When designing or renovating a home with a crawlspace, two primary options exist under the Montana Energy Code: vented non-heated crawlspaces with floor insulation and unvented heated crawlspaces with insulated foundation walls. Each option has unique design requirements to ensure energy efficiency, durability, and indoor air quality.

1. Vented Non-Heated Crawlspace with Floor Insulation

This design treats the crawlspace as an unconditioned space by insulating the floor above it and providing ventilation to the outdoors. Key construction details include:

• Floor Insulation: Use insulation levels specified in the energy code (e.g., R-30 or REScheck™ standards). Proper air sealing is necessary to prevent air leakage.

• Ventilation:

  • 1 sq. ft. of venting per 150 sq. ft. of crawlspace area.

  • Reduced to 1 sq. ft. per 1,500 sq. ft. if a Class I vapor retarder is installed.

  • Vents must be placed within 3 feet of each corner of the building.

• Sealing and Insulation:

  • Seal plumbing, electrical, and HVAC penetrations through the floor.

  • HVAC ducts must be sealed and insulated to at least R-6.

  • Mechanical system piping carrying liquids outside the range of 55°F–105°F must be insulated to at least R-3.

This option is cost-effective but less efficient in maintaining consistent indoor temperatures and controlling humidity compared to unvented designs.

2. Unvented Heated Crawlspace with Insulated Foundation Walls

An unvented crawlspace is conditioned to remain part of the home’s thermal envelope. This approach offers improved energy efficiency and air quality but requires more attention to detail during construction:

• Foundation Wall Insulation:

  • Insulate crawlspace walls to R-19 (cavity), R-15 (continuous), or REScheck™ standards. Insulation must be permanently attached and cover the entire wall height.

• Vapor Retarder:

  • Cover exposed earth with a Class I vapor retarder.

  • Overlap joints by 6 inches, seal or tape them, and extend the edges 6 inches up the foundation wall. Secure and seal the retarder to the wall.

• Airflow Requirements:

  • Continuous airflow must be provided via one of two options:

    • Exhaust Air: Exhaust fan rated at 1 cfm per 50 sq. ft. of crawlspace.

    • Supply Air: Deliver 1 cfm of airflow per 50 sq. ft. through a heat recovery ventilator, HVAC system, or supplemental fan.

  • Include an air pathway (e.g., duct or transfer grille) between the crawlspace and common areas.

This design also supports radon mitigation systems if needed, although radon mitigation is not mandated by the energy code.