Below-grade exterior assemblies are any walls and slabs in contact with or extending below the ground surface, such as basement walls and  slabs, where they are subject to various structural and environmental loads. Below-grade exterior assemblies, particularly the insulation within these assemblies,  can have a critical impact on the durability and comfort of basements and other below-grade spaces. 

Designing for a below grade assembly requires unique product performance  characteristics. Whether insulating slab-on-grade, or on the foundation wall,  stone wool insulation provides the required durability, moisture resistance and long  term thermal performance needed for your project.

Below Grade Insulation Applications

Why insulate below grade?

Below grade applications are components of the full building envelope and can provide a path for heat to travel if not properly insulated.

Incorporating insulation as part of a complete below grade assembly can provide: 

  • Improved thermal performance and below grade indoor comfort
  • Increased moisture control and reduced interior condensation risk
  • Protection of concrete from freeze-thaw cycling
Below grade foundation wall installation
Comfortboard 80
Simcoe, Ontario

Stone Wool in Below Grade Applications

  • Thermal Efficiency – stone wool provides consistent, long term thermal performance, reducing heat transfer through below grade walls and floors, even with the presence of rainwater, snowmelt, or freezing conditions
  • Moisture resistance - stone wool resists the absorption of water, and can be used to separate below-grade walls and slabs from the soil to reduce water loads on the structure and reduce condensation risk 
  • Vapor Permeable - stone wool will not trap moisture, and will allow assemblies to drain and dry out. The product is inorganic, and will not promote the growth of mold, mildew or fungi
  • Compressive strength – stone wool is able to resist soil and structural loads, and maintains its structural integrity in backfill applications and below concrete slabs  
  • Sustainability – Manufactured from one of the world’s most abundant raw materials, stone wool maintains its performance over the lifespan of the building

Slab-on-grade Insulated Assemblies

Insulated slab-on-grade assemblies can improve the thermal performance of the
enclosure and improve occupant comfort by keeping the floor surface warmer. They can also be paired with a variety of heated flooring systems to improve energy efficiency and reduce the sizing of heated flooring systems.

Without slab insulation, the direct contact from the floor to the ground makes it an easy route for winter cold to get in.

ROCKWOOL Comfortboard® 80 has been ICC-ES certified for use in slab-on-grade insulation applications. View the report for more information. 

Nick Schiffer custom home builder from NS Builders in Massachusetts

Exterior-Insulated Foundation Wall

Protecting your foundation wall from heat transfer and moisture can be critical to the long term performance of the foundation wall and the full building envelope. Incorporating stone wool insulation onto the exterior side of the foundation wall is a proven and viable solution to protect and strengthen the assembly.

Exterior-insulated wall assemblies are best suited to new construction projects  since they require access to the exterior of the foundation wall.

Stone wool insulation can provide the required thermal and moisture control required for many exterior insulated assemblies and can be installed with a number of membrane types and materials

Below grade foundation wall installation
Comfortboard 80
Simcoe, Ontario

Installing stone wool insualtion on the exterior side of a foundation wall will provide:

  • Consistent thermal performance, even when exposed to moisture
  • Reistance to moisture absorption, bulk water will largely drain away and not be absorbed into the insulation
  • Inorganic material to avoid rot, corrosion, or mold or fungi growth

ROCKWOOL stone wool insualtion can be used throughout the building envelope as continuous insulation. Find out more about available products below.

Interior-Insulated Foundation - Basement Wall

Insulating basement walls from the inside of the foundation wall is a quick and easy way to improve the comfort and performance, turning the basement into usable living space.

Interior-insulated wall assemblies are often the best and most practical solution for existing basement finishing, renovation, and retrofit projects.

ROCKWOOL offers two stone wool insulation products that can be used in a basement wall assembly. ROCKWOOL Comfortboard® rigid stone wool board can be installed directly against the basement wall to provide a continuous layer of insulation, and ROCKWOOL Comfortbatt® is installed in the stud cavity.

Nick Schiffer custom home builder from NS Builders in Massachusetts

Frequently Asked Questions

ROCKWOOL Comfortboard®

Comfortboard is a rigid, high-density stone wool board designed for use in below grade applications, including under slab and foundation wall insulation applications. Comfortboard offers improved energy efficiency with minimal penetrations to reduce thermal bridging, increased fire resistance, enhanced acoustical comfort, along with vapor permeance and moisture resistance for a durable enclosure.


Available as either Comfortboard 80 or Comfortboard 110 to meet specific structural requirements, these products may also be used above grade as an exterior continuous insulation as a screw through insulation cladding support system, and for sloped roofing applications.



Below grade foundation wall installation
Comfortboard 80
Simcoe, Ontario

ROCKWOOL Comfortbatt®

Comfortbatt is a semi-rigid stone wool thermal batt insulation used for stud cavity applications in above and below grade assemblies. Noncombustible and fire resistant, Comfortbatt will not develop toxic smoke or promote flame spread, even when exposed directly to a fire. It also offers water and moisture resistance and excellent sound absorbency, providing an effective way to improve a home’s efficiency and comfort. 


Comfortbatt features a unique flexible edge that allows for easy installation and helps prevent over compression to maintain its R-value. Simply insert, compress and release the batt into the stud cavity to allow it to completely fill the space and maximizing performance.

Nick Schiffer custom home builder from NS Builders in Massachusetts

Looking for answers? Let us help!