Venting and Insulating a Crawlspace

By Robert Robillard on Crawlspace

Crawlspaces: To Vent or Not to VentVented Crawl Spaces Can Cause Problems

Crawl spaces are common in the building trade, and especially with new additions. While geographic areas, water tables and rock ledge may play a factor in deciding whether to install a crawl space versus a full basement, many builders like them because they’re easier, faster and cheaper to install.  I’ve never been a fan of crawl spaces and always prefer to install a full basement on new additions to the existing homes.

Many older crawl spaces have dirt floors, and the best scenario is a poured concrete floor.  If the crawl space is enclosed it typically has wall vents, which are designed to allow outside air to circulate. Over time, these vented crawl spaces can develop moisture problems.

Venting and Insulating a Crawlspace do not typically belong together – this article will explain why.

Venting and Insulating a Crawlspace

Careful design and construction to mitigate moisture problems typically involves encapsulating and insulating the space – this is often the best approach. Closing the vents off, encapsulating the crawl space, a practice that has gained favor among today’s energy efficient builders, and conditioning the air involves several careful steps we discuss below.

Moisture in Crawl Spaces:

In the summer, warm and moist outside air enters the crawl space and condenses on cooler crawl space surfaces. Over time mold grows and the wood framing rots, which then attracts carpenter ants or termites.

Over the years building codes have required crawl spaces to be vented to prevent the buildup of moisture, mold and rot. The question many people ask is does ventilation really provide the drying and moisture control desired. Lately building science experts have pointed out better methods to build crawl spaces.

Advantages of Eliminating the Vents

Choosing not to ventilate a crawlspace with outdoor air has some advantages:

  1. Energy savings and control
  2. Prevention of moisture problems – the space will remain dryer that a vented space
  3. Protects pipes from freezing
  4. Bring ducts within the conditioned envelope of the home [energy efficient]

If properly designed and constructed, an unvented, sealed and HVAC conditioned crawlspace can prevent mold, mildew and pest problems; improve the house’s energy performance; and prevent rot in the structure.

A sealed, conditioned crawlspace can be a great location for mechanical equipment and ductwork. HVAV equipment takes up valuable space and the system’s efficiency can be increased by eliminating hot or cold outdoor temperature swings.

What’s the Best Crawlspace Floor?

The best floor covering is a combination of 6 polyethylene liner, with sealed edges, covered with a concrete slab. Polyethylene covering is one of the most effective methods for controlling moisture. The membrane provides both a vapor retarder and air barrier and if covered with a concrete pour is both durable and long lasting.

Crawlspaces: To Vent or Not to Vent

12 Mil Tuff Scrim Poly

There are other commercially available products that can act as a stand alone floor covering and are more durable than 6 mil plastic.  These liners are typically 12 up to 20 mil thick and are marketed as reinforced crawl space liners.  One brand, 12 mil Antimicrobial Poly, by Tuff Scrim is a white 12 mil Antimicrobial extrusion laminated reinforced film.  This product has antimicrobial additives that prevent the growth of bacteria, fungi, mold, and mildew.

Products like Tuff Scrim, Guardian Liner, Vapor Barrierand others offer heavy duty sheeting that is significantly stronger and more tear resistant than standard 6 mil poly.  20 mil liners are thick and strong enough to allow for medium to heavy storage, heavily traveled areas and to cover over with stone.

A stand alone concrete floor slab will also suffice as a membrane, unless the ground under the slab is very wet.


Insulating A Crawlspace

The goal is to build a sealed, conditioned crawlspace and the most important reason for sealing a crawlspace is to avoid condensation in humid climates or humid seasonal conditions.

Most building codes permit the construction of unvented crawl spaces. In the 2006 International Residential Code, requirements for unvented crawl spaces can be found in Section R408.3. If an unvented crawl spaces has a dirt floor, the code requires exposed earth to be covered with a continuous vapor retarder with taped seams. The code lists two options for conditioning unvented crawl spaces;

  1. Install a continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.
  2. Install a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area.

Encapsulate with Spray Foam:

Many crawl space floor joists are insulated with fiberglass batt insulation, which tends to absorbs moisture, and also can fall out, if not properly installed. Insulating the perimeter walls is one way to avoid this or another option is to fill the entire floor joist bay and encapsulate the entire floor joist system [sills, rim joists and joists] with closed cell, spray foam insulation.

Tips to Creating an Unvented Crawl Space

  1. Keep crawl space dry by correcting any grading problems on the exterior so that the grade slopes away from the foundation.
  2. Remove rocks and debris from the crawl space floor, and rake the dirt smooth to prevent your  poly vapor barrier or other barrier from tearing.
  3. Install a Tu-Tuf poly over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall.
  4. Attach the top of the vapor barrier to the wall with horizontal battens, secured to the wall with masonry fasteners.
  5. Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in fiberglass mesh tape.
  6. Consider installing a 2 in. or 3 in. thick concrete slab to protect the vapor barrier.
  7. Insulate the interior of the walls and rim-joists with R-5 to R20 rigid foam [depending on climate,] Thermax, [a polyisocyanurate foam] or spray polyurethane foam.
  8. Install an exhaust fan or a HVAC register to meet code requirements for conditioning the crawl space.
  9. Try to make the crawl space floor higher than the exterior grade.
  10. If the crawl space is subject to water entry, be sure to slope the floor to a sump equipped with a drain or a sump pump.
  11. Install gutters and downspouts, or install generous eave overhangs coupled with positive surface drainage.
  12. Slope the exterior grade away from the house to create positive surface drainage around the house
  13. Avoid bathroom or dryer vents or install any condensate drains into crawlspaces
  14. Open any dividing walls between the full basement and the crawlspace, run a dehumidifier.


The best way to deal with the air in a crawl space is to encapsulate it and install a dehumidifier or a supply air vent from the HVAC system.

Crawl spaces are difficult places to work and even more difficult to seal off. If you plan on sealing off and encapsulating a crawl space I recommend hiring a reputable company that specializes in this type of work.

If you enjoyed this post, make sure you subscribe to my RSS feed!

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

All posts by Robert »

Not what you're looking for?

Search for more articles here. Enter keywords like roof leak, bookcase, deck, etc to find your topic.

© Copyright 2019 A Concord Carpenter · All Rights Reserved