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200 micron Polyethylene DPM
Professional calculator for polyethylene DPM and moisture barriers
Calculate sheet quantities, overlaps, tape requirements, and costs for vapour barriers in floors, walls, and roofs for 2026 projects.
Calculate moisture protection materials for construction projects
Calculate exact polyethylene sheeting requirements including overlaps, waste factors, and sealing materials. Our calculator accounts for roll widths, area coverage, and seam overlaps to provide precise material takeoffs for slab foundations, subfloors, and wall applications.
Design vapour barriers for concrete slabs (DPM), subfloor moisture protection, wall cavity barriers, crawl space encapsulation, and roof underlayment. Each application has specific thickness and overlap requirements for effective moisture control.
Compliant with AS/NZS building codes and international standards for damp-proof membranes. Proper vapour barrier installation prevents ground moisture, condensation damage, mould growth, and structural deterioration in buildings.
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Vapour barriers, also known as vapour retarders or damp-proof membranes (DPM), are impermeable sheets installed to prevent moisture migration through building elements. Typically made from polyethylene plastic ranging from 100 to 400 microns thickness, vapour barriers protect against ground moisture rising through concrete slabs, condensation forming in wall cavities, and water vapour damaging building materials. The Australian Building Codes Board mandates vapour barrier installation in specific applications to ensure building durability and occupant health.
The effectiveness of vapour barrier systems depends on three critical factors: material thickness and permeability, complete coverage without gaps or tears, and proper sealing of all seams and penetrations. A single small tear or unsealed joint can dramatically reduce barrier performance, allowing moisture to bypass the protection. Installation under concrete slabs, in subfloor areas, and within wall assemblies requires careful attention to overlap distances, sealing methods, and protection from construction damage during subsequent building stages.
Vapour barrier prevents ground moisture rising into concrete slab and building structure
Damp-proof membrane under concrete slabs prevents ground moisture rising into concrete and floor coverings. Minimum 200-micron polyethylene required per Australian standards. Install on compacted sand blinding layer, lap joints minimum 150mm, seal seams with tape. Critical for timber flooring, carpet, and moisture-sensitive finishes.
Suspended timber floors benefit from ground-level vapour barriers reducing moisture evaporation into subfloor space. Use 200-250 micron sheeting, extend up foundation walls 150mm, overlap seams 300mm. Complements subfloor ventilation to control moisture and prevent timber decay.
Wall vapour barriers control moisture movement through external walls in cold climates. Position on warm side of insulation to prevent condensation. Use 200 micron minimum, tape all joints, seal around penetrations. Required in climate zones with significant heating degree-days to protect wall insulation from moisture damage.
Complete crawl space moisture protection uses heavy-duty 250-300 micron barriers covering ground and extending up walls. Sealed system dramatically reduces moisture entry, improving air quality and structural protection. Popular in humid climates and areas with high ground water tables requiring comprehensive moisture management.
Starting point for all vapour barrier calculations
Add material for seam overlaps - typically 150mm standard, 300mm for subfloor applications
Include 10-15% wastage for cutting, fitting, and site waste
Double-sided polyethylene tape for permanent moisture seal at all joints
| Thickness | Applications | Permeability | Cost (2026) |
|---|---|---|---|
| 100 micron | Temporary protection, light duty barriers | Low resistance (not DPM rated) | $0.50 - 0.70/m² |
| 200 micron | Standard DPM under slabs, wall barriers | Meets AS 2870 DPM requirements | $0.80 - 1.20/m² |
| 250 micron | Heavy duty slabs, subfloor barriers | Enhanced puncture resistance | $1.00 - 1.50/m² |
| 300 micron | Commercial slabs, crawl space encapsulation | High puncture and tear resistance | $1.20 - 1.80/m² |
| 400 micron | Industrial floors, aggressive ground conditions | Maximum durability and protection | $1.50 - 2.20/m² |
Proper substrate preparation is essential for vapour barrier integrity. Remove all sharp objects, stones, and debris that could puncture the sheet. Install a 50-75mm sand blinding layer over compacted sub-base, screeded smooth to create uniform support. The sand layer protects the polyethylene from puncture by sub-base aggregate and provides flat surface for even sheet placement. Check for protruding reinforcement bar chairs, formwork ties, or other sharp elements before laying sheeting.
Unroll polyethylene sheets parallel to longest dimension to minimize seam count. Overlap sheets minimum 150mm for standard applications, 300mm for subfloor or high-moisture environments. Stagger end laps from side laps by minimum 1 metre to avoid four-layer intersections. Tape or seal all laps immediately after placement - unsealed laps can separate during subsequent construction activities allowing moisture pathways.
All seams, penetrations, and terminations must be properly sealed to maintain vapour barrier continuity. Use double-sided polyethylene tape rated for permanent moisture barrier applications - standard packaging tape or duct tape degrades and fails. For pipe and column penetrations, cut cross-slits in sheet, fold up around penetration, and seal with mastic or butyl tape. At walls and footings, fold sheet up vertical surface minimum 150mm and seal to structure.
Critical: A vapour barrier with gaps, tears, or unsealed seams provides minimal protection. Even small openings allow significant moisture transmission defeating the purpose of installation.
Damp-proof membranes under concrete slabs require 200-micron minimum polyethylene per AS 2870 and AS 3600 standards. Install DPM directly on sand blinding layer, never directly on compacted aggregate which can puncture the sheet. Overlap all joints 150mm minimum, seal with DPM tape. Turn sheet up at slab edges and seal to edge formwork or footings. Some engineers specify placing DPM between two sand layers (50mm below, 50mm above) for maximum puncture protection.
For suspended slabs, DPM may be placed on underside of formwork to prevent moisture absorption by plywood forms. For slabs-on-ground in high moisture areas or where ground water is present, consider upgraded 250-300 micron sheeting and full seam heat welding for enhanced protection. Properly installed DPM prevents rising damp, protects floor coverings from moisture damage, and reduces concrete curing water loss improving concrete strength.
Ground-level vapour barriers in subfloor areas reduce moisture evaporation by 70-90%, significantly improving timber floor durability and indoor air quality. Use 200-250 micron sheeting, extend minimum 150mm up foundation walls, seal to wall structure with mastic or mechanical fasteners. Overlap joints 300mm and seal with tape. For complete crawl space encapsulation, install continuous barrier across ground and up walls to subfloor level, creating sealed envelope.
Encapsulated crawl spaces maintain lower relative humidity, reduce timber decay risk, eliminate musty odors, and can reduce heating/cooling costs by conditioning the crawl space. Combine with perimeter drainage and optional dehumidification for comprehensive moisture management in humid climates or problem sites. Inspect crawl space barriers annually for damage, tears, or standing water indicating drainage problems.
Wall vapour barriers control moisture movement through external walls in heated/cooled buildings. In cold climates, install barrier on warm (interior) side of insulation to prevent moisture-laden interior air from reaching cold exterior sheathing where it would condense. In hot-humid climates, some building scientists recommend avoiding interior vapour barriers to allow inward drying of wall assemblies.
Cold Climate (heating dominated): Install vapour barrier on interior side of wall insulation. Prevents warm, moist interior air from reaching cold exterior sheathing where condensation would occur. Use 200 micron polyethylene, tape all seams, seal around electrical boxes and penetrations.
Hot-Humid Climate (cooling dominated): Consider vapour-permeable "smart" membranes rather than impermeable polyethylene. Allows walls to dry inward during summer while providing winter vapour control. Consult building scientist for climate-appropriate wall design.
| Project Type | Area | Specification | Total Cost (2026) |
|---|---|---|---|
| Small Slab DPM | 100m² slab | 200µ poly + tape | $120 - 180 |
| Medium House Slab | 180m² slab | 200µ poly + tape | $200 - 300 |
| Large House Slab | 250m² slab | 250µ poly + tape | $300 - 450 |
| Subfloor Barrier | 150m² subfloor | 250µ poly + adhesive | $250 - 400 |
| Crawl Space Encapsulation | 100m² ground + walls | 300µ poly + sealing | $400 - 650 |
| Wall Barriers (House) | 200m² wall area | 200µ poly + tape | $220 - 350 |
Cost Components: Prices include polyethylene sheeting, sealing tape or adhesive, and installation labour. Material costs vary by thickness and roll width. Installation labour ranges $0.50-1.50/m² depending on complexity, access, and site conditions. Add 20-30% for difficult access, complex layouts with many penetrations, or crawl space work requiring confined space entry.
Minimum 200-micron polyethylene is required under concrete slabs per AS 2870 Australian standards. This thickness provides adequate puncture resistance and impermeability to prevent ground moisture rising through the slab. For residential construction, 200 micron is standard. Upgrade to 250-300 micron for commercial slabs, high ground water areas, or when extra durability is needed against construction damage. Industrial applications may specify 400 micron for maximum protection.
Standard overlap is 150mm minimum for concrete slab DPM and wall barriers. Increase to 300mm overlap for subfloor moisture barriers and crawl space applications where higher moisture loads are expected. All overlaps must be sealed with double-sided polyethylene tape or liquid adhesive rated for permanent moisture barriers. Unsealed overlaps dramatically reduce barrier effectiveness. Stagger end laps from side laps by minimum 1 metre to avoid four-layer joints.
Only use polyethylene specifically rated as DPM (damp-proof membrane) or vapour barrier. Standard "builders plastic" or drop sheets may be inadequate thickness, contain UV stabilizers that degrade in ground contact, or have insufficient puncture resistance. Proper DPM polyethylene is manufactured to building standards with controlled thickness (200-400 micron), high puncture resistance, and long-term stability. For building code compliance and effective moisture protection, always specify DPM-rated polyethylene from building suppliers.
Minimum requirement is 50-75mm sand blinding layer below the vapour barrier to protect it from puncture by sharp aggregate in the sub-base. Some engineers specify "sandwich" installation with 50mm sand below and 50mm sand above the DPM for maximum puncture protection. The top sand layer protects during reinforcement placement and concrete pour. For standard residential slabs, sand below only is typically adequate. Use top sand layer for commercial projects, or when site conditions require personnel and equipment traffic over the DPM before concrete placement.
Calculate: (Floor Area × 1.15 for overlaps and waste) / (Roll Width × Roll Length). Example: 150m² slab with 4m wide × 25m long rolls = (150 × 1.15) / (4 × 25) = 173 / 100 = 1.73 rolls, round up to 2 rolls. Standard roll sizes are 4m × 25m (100m²), 4m × 50m (200m²), or custom lengths. Always add 15-20% for overlaps, cutting waste, and site conditions. Order extra rolls rather than running short - partially used rolls are difficult to return.
Use double-sided polyethylene tape specifically designed for DPM sealing. These tapes have aggressive adhesive that bonds permanently to polyethylene and resists moisture degradation. Width should be 50-75mm for adequate seam coverage. DO NOT use standard packing tape, duct tape, or masking tape - these fail in ground moisture conditions. Specialist DPM tapes cost $15-25 per 50m roll but provide essential permanent seal. Apply tape with pressure roller for full adhesion. Alternative sealing methods include liquid polyethylene adhesive or heat welding for large commercial projects.
Depends on your climate zone. In cold climates with significant heating loads, install 200 micron polyethylene on interior (warm) side of insulation to prevent condensation. In hot-humid climates, avoid impermeable interior vapour barriers as they can trap moisture and prevent wall drying. Consider "smart" vapour-variable membranes that adjust permeability with humidity. Mixed climates require careful analysis - consult building scientist or hygrothermal modeling. Wall vapour barrier design is complex and climate-specific, unlike slab DPM which is universally beneficial.
Yes, small tears and punctures can be repaired before concrete placement. Clean area around damage, cut patch from same thickness polyethylene extending 150mm beyond tear in all directions. Apply DPM tape around patch perimeter or use liquid adhesive for full-surface bond. For large tears (over 300mm), replace entire sheet section with proper overlaps to adjacent sheets. Inspect entire barrier carefully before concrete pour - multiple small tears indicate possible widespread damage requiring sheet replacement. Never attempt to repair DPM after concrete placement.
Review NCC requirements for damp-proof membranes and vapour barriers in residential and commercial construction. Includes climate zone specifications and installation standards.
Access NCC →AS 2870 and AS 3600 provide technical specifications for DPM thickness, installation methods, and performance requirements for concrete slab construction.
View Standards →Research-based guidance on vapour barrier design for different climates, wall assemblies, and moisture management strategies from building science experts.
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