Case study · Subfloor ventilation
A damp subfloor, measured before and after
A rural home, Coffs Harbour region, NSW · Subfloor ventilation upgrade
You can usually tell when something's not right: a musty smell, a room that never quite feels dry, a floor colder than it should be. But you can't see under the floor, and getting it wrong is expensive. So we didn't assume. We measured.
The readings indicated a subfloor that wasn't breathing well: CO₂ well above the outdoor reference, and elevated moisture recorded in the timber. After a subfloor ventilation upgrade, we came back and re-measured the exact same points, on the same gear, under matched closed-up conditions, with the day's conditions recorded both times. The numbers moved, and we can show you both sets.
Subfloor CO₂ (air movement)
Fresh outside air ~506 ppm reference (dashed)
14 Apr
4 Jun
Verified Improvement: dropped ~70-83%, back to about fresh outside air
Subfloor timber moisture
~20% decay-risk line (dashed)
14 Apr
4 Jun
Verified Improvement: now below the ~20% timber decay-risk threshold
Before worksAfter works
01Measure: what we recorded first
Rather than open things up and form an opinion, we started with instruments. At the first assessment we recorded, point by point:
| What we measured | Reading | What it indicated |
|---|---|---|
| Subfloor CO₂ | 2,802 ppm | Consistent with air sitting still: very limited ventilation indicated |
| Outside air (control) | 506 ppm | Fresh-air reference on the day |
| Kitchen bench CO₂ | 573 ppm | Normal for a lived-in room |
| Subfloor timber moisture | ~29% MC | In the range where timber decay risk sits |
| Subfloor air humidity | 73% RH | Elevated: moisture held in the subfloor air |
| Timber / surface humidity | 85.8% RH | Higher again at the timber: moisture held in the structure, not just the air |
| Floor perimeter (moisture) | Elevated to high | Across several points: not one spot, a pattern |
Instruments: Aranet4 (CO₂, humidity, temperature), Tramex MEX5 (timber & material moisture), thermal camera at baseline.
02Understand: what the readings were telling us
The recorded pattern didn't point to a single leak. Moisture readings were elevated across wall junctions and the subfloor, consistent across different materials and locations, and it concentrated where air moved inside the home, not where water would enter from outside.
Put together with a subfloor reading 2,802 ppm of CO₂ and 73% humidity, the readings pointed one way: a subfloor that wasn't ventilating well, with an elevated moisture load indicated up through the structure. The pattern indicated a system issue, not one isolated thing to chase.
03Recommend: a targeted, measurable improvement
The recommendation followed the evidence: open the subfloor up so air can move through it. New openings were installed to establish cross-flow ventilation: air in one side, out the other, to carry moisture out instead of letting it sit.
Crucially, we set this up so it could be verified, not just assumed. Same measurement points marked, so we could come back and compare like for like.
04Verify: we came back and re-measured
On 4 June we returned and re-measured the same points, with the same instruments, under the same closed-up conditions as the original assessment. The two visits sat in different months, so the day's temperature and humidity were recorded alongside each set of readings, and the comparison is read with that context.
| Metric | Before (14 Apr) | After (4 Jun) | Result |
|---|---|---|---|
| Subfloor CO₂ | 2,802 ppm | ~480-854 ppm | Verified Improvement: ~70-83% lower |
| Subfloor timber moisture | ~29% MC | 13.2% MC | Verified Improvement: below ~20% decay-risk |
| Kitchen CO₂ | 573 ppm | 483-502 ppm | Stable / low |
| Outside control | 506 ppm | 474-490 ppm | Reference held |
05What we won't overclaim
We also achieved cross-flow ventilation through the subfloor and measured it on the day. But the verification day was windy, and we didn't have a matching windy-day airflow reading from before the works to compare against. Airflow changes a lot with wind, so we report the cross-flow as achieved and measured, not as a clean before/after number.
That distinction matters, and it's how we work: we'll only put a "before and after" on a figure when the two readings were taken under genuinely comparable conditions. Everything else is reported as exactly what it is.
06What happened next
With the subfloor now recording substantially lower moisture and CO₂, the home was ready to move on to the flooring stage, backed by measured evidence. The owners also have the option to keep the subfloor tracked through the seasons, so any change is caught early rather than discovered late.
The reference points we measure against
We don't invent our thresholds. Two from this case:
Timber moisture ~20%. Wood-decay fungi generally need timber above about 20% moisture content to take hold; kept below that, the risk is low. The subfloor timber came back at 13.2%, under that line.
CO₂ as a ventilation read. Indoor CO₂ is a recognised indicator of how well a space is exchanging air with the outside (~400+ ppm outdoors). A high reading means the air is sitting still: it is a ventilation measure, not a health claim. The subfloor's 2,802 ppm indicated one thing: air that was barely moving.
Sources: US Forest Products Laboratory; ASHRAE position document on indoor carbon dioxide.
Your home might be telling you something different.
A damp bathroom, condensation on the windows, a room that won't warm up. The subject changes; the method doesn't. We measure what's happening, work out why, and, if you go ahead with works, come back and check it moved.
Measure → Understand → Recommend → Verify
The Joe Builds way, in one job
We measured what the home was doing, worked out why, recommended targeted works, then came back and verified the change with numbers, under matched conditions. No hunches. No "trust us." Just recorded evidence, before and after.
Start with a measured read of your homeEvery figure in this case study comes from recorded on-site measurements. Home and owner details have been kept private.