Basement Insulation – Best Practice To Avoid Mold

By Robert Robillard on Insulation, Remodeling

Basement Insulation – Best Practices

A basement remodel project is one of the most popular choices when choosing a home improvement project.  In fact, you can potentially achieve upward of 70% return on investment (ROI) when done correctly.

The ability to expand your living space while staying within your home’s existing footprint can have huge monetary gains as well as the significant quality of life improvements.  However, one common foe that can be a potential disaster and must be mitigated across all basement living spaces is moisture. To avoid this you need to understand the basement insulation best practices.


Basement Moisture Sources


Moisture in basements can be attributed to multiple sources.

  1. Interior sources such as poorly vented dryer vents, water leaks or humidifiers
  2. Exterior liquid water such as groundwater or poor drainage leading to rainwater propagating through porous surfaces and cracks
  3. Exterior water vapor entering through air movement or propagating through permeable materials

Interior Water

Interior sources of water are the easiest to identify and eliminate.  When renovating a previously unfinished basement into a living space, great care must be taken to identify potential sources of interior water and to mitigate the risks.  Any plumbing that introduces water into space needs to be secure and well ventilated.  This includes sinks, bathrooms, and clothing washers/dryers.   New construction concrete is a significant source of moisture and may take years to completely reach equilibrium depending on climate and conditions.  Even people and their exothermic bodies are a significant source of moisture.  Consider dehumidifiers or air conditioners as potential mitigation to negate these sources of moisture.

Exterior Liquid Water

Exterior liquid water is a common problem that needs to be addressed before any renovation steps are taken.  Any cracks or obvious entry points for liquid water need to be sealed and repaired.  If the basement floor is below the fluctuating water table, a sump pump will be a necessity. Lastly, the most common source and sometimes easiest to mitigate is to look outside see how the home physically forces water to flow around it.  Improper landscaping, lack of gutters, or poorly directed gutters can often lead to large quantities of liquid water pooling against basement foundations.  Ensure all roof runoff is directed away from the foundation and consider re-grading the surface to help direct the water away from the home.

Water Vapor

Exterior water vapor is the most challenging source of basement moisture to combat.  It is relatively easy to understand that any air entering the basement space will contain some components of water vapor.  The relative humidity and temperature of that air will determine the quantity of water introduced.  The simplest way to prevent this water from entering the space is to prevent air movement into the basement by sealing all potential air gaps.

In layman’s terms:

Water vapor escapes from the concrete wall, and into the basement room. Concrete continuously lets off water vapor, and it is trying to either dry to the outside or into your finished basement.


Water vapor is not limited to air movement as the only source of entry.  Any material that contacts your basement space will have some level of permeability to water vapor.  Water vapor will always propagate from an area of higher vapor pressure (high relative humidity) to one of lower vapor pressure (lower relative humidity). Unfortunately for basement spaces is the fact that areas below grade are always subject to an exterior environment that is close to 100% relative humidity.  This fact implies that a below-grade basement surface will always see a net water vapor movement into the conditioned space.  Living spaces with exterior walls that are above grade during the winter months in moderate to high latitudes will see an opposite effect as the interior’s higher relative humidity space is heated to a higher temperature than the exterior and thus sees vapor move in the outward direction. The bi-directional nature of water vapor movement in above-grade walls can allow for sufficient drying of the structural material to occur.  However, the unidirectional movement of water vapor in a below-grade basement can provide a continuously damp environment that is if favorable to the growth of mold.

Best Practices for Basement Insulation


As a remodeling carpenter, I’ve seen many iterations of basement insulation, and most were wrong!!

The wrong way to approach basement insulation is to treat it like an above grade, living space, with 2×4 walls and fiberglass insulation. I’ve seen regular Kraft-Face insulation in walls with a 1-inch gap, as well as 6-mil plastic poly behind fiberglass, against the foundation, as a “vapor barrier.” Both are wrong and will result in mold growth.

Fiberglass [insulation] does not support mold growth, but its paper backing does! Fiberglass insulation acts as a filter to catch dust and debris from air leakage into the house. The dirt caught in the fiberglass grows mold.

You need less water to cause a mold problem in a fiberglass insulated wall than you do in a cellulose insulated wall because of this concentration effect. One cavity insulation versus another doesn’t matter much from a mold risk perspective. Cellulose insulation is paper based, but it is also treated with chemicals (borates) that suppress mold growth. In addition some fiberglass insulation contains urea based binders that do support mold growth. From a mold risk perspective it doesn’t matter one way or the other. The best thing to do is not put pipes in the exterior walls.   Source:

Mold spores can look like a green “furry growth,” black stains, or specks of black, white, orange, green, or brown. Similar to a cooking recipe, mold spores need three ingredients in order to grow:

  1. Temperature between 47-120 degrees F.
  2. Food (organic material).
  3. Moisture/Water. [High humidity]  **

Take away one of the ingredients and mold will not grow. In reality, we want warm temperatures and will have organic materials and fabrics in our living space. The mission then is to prevent the moisture string the basement. No moisture no mold! [READ: Water Damage and Mold]

Examples of organic materials [nutrients] for mold growth include

  • Carpet
  • Fabric
  • Furniture upholstery
  • Paper and books
  • Drywall paper
  • Ceiling tiles
  • Wallpaper
  • Wood
  • Dust
  • Kraft faced fiberglass insulation

De-humidification and good circulation throughout the home is important to eliminate dampness or potential moisture; especially in basements, and crawlspaces.

Basement Insulation – Thermal Considerations

Basements have a unique thermal condition when compared to the rest of the above-grade dwellings.  During the winter months in most climates, the home experiences heat loss through the exterior walls from a heated interior to a cold exterior.  The inverse is true for the summer months.  Energy is spent conditioning and cooling the interior space, in contrast, the warmer and usually more humid exterior(think inward water vapor transfer).

In a below-grade basement, the thermal gradient is almost always directed outward as the temperature below the frost line remains relatively constant and significantly cooler than the conditioned space year-round.


The basement space will also typically contain an above-ground area that is comprised of the first-floor joist bays and in particular the perimeter rim joists.  The rim joists are often overlooked and under-insulated locations in a home.  Despite it being a relatively small area, recent studies have shown that >19% of your home’s heat energy costs could be lost through an uninsulated rim joist.

Bathroom Remodeling Insulation and Ventilation


Basement insulation is commonly thought of as a solution to prevent energy or heat loss.  The goal of thermal insulation is to create as steep a thermal gradient in as little space as possible. There are many different types of thermal insulation and the performance characteristics of these materials vary greatly as well.  The most common types of insulation used in modern building practices consist of:

  • Fiberglass batts
  • Mineral wool batts
  • Blown-in cellulose
  • Preformed foam polystyrene
  • Polyurethane or polyisocyanurate boards
  • Polyurethane spray foam insulation. [Open and Closed Cell]

R-Value – Insulation Performance

To standardize the insulating performance of the various types of insulation, the R-Value rating is used.  R-value is an indication of a material’s resistance to heat flow through conduction (physical contact).  The higher the R-value, the better performing the material is as a thermal insulator.  Below is a chart summarizing typical R-values per inch depth of various common insulation materials.

R-value of Materials and Depths
MaterialR-value/in3 1/2"5 1/4"10" 12"15"
Fiberglass (batt)3.1 - 3.410.8 - 11.916.3 - 17.831.0 - 34.037.2 - 40.846.5 - 51.0
Fiberglass blown (attic)2.2 - 4.37.7 - 15.011.5 - 22.622.0 - 43.026.4 - 51.633.0 - 64.5
Fiberglass blown (wall)3.7 - 4.312.9 - 15.019.4 - 22.637.0 - 43.044.4 - 51.655.5 - 64.5
Mineral Wool (batt)3.1 - 3.410.8 - 11.916.3 - 17.831.0 - 34.037.2 - 40.846.5 - 51.0
Mineral Wool blown (attic)3.1 - 4.010.8 - 14.016.3 - 21.031.0 - 40.037.2 - 48.046.5 - 60.0
Mineral Wool blown (wall)3.1 - 4.010.8 - 14.016.3 - 21.031.0 - 40.037.2 - 48.046.5 - 60.0
Cellulose blown (attic)3.2 - 3.711.2 - 12.916.8 - 15.032.0 - 37.038.4 - 44.448.0 - 55.5
Cellulose blown (wall)3.8 - 3.913.3 - 13.619.9 - 20.838.0 - 39.045.6 - 46.857.0 - 58.5
Polystyrene Board3.8 - 5.013.3 - 17.519.9 - 26.238.0 - 50.045.6 - 60.057.0 - 75.0
Polyurethane Board5.5 - 6.519.2 - 22.728.9 - 34.155.0 - 65.066.0 - 78.082.5 - 97.5
Polyisocyanurate (foil-faced)5.6 - 8.018.2 - 28.029.4 - 42.056.0 - 80.067.2 - 96.084.0 - 120.0
Open Cell Spray Foam 3.5 - 3.612.2 - 12.618.4 - 18.935.0 - 36.042.0 - 43.252.5 - 54.0
Closed Cell Spray Foam6.0 - 6.521.0 - 22.731.5 - 34.160.0 - 65.072.0 - 78.090.0 - 97.5

The Perfect Basement Insulator

The perfect insulator would be a material that has infinite R-value, zero vapor permeability, is thin and flexible, and inexpensive.  This perfect insulator doesn’t exist so we’ll focus on the next best thing.  Closed Cell Polyurethane Spray Foam (CCSPF).

Closed-cell spray foam is as the name suggests a polyurethane foam that is sprayed on by a nozzle.  A mixture comprising isocyanate and polyol resin mix at the tip of the applicator.  The mixture quickly adheres to the surface and rapidly expands.  High-density closed-cell mixtures have a closed cellular structure that is impervious to water and air movement.  The foam becomes rigid once dry and has a fantastic R-value > 7.0 to 7.5 R-value/inch.

Due to the closed-cell nature of the CCSPF, the foam acts as a vapor barrier as well.  This along with roughly double the R-value rating per inch are the biggest advantages over Open Cell Spray Foam.  The only downside to CCSPF, when compared to Open Cell, is the cost.  Closed-cell spray foam is one of the most expensive forms of insulation on the market.

The CCSPF is also not a water stop.  Spray foam should never be used to plug or damn a potential liquid water ingress point.  CCSPF is also very difficult to remove once applied, so proper planning and project management are critical as it will be very difficult to run wires and other utilities once in place

Basement Spaces and Closed Cell Spray Foam

As we previously discussed the basement has some unique challenges when considering a choice of insulation.  Moisture and vapor movement require the use and proper application of a vapor barrier to prevent mold growth.  Fiberglass batts and open-cell spray foam would require an additional vapor barrier application.  Rigid foam boards like CCSPF can serve as vapor barriers and thermal insulation.  However, it can be difficult to achieve 100% coverage between your interior construction materials, utilities, and the concrete foundation.  Small irregular spaces like rim joist bays can be extremely difficult to seal completely with foam boards and batt insulation.  Spray foam can be used to cover all irregular spaces and achieve a complete insulation layer.

Spray Foam Application and Best Practices

Spray foam insulation is a job that’s best left for spray foam professionals.  Period.  Spray foam kits are becoming increasingly popular in the United States which can be a convenient option for an enthusiastic DIYer.  However, the mixture balance between the two chemicals must be perfect and an untrained applicator may not notice the incorrect balance.  Improper mixing can also lead to prolonged periods of potentially harmful and uncomfortable off-gassing.

Given the cost of the materials and unforeseen potential cost of a poor insulation application, it’s best left to a trained professional. With regards to the kits, the cost is typically the same per board foot as a professional installation. The kits don’t offer the same density as a spray applied/professional install.

In a typical timber wall stud framed finished basement, it is recommended to space the sill plate and wall studs at 1-2” from the foundation wall.  Once the frame is up, spray foam application can begin.  By leaving a 1-2” gap the applicators will have enough room to apply the spray foam between the wood studs and the foundation wall.  This will create a thermal break preventing conduction heat loss through the studs as well as creating an uninterrupted vapor barrier.

Off-gassing is a byproduct of the chemical reaction between the two components being mixed together.  Proper protective equipment must be worn by all workers on site when the foam is being applied.  In addition, it is recommended that all residents of the home vacate the premises for a full 24 hours after application to allow the off-gassing process to complete.

Make Basement Air Tight

The simplest way to prevent this water vapor from entering the space is to prevent air movement [moisture] into the basement by installing a vapor barrier. When properly installed, closed-cell foam (spray or board) acts as a vapor barrier and insulates.

Circa 2018 I-Joist and Rim Joist Area [uninsulated]

Insulating A Basement – Best Practice

There are three ways that we approach basement masonry wall insulation to prevent air vapor movement and mold growth. The three methods below are considered best practice basement insulation techniques. They are listed in a top-down ranking based on the best performance and highest cost.

  1. Closed-cell insulation – completely filling the stud wall cavity  
  2. Closed-cell insulation – “flash and batt” method
  3. Rigid foam board –  1″ rigid insulation with seams taped [foam joint tape] and edges sealed with spray foam.  

The links above will take you to topic-specific articles dealing with that specific basement insulation approach.

Rim Joist

Insulate The Rim Joist

In addition to preventing vapor transmission, we make sure to prevent heat loss and air leakage by sealing the house rim joist area.   The rim joists are often overlooked and under-insulated locations in a home. For more information on how to insulate the rim joist area check out our detailed article “Sealing and Insulating Rim Joists”

Framing the Basement Walls

Once you have insulated and sealed off the masonry you can install the partition walls. If using spray foam insulation to seal and insulate you can install your partition walls ahead of time. Ensure that you space the wall away from the masonry 1-2 inches to create a capillary gap between the masonry and wood wall or metal studs.

Basement Insulation – Best Practice To Avoid Mold – VIDEO


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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzz As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising the bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob

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