Moisture is the main consideration in developing a basement insulation strategy. Excess moisture in a basement can cause mold, mildew, odors, fungus growth, decay and rot. Any added insulation must not impede the ability of the interior walls to dry. Foundations below grade must be able to dry on the inside. Almost all basements can be insulated if moisture and air flow are controlled.
Moisture penetrates a basement from several sources:
- Construction materials - they must dry before the basement is sealed
- Interior diffusion from humidity in warm air
- Exterior diffusion - hydrostatic pressure against the basement wall from rainfall, snow melt or irrigation
- Air infiltration bringing moisture with it
- Capillary rise - wicking up from the soil below
- Groundwater leakage through the foundation
The underground environment that walls below grade exist in presents unique challenges. The moisture content of soil below three feet deep will almost always be higher than the moisture content of the air inside the basement. This difference in vapor pressure causes moisture to be driven from the soil into the basement interior by diffusion through the wall. The direction of vapor movement in the portion of the basement wall that is above ground will oscillate with the seasons. Significant amounts of moisture from the soil below the slab will also wick upward into the basement through the slab.
Condensation is a significant problem in basements. Underground soil temperatures can be much colder than either the outside air or the indoor air in the basement. When the warmer air of the basement reaches the colder surface of the wall next to the ground, condensation will occur on the interior of the wall. Thus, any interior insulation and wall assembly will be subject to potentially significant amounts of moisture from vapor driven from both the exterior and the interior depending on the season.
Air infiltration can be a significant source of moisture in a basement. A lot more moisture 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 as air-transported moisture tends to be concentrated. As a result, air-transported moisture can quickly lead to mold, mildew and decay in interior wall assemblies.
While an effective air barrier is required, vapor barriers are typically not needed on the interior of basement insulation and wall assemblies as basement walls with an interior vapor barrier will never dry if they become wet. A vapor barrier on the interior side of the basement wall assembly inhibits drying of the wall from exterior sources more than it prevents the wall from becoming wet from the inside. Even basement walls that initially appear dry may become wet once insulated if the insulation prohibits the wall's ability to continuously evaporate soil-sourced moisture.
Any interior basement insulation must be able to dry to the inside should it become wet as the portion of the wall below grade will not ever be able to dry to the exterior. The insulation system must prevent warm inside air from reaching the cool foundation wall to prevent condensation. Reflective insulation is an effective interior air barrier. In addition, the insulation must be moisture tolerant and should not support mold, mildew or fungus growth or deteriorate when wet.
Don't use Batts or Blankets
Moisture problems in basements have lead experts to recommend against using fiberglass batt or blanket insulation 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 but is no longer recommended due to excessive problems with mold, decay and odors." (Yost and Lstiburek, 2002)
To maintain its effectiveness, fiberglass must not be exposed to high-moisture conditions such as a damp basement. Moisture will negate the insulation value of fiberglass batts and promote mold and mildew which degrade indoor air quality. Installing a vapor barrier over the interior of the fiberglass will prevent the foundation walls from drying to the interior, a problem with mitigating capillary rise, groundwater leakage and construction moisture. Batts with no vapor barrier will not be effective either as interior humidity will still be a problem.
Current recommendations for basement interior insulation involve placement of a layer of rigid foam or reflective insulation (or both) against the foundation wall. This will keep wood framing and drywall away from the damp masonry wall.
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