Air and moisture can get into a home a number of ways. Convective transfer involves moving air, such as a draft around a window or door, electrical boxes, or other wall penetrations. Diffusion refers to moisture moving through a material, similar to a sponge soaking up water. Cavity wall construction typically with fiberglass batt placed into the cavities between studs allows air to pass through and requires an additional, separate vapor barrier, typically polyethylene sheet (Visqueen) or kraft paper facing on the fiberglass batts. Continuous insulation (EPS foam or XPS foam) can also be used as a vapor barrier. Vapor barriers, or more properly worded, retarders, must be sealed properly to prevent air infiltration and vapor transmission.
Insulating Concrete Forms (ICFs) do not require a separate vapor barrier or retarder. ICF walls are mass walls, characterized by a solid structure from the interior to exterior, with no voids or cavities. The EPS and the concrete are bonded together at the inner face of the form, eliminating airflow through the wall. BuildBlock’s EPS at 2.5” of thickness per side, provides a perm rating of .408 and classifies the ICF form as a Class II vapor retarder. The monolithic concrete core also serves as an air barrier and has its own perm rating at 6 inches thickness of .533. (Estimated according to the average perm rating of 3.2 per inch of concrete; per ASHRAE).
The concrete core of a BuildBlock ICF wall effectively seals the wall from air infiltration. Being a poured product, it will fill any openings or gaps completely, eliminating air movement. The EPS forms will bond to the concrete, creating a seal, and breaking the path for moisture to penetrate the wall. Additionally, the insulative effects of the EPS can serve to maintain a more stable inner wall temperature, lessening the condensation seen in cavity walls. Any condensation that does occur, has no effect on the EPS or the concrete. Concrete and EPS are both inorganic and do not serve as a food source for mold or mildew, and the highly alkaline environment of the concrete further reduces the incidence of mold and mildew growth.
Applying additional vapor barriers can potentially trap moisture, and allow it to run down or evaporate up into more moisture sensitive areas of the structure. A properly sized HVAC unit will ensure that any moisture present will be quickly reduced to a level below 50% humidity, thus stopping mold and mildew growth.
Below is additional information from the IRC 2012 regarding Vapor Barriers.
R702.7 VAPOR RETARDERS.
CLASS I OR II VAPOR RETARDERS ARE REQUIRED ON THE INTERIOR SIDE OF FRAME WALLS IN CLIMATE ZONES 5, 6, 7, 8 AND MARINE 4.
1. BASEMENT WALLS.
2. BELOW GRADE PORTION OF ANY WALL.
3. CONSTRUCTION WHERE MOISTURE OR ITS FREEZING WILL NOT DAMAGE THE MATERIALS.