BSC Information Sheet 511Basement Insulation

for All Climates

This Information Sheet has been prepared by Building Science Corporation for the Department of Energy’sBuilding America Program, a private/public partnership that develops energy solutions for new and existinghomes. The views and opinions of authors expressed herein do not necessarily state or reflect those of theUnited States government or any agency thereof.

For more information about Building America go to www.buildingamerica.gov

Rigid Insulation Wraps Concrete Surfaces

Cold concrete foundation wall must be protected from interiormoisture-laden air in summer and winter

Rim joist assembly must be insulated with air impermeable insulationor insulated on the exterior

Rigid insulation completely wraps exposed concrete preventinginterior air from contacting potential concrete condensing surface

Seams in rigid insulation and joints to other materials sealed toprovide air barrier

Rigid insulation is vapor semi-impermeable or vapor semi-permeable(foil facing or plastic facing not present)

Rigid insulation provides bond break between foundation wall andslab when insulation is installed before slab is poured

Basement InsulationBasements are part of a home, within thebuilding boundary — despite repeated attemptsover the years to disconnect them from theliving space. Because of this, basements shouldbe designed and constructed to be dry andconditioned. This is particularly important forbasements that contain mechanical equipment –a situation that is practically guaranteed inbuildings that have a basement. Mechanicalsystems must not be installed outside of a homein unconditioned space unless there is nopractical alternative.

Basements need to be dry for reasons of indoorair quality, pest control, and durability of thebuilding. A dry basement or crawlspace is lesslikely to have pests and termites. If a basementis being used for storage or as living space, itneeds to be kept dry to avoid mold and dustmites.

The most important strategies for keepingbasements dry are those of groundwater andrainwater control. Refer to BSC InformationSheet 1, “Groundwater Control” for furtherguidance. Once these groundwater controlstrategies are employed, the basement should beinsulated to minimize cold surfaces that cancondense water and elevate local relativehumidity.

Basements should be insulated on theirperimeters — they should not be insulatedbetween floors.

WallsBasement walls should be insulated with non-water sensitive insulation that prevents interiorair from contacting cold basement surfaces –the concrete structural elements and the rimjoist framing. Allowing interior air (that isusually full of moisture, especially in the humidsummer months) to touch cold surfaces willcause condensation and wetting, rather than thedesired drying. The structural elements ofbelow grade walls are cold (concrete is in directcontact with the ground)– especially when

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insulated on the interior. Of particular concern are rim joistareas – which are cold not only during the summer but alsoduring the winter. This is why it is important that interiorinsulation assemblies be constructed as airtight as possible.

The best insulations to use are foam based andshould allow the foundation wall assembly to dryinwards. The foam insulation layer should generallybe vapor semi impermeable (greater than 0.1 perm),vapor semi permeable (greater than 1.0 perm) orvapor permeable (greater than 10 perm) (Lstiburek,2004). The greater the permeance the greater theinward drying and therefore the lower the risk ofexcessive moisture accumulation.

Up to two inches of unfaced extruded polystyrene(R-10), four inches of unfaced expanded polystyrene(R-15), three inches of closed cell medium densityspray polyurethane foam (R-18) and ten inches ofopen cell low density spray foam (R-35) meet thesepermeability requirements.

In all cases, a capillary break should be installed onthe top of the footing between the footing and theperimeter foundation wall to control “rising damp”.It is also necessary to install a capillary break betweenthe foundation wall and framing.

In certain situations, foil-faced insulations may beused on the interior of foundation walls. However,such requires careful attention to supplementalmoisture management strategies. Consult the Builder’s

Guide listed among the Suggestions for FurtherResearch below for specific guidance.

In most basement wall situations, the foam plasticinsulation material will need to be covered by afire/ignition barrier. _” gypsum board usuallyprovides sufficient ignition barrier (check your localbuilding code). When this ignition barrier issupported on a stud wall, the cavities of this wall maybe filled with supplemental insulation. It is importantthat the airtight foam insulation assembly becontinuous behind the framed wall. No interiorvapor barriers should be installed in order to permitinward drying.

FloorsBasement floor slabs are best insulated underneathwith rigid insulation: both extruded or expandedpolystyrene have been widely used with success.Although the energy savings of sub-slab insulation

are not as significant as basement wall insulation,such insulations do offer a significant improvementin comfort and moisture damage resistance (includingagainst summertime condensation).

When slab insulation is provided, a sheetpolyethylene vapor barrier should be located over therigid insulation and in direct contact with theconcrete slab. As the slab will only be able to dryupward, the slab should be allowed to dry beforefinishes are applied. Impermeable interior floorfinishes such as vinyl floor coverings should also beavoided.

A sand layer should never be installed between thesheet polyethylene vapor barrier and the concreteslab. Sand layers located between the slab and thevapor barrier can become saturated with water, whichare then unable to dry downwards through the vaporbarrier. In this scenario, drying can only occurupward through the slab which typically results indamaged interior floor finishes (Lstiburek, 2002).

Interior Rim Joist Insulation

Air barrier needed to protect the rim joist from interiormoisture-laden air in summer and winter

Seams in rigid insulation and joints to other materials sealedto provide air barrier

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Rigid Insulation with Supplemental InsulatedFrame Wall Assembly

Rigid foam insulation board assembly must providecontinuous air barrier and capillary break around concretefoundation

Rigid insulation assembly must be continuous behind woodframe wall

Rigid insulation is vapor semi-impermeable or vapor semi-permeable (foil facing or plastic facing not present)

Wood frame wall cavity to be insulated with unfacedfiberglass or damp spray cellulose

No interior vapor barrier installed

Interior Spray Foam Encapsulates ConcreteSurfaces

Least risky interior wall insulation approach

Cold concrete foundation wall must be protected frominterior moisture-laden air in summer and winter

Rim joist assembly must be insulated with air impermeableinsulation

Interior air cannot access concrete condensing surface orrim joist condensing surface due to spray foam layer

Spray foam insulation layer is vapor semi-permeablepermitting inward drying

Spray foam must be covered with fire/ignition barrier

Suggestions for Further Research:

“Understanding Basements,” Building Science Digest-103, www.buildingscience.com.

“READ THIS: Before You Design, Build, or Renovate,” Building Science Primer-040, www.buildingscience.com.

Lstiburek, Joseph W.; Builder’s Guide Series, Building Science Press, 2006.

Lstiburek, Joseph W.; “Investigating and Diagnosing Moisture Problems,” ASHRAE Journal, December 2002.

Lstiburek, Joseph W.; “Understanding Vapor Barriers,” ASHRAE Journal, August 2004.