Material · Dimensional stability
Moisture and Dimensional Stability: What Humidity Does to a Door
Every material in a door is in constant negotiation with the water vapor around it, and how it handles that negotiation is the best predictor of whether the door stays flat and keeps working. Here is the honest account for both solid wood and a premium MDF core, including where each one fails.
Solid wood is hygroscopic. It absorbs moisture when the air is humid and releases it when dry, changing dimension mostly across the grain[5]. That movement is uneven across a board, producing cupping and bowing, and it is seasonal, which is why solid wood doors bind in a wet summer and gap in a dry winter[5]. Under sustained load with humidity cycling, wood shows mechano-sorptive creep and can take a set[5]. This does not make solid wood bad; it makes it a material that must be detailed to move.
MDF behaves differently. It has no grain, so it does not cup along a grain direction, and its response is more uniform across the panel[2][5]. Its vulnerability is different: unsealed MDF absorbs water at exposed faces and edges and swells in thickness, and that thickness swell does not fully reverse[4][5]. The relevant measurement is thickness swell per ASTM D1037, typically after a defined water-soak[4]. The rules follow from the mechanism: seal the surfaces, pay special attention to edges and penetrations, and specify a moisture-resistant grade where the environment demands it. Moisture-resistant MDF grades are formulated to resist thickness swell better than standard grades in damp conditions[2][5]. They are not waterproof, and no fiberboard should be treated as if it were.
Sealing is not cosmetic; for MDF it is functional. Because thickness swell begins at exposed surfaces, a continuous sealed film is what keeps ambient humidity from reaching the fiber[4][5]. Edges and cut penetrations are the most exposed and the most important to seal well, since they wick moisture fastest. A well-sealed panel and a poorly sealed panel from the same board can behave like two different materials in the same room.
Vaultwood Core™ is specified as a premium MDF core at 45 to 49 lb/ft³, with the properties that support dimensional stability in a finished door: modulus of elasticity roughly 450,000 to 500,000 psi for stiffness, modulus of rupture over 4,500 psi reaching to 5,500, and internal bond at or above about 90 psi[11]. It meets EPA TSCA Title VI and CARB Phase 2, with no-added-formaldehyde options[7][8][11]. As with any MDF, moisture performance in service depends on being finished and sealed correctly and on selecting the right grade. We state this plainly because a sealed, appropriately specified panel is where the material performs as intended[4][5][11]. Published moisture and thickness-swell figures for other materials vary by species, grade, and manufacturer and should be confirmed against a specific grade sheet.
Failure conditions, plainly
Standing water or sustained direct wetting will eventually defeat any fiberboard, so do not use interior MDF grades in exterior or wet-service locations; unsealed or poorly sealed edges are the most common failure point; solid wood fails differently, cupping, warping, and creeping under load with humidity cycling when not detailed to move[5]; and neither material should be specified without matching the grade and the finishing system to the actual exposure.
The honest summary: solid wood moves seasonally and must be allowed to; MDF stays uniform but must be sealed and grade-matched to keep water out. Pick the material whose failure mode you can most easily design against.