Sub-slab vapor barriers are essential to prevent moisture migration into buildings and form the first line of defense against soil gases like radon. This article outlines proper installation, product selection, and detailing strategies for high-performance sub-slab systems.
Purpose of Sub-Slab Vapor Barriers
A properly detailed vapor barrier blocks ground moisture and radon entry into the conditioned space. Barriers also protect flooring materials, reduce humidity loads on HVAC, and comply with building code requirements for moisture protection (e.g., IRC R506, IBC 1907).
Installation Guidelines
- Place vapor barrier directly under slab with minimal damage during rebar placement
- Overlap seams a minimum of 6 inches and seal with compatible tape or mastic
- Extend membrane to perimeter wall and seal to foundation waterproofing
- Protect barrier during pour with reinforcing fabric or sand layer where necessary
Radon Control Strategies
Install a passive or active sub-slab depressurization system per ANSI/AARST standards. Include vent pipe(s) in the gravel base layer and connect through the roof. Seal all slab penetrations and pipe joints. Where radon is expected to exceed 4 pCi/L, activate the fan and label the system per EPA guidance.
Quality Assurance
Inspect vapor barrier before concrete placement for tears, punctures, or gaps. Perform smoke testing or pressure testing for radon system functionality. Record seam overlaps and photograph all sealed penetrations. For LEED or Passive House certification, use a 10-perm or lower rated barrier with manufacturer documentation.
Disclosure: As an Amazon Associate, EnvelopeIQ earns from qualifying purchases.
Interface with Foundation Insulation
Where rigid foam insulation is installed vertically at the slab perimeter, ensure the vapor barrier overlaps behind or under the insulation. Use compatible construction adhesive or tape to fasten the barrier to the foam board. This maintains continuity with above-grade air/vapor barriers and eliminates moisture migration pathways at the slab edge.
Slab Penetration Sealing Details
Seal all pipe, conduit, and sleeve penetrations through the vapor barrier using flexible boots, sealant collars, or pre-molded flashing panels. Avoid cutting tight holes that stress the barrier. For grouped penetrations, install a membrane patch over and seal using mastic or pressure tape. Use compatible sealants to maintain integrity over time and during concrete pour expansion.
Expanded Radon Control Integration
- Install a 4″ perforated pipe loop in the gravel base layer before placing the barrier
- Use a vertical vent riser through conditioned space to reduce condensation risk
- Label the riser and electrical junction box for future fan activation
- Confirm system functionality with a digital manometer after fan startup
Comply with ANSI/AARST CC-1000 or EPA Building Radon Out protocols depending on project jurisdiction and size.
