Basement & Crawlspace Air Sealing and Insulating Project

Sealing air leaks and adding insulation to the basement or crawlspace can yield substantial benefits if done correctly. While these improvements are generally considered moderate to difficult do-it-yourself (DIY) projects, whether you are a homeowner, or a qualified home performance contractor, there is a wealth of available information outlining best practices and project guidance.

Below, we have included just some publicly-available resources that contain guidance air sealing and insulating the following:

  1. Unfinished Basements
  2. Finished Basements
  3. Enclosed Crawlspaces
  4. Other Resources

IECC Climate Zones

Note: When planning any home improvement, it is important to consider your area's specific climate and weather conditions. Some of the below resources reference the IECC Climate Zones when providing climate-specific guidance. Learn which Climate Zone you are in, and view a larger map at https://basc.pnnl.gov/images/iecc-climate-zone-map.

Unfinished Basements

  • Air Sealed, Insulated Basements (PDF, 781 KB). U.S. Department of Energy's Building America Program, with content prepared by Steven Winter Associates, Inc., updated November 2017.
    • This guide describes how to insulate a basement to improve the overall thermal performance of the building and provide drier, usable conditioned space for home occupants and for HVAC equipment.
  • BSC Information Sheet 408: Critical Seal (Spray Foam at Rim Joist) (PDF, 256 KB). Prepared by Building Science Corporation for the Department of Energy's Building America Program.
    • This Information Sheet describes common problems air leakage at the rim and band joist, and guidance on application spray foam for remediation, factoring in condensation control, code acceptance, and more.
  • BSC Information Sheet 511: Basement Insulation for all Climates (PDF, 309 KB). Prepared by Building Science Corporation for the Department of Energy's Building America Program.
    • This Information Sheet describes how basements should be designed and constructed to be dry and conditioned. Includes installation guidance and diagrams.
  • Renovating Your Basement for Moisture Control and to Improve Energy Efficiency (PDF, 384 KB). Prepared by Building Science Corporation for the US Department of Energy's Building America Program, 2003; revised 2007.
    • This report explains how heat loss through uninsulated basement walls can account for up to one-third of the heat loss from an average home, and how to properly install insulation on basement walls, especially when combining the project with "finishing the basement."

Basement framing man adding insulation to ceiling

Finished Basements

  • Air Sealed, Insulated Basements (PDF, 781 KB). U.S. Department of Energy’s Building America Program, with content from Steven Winter Associates, Inc., updated November 2017.
    • This guide describes how to insulate a basement to improve the overall thermal performance of the building and provide drier, usable conditioned space for home occupants and for HVAC equipment.
  • ​Water Management of Existing Basement Floor (PDF, 67 KB). Prepared by Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team, updated December 2017.
    • This guide describes how to retrofit a basement floor or slab to reduce moisture issues.

Enclosed Crawlspaces

  • BSC Information Sheet 512: Crawlspace Insulation for all Climates (PDF, 354 KB). Prepared by Building Science Corporation for the Department of Energy's Building America Program.
    • This Information Sheet describes how crawlspaces should be insulated and have a continuous sealed groundcover of vapor diffusion resistant materials. Includes installation guidance and diagrams.
  • Designing Closed Crawlspaces (PDF, 618 KB). Dastur, Davis, Warren. Advanced Energy, February 2012.
    • This quick reference guide summarizes of design implications and recommendations for builders, code officials and consumers who are considering designing or installing a closed crawl space. Includes two sample designs.
  • Guide to Closing and Conditioning Ventilated Crawlspaces (PDF, 1.2 MB). Bruce Dickson, IBACOS, Inc. Prepared for the National Renewable Energy Laboratory (NREL) on behalf of the U.S. Department of Energy's Building America Program, January 2013.
    • This how-to guide explains the issues and concerns with conventional ventilated crawlspaces and provides prescriptive measures for improvements that will create healthier and more durable spaces.
  • Technology Fact Sheet: Crawlspace Insulation (PDF, 215 KB). Written and prepared for the U.S. Department of Energy by Southface Energy Institute and U.S. Department of Energy's Oak Ridge National Laboratory (NREL), December 2000.
    • Building science and how-to technical guide for sealing, moisture-protecting, and insulating crawlspaces to increase comfort and durability of a home.

Enclosed crawlspaces

Vapor retarder applied to floor or crawlspace. Source: Guide to Closing and Conditioning Ventilated Crawlspaces. Bruce Dickson, IBACOS, Inc. Prepared for The National Renewable Energy Laboratory on behalf of the U.S. Department of Energy's Building America Program, January 2013.

Open Crawlspaces

Additional Resources

  • Insulation Fact Sheet (PDF, 1.1 MB). Prepared by Oak Ridge National Laboratory for the U.S. Department of Energy's Building Technology Program, 2008.
    • This fact sheet will help homeowners understand how insulation works, what different types of insulation are available, and how much insulation makes sense for in a specific climate.

Basement Joists

DIY Project: Sealing and Insulating Rim Joists

Considering a sealing and insulating project for your basement or crawlspace?

Sealing air leaks and adding insulation in the basement are generally considered moderate to difficult do-it-yourself (DIY) projects, but the benefits can be substantial. If you are doing a major home renovation project, now may be a great time to tackle this project too.

The good news is that even if you're not comfortable taking on this project yourself, there are many qualified contractors who can help you get the work done.

Use the information below to determine if this project is a good fit for your home, what common problems it can help address, what the potential benefits are, what tools and materials you'll need to complete the project if you choose to do it yourself, important safety considerations, step-by-step instructions for getting the job done and when to seek professional help.

Project Instructions

Tools, materials, and safety

Materials and Equipment

  • Expanding spray foam
  • Caulk and caulk gun
  • Old clothes
  • Box cutters
  • Flashlight/headlight
  • Safety glasses, gloves and dust mask/protective face mask

Tips and Safety Considerations

  • Gather all your tools and supplies before you begin to minimize trips into and out of your basement or crawlspace.
  • Be sure that the work area is well lit. Use a drop light for most of the space if necessary and keep a flashlight or headlight handy.
  • During cold weather, try to pick a temperate day and wear warm clothes.

Addressing Indoor Air Quality

Sealing the home can trap indoor air pollutants such as carbon monoxide, radon and volatile organic compounds (VOCs) and create unsafe conditions. Properly addressing this may require additional ventilation fans to maintain safe air quality in your home and repairs to reduce or eliminate the sources of indoor air pollutants. Here are some things to consider before starting your home sealing project:

warningRadon

If you are in a high radon area, consider contacting a contractor to conduct a radon test and install radon mitigation systems before sealing your home. Find out more at EPA's Consumer Guide to Radon Reduction.

warningCombustion Safety

If you have an appliance that uses oil or gas, consider hiring a Home Performance Contractor to test those appliances to ensure they are properly drafting combustion gasses before and after sealing your home. Sealing in some cases can cause naturally venting combustion appliances to backdraft gases back into your home, creating unsafe conditions.

Learn More!

Sealing your home can trap indoor air pollutants. To properly address this may require additional mechanical ventilation to maintain safe air quality in your home.

For more information on safe ventilation methods, visit EPA's Indoor Air Quality for Homes Page.

Step-by-Step Instructions

Sealing Your Basement or Crawlspace

Look for common locations of air leaks in basements and crawlspaces:

  • Between rim joists and under the sill plate
  • Around windows
  • At wiring holes
  • Around plumbing pipes
  • Around the door to crawlspace, if attached to outside of house
  • Around foundation at the sill plate, if not sealed properly
illustration showing where air can leak around rim joist
 

Step 1. Seal any gaps or cracks in basement wall, ceiling or floor. It is best to seal up the top and bottom of the inside of the rim joist cavity. This is especially important at areas such as bay windows that hang off the foundation. Use caulk for any gaps or cracks ¼ inch or less and spray foam for anything larger. It is also very important to seal any holes for wires, pipes or other service areas that may lead to other floors of your home.

Step 2. Cut insulation and insert accordingly. Insert and secure all insulation between holes in rim joists. If using batts, cut the insulation to fit and place against the rim joist. If using rigid foam insulation, foam around the edges to hold the insulation in place. After installing the rigid foam insulation or fitting batts into rim joists, seal any remaining holes and cracks to make your basement airtight.

Get Started on Your Next Project

couple installing thermal insulation

man installing insulation in attic

man using caulking gun to seal window

handyman sealing plastic

Seal & Insulate

Resources