Choose between negative-side and positive-side waterproofing for a below-grade wall

Steps

1. Prefer positive-side waterproofing (applied on the water-facing side, i.e., exterior of a foundation wall before backfill) whenever the exterior is accessible during construction, since hydrostatic pressure pushes water against/into the membrane and keeps water out of the structural material entirely.
2. Reserve negative-side (interior) waterproofing for cases where the exterior is genuinely inaccessible (existing structure with no excavation possible, party wall, or a failed positive-side system) — use products specifically rated for negative-side hydrostatic use, since most exterior-designed membranes aren't built to resist being pushed off the substrate from the wrong direction.
3. Treat blindside conditions (membrane installed against a temporary shored wall before the permanent wall is poured) as a distinct third category requiring its own detailing rather than a simple positive/negative choice.
4. Base the decision on accessibility and project timing, not performance alone — positive-side is generally higher-confidence long-term, but often impossible on renovation work, where negative-side (paired with interior drain tile and a sump) becomes the practical option.
5. Confirm the specific product's data sheet (not marketing copy) actually rates it for negative-side hydrostatic performance before specifying it that way.