Introduction
A capillary break under slabs is a layer or treatment that stops water from wicking from the soil into the concrete. In plain terms, it keeps moisture away from the slab to prevent dampness and cracking. Choose a method and materials that fit your climate and slab design.
Foam-glass options can be used as a capillary break under slabs, but you should verify material compatibility with your project. Edge details matter for an effective break, including proper flashing, backer, and a clean joint at the wall edge. Check product labels and local building rules for installation steps, and tailor methods to your foundation type.
Key takeaways
- Capillary break stops rising moisture under slabs, protecting insulation and finish floors.
- Foam-glass offers cellular barrier options under slabs; verify manufacturer’s guidance.
- Edge details at foundation walls and sill plates prevent capillary ascent effectively.
- Installing foam-glass step-by-step best practices—check labels and local rules.
- Follow safety QA, avoid gaps, consult GBA Detail Library and become GBA Prime.
- Explore Related Questions, log in or create an account to post an answer.
Table of Contents
- Introduction
- Key takeaways
- Why Capillary Breaks Matter for Slabs
- Capillary-Break Material Options — Overview and Selection Criteria
- Foam-Glass (Cellular Glass) Under Slab — Types and Performance
- Edge Details That Reliably Stop Capillary Rise
- Installing Foam-Glass Under Slabs — Step-by-Step Best Practices
- Comparative Performance, Durability, and Lifecycle Considerations
- Safety, Quality Assurance, and Common Installation Mistakes
- Planning Checklist, Tools, and Material Specs
- Conclusion
- FAQ
Why Capillary Breaks Matter for Slabs
A capillary break stops water from wicking up from the subgrade into the slab. It works in concert with foam-glass and other insulating breaks to disrupt capillary suction at the edge. The break sits near the edge details and vapor retarders to maintain a continuous moisture-management detail.
Moisture problems to prevent include mold growth, efflorescence on finishes, and adhesive or flooring failures. It also helps reduce risks of thermal degradation or insulation delamination in slab-on-grade and thickened-edge configurations. This detail fits into a broader strategy that coordinates drainage, grading, and below-slab moisture control while remaining accessible at interior penetrations.
What a capillary break does
A capillary break is like a barrier that stops moisture from wicking up into your slab. It works by interrupting the tiny tubes of water that form in porous materials, like concrete.
Here’s how it works: Water naturally moves upwards through these tiny tubes due to surface tension and adhesion. A capillary break sits between the subgrade and your slab, breaking this upward path.
It’s like a dam in a river, stopping the flow of water into your slab.
When you need a capillary break
A capillary break is crucial when there’s a risk of moisture migrating up into your slab. Here are some triggers:
High groundwater levels, fine-grained subgrades, and sensitive floor finishes like hardwood or tile all call for a capillary break.
Radiant heat systems also benefit from one. They can’t function properly if the slab is damp, and moisture can cause delamination of the insulation.
Project conditions might change your design approach. For instance, thickened-edge slabs need different details than standard slab-on-grade configurations.
Capillary-Break Material Options — Overview and Selection Criteria
Define and compare the main capillary-break material families, including granular stone, geotextiles, membranes, cellular/foam glass, and recycled glass aggregates. Note typical use cases and limitations for slab-on-grade assemblies. Each option behaves differently under load and moisture exposure.
Selection criteria include drainage capability, compressive strength, moisture storage, compressibility, chemical stability, and compatibility with vapor barriers and insulation layers. Consider climate, soil conditions, and whether the break is needed above or below insulation. Think about interaction with edge details and pour sequence for best results.
Granular options (crushed stone, coarse sand)
Granular materials like crushed stone and coarse sand work by creating a capillary break and promoting drainage. They’re graded to allow water to pass through while preventing moisture from wicking up into your structure.
Ideal particle gradation is crucial here. You want a mix that allows for good compaction, with no large voids or excessive fines. Check the product label or ask the supplier for gradation details.
Pros of granular options include their durability and low cost. They’re also easy to compact, which helps create a solid base. Cons? They can be heavy and messy to handle. Lightweight alternatives might be easier to work with, but they often don’t perform as well in the long run.
Remember, no matter what you choose, always check base compaction before laying your slab or wall. A poor base will lead to a poor final product, every time.
Membranes and geotextiles
Sheet membranes and geotextiles are your go-to for stopping water from climbing up your walls. They’re not as heavy or messy as granular options, but they’ve got their own tricks.
Membranes: These are like a plastic blanket that stops water dead in its tracks. You’ll want them when you’ve got a solid base and no chance of water seeping through. They’re great for stopping water from getting into your walls, but they won’t let water out if it’s already there.
Geotextiles: These are like a tough, breathable fabric that lets water out while keeping dirt in. They’re perfect for drainage layers and separating different materials. But remember, they won’t stop water from coming up your wall on their own.
Installing them is easy enough. Just lay ’em down flat, make sure there’s no gaps or tears, and secure the edges with staples or nails. Check your manufacturer’s instructions for specifics. And always check base compaction before you start – you don’t want any surprises later.
Engineered options (foam glass / cellular glass, recycled glass aggregate)
Foam-glass aggregate is a top-notch capillary break material. It’s lightweight and won’t soak up water like some other stuff.
Advantages: It insulates better than most, it’s strong – can take a beating without crumbling, and it lasts. No rot or rust to worry about here.
Recycled glass aggregate is another option. It’s eco-friendly, but not as light or insulating as foam-glass. Check the label for specifics on what you’re buying.
Foam-Glass (Cellular Glass) Under Slab — Types and Performance
The foam-glass product family includes blocks, boards, and loose-fill forms. Each type suits different edge conditions and backfill strategies under slabs. Identify the form that aligns with your project layout and handling needs.
Important properties to check are bulk density, compressive strength, water absorption, and particle size distribution. These factors affect moisture control, load transmission, and insulation performance. Use supplier datasheets and certification marks to verify claims and compatibility with concrete and edge details.
Product types and sizing
Cellular glass, also known as foam-glass, comes in various forms for under-slab applications. Each has its use:
Loose aggregate is best for small slabs or where flexibility is needed. It’s poured and leveled like gravel.
Graded fill is compacted, providing better stability for larger slabs. It’s ideal when precise leveling is crucial.
Rigid blocks/panels offer high compressive strength and are perfect for heavy loads or where minimal settling is vital. They’re laid like large interlocking bricks.
Key performance characteristics to verify
Before buying, check these lab and manufacturer data:
Compressive strength ensures the product can bear slab loads without crushing. Look for values around 100-200 psi.
Permeability/drainage behavior helps manage moisture. Check if it’s free-draining or not, and consider its permeability coefficient.
Water absorption should be low (less than 5%) to prevent capillary action. High absorption can lead to slab issues.
Thermal resistance is important for heated slabs. Look for R-values similar to or higher than the slab’s insulation.
Long-term stability ensures the product won’t degrade over time, affecting slab performance.
Edge Details That Reliably Stop Capillary Rise
Capillary-break requirements differ for monolithic slabs, thickened-edge slabs, grade beams, and inverted thickened-edge slabs. The break must tie into the foundation wall and slab edge to block moisture and curb cold bridging. Each configuration demands careful coordination with edges and insulation.
Materials and thickness are chosen for compatibility with concrete pours, reinforcement, drainage, and edge forms. Ensure continuity with vapor barriers and air barriers at the edge for a robust moisture-control detail.
Typical perimeter detail for monolithic slab
The capillary break under a monolithic slab should tie into the foundation wall and slab edge to prevent moisture and cold bridging. Start by placing a continuous layer of rigid foam or foam glass (1-2 inches thick) along the entire perimeter, directly on top of the vapor barrier.
Ensure the foam extends from the exterior face of the foundation wall up to the bottom of the slab. This creates a thermal and moisture break at the wall-edge transition. Use edge forms that allow for proper placement and support of the capillary break material.
Do not place reinforcing steel on top of the capillary break. Instead, position it above the foam to maintain the break’s continuity and effectiveness.
Thickened edge, grade beam, and inverted-edge considerations
For thickened edges or grade beams, the capillary break must remain continuous and effective. Use foam glass or rigid foam that matches the thickness of the slab or beam.
In the case of an inverted thickened edge, place the capillary break material on top of the reinforcing steel before pouring concrete. This ensures the break extends throughout the entire thickness of the slab.
Do not reduce the thickness of the capillary break to accommodate reinforcement or other elements. Maintain its full thickness for optimal performance.
Connections to vertical elements and flashings
The capillary break must be properly sealed and flashed where it meets foundation walls, damp-proofing, and above-grade transitions. Before installing the capillary break, apply a continuous bead of sealant along the top edge of the foundation wall.
Press the capillary break firmly against this bead to create a tight seal. For above-grade transitions, use flashing tape or liquid flashing membrane to connect the capillary break to the exterior cladding or water-resistive barrier.
Do not rely solely on paint or caulk for sealing. Use durable, long-lasting materials designed for flashing and sealing applications.
Installing Foam-Glass Under Slabs — Step-by-Step Best Practices
Subgrade prep should establish a reliable baseline for drainage and moisture control. Remove organics and debris so foam-glass settles evenly and remains stable. Check moisture conditions before laying the separation layer.
Lay foam-glass boards tightly with minimal gaps and trim to fit edges without compressing boards. Coordinate placement with the vapor retarder and insulation, maintaining continuity at edges and penetrations. Protect boards from damage prior to pour and avoid traffic on the surface.
Step-by-Step Installation Process
The following is a clear, numbered sequence of practical steps for installing foam-glass under slabs. This will ensure a successful DIY job and protect your concrete from moisture issues.
- Prepare the site: Check safety, remove debris, and set up any necessary equipment.
- Verify subgrade: Ensure it’s level, dry, and compacted according to specs (see Site prep and subgrade verification).
- Lay foam-glass: Place boards in a tight, butt-jointed pattern with minimal gaps. Trim edges without compressing boards.
- Level and align: Ensure foam-glass is level and aligned with vapor barrier and insulation system (see Integrating with vapor barriers, insulation, and radiant tubing).
- Protect before pour: Keep foam-glass clean, dry, and free from damage until concrete placement.
Site prep and subgrade verification
Before placing any capillary-break material, it’s crucial to verify the subgrade. This ensures a solid foundation for your foam-glass and concrete slab.
Check drainage: Ensure water can drain away from the site to prevent standing water. If necessary, install French drains or other drainage solutions.
Remove organics/debris: Clear the area of any organic material, vegetation, or debris that could cause settlement or contamination.
Compaction: Compact the soil to at least 90% of its maximum dry density. This prevents settlement and ensures a stable base for your slab.
Placing, leveling, and jointing foam glass
Proper placement of foam-glass is key to its effectiveness. Here’s how to do it right:
Loose vs panelized: For loose fill, spread evenly with a rake or shovel. For panels, lay in a tight, butt-jointed pattern with minimal gaps.
Compaction: While foam-glass doesn’t require compaction like aggregate, ensure it’s level and well-distributed. Use a straight edge to check and adjust as needed.
Joints: Maintain tight joints between panels. If using loose fill, ensure it’s evenly distributed with no voids or low spots.
Integrating with vapor barriers, insulation, and radiant tubing
Proper integration of foam-glass with other systems ensures a successful slab installation. Here’s how to do it:
Vapor barrier: Ensure the vapor barrier overlaps foam-glass panels by at least 6 inches. Seal any penetrations or gaps.
Insulation: Place insulation on top of foam-glass, maintaining continuity and proper R-value. Use spacers to keep it off the slab for air circulation.
Radiant tubing: If using radiant heating, ensure tubing is properly spaced and supported above the foam-glass. Maintain an air gap between tubing and insulation to prevent heat loss.
Comparative Performance, Durability, and Lifecycle Considerations
Compare foam-glass with traditional capillary breakers like stone, sand, and membranes across how they manage moisture and ease of installation. Consider short-term behavior versus long-term durability and maintenance needs. This helps guide material selection for your project scope.
Evaluate thermal performance, including insulation value and potential for thermal bridging near edges. Assess how each option handles shrinkage, settlement, and exposure to moisture cycles. Lifecycle considerations include expected lifespan and replacement implications to inform decisions.
Moisture-control and drainage effectiveness
When choosing a capillary break under your slab, consider how well it handles moisture. Here’s what you need to know:
Capillary Suction: Foam-glass doesn’t absorb water like stone or sand. It prevents capillary rise effectively due to its closed-cell structure.
Bulk Water: Membranes can handle bulk water well, but they’re susceptible to punctures during installation. Foam-glass and aggregate options are more robust but may allow some water absorption.
Lateral Drainage: Foam-glass doesn’t wick moisture laterally like sand or stone. It allows water to drain through its open structure, preventing ponding. Membranes can also manage lateral drainage if properly sloped and installed.
Structural and thermal durability
Durability under load and insulation are crucial for your slab’s capillary break. Here’s how each option performs:
Load Bearing: Foam-glass has a compressive strength similar to concrete, making it a reliable load-bearing option. Stone and sand can also handle loads but may settle over time.
Creep Risk & Freeze-Thaw Performance: Foam-glass doesn’t creep or degrade under repeated loading or freeze-thaw cycles due to its glass composition. Membranes can degrade over time, while stone and sand may shift or break down.
Insulation Contribution: Foam-glass has a higher R-value than other options, helping maintain slab temperature stability year-round. It also reduces thermal bridging around edges when properly installed.
Cost drivers and how to estimate lifecycle cost
Estimating the total cost of your capillary break involves more than just material prices. Here’s what to consider:
Material Unit Cost: Foam-glass is typically more expensive per square foot than stone, sand, or membranes. However, it may offer long-term savings due to its durability and low maintenance.
Freight & Labor: Freight costs can vary based on your location. Labor costs depend on installer experience and site access. Get local quotes for accurate estimates.
Lifecycle Comparison: Consider the expected lifespan of each option, replacement/repair implications, and potential long-term performance risks. Foam-glass may have a higher upfront cost but could save you money in the long run by reducing maintenance needs.
Safety, Quality Assurance, and Common Installation Mistakes
Follow safety protocols for handling foam-glass and other capillary-break materials, including appropriate PPE and storage to prevent breakage. Maintain good ventilation and awareness of dust or particulates. Be prepared for emergency procedures on site.
Inspection and testing checkpoints help verify substrate readiness, continuous capillary-break depth, and moisture transmission potential. Document material verification, adherence to specs, and corrective actions to support QA. Be aware of common mistakes that cause capillary failure or delays and plan accordingly.
Safety during handling and placement
Wear gloves, goggles, and a respirator when handling foam-glass or other capillary break materials. Dust can be harmful if inhaled.
Keep material storage areas clean and organized to prevent breakage and dust release. Follow manufacturer guidelines for safe storage and handling.
Use proper lifting techniques and equipment to avoid injury. Do not exceed manual handling limits.
QA checks, testing, and as-built documentation
Conduct field checks to ensure capillary break depth, thickness, and continuity. Verify the vapor retarder is intact.
Observe substrate flatness/level before placement. Check moisture and vapor transmission potential.
Document all inspections, tests, and corrective actions. Keep photos and submittals for warranty purposes.
Common mistakes to avoid
Here are some frequent errors that can lead to capillary failures or delays:
- Placing capillary break above vapor barrier: This reverses the intended moisture flow, causing problems.
- Insufficient thickness: Too thin and it won’t provide proper capillary action. Follow manufacturer specs.
- Failing to protect fill before pour: Unprotected fill can contaminate or damage the capillary break.
- Poor edge continuity: Inadequate detailing at slab-perimeter transitions can lead to moisture intrusion.
Avoid these mistakes by following best practices and adhering to design specs.
Planning Checklist, Tools, and Material Specs
Define the pre-construction scope by confirming slab type, climate considerations, and moisture risk to tailor capillary break depth and foam-glass placement. Align plans with project specs and codes. Clarify compliance requirements early.
List must-have tools and consumables, including measuring devices, chalk lines, foam-glass sheets, adhesive, vapor barrier, and safety gear. Request material specs and standards such as foam-glass grades, insulation thickness, and vapor-permeance data from suppliers. Include edge-detail guidance and documentation steps for verification.
Tools and materials checklist
Before you start, make sure you have all the necessary tools and materials to prep your site and place the foam-glass capillary break.
- Tape measure: To ensure accurate measurements for slab dimensions and capillary break placement.
- Level: For checking and maintaining a consistent slab level during installation.
- Chalk line: To mark out slab edges, joint locations, and foam-glass placement.
- Knife or utility blade: For cutting foam-glass sheets to size and trimming excess material.
- Foam-glass sheets: Pre-measured for your slab dimensions to ensure a proper fit.
- Adhesive: To bond the foam-glass to the subgrade, preventing movement during slab placement.
- Vapor barrier membrane: To prevent moisture from entering the slab and causing damage.
- Mechanical fasteners: For securing protection boards and vapor barriers in place.
- Calibration tools (depth gauge, straight edge): To ensure foam-glass is placed at the correct depth and level.
- Safety gear (hard hat, safety glasses, gloves): Always protect yourself when working on-site.
Quick rule: Missing any of these can lead to installation delays or a poor-quality capillary break that fails to protect your slab.
Material specs and standards to request
When ordering foam-glass and related materials, ensure you’re getting the right stuff for your job. Here’s what to ask for:
- Foam-glass product grade: Confirm it meets or exceeds required compressive strength and absorption standards.
- Insulation thickness: Ensure it matches your project’s R-value requirements.
- Capillary break thickness: Verify it aligns with your planned slab depth and expected moisture risk.
- ASTM/CSA/ISO references: Ask for these standards’ numbers to confirm product compliance.
- Vapor barrier permeability: Request a value of 0.1 perm or less to ensure effective moisture protection.
- Drainage composite (if used): Confirm compatibility with foam-glass and edge details.
- Rebar layout consideration: Ensure foam-glass won’t interfere with rebar placement or reinforcement.
- Recyclability/chemical resistance confirmation: Verify these properties to maintain a sustainable, durable capillary break.
Quick rule: Inadequate materials can lead to poor performance and costly repairs down the line.
Visual checkpoints and inspection guide
Regularly inspect your capillary break installation to ensure it’s up to par. Here are quick visual checks to perform:
- Uniform coverage: Check for consistent foam-glass placement across the entire slab area.
- Clean separation from soils: Ensure no soil or debris is trapped between the foam-glass and subgrade, which could lead to moisture issues.
- Continuous laps/seams: Verify that foam-glass sheets are properly overlapped and sealed at joints to prevent gaps.
- Protected edges: Confirm that foam-glass edges are secure and protected from damage during slab placement.
- No visible gaps or voids: Look for any spaces between the foam-glass and subgrade, formwork, or adjacent materials.
Quick rule: Regular inspections help catch issues early, saving time and money in the long run.
Conclusion
Capillary break work protects the slab from moisture wicking, preserves durability, and keeps finishes looking right, so you don’t spend money fixing leaks or redoing coatings after the concrete sets. Tactics from edge detailing to foam-glass under-slab placement are practical, repeatable, and pay off with a long, clean performance.
The plan now is to check the edge detail readiness, confirm foam-glass type and thickness matches the plan, measure and mark the slab area, install the break with clean joints, carry out any required tests or inspections, and follow safety rules at every step while keeping the work area well ventilated and protected. Start with a small test patch if you’re unsure, verify clearances and overlaps, then scale up to the full area using the same sequence, and keep your notes for QA.
Common mistakes to avoid are skipping edge details and not stopping capillary rise at the perimeter, using the wrong material or thickness, and rushing the install without PPE or proper joint sealing. Stay disciplined: wear eye protection and gloves, keep the work area dry during placement, and never cut or compress foam-glass beyond the design tolerances. Safety first means you slow down enough to do it right and prevent costly reversals.
If the slab is large, or if there are complex edge conditions, and you’re unsure about pass/fail tests or material specs, call a professional to confirm the plan before you proceed. Otherwise, stay methodical, follow the plan, and you’ll have a durable, dry slab with clean edges and a finish you won’t have to revisit. You’ve got this—do the work one precise step at a time.
FAQ
What is a capillary break and why does it matter under a slab?
A capillary break stops moisture from wicking up from the soil into the concrete. It helps prevent dampness, mold, and potential damage to insulation and finishes. It’s a simple layer or barrier that keeps capillary action from pulling water into the slab edge.
What are common materials for capillary breaks, and how do foam-glass options fit in?
– Foam-glass (cellular glass) is one option that creates a rigid, moisture-resistant barrier under slabs. Other choices include plastic or rubber sheets and granular materials, but each has its own install method and durability. Read the label or manufacturer instructions to see what works best for your project conditions.
What edge details reliably stop capillary rise?
Install a continuous barrier up the edge where the slab meets the wall or frost-wall. Keep the barrier tight to the formwork and extend it past the edge, avoiding gaps. Use sealant at joints and double-check that no path exists for moisture to bypass the barrier.
How do I implement capillary breaks around foundation walls or sill plates?
Place the barrier so it abuts the foundation wall and sits below the slab edge. Ensure the barrier covers all potential moisture paths and that it remains intact during concrete pour. If you’re using foam-glass, follow the product’s placement guidelines and any required spacers or supports.