The result
The site team kept receiving the same complaint during internal QA and early occupation:
Rooms felt stale, windows were streaming, and trickle vents seemed to “do nothing.”
Although vents were fitted, the homes were behaving as if no background ventilation existed.
ATSPACE carried out background ventilation testing, traced the issue to a repeat installation fault at the vent airway, and provided a fast, repeatable fix.
Once corrected, airflow and vent operation improved across all affected plots, reducing moisture risk and preventing a wider customer‑care issue.
Project snapshot
Service: Background ventilation testing + defect investigation
Client: Main contractor + window installation contractor
Site: Fernwood Grove, Plots 31–46, 7 Maple Rise, Derby DE21 4LQ
Building type: New build houses, two storey, high‑airtightness fabric
Ventilation strategy: System 3 MEV with background ventilators
Programme stage: Late completion + early occupation
Compliance driver: Approved Document F + post‑completion QA
ATSPACE delivery: Vent function checks, airway checks, fault investigation, fix plan, verification checks, written summary
Team: ATSPACE compliance engineer + site coordinator
What the site was seeing
Symptoms were consistent across plots:
- trickle vents stiff or jammed
- vents opened but airflow felt weak or absent
- rapid condensation in bedrooms on cold mornings
- rooms feeling stuffy even with MEV fans running
With high‑airtightness homes, background ventilators matter even more — if they fail, the entire ventilation strategy is compromised.
Why this matters commercially
Non‑functioning trickle vents create:
- mould and condensation complaints
- reputational risk
- high volumes of repeat callbacks
- disputes between installers, decorators and M&E
- residents closing or disabling ventilation systems due to discomfort
The contractor needed a definitive cause and a fix that could be rolled out quickly across the phase.
What ATSPACE did
Step 1: Confirm ventilation strategy and expected provision
We confirmed the homes relied on trickle vents as part of the System 3 MEV strategy — making vent performance critical.
Step 2: Test a representative sample
We tested multiple plots across streets and house types to check if the issue was isolated or systemic.
Step 3: Functional vent checks
In every affected room, we checked:
- smooth opening/closing
- slider obstruction from paint or sealant
- visible blockages at the outlet
Step 4: Airway checks (the real cause)
This is where most issues originate. We checked:
- whether the vent airway was clear
- external slot unobstructed
- internal finish not blocking airflow
- no protective film/foam left in place
- vent fitted correctly for frame type
Step 5: Identify the repeat installation issue
We found a consistent installation fault:
The vent airway was partially obstructed at the window‑head detail.
This made the vent look correct but operate incorrectly.
Step 6: Provide a repeatable fix + verify
We issued a simple, fast fix that could be applied across affected plots, then returned to verify airflow improvement.
The root cause and the fix
Root cause
The vents were installed, but the airflow path was blocked by a repeated finishing detail at the frame head.
The vent “looked fine,” but in reality the airway was choked.
Fix
The contractor:
- corrected the interface detail
- ensured the airway was fully open
- confirmed the vent outlet was not sealed by trim
- checked operation after fix
The method was then rolled out across all plots with the same detail.
Outcome
After correction, the site saw immediate improvements:
- vents operated smoothly
- airflow increased noticeably
- reduced condensation risk
- fewer early‑stage complaints
- repeat defect removed from the build process
The window installer and finishing teams also gained a clear QA checkpoint to prevent the issue recurring.
Common mistakes this project avoided
- replacing vents even though vents weren’t the problem
- blaming resident behaviour instead of checking airflow
- sealing visible edges while leaving the airway blocked
- treating condensation as a heating issue rather than a ventilation issue
- fixing only one plot without addressing repetition across the phase
CTA
If your trickle vents are fitted but not working, ATSPACE background ventilation testing will identify whether the problem is the vent, the airway, or the finishing detail — and provide a fast, repeatable fix plan.
Ask for:
- investigation of trickle vents not working
- background ventilation testing across sample or full plots
- repeat‑defect fix plans for window/finishing details
- verification checks after corrective works
Frequently asked questions
Why do trickle vents sometimes feel like they do nothing?
Because the airway can be blocked by finishing materials or incorrect installation, even when the vent looks correct.
Can a blocked trickle vent cause condensation?
Yes — without background ventilation, moisture builds quickly, especially overnight.
Is the issue usually the installer or the finisher?
It can be either. Testing must check the full air‑path, not just the vent.
Can the same fault affect multiple plots?
Yes — if it’s a repeated detail, it usually affects every plot built with that detail unless corrected.
