
Finally, the outer leaf is built, taking care that it does not touch the insulation.
3. The 2 Most Destructive Builder Mistakes
If you're supervising your build, watch out for these two very common "crimes":
❌ Mistake #1: Mortar Debris in the Cavity
As the mason applies mortar to build the outer leaf, chunks often fall into the air gap. If this mortar builds up and bridges the outer brick to the inner XPS, it creates a "moisture bridge". Rainwater will pass through the mortar directly to your inner wall.
❌ Mistake #2: XPS Left Loose in the Middle
If the XPS board is not properly bonded to the inner leaf but left floating in the middle of the cavity, air currents form on both sides of the insulation. This effectively cancels 50% of the material's performance!
4. The 10x10 Model Experiment
Let's take the 10x10 Model and build its walls correctly: Inner brick + 7cm XPS (bonded internally) + 3cm air gap + Outer brick.
📊 Result
The masonry U-Value reaches an excellent 0.38 W/m²K.
☀️ Summer Cooling Champion
The air gap acts as an extra "cushion" of protection. In summer, the heat absorbed by the outer brick is expelled through the ventilated cavity before it even warms the XPS. This makes the correctly built cavity wall the ultimate champion for summer coolness.
Related Articles
Cavity Wall Core Insulation: How It Works & Advantages Thermal Bridges in Cavity Walls: Insulating Columns & Ring Beams Cavity Wall Moisture Management: Weep Holes & Ventilation Insufflation Method: Insulating Existing Wall Cavities
Insulation Systems: Complete Solutions for Every Building
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