Proper foundation and insulation techniques can turn your basement into truly usable, comfortable living space. For the purposes of this article, a basement is defined as a 7 to 10 foot foundation wall with no more than the upper 25 percent exposed above grade. Many people insulate and finish their basement so it can be heated and cooled to the same temperatures as above grade spaces - typically 68 to 78 degrees. Un-insulated basement temperatures usually fluctuate between 55 and 70 degrees based on heat transfer between the basement and the rooms above, the outside soil temperature and basement air ducts or water pipes. Heat lost from the house into an un-insulated basement can be as great as heat losses to an un-insulated attic leading many jurisdictions to create building code requirements that include basement insulation.

While basement ceilings can be insulated as a low cost option to save energy by keeping heat inside the house above, this does not impact the usefulness of the basement as additional living space. Ceiling insulation can also be undesirable in Northern climates if ducts and pipes in the basement are left exposed to freezing temperatures. To create additional living space, basement walls can be insulated on the interior, exterior or in the middle of the wall. As the middle option is only available during new construction, we will look at the interior and exterior options which are applicable to both new construction and retrofit.

Both rigid insulation and reflective insulation can be added on the exterior of a masonry basement wall. This approach has several advantages over interior insulation. The most important advantage to exterior insulation is that it minimizes moisture and condensation problems. Exterior basement insulation can be installed as a continuous layer with no thermal bridges. It protects the waterproofing applied to the foundation wall and will not shrink the size of the interior basement floor area.

Installing insulation on the interior has some advantages over exterior insulation if moisture in the basement is not a problem. The total cost of interior insulation is usually less expensive than exterior insulation if the cost of the interior finish materials is not included. Exterior insulation can cost more because the insulation must be completely covered with a weatherproof sheath. However, insulating the inside alone leaves the wall without a finished surface. Interior insulation is also less expensive that retrofitting an existing basement with exterior insulation as no excavation of the soil outside the foundation is necessary to add the insulation. With a preservative treated wood foundation, insulation is placed either inside the stud cavities or over the framing members which is the same as the insulation technique in an above-grade wood frame wall.

Insulation is rated by its ability to resist convective heat flow in units called R-value. R-value gives the insulation resistance per inch of material. Construction materials with higher R-value ratings are more effective insulators than materials with lower ratings for the same thickness. The R-value is a function of the material type, thickness and density. The R-value of an insulation system is calculated by adding the R-values of the individual components together to achieve the recommended insulation protection based on climate.

The US Department of Energy recommends R-value levels for basements based on a combination of climate conditions, placement of the insulation and time to payback the cost of installation. For most locations, R-11 is recommended for both interior and exterior installation on masonry walls and R-19 for wood walls. In the coldest climates, R-19 to R-30 is recommended.

Heat flows from a hot or warm medium to a cold medium in three ways:

• By radiation from a warm surface to a cooler surface through an air space through infra-red heat rays
• By conduction through solid or fluid materials resulting from direct contact
• By convection, which involves the physical movement air - warm air rises

Heat moves through wall cavities, between roofs and attic floors or between floors and basements by a combination of radiation, conduction, and convection with radiation being the dominant method of heat transfer. Research shows that control of radiant heat transfer is the core of heating/cooling climate control.

Reflective Insulation

While other types of insulation are made to resist or impede the flow of warm air, reflective insulation reflects radiant (infra-red) energy back inside the house so it does not escape. Reflective insulation is particularly effective against the downward flow of heat.

Reflective insulation is commonly made of two layers of aluminum foil sheeting with foam or plastic bubbles in between creating an air space to also resist convective heat transfer. The aluminum foil component in reflective insulation will reduce radiant heat transfer by as much as 97%.

The benefits of using reflective insulation instead of conventional fiberglass insulation are numerous.

• Clean, Lightweight, Flexible & Very Strong
• Very thin - can be used effectively in small spaces
• Easy to install, installation requires no special tools or clothing
• Easier to cut than fiberglass batt and bubble wraps
• Convenient flange tabs on sides
• Can be stapled, nailed, glued or sewn
• Non-hazardous/Non-toxic/Non-carcinogenic/No fibers to breath or cause skin irritation
• Provides up to 19 DB of soundproofing
• Works in temperature extremes as low as (minus) -20 degrees Celsius and as high as 80 degrees Celsius (and if an air space is added it can go up to 300 degrees Celsius)
• Vapor Barrier, waterproof, non-absorbent - reduces or eliminates condensation when properly installed
• Radiant barrier - reflects 97% of radiant heat
• Does not promote mold, mildew or fungus growth
• Does not provide nesting for birds, rodents or insects
• Provides Class A/Class 1 fire rated protection - meets fire and smoke safety requirements of most federal, state, and local building codes
• Flexible at low temperatures and withstands fluctuations in outdoor temperatures
• Reflective insulation has higher R-values (or resistance to heat transfer) than any other type of insulation per inch. ¼" thick reflective insulation has more insulation value than 6 inches of common fiberglass batt

Rigid Insulation

Rigid foam board insulation is a popular mass insulation product used to insulate basements against the movement of conductive and convective heat transfer. A high insulating value for relatively little thickness makes rigid foam ideal for insulating exterior walls. The water resistant nature of foam makes it well suited for use under slabs and in the ground around foundations.

Rigid insulation is made of air-entrained plastic that is either extruded or pressed into sheets. There are three types of rigid foam insulation: expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso), each varying in cost and R-value. Rigid foam can be installed in combination with reflective insulation to add either a radiant barrier, air infiltration barrier or radon barrier.

The benefits of using rigid insulation in a basement are many.

• Density - Density provides high R-value with minimum thickness making rigid insulation more resistant to air and water vapor movement than fiberglass batts or cellulose.
• Low weight makes rigid insulation boards easy to install and less expensive to ship.
• Resists outside air infiltration when joints are sealed with tape or caulk.
• New products are made without ozone depleting chemicals for virtually no global warming impact.
• Can be installed with full coverage over studs instead of just between them to eliminate the heat loss path through framing members.
• Non-hazardous to install - no fibers or fumes to inhale, non-irritating to skin.
• No deterioration of R-value over time - rigid insulation does not lose R-Value over its service life.
• Green - A manufacturing study showed that the energy required producing polystyrene foam insulation is 24 percent less than the energy required to make the equivalent R-value of fiberglass insulation.
• Rigid insulation "breathes" instead of trapping moisture like fiberglass or cellulose and therefore does not require the venting methods used for other insulation materials to prevent trapped moisture within walls.
• Highly resistant to mold.
• Not a food for insects.
• Good acoustical insulation properties.

Rigid and Reflective Insulation in a Foundation Wall (listed from outside to inside) Concrete Wall Rigid Insulation Reflective Insulation Wood Strapping or Studs (airspace) Drywall

Click here for a comprehensive article on rigid foam board insulation.

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.

To create a conditioned, comfortable basement, insulation can be installed on either the exterior or interior of the basement wall.

Exterior Installation

Exterior basement insulation has numerous advantages for both retrofit applications and new construction

• The below-grade portion of the wall will be able to dry inward continuously - this is the biggest advantage.
• The foundation itself will be within the insulated portion of the house. This will help control temperature fluctuations.
• For existing basements, outside walls are usually more continuous than the interior and are easier to insulate.
• You can correct moisture problems in masonry, rubble and brick foundations including water leakage. Basements with water leakage, dripping and other moisture problems must be insulated from the outside.
• Repairs to the foundation or drainage systems and waterproofing can be done at the same time.
• No inside floor area is lost.

The disadvantages to exterior basement insulation are all in the additional cost of materials plus excavation for retrofit installations.

• The top portion of the basement wall will be above grade requiring an extra layer of flashing to keep water from getting in behind the insulation. A protective cover is also required.
• The entire house structure must shift outward to compensate for the additional thickness of the exterior basement insulation.
• Insulation must be covered and protected during the construction process until the final exterior sheathing is installed.
• For retrofits, excavating around the foundation can be a significant expense.

Despite the drawbacks, exterior insulation is currently the preferred method for insulating a basement wall during new construction and also for retrofitting rubble and brick foundations or foundations with groundwater leakage or other serious moisture problems. Basements with a history of damp or dripping walls must be insulated on the exterior.

Interior Insulation

Despite the advantages of exterior insulation, basements are most often insulated from the inside for both financial and practical reasons. Interior insulation is less expensive than the same R-value of exterior insulation due to the additional labor and material cost of adding the flashing and protective sheathing. In addition, interior insulation saves the cost of excavating the soil around an existing foundation to add insulation on the outside. Finished basements also add value to a home. Interior insulation is routinely added as a part of finishing the basement.

Reflective Insulation with Rigid Foam Board Insulation Concrete Wall Rigid Insulation Reflective Insulation Wood Strapping (air space) or Studs Drywall

Reflective Insulation Alone Concrete Wall Asphalt felt behind the wood strapping Wood strapping (air space) Reflective Insulation Wood Strapping (air space) or Studs Drywall

Under the Slab

Reflective insulation is used to provide a thermal break, a vapor barrier, and a radon barrier under the slab on top of the gravel drainage bed. Insulation under the slab will make the basement floor warmer. To create a vapor barrier, the insulation should be sealed to the foundation wall. Reflective insulation is quicker and easier to install under the slab than foam board.

Click here for installation instructions for insulating the basement ceiling and insulating under the slab.

US Department of Energy,
"Insulation Fact Sheet, R-Value Recommendations for New Buildings",
1997.

Carmody, John; Christian, Jeffrey; Labs, Kenneth,
"Builder's Foundation Handbook",
Oak Ridge National Laboratory, May 1991.

Yost, Nathan and Lstiburek, Joseph,
"Basement Insulation System",
Building Science Corporation, 2002.