Crawlspace Insulation

March 17, 2021
by Charlie Priest

Crawlspaces should be designed and constructed as mini-basements.  Crawlspaces should not be vented to the exterior (see FAQ on Crawlspace Venting). They should have their floors uninsulated, the ground vapor sealed, their walls insulated and air sealed, and their air conditioned with indoor air.

Traditional vented crawlspaces are often damp, mouldy and inhabited with pests. They have almost universally been found to be well connected to indoor air through many small unintentional air leaks in the floors, partitions, and ducts.   Therefore, to ensure both durability and indoor air quality (and save energy), a crawlspace must be kept dry, conditioned to control temperature and humidity, and sealed tight to be pest free.  This is particularly important for crawlspaces that contain mechanical equipment—a situation that is practically guaranteed in buildings that have a crawlspace. Mechanical systems should not be installed outside of a home in unconditioned space unless there is no practical alternative.

Crawlspaces should have a continuous sealed groundcover of vapor diffusion resistant materials, such as taped polyethylene or, preferably, a thin poured concrete slab over polyethylene with perimeter and control joints sealed. When the crawlspace ground level is below the ground level of the surrounding grade they should have perimeter drainage just like a basement. There must be good drainage away from crawlspaces (refer to “Groundwater Control”). Crawlspace design and construction should also provide drainage for potential plumbing leaks or flooding incidents by sloping the ground level to a drain.
Crawlspaces should be insulated on their perimeters — they should not be insulated between floors. Crawlspaces insulated on the perimeter are warmer and drier than crawlspaces insulated between the crawlspace and the house.

Crawlspace walls should be insulated with non-water sensitive insulation that prevents interior air from contacting cold basement surfaces—the concrete structural elements and the rim joist framing. Allowing interior air (that is usually full of moisture, especially in the humid summer months) to touch cold surfaces will cause condensation and wetting, rather than the desired drying. The structural elements of below grade walls are cold (concrete is in direct contact with the ground)—especially when insulated on the interior.  Of particular concern are rim joist areas—which are cold not only during the summer but also during the winter.  This is why it is important that interior insulation assemblies be constructed as airtight as possible.

The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004). The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation.

Up to two inches of unfaced extruded polystyrene (R-10), four inches of unfaced expanded polystyrene (R-15), three inches of closed cell medium density spray polyurethane foam (R-18) and ten inches of open cell low density spray foam (R-35) meet these permeability requirements.
In crawlspaces where the insulation material will need to be covered by a fire/ignition barrier, it may be acceptable to use fire-rated foil-faced insulations. However, such requires careful attention to supplemental moisture management strategies. With vapor impermeable facings on interior insulation, it is possible that water may accumulate between the insulation facing and the inside surface of the foundation wall. The airtightness of the assembly is, therefore, extremely important to prevent the exchange of air between this damp interface and anywhere else in the building.

Consult the resources listed below for specific guidance.
In cold climates or in very low-energy buildings, installing insulation below the ground cover is recommended. When a slab is poured over the ground in this application the temperature and humidity conditions in the crawlspace become very stable and essentially identical to the interior.
To remove any small incidental sources of moisture, it is important that some air circulate from the living space into the crawlspace. It is for this reason the approach is called a “conditioned crawlspace” not a “unvented crawlspace.” Flows of 50 cfm per 1000 sf when the mechanical system operates are recommended.

In all cases, a capillary break should be installed on the top of the footing between the footing and the perimeter foundation wall to control “rising damp.” It is also necessary to install a capillary break between the foundation wall and framing.

If it is not possible to treat the crawlspace as a part of the house such as in flood zones in costal areas or in dry climates where it is not necessary, it is important to construct the house such that the crawlspace is isolated from the house—outside of the building boundary. These situations should follow recommendations for homes built on piers.


Crawlspace Insulation Details

Figure 1

Rigid Insulation Wraps Concrete Surfaces

Figure_01

  • Cold concrete foundation wall must be protected from interior moisture-laden air in summer and winter
  • Rim joist assembly must be insulated with air impermeable insulation or insulated on the exterior
  • Rigid insulation completely wraps exposed concrete preventing interior air from contacting potential concrete condensing surface
  • Seams in rigid insulation and joints to other materials sealed to provide air barrier
  •   Rigid insulation is vapor semi-impermeable or vapor semi-permeable (foil facing or plastic facing not present)
  • Rigid insulation provides bond break between foundation wall and slab when insulation is installed before slab is poured

Figure 2

Interior Rim Joist

Figure_02

  • Air barrier needed to protect the rim joist from interior moisture-laden air in summer and winter
  • Seams in rigid insulation and joints to other materials sealed to provide air barrier

Figure 3

Rigid Insulation with Supplemental Insulated Frame Wall Assembly

Figure_03

  • Rigid foam insulation board assembly must provide continuous air barrier and capillary break around concrete foundation
  •  Rigid insulation is vapor semi-impermeable or vapor semi-permeable (foil facing or plastic facing not present)
  • Protective membrane adhered to top of foundation wall and wrapped over top of insulation
  • Insulation material non-moisture sensitive and not subject to degradation with ground contact

Why Homeowners Often Avoid Inspections

Home inspections are a critical part of the home buying and selling process, providing valuable insights into a property's condition and potential issues. Some homeowners may choose to avoid inspections for various reasons. By understanding these factors, both buyers...

The Importance of Using a Line Voltage Indicator (Wiggy) in Home Inspections

When it comes to home inspections, ensuring safety is paramount. Among the many tools and devices utilized during the inspection process, the line voltage indicator, commonly known as a "Wiggy," plays a crucial role. Tennessee Inspection Services recognizes the...

Pinless vs. Pinned Moisture Meters: Choosing the Right Tool for Effective Home Inspections by Tennessee Inspection Services

During a home inspection, moisture meters play a vital role in identifying potential issues and evaluating the condition of a property. Tennessee Inspection Services understands the significance of moisture meters and utilizes them to enhance the accuracy of their...

Types of Pests That You Need to Keep Out of Your Home

Keeping pests out of your home is essential for maintaining a safe and comfortable living environment. Certain pests pose significant risks, ranging from health concerns to structural damage. Understanding the types of pests that can infiltrate your home and taking...

Pests To Be On The Lookout For When Buying Or Selling A Home

In the realm of home buying and selling, one of the major concerns is pests. These tiny invaders can cause significant damage to the property, degrade its value, and even create health risks. Comprehensive pest inspections are a crucial step in safeguarding your...

The Importance of Using Insulated Screwdrivers During Home Inspections

When it comes to conducting a thorough and safe home inspection, attention to detail is crucial. As a professional home inspector at Tennessee Inspection Services, we understand the importance of using the right tools for the job. In this blog post, we will discuss...

What You Can Do About Fire Risk in Your Home

We offer certified inspections for residential and commercial properties to both buyers and sellers. We emphasize the significance of fire safety in your home and provide measures to decrease fire risk and create a safer living environment. Ensure Electrical Safety...

The Consequences of Having Small Gaps in Your Roof

The integrity of your roof is critical for the overall protection of your home. Small gaps may seem insignificant but can lead to significant problems over time, including water leaks, pest infestations, and energy inefficiency. Understanding these potential issues...

The Crucial Role of Gas Leak Detectors in Home Inspections

Home inspections play a vital role in ensuring the safety and well-being of homeowners. Among the numerous potential hazards that need to be assessed, gas leaks pose a significant threat. Gas leak detectors are indispensable tools for Tennessee Inspection Services and...

Demystifying Outlet Testers: Understanding Their Role in Home Inspections

When it comes to ensuring the safety and functionality of your home's electrical system, outlet testers play a vital role. During home inspections, these simple yet powerful devices are used to assess the condition of electrical outlets and detect potential issues. In...

Read Also

Let’s Talk About Your Inspection

Send Us an Email

charlie@tninspectionservices.com

Call Us

731-207-0601

Contact Us