How Below Grade Polyiso Insulation Can Help You Achieve Net Zero Energy or Passive House Standards

Net Zero / Passive House Building Standards for Lowest Energy Use & Carbon Emissions

If you are looking for a way to reduce your energy consumption and greenhouse gas emissions, you may want to consider building or retrofitting your home to net zero energy or passive house standards. These are two of the most rigorous and ambitious performance standards for buildings in the world, aiming to achieve ultra-low or zero energy use and carbon emissions.

Net zero energy buildings are buildings that produce as much energy as they consume on an annual basis, usually by using renewable energy sources such as solar panels or wind turbines. Passive house buildings are buildings that use very little energy for heating and cooling, usually by using high levels of insulation, airtightness, heat recovery ventilation and passive solar design.

Both net zero energy and passive house standards require careful attention to the building envelope, which is the physical barrier between the indoor and outdoor environment. The building envelope plays a crucial role in controlling heat flow, air flow and moisture flow, affecting the thermal comfort, indoor air quality and durability of the building.

One of the most important components of the building envelope is below grade insulation, which is insulation that is installed below ground level on the exterior facing side of the building. Below grade insulation can help prevent heat loss through the foundation and slab, prevent condensation and moisture infiltration from the soil, and protect the building from frost heave and soil pressure.

One of the best solutions for below grade insulation is polyiso insulation. Polyiso insulation is a rigid foam board that consists of a closed-cell polyisocyanurate core sandwiched between two protective facers. Polyiso insulation is commonly used on commercial and residential roofs and walls because of its high R-value per inch of thickness, its capacity to serve as a vapor retarder, low water absorption, high compressive strength and durability, and ease of handling.

But did you know that polyiso insulation can also be used in below grade applications? Depending on the facer type used, polyiso insulation can be applied on the exterior side of basement or foundation walls and under concrete slabs on grade to help you achieve net zero energy or passive house standards. In this article, we will discuss the main features and benefits of polyiso insulation below grade and provide some tips and examples for its installation.

Polyiso Insulation for Exterior Below Grade Walls

One of the most common applications of below grade insulation is foam boards on the outside of waterproofed basement walls. This can help reduce heat loss through the foundation and prevent condensation on the interior side of the wall. Polyiso insulation is an ideal choice for this application because of its high thermal performance, water resistance, compressive strength and durability.

Polyiso insulation can be installed on the exterior side of basement or foundation walls using either aluminum foil-faced or coated glass-faced boards. These facers are non-absorbing and suitable for moist environments. Aluminum foil-faced polyiso can be applied directly against the soil, while coated glass-faced polyiso requires a protective membrane or coating to prevent degradation from ultraviolet rays.

Polyiso insulation helps achieve net zero energy or passive house standards by reducing heat loss, preventing condensation, protecting the waterproofing or damp-proofing and eliminating thermal bridging. By doing so, it can:

• Create a continuous thermal barrier that reduces heat loss through the foundation and improves energy efficiency
• Keep the wall warm, reducing the potential for condensation on the interior surface of the wall and improving indoor air quality
• Resist water absorption, reducing the risk of moisture infiltration through cracks or gaps and improving durability
• Protect the waterproofing or damp-proofing from damage caused by backfill and improving reliability
• Eliminate thermal bridging through studs or other framing members and improving thermal comfort

Some examples of net zero energy or passive house buildings that use polyiso insulation for exterior below grade walls are:

• The Sungazing House in Salt Lake City, Utah: This custom residence used 5.5-inch-thick foil-faced polyiso boards on its exterior below grade walls to achieve an R-value of 13.1
• The Net Zero Energy Home in Ashland, Massachusetts: This single-family home used 2-inch-thick coated glass-faced polyiso boards on its exterior below grade walls to achieve an R-value of 13
• The Passive House in New York City: This multi-family building used 4-inch-thick foil-faced polyiso boards on its exterior below grade walls to achieve an R-value of 26

There are several advantages of insulating the exterior versus the interior side of the wall, such as:

• Preserving usable space inside the basement or foundation
• Providing continuous insulation without thermal bridging through studs or other framing members
• Creating a warm wall solution that minimizes condensation on the interior surface of the wall
• Protecting the waterproofing or damp-proofing from damage caused by backfill
• Eliminating freeze/thaw cycles on structural elements

When installing polyiso insulation below grade, it is important to follow some best practices, such as:

• Following manufacturers’ recommendations and installation instructions
• Ensuring proper drainage and backfilling to prevent water accumulation and soil pressure
• Sealing all joints and edges with compatible tape or sealant to prevent air infiltration and moisture intrusion
• Protecting exposed edges from physical damage

Polyiso Insulation for Under Slab Applications

Another application of below grade insulation is under concrete slabs on grade. This can help improve thermal performance and prevent frost heave in cold climates. Frost heave occurs when water in the soil freezes and expands, causing upward movement of the soil and slab. This can result in cracking, buckling and unevenness of the slab surface.

Polyiso insulation can be used under concrete slabs on grade to provide a thermal barrier and a frost protection layer. Polyiso insulation offers a high R-value per inch when compared to other products, which allows for the use of less material to achieve equivalent performance.

Polyiso insulation helps achieve net zero energy or passive house standards by reducing heat loss, preventing moisture problems and protecting the slab and foundation from frost heave. By doing so, it can:

• Create a continuous thermal barrier that reduces heat loss through the slab and improves energy efficiency
• Keep the slab warm, reducing the potential for condensation and moisture problems and improving indoor air quality
• Provide a frost protection layer that prevents frost penetration into the soil and improves durability
• Resist water absorption, reducing the risk of moisture infiltration through cracks or gaps and improving reliability
• Offer high compressive strength, resisting deformation under heavy loads and improving stability

Some examples of net zero energy or passive house buildings that use polyiso insulation for under slab applications are:

• The Net Zero Energy Home in Ashland, Massachusetts: This single-family home used 2-inch-thick coated glass-faced polyiso boards under its basement slab to achieve an R-value of 13
• The Passive House in New York City: This multi-family building used 4-inch-thick foil-faced polyiso boards under its ground floor slab to achieve an R-value of 26
• The Parking Garage in Denver, Colorado: This four-level parking garage used 2-inch-thick foil-faced polyiso boards under its parking slabs to prevent cracking and buckling due to frost action and thermal stress

The amount and type of insulation required under slab depends on several factors, such as:

• Climate zone: The colder the climate, the more insulation needed to prevent heat loss and frost penetration
• Soil type: The more moisture content and frost susceptibility in the soil, the more insulation needed to prevent frost heave
• Building use: The more occupied or heated the building, the more insulation needed to improve thermal comfort and efficiency
• Design load: The more weight or traffic on the slab, the more compressive strength needed in the insulation<

As you can see, polyiso insulation can provide many benefits for your building below grade. By using polyiso insulation on your exterior below grade walls and under your slabs on grade, you can:

• Eliminate thermal bridging, improving thermal efficiency
• Resist water absorption, reducing the risk of moisture infiltration
• Protect the waterproofing or damp-proofing from damage caused by backfill
• Keep the wall and slab warm, reducing the potential for condensation and frost heave
• Save material and installation costs due to its high R-value and compressive strength

If you are interested in learning more about polyiso insulation and its below grade applications, please contact Green Insulation Group or visit our website for more information. We would be happy to provide you with a quote for your project and answer any questions you may have.