Once again, The Word invites you to travel into the dark realm of subjects that may be of interest to home inspectors. The Word hopes you will find this trip informative and maybe a little entertaining.

Our subject this month is crawlspace ventilation. The Word finds this subject interesting because he can’t find any documentation explaining how the current rules for crawlspace ventilation were developed, nor can he find any evidence-based support for the rules.

In the eight years since the original version of this article was written, there appears to be a growing consensus that the current rules for crawlspace ventilation do not make sense. Pipes are more likely to freeze in a ventilated crawlspace in the winter. Crawlspace ventilation may do more harm than good in warm or moist climates in the summer by bringing in more moisture than it removes.

Current Crawlspace Ventilation Rules

While the current crawlspace ventilation rules may not make sense, if the crawlspace is ventilated, and most are, the current rules should be followed. The current crawlspace ventilation rules are pretty simple. That’s not surprising; they were developed in a simpler time. There should be at least 1 square foot of net free ventilation opening area for every 150 square feet of crawlspace ground surface. This ratio may be reduced to 1 square foot to 1,500 square feet if the crawlspace ground surface is covered with a Class I vapor retarder, usually 6-mil polyethylene.

There should be a ventilation opening within 3 feet from every building corner. These openings should be covered with screens or grates that have openings of not more than ¼ inch. These screens or grates take up space in the opening, so the net free opening area equals the total opening area minus the area of the screens or grates. Screens and grates can reduce the opening area by one-third or more.

What’s Wrong with the Current Crawlspace Ventilation Rules?

It is said that they don’t build buildings like they used to. That’s true. Older buildings were built using natural materials. Buildings are now built with engineered materials such as oriented strand board (OSB), which are more susceptible to damage when exposed to moisture.

Older buildings were not built to be energy efficient. These buildings constantly exchange air, water vapor and heat with the outdoors (including a ventilated crawlspace). This exchange system occurs through holes, openings and joints between materials that can add up to the size of a large window that is open 24/7. This exchange system is the environment in which the current crawlspace ventilation rules were adopted. These rules can work if the exchange system isn’t changed.

Here’s a common-sense fact about systems. When you change a system, you probably change how that system works. You change the building’s air, water vapor and heat exchange system when you install vapor-impermeable floor covering materials such as vinyl and tile, and when you install insulation between the crawlspace joists. The exchange system is now different from what worked under the old crawlspace ventilation rules. The changed system will work differently, and probably worse. Let’s find out why.

First, Let’s review some building science:

  • The dew point is the temperature at which water vapor in the air condenses into liquid water.
  • Warmer air holds more water vapor than cooler air.
  • Heat flows from hot to cold.
  • Water vapor flows from higher vapor pressure to lower vapor pressure.
  • A crawlspace ventilation system required by current rules doesn’t remove moisture from the crawlspace very well; the ventilation openings may allow more water vapor into the crawlspace than is removed. This is particularly true in humid climates.
  • In the winter, heat from the building often keeps the uninsulated floor sheathing and floor joists warm and above the dew point; this situation is reversed in the summer when the building is air conditioned.
  • The crawlspace ground surface is often colder than the uninsulated floor joists during the winter. Thus, if any area of the crawlspace might be at or below the dew point, it will usually be the crawlspace floor; water will probably condense there first.
  • Water condensing on the crawlspace floor is usually no big deal.

Now, let’s change the system by adding insulation between the floor joists. During the winter, insulation changes the heat exchange that kept the floor joists warm and above the dew point. The floor joist bottoms and the bottom of the insulation may now exchange heat with the colder ground instead of with the warmer building. Liquid water will condense on the floor joists and on the insulation if their temperature falls below the dew point. During the summer, if the building is air conditioned, this heat exchange is reversed. Hello, wood rot, mold and ineffective wet insulation!

To make things even more interesting, let’s throw a vapor retarder such as a vinyl floor covering or tile into the system. Heat and water vapor will flow from the warmer or wetter crawlspace into the cooler or dryer building in the summer, and will reverse flow in the winter. Floor coverings such as wood and carpet allow some heat and water vapor transfer, mitigating some of the condensation potential. The vinyl or tile floor covering is a vapor retarder on the wrong side of the system (in the summer) that effectively stops this transfer and allows water to condense on the underside of the floor covering. This is what sometimes allows mold growth that causes floor covering discoloration.

What to Do?

As we now know, changing the crawlspace system, even with good energy-efficiency intentions, can have bad consequences. But the energy inefficiency of an uninsulated crawlspace is not acceptable either. What to do? Fortunately, there is an answer: the unventilated crawlspace.

Unventilated Crawlspaces to the Rescue

Unventilated (unvented) crawlspaces go by several names including closed, sealed, encapsulated and semi-conditioned. The International Residential Code (IRC) uses the term unvented. The Word prefers unventilated and he sometimes uses the term closedSealed and encapsulated are not the best terms because they may not be accurate. Semi-conditioned can be used if the insulation is installed on the crawlspace walls.

The option to build unventilated crawlspaces has been available in the IRC for several years. The Word is encountering more of these crawlspaces as the memo about the problems with ventilated crawlspaces gets to more people. The physics supporting unventilated crawlspaces is difficult to challenge; however, like any system, success requires proper implementation.

Here are the basic requirements:

An uncovered crawlspace floor will almost always emit water vapor into the crawlspace, so it must be sealed as well as possible. A Class I vapor retarder must cover the entire floor. Six-mil polyethylene is the minimum material. A reinforced material in the 10- to 12-mil range is  a better choice for long-term performance, especially if the crawlspace contains equipment that requires maintenance and replacement. Six-mil polyethylene won’t last long if subjected to traffic.

Seams in the material should be lapped at least 6 inches and sealed with a compatible sealant. The material should turn up the crawlspace wall at least 6 inches, with the end secured and sealed to the wall. The same is true for piers and columns (Photo 1). The sump pump and radon mitigation system should also be covered and sealed, and a means should be provided for access (Photo 2). The crawlspace door should be insulated and weather stripping should be installed if the crawlspace is a semi-conditioned space (Photo 3).

Covering the crawlspace floor will not completely eliminate water vapor in the crawlspace, so some method of ventilation or dehumidification is required. Options include the following:

  • install a mechanical exhaust (fan) to the outdoors at a rate of 1 cubic foot per minute per 50 square feet of crawlspace floor area; include a pressure relief opening to the interior if using this option
  • provide conditioned air at a rate of 1 cubic foot per minute per 50 square feet of crawlspace floor area; do not exceed this rate or condensation issues could occur
  • provide dehumidification at a rate of at least 70 pints per day per 1,000 square feet of crawlspace floor area


Crawlspace insulation should be installed per IRC Chapter 11, the International Energy Conservation Code or local regulations. The minimum insulation R-value depends on where the insulation is installed and on the climate zone. The insulation may be installed between the floor joists. The insulation may be installed on the inside or on the outside of the crawlspace walls, or on both the inside and the outside.

Insulation installed between the floor joists is usually fiberglass batts. Like all components, fiberglass batt insulation should be installed per manufacturer’s instructions. The most common problems that The Word sees include the following:

  • The insulation is not in contact with conditioned space (Photo 4).
  • The insulation is significantly compressed.
  • The insulation is not cut to fit at full depth between the floor joists (Photo 5).

In all of these cases, the insulation R-value is significantly reduced and if not in contact with conditioned space, the R-value is zero.

Insulation on the framed crawlspace walls may be fiberglass batts, foam sheets or spray foam. Insulation on masonry and concrete crawlspace walls should be foam sheets or spray foam. The Word prefers closed-cell spray foam on the walls because it seals against both air and water vapor infiltration better than other alternatives, such as open-cell foam.

A gap of about 3 inches should be left between crawlspace wall insulation and framing. This allows for termite tube inspection.

Thermal Barriers and Ignition Barriers

Foam insulation is great stuff (hence the brand name of a type of this insulation). Foam insulation is available as sheets and as spray foam. Spray foam fills in almost all gaps, so it is a great air barrier. Closed-cell spray foam is a vapor retarder and a liquid water barrier, depending on thickness and on how it is installed. Open-cell spray foam is usually neither a vapor retarder or a liquid water barrier, but it is still a great air barrier and it is good insulation material.

A problem with all types of foam insulation is that they can ignite easily and they release toxic fumes when they burn. For these reasons, foam insulation must be covered with something to reduce the chance that it will ignite, or it must be tested and installed in a manner that it resists ignition of the foam.

The general rule is that foam insulation should be separated from the building interior by a thermal barrier. Common thermal barriers are ½-inch-thick drywall and 23/32-inch-thick wood structural panels. This isn’t a problem when foam insulation is installed where it will be covered with something like drywall. Foam insulation usually isn’t covered in crawlspaces and attics, so something must be done in these areas to reduce the chance of ignition.

The ignition risk of foam insulation in crawlspaces and attics can be dealt with in one of two ways. The foam insulation can be covered with an ignition barrier such as mineral fiber insulation or wood structural panels. This ignition barrier option is often not practical or cost-effective for large areas. The other option is to install the foam insulation in a manner that resists ignition. This option may involve covering the foam insulation with a coating that is tested to help the foam insulation resist ignition.

The problem with the ignition-resistant option is that The Word is not aware of a method that home inspectors can use to visually determine if the exposed foam insulation is ignition-resistant. There is a good chance that exposed foam sheets are not ignition-resistant (Photos 6 and 7). The ignition-resistance of exposed spray foam is more difficult to visually determine. The Word, therefore, disclaims inspection for ignition-resistance of spray foam insulation and recommends evaluation to determine if the spray foam insulation has been installed according to manufacturer’s instructions and local regulations.

The Bottom Line

The Word can think of no advantages, and many disadvantages, of ventilating crawlspaces. Crawlspaces in most climate zones should, therefore, be unventilated. This is what The Word recommends to clients in his market.

Inspectors should note that, in some jurisdictions, converting a ventilated crawlspace to an unventilated crawlspace requires a permit. Recommending a permit check when encountering a retrofit unventilated crawlspace is usually prudent.

Memo to the crawlspace gods (or monsters that may reside therein): The Word does not reside on Mt. Olympus (just at its base) and welcomes other viewpoints. Send your lightning bolts or emails to [email protected] The thoughts contained herein are those of The Word. They are not ASHI standards or policies.