Victorian Suspended floor, new subfloor

Moisture Control Comes First If the void is wet year-round, the priority should be: •Ensuring air bricks are clear and cross-ventilation is effective. •Ground covered with 1200g DPM (if not already done). •All timber isolated from masonry with DPC (which you’ve correctly done). Chipboard alone will not solve a moisture issue. Subfloor Choice – P5 vs Ply/OSB •P5 chipboard is moisture-resistant, but not moisture-proof. In persistently damp environments it can swell over time if conditions aren’t properly controlled. •Structural plywood (Class 2 or WBP) is more dimensionally stable and performs better long-term in variable moisture conditions. •OSB3 is acceptable but I generally favour plywood for higher-end glued hardwood finishes. For glued herringbone, the subfloor must be: •Structurally rigid •Low moisture content •Stable with minimal movement Plywood typically provides a more reliable base for adhesive bonding. Membrane Strategy: I would not omit membranes in a damp void: Recommended build-up: •Breathable membrane under insulation (to prevent sagging and allow airflow). •Rigid insulation between joists (not quilt if void is damp). •Vapour control layer on the warm side (above insulation, below subfloor). Skipping this increases risk of interstitial condensation and timber movement. Professional Opinion: In a wet void, I would install: •Treated structural joists (as done) •Rigid PIR between joists •Vapour control layer •18–22mm structural plywood screwed (not just glued). For a glued hardwood herringbone finish, stability is critical. Plywood gives better long-term security than chipboard in these conditions. If the void remains wet despite ventilation, I would investigate that first,because no subfloor choice will compensate for chronic moisture.

This is a classic "old school vs. new school" debate in floor construction, but given your specific situation—a deep, wet void year-round—your instincts are actually much sharper than the specialist's "standard" recommendation. Here is a breakdown of why you should be cautious with P5 chipboard in a damp environment and how to best prep for a high-end herringbone finish. 1. The P5 Chipboard Problem P5 is "moisture resistant," but it is not moisture-proof. • The Risk: In a wet void, humidity is constant. P5 chipboard is made of wood particles and glue; if it absorbs ambient moisture from the air below, it can swell, particularly at the tongue-and-groove joints. • The "Telegraphing" Effect: Since you want herringbone hardwood, any slight movement or "peaking" at the chipboard joints will telegraph straight through to your expensive hardwood, causing it to lift or creak. • The Specialist's Logic: They want to glue the board to the joists to prevent squeaks. While valid, you can glue Plywood or OSB3 just as easily. 2. Plywood vs. OSB3 vs. P5 If you are investing in a glued hardwood floor, the subfloor is the most important part of the job. Stick with P5 all day long.

​Hello 👋 With a wet void, moisture management is critical. P5 chipboard is moisture-resistant but not waterproof; in a deep, damp environment, Marine Grade Plywood is the superior professional choice for longevity. You must prioritize cross-ventilation via clear air bricks to stop moisture rising. I strongly advise against skipping the membrane; a vapour barrier on the warm side of your insulation is vital to prevent interstitial condensation from rotting your new joists from the inside out.