Building a post-on-pad foundation
Post on pad foundations are a common sight in Fairbanks, as they represent one of the least expensive approaches to building on unstable soils – of which we have no shortage. Usually the floor is raised several feet off of the ground, allowing air to flow freely underneath and preventing the heated building from disturbing the frozen ground.
One of the common questions of building post-on-pad foundations is whether to install a vapor barrier in the floor. While it’s standard practice to install plastic sheeting on the inside of exterior walls and ceilings, to prevent moisture from entering the insulated spacings and condensing during the winter, the floor is a different story. It may seem logical to install plastic sheeting over the tops of the floor joists before laying down the subfloor sheathing, yet it can do more harm than good.
If any rainwater leaks through the joints in the subfloor before the roof is on, it will be stopped by the plastic, and the floor may not be able to dry out quickly enough to avoid mold and decay. The same holds true if liquids are spilled on the floor once the house is finished, or if a major plumbing leak occurs. The plastic also prevents the use of subfloor adhesive between the joists and sheathing, which is designed to prevent squeaks in the floor. Modern subfloors are usually sheathed with industry-standard ¾-inch tongue and groove exterior-rated plywood, or oriented strand board.
With post-on-pad construction, the subfloor sheathings are comparatively less at risk for moisture issues to begin with. That’s partly because warm indoor air leaves at the top and is replaced by outside air drawn in at the bottom, so water vapor moves upwards – away from the floor. In addition, the combination of thickness and types of glues used in ¾ inch plywood and OSB subfloor sheathings means they are less likely to absorb any moisture that might be forced into the house.
To minimize air leakage through the floor, the unsupported seams can be caulked with an adhesive sealant, such as a silicone, that bonds well with wood. The decision of whether to use or omit a plastic vapor retarder in floors using post and pad construction ultimately rests with the engineer or the local code official who may have reasons specific to the project or building site.