April 2, 2012. Basements and crawlspaces have different insulation requirements than the house above.

Due to the underground environment, moisture is the primary consideration in developing either a basement or crawlspace insulation strategy. Excess moisture can cause mold, mildew, odors, decay, fungus growth and wood rot.

The underground environment of walls below grade presents unique challenges. The moisture content in soil below three feet deep is almost always higher than the moisture content of the air inside. This difference in vapor pressure causes moisture to be driven from the soil into the basement or crawlspace interior by diffusion through the wall. A significant amount of moisture from the soil below the slab will wick upward into the basement through the slab or directly into the crawlspace.

Moisture penetrating the basement and crawlspace can come from multiple sources:

• Interior diffusion from humidity in warm indoor air
• Hydrostatic pressure against the foundation wall from rainfall, snow melt or irrigation creates exterior diffusion
• Air infiltration bringing moisture in.
• Capillary rise wicking moisture up from the soil below
• Groundwater leakage through the foundation 

Condensation is also a significant problem in basements and crawlspaces. Underground soil temperatures can be much colder than either the outside air or the indoor air in a basement. The warmer the air of the basement constantly expands. When this warm air reaches the cooler surface of a wall next to the ground, condensation will occur on the interior of the wall. Any interior insulation will be subject to potentially large amounts of moisture driven from both the exterior and the interior depending on the season.

Air infiltration can be a major source of moisture in a basement. A lot more water vapor can move through a very small opening where the vapor pressure is greater on the exterior than can move through the wall surface by diffusion. Air-transported moisture can quickly lead to mold, mildew and decay in interior wall assemblies.

Fiberglass insulation of blanket insulation should not be used in a basement! "Framed stud walls on the basement interior with batt insulation between the studs and a vapor barrier over the studs was used for many years as the building standard -- This is no longer recommended due to excessive problems with mold, decay and odors." (Yost and Lstiburek, 2002)

To maintain its thermal effectiveness, fiberglass insulation cannot be exposed to moisture. Moisture will degrade the insulation R-value of fiberglass batts and promote mold and mildew which lower indoor air quality. Installing a vapor barrier over the interior of fiberglass insulation will not prevent the foundation walls from drying to the interior, migrating capillary rise and ground water leakage. Batts with no vapor barrier will not be effective either as interior humidity will still cause loss of insulation R-value.

Basements can be insulated on either the interior or the exterior. Basements with water leakage and dripping in masonry, rubble or brick foundation must be insulated from the outside. 

Prodex Total insulation will prevent the inside air from reaching the cool foundation wall to prevent condensation. Prodex does not support mold, mild or fungus growth. It does not lose thermal effectiveness (R-value) when wet.

Prodex Total can also be used under a basement slab. It provides a thermal break, vapor barrier and radon barrier when installed under the slab on top of the gravel drainage bed. Prodex Total Insulation under slab will also make the basement floor warmer. To create a vapor barrier see radon installation page. Prodex is an effective barrier against the upward migration of radon through joints, cracks and other openings in the building foundation into your living space. Radon is radioactive gas found in soils, rock and groundwater. It is formed by natural decay of radium and uranium isotopes. Radon levels vary depending on the geologic makeup of your location. Radon is naturally emitted from the ground where it is diluted to an insignificant level by the atmosphere. Inside a sealed and insulated building, radon can concentrate into high and  potentially dangerous levels.

Your home can act like a vacuum drawing radon up through the crawlspace in two ways. First, the air pressure inside is usually lower than pressure in the soil around the foundation. Second, warm air will rise to leak out through attic vents creating a small amount of suction that can pull radon gas up through the crawlspace.

If you do not intend to use the basement as an extension of your living space, it can be insulated in the same manner as a crawlspace. Proper insulation of your crawlspace will:

• Keep your floors noticeably warmer
• Reduce air infiltration and drafts
• Save energy and money by keeping heat and air conditioning inside
• Minimize problems with condensation, wood rot, mold and mildew
• Provide waterproofing
• Seal entry paths against termites
• Control radon infiltration

A poorly insulated crawlspace can account for 15 percent or more of a building's total heat loss. Prodex is most effective against downward radiant heat transfer making it a builder's choice for crawlspaces as a radiant barrier. Heat radiating downward from your floor into the crawlspace will be reflected back inside, saving energy and maintaining constant room temperatures. The most recent advance in crawlspace insulation is Prodex Total as a vapor barrier, radon barrier and insulation combined in one product.

Today's building codes call for un-vented crawlspaces. Crawlspace walls should only be insulated if the crawlspace is un-vented and the floor above the crawlspace is un-insulated. While walls can be insulated on either the interior or the exterior, interior insulation is the preferred approach if you want the insulation to double as a vapor and radon barrier. Crawl spaces are most commonly insulated on the ceilings and the floor and not on the walls. Click for crawlspace installation instructions. Installation technique depends on whether you have a vented or un-vented crawlspace. The US Department of Energy recommends an un-vented crawlspace for all new construction. A vented crawlspace will allow winter cold air into the crawlspace often with a dew point that is lower than the crawlspace. This helps prevent condensation.