Vapor barriers or vapor retarders are incapable of allowing foundation walls to dry to the interior. This is an issue. Simply leaving off interior vapor barriers and vapor retarders will not work due to the issues associated with interior vapor diffusion. Additionally, these two methods are incapable of being constructed in an airtight manner using typical production trades and materials and therefore are unable to address the air leakage wetting mechanism. The problems with these two common approaches to interior basement insulation manifest themselves in mold, decay and odors. Approaches to basement construction must not result in mold, decay and odors. The experience acquired by the Building Science Consortium has been reflected in changes that have been made to the Builder’s Guide Cold Climate (Lstiburek, 2001) as well as those for Builder’s Guide Hot-Dry & Mixed-Dry Climate (Lstiburek,2000) and Builder’s Guide Mixed-Humid Climate (Lstiburek,2001).
****** ALL ****** recommended basement interior insulation strategies involve placement of a layer of rigid foam insulation against the foundation wall.
The moisture sensitive interior wood framing and paper faced gypsum board are no longer in contact with the major moisture source – the concrete or masonry foundation wall such as can occur with water trapped behind POLYETHYLENE installed directly against foundation wall.
Moisture dynamics must be considered in detail before insulating a basement wall. Materials used to insulate a basement wall must be selected based on their ability to control the flow of moisture and air as well as heat. Selecting the wrong type of insulation or placing it in the wrong wall assembly often leads to moisture accumulation with subsequent material deterioration and growth of mold. A damp or wet basement that is improperly insulated will lead to deterioration of the building and promote conditions that worsen indoor air quality. A basement wall will remain dry only if it is built to handle all the different ways in which water can move into and through basement walls. Since walls will at times get wet in spite of good design and construction, BASEMENT WALLS MUST ALSO BE ABLE TO DRY. Drying typically means TOWARDS THE INTERIOR.
Basement walls can be wetted by liquid water (bulk flow and capillary suction) and water vapor. However, once materials become wet, they can typically dry only by the removal of water vapor either by evaporation or diffusion. Evaporation requires energy but insulation decreases the flow of energy. Insulated walls cannot dry as easily as uninsulated walls.
The rate at which water vapor moves through materials is “permeability”. Individual water molecules can move easily through permeable materials even if the materials do not permit air flow through them. Other materials are said to be semi-permeable to water vapor because they permit the passage of water molecules at a much slower rate. Materials that allow very little water vapor to pass through them are classified as impermeable. Air transport of water vapor requires an air pressure differenceas well as a pathway or opening between the areas of differing air pressure.
******A vapor barrier on the interior would prevent the walls from drying should they ever get wet.******
Insulating only on the interior side of basement walls presents problems because of ground water and the alternating direction of the vapor drive discussed above. The fact that ground temperature at various depths frequently is much colder than either exterior or interior air temperatures means that condensation can occur on the interior surface of the foundation wall. The interior basement insulation and the finished wall assembly are subjected to potentially significant moisture loads from vapor driven from both the exterior and the interior at different times of the year. While the building industry in the United States has become preoccupied in the past decade with vapor diffusion and vapor barriers in building assemblies, the problem of air-transported water vapor is often ignored. This is unfortunate because air-transported moisture is generally much more of a problem than is the diffusion of water vapor. Air transported moisture can quickly lead to deterioration in moisture sensitive materials. The entire consideration of water vapor has been complicated and confused by the fact that some materials can block the flow of air (an air barrier) as well as the flow of vapor (a vapor barrier). Some research in basement insulation systems has attributed moisture accumulation to vapor diffusion when airflow was not controlled. An effective air barrier is required in basement walls.
However, vapor barriers are typically not needed – particularly on the interior of basement assemblies.
The almost indiscriminate use of vapor barriers (polyethylene or vinyl wall coverings) over the past decade has caused many building failures and facilitated the growth of mold in many buildings. The permeability of materials must be considered before placing them in a particular location within a wall assembly. Otherwise water vapor may become trapped within a wall assembly where it can condense when the temperature is low enough. Any interior basement insulation strategy must successfully handle both the internal and external moisture loads. One proposed solution to this dilemma is to install a vapor barrier on both sides of the interior insulation system. The barrier against the foundation wall is often called a moisture barrier. The main problem with a double vapor barrier wall is that it cannot dry to either the inside or the outside should it ever get wet. In addition, it requires a perfect air barrier on the interior to prevent warm interior air from contacting and condensing on the cold foundation wall where it may be trapped. This type of construction should be avoided.
****** The major change in the past 20 years is the realization that a vapor barrier (usually polyethylene) on the interior side of the basement wall assembly inhibits drying of the wall more than it prevents wetting of the wall. *******
In testing walls that dried the fastest were the ones that did not have a moisture barrier against the foundation wall allowing the wall to dry to the exterior. Unfortunately this design would also allow the wall to become wet from the exterior likely causing condensation on the interior vapor barrier.
Many superficially dry walls will not remain dry when they are insulated. Many walls are dry because of “their ability to continuously evaporate soil-sourced liquid water to the inside.”
Basement wall assemblies with an interior vapor barrier can never dry if they become wet.
The widespread use of a double vapor barrier basement wall has resulted in many failures in some cases within one year of construction (Ellringer, 2002). Extruded polystyrene and cavity batt insulation, with and without a vapor barrier, covered by gypsum board were compared with walls having only a thicker layer of extruded polystyrene and an empty frame wall covered with gypsum board. The walls with an interior vapor barrier did not get wet from the interior during the winter but they did trap moisture during the summer when moisture is moving inward.
Without the vapor barrier, the fiberglass batts would remain dry if interior humidity is not excessive during the summer. Such low interior levels of relative humidity during summer conditions typically can only be achieved with active dehumidification provided by air conditioning or a dehumidifier. Walls with 3.5 inches of extruded polystyrene (XPS) and no vapor barrier performed the best. However,walls with 0.75 inches of extruded polystyrene and 3.5 inches offiberglass batt insulation in the cavity would perform well as long as interior humidity was controlled below 50 percent during the summer. Increasing the extruded polystyrene to 1.0 or 1.5 inches would improve performance even with higher interior relative humidity during the summer months. This part of the analysis assumed that the concrete wall had arelative humidity of 100 percent at the exterior temperature.
Any interior basement insulating wall system must have the following properties: It must be able to dry to the interior should it become wet since the below grade portion of the wall will not be able to dry to the exterior during any time of the year. This precludes an interior polyethylene vapor barrier or any impermeable interior wall finishes such as vinyl wall coverings or oil/alkyd/epoxy paint systems. The wall assembly must prevent any significant volume of interior air from reaching the cool foundation wall. Thus it must have an effective interior air barrier or a method of elevating the temperature of potential condensing surfaces(such as rigid insulation installed directly on the interior of concrete or masonry surfaces).Materials in contact with the foundation wall and the concrete slab must be moisture tolerant; that is the materials should not support mold growth or deteriorate if they become wet - some materials may tolerate being wet without blocking the passage of liquid water through the materials. A capillary break must be placed between these materials and moisture sensitive materials.
If a frame wall is placed interior to the rigid insulation, cavity insulation without a vapor barrier or retarder can be installed between the studs.
Wall Insulation with Foam Sheathing Covered with Gypsum Board either expanded or extruded polystyrene insulating sheathing can be attached directly to the foundation wall. Since extruded polystyrene is more moisture tolerant it should be used if there is any question about the effectiveness of the external drainage system. If additional insulation is desired, cavity insulation can be installed in a frame wall built interior to the foam insulationand covered with 0.5 inch gypsum board or other thermal barrier. |
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