Poorly or incorrectly insulated foundation slabs can present several problems for homes, including energy loss, moisture control issues, and indoor air quality challenges. Energy loss through slabs is primarily a result of heat conducted outward through the perimeter of the slab and into the surrounding soil. Moisture can become an issue inside the house if the relative temperature difference between the slab and indoor air temperatures become too great and condensation or high localized relative humidity issues occur. With condensation, mold may have a chance to grow and create indoor air quality issues. While it is common practice to install insulation, specifically rigid insulation, during the construction of the slab, it is often installed incorrectly or incompletely. However, by properly installing insulation that extends to the top of the slab in either a monolithic slab with a grade beam or a slab independent of the foundation design, thermal bridging, moisture, and air quality issues can all be addressed and minimized (Ueno and Lstiburek 2012).
Figure 1 - Properly installed rigid insulation. This image shows monolithic slab construction with properly installed rigid insulation that extends to the top of the slab and provides a complete thermal break.
To address the problems associated with thermal bridging and potential moisture issues that can occur with improperly insulated slab foundation systems, follow the steps below. The strategies for insulating the slab edge depend upon whether the slab on grade is:
- Monolithic with a grade beam
- Slabs independent of the foundation wall
For either design style it is important to review the plan for slab insulation with pest control and local building officials to ensure code compliance. Material selection is also important. Insulation levels should meet or exceed state requirements in accordance with the International Energy Conservation Code (IECC). Finally, only use insulation approved for below-grade use.
Figure 2 - Properly installed vapor barrier. The polyethlene vapor barrier shown above is properly positioned to work in conjunction with the rigid foam insulation that will be installed on the exterior of the slab after the wood forms have been removed.
Monolithic with a Grade Beam
When the slab is monolithic with a grade beam, the insulation must be installed to the exterior of the slab edge/grade beam and continue vertically to the bottom of the grade beam as shown below. Different regions may have different code requirements, so be sure to check the local codes to make sure this meets the requirements. The insulation material must be appropriate for ground contact. XPS, rigid fiberglass, and rock wool are examples of acceptable materials. The exterior insulation will need to be protected from impact damage during construction and, subsequently, the above-grade portion must be protected from UV and impact damage (BSC 2009).
General steps for installation of slab insulation in monolithic with grade beam construction:
- Install rigid insulation on the exterior that extends to the bottom of the grade beam.
- Secure a protection board over the above-grade portion of the rigid insulation.
- Make sure the protection board is of non-water sensitive material and coated to control absorption of water.
- Install a protective membrane adhered to the slab and wrapped over the top of the insulation.
- Ensure that insulation material is non-moisture sensitive and not subject to degradation with ground contact.
Figure 3 - Monolithic slab with a grade beam. The diagram above shows the monolithic slab with rigid insulation properly installed on the exterior of the slab and extending fully to the top of the concrete.
Important notes if insulation is installed on the exterior of the slab (DOE 2000):
Install the insulation from the top of the slab to the bottom of the frost line unless a termite inspection gap is required.
Encapsulate or cover the exterior face of the insulation with a protective membrane to serve as a capillary break and to protect the insulation from termites.
Cover the above-grade portion of the insulation exposed to outside air using a stucco coating, pressure-treated wood, brick, or aluminum flashing. When covering insulation, be conscious of how to detect termites in areas prone to termite infestation. Some states in termite-prone areas have addressed this issue by requiring a termite inspection gap near the top of the slab insulation.
Independent of the Foundation Wall
When the slab is independent from a perimeter foundation wall, insulation may be installed either on the exterior of the foundation wall or between the foundation wall and the slab. In order for the slab to be independent of the foundation wall, a bond break is needed between the slab, which is supported on grade, and the foundation wall that supports the exterior wall structure and its loads. Insulation at the vertical slab edge and under the slab perimeter provides this bond break. The graphic shows an example of a slab on grade that is structurally and thermally isolated from the perimeter foundation wall. Limiting factors on the width of the slab edge insulation in this situation are determined by the attachment of floor finishes and the width of the foundation wall needed to support the wall structure.
Figure 4 - Slab independent of foundation wall. The diagram above shows the proper placement of rigid insulation when the concrete slab is placed independent of the foundation wall.
General steps for installation of slab insulation independent of the foundation wall:
Make sure the slab is insulated vertically at the edge and horizontally at the perimeter or under the entire slab.
Install the rigid insulation to provide a bond break between the slab and foundation wall.
Ensure the protective membrane is adhered to both the slab and top of foundation wall.
Whenever insulation is used in contact with ground or near ground, appropriate insect control measures must be used (BSC 2009).