The Result
Plot 17 was heading for a failed airtightness test. The build looked finished, but key junctions were not forming a continuous airtight layer. ATSPACE provided on‑site airtightness design advice, targeted the real leakage routes, and helped the team close them out in a practical way. Plot 17 passed its Part L airtightness test first time, with no retest delay and no disruptive strip‑out.
Project Snapshot
Service: Air leakage on‑site design advice
Client: Regional housebuilder
Site: Oakfield Grange, Waverton Road, Chester CH3
Build type: 3‑storey new‑build townhouse
Plot: 17
Programme stage: Late second fix, pre‑test
Compliance driver: Approved Document Part L
ATSPACE delivery: On‑site design advice visit, leak path checks, close‑out actions, recheck prior to test
What Was Happening on Plot 17
The site team had good airtightness results on earlier plots, but Plot 17 differed. It had more services, a slightly altered layout, and extra penetrations added late. It was also the first plot of this house type to reach test stage — and first‑of‑type plots often reveal design coordination gaps.
From a quick walk round, everything looked fine. The problem was continuity behind the finishes.
If Plot 17 failed, the programme would slip — and every similar plot behind it would be at risk.
Why Airtightness Fails Even When the Build Looks Finished
Airtightness failure rarely comes from one big leak — it comes from multiple small gaps that add up.
Typical causes on plots like this:
- Service penetrations boxed in but not sealed
- Party wall drylining stopping short of the airtight layer
- Loft hatches with uneven compression
- Gaps behind meter/consumer unit zones
- Floor edge junctions and stair void interfaces
- Penetrations in kitchens/bathrooms hidden behind units
Once finishes go in, access disappears — making fixes costly.
What ATSPACE Was Asked To Do
The brief:
- Confirm if Plot 17 was genuinely test‑ready
- Identify the likely leakage paths
- Provide practical airtightness design advice
- Reduce retest risk
- Create a repeatable method for future plots
What ATSPACE Did On Site
1. Focused airtightness design advice walkthrough
We followed the airtightness line through the building — not the finishes.
Key areas checked:
- Under‑stairs services and boxing
- Soil stack penetrations at floor/ceiling junctions
- Loft hatch and ceiling penetrations
- Meter cupboard interfaces
- Kitchen/utility penetrations behind units
- Party wall junctions (1st & 2nd floors)
- Bathroom/ensuite pipe boxing
- External door thresholds and internal seal continuity
2. Identify the leakage routes most likely to move the result
Three issues were driving the risk:
1) Soil stack and waste routes
Boxing present, airtight layer not continuous behind it.
2) Meter/consumer unit zone
Incoming services had gaps hidden behind the cupboard.
3) Loft hatch
Seal compression inconsistent — visually good but likely to leak under pressure.
3. Provide clear close‑out actions (not generic advice)
Actions tailored to what the trades could do immediately:
- Seal the airtight layer behind boxing, not just at visible edges
- Close gaps around services before finishes blocked access
- Improve loft hatch seal compression and closure pressure
- Seal behind meter cupboards and around incoming services
- Recheck critical junctions after late M&E works
4. Recheck before the test slot
Once close‑outs were done, we reviewed the critical points again — avoiding last‑minute surprises.
Problems Faced On Site (And How They Were Handled)
Problem 1: Trades assumed boxing = airtight
Fix: New rule — anything boxed must be sealed behind before closure.
Problem 2: Incoming services fell between trades
Fix: A named owner for service entry close‑out + added checks before test booking.
Problem 3: Loft hatch performance not being physically checked
Fix: Team added compression/closure pressure checks — not visual only.
The Outcome
Plot 17 passed its airtightness test first time.
Just as important, the team gained a repeatable method for similar plots.
Measured outcomes:
- No failed test
- No retest delay
- No disruption to finishing trades
- A clear set of repeatable close‑out checks
What This Proves
On‑site airtightness design advice works because airtightness fails at interfaces, not on drawings.
Identifying leakage routes early avoids:
- Programme delay
- Costly rework
- Access loss
- Repeat defects
- Trade conflict
Common Mistakes This Project Avoided
- Booking tests based on appearance, not readiness
- Leaving boxing close‑out until after a failure
- Assuming fire stopping = airtight
- Treating loft hatches as minor
- Allowing late penetrations without resealing
CTA
If you have a plot approaching test day and you’re not confident it will pass, ATSPACE can provide on‑site airtightness design advice to identify leakage risks early.
Ask for:
- On‑site airtightness design advice visits
- Readiness checks before test booking
- Repeatable close‑out guidance for house types
- Support to increase first‑time pass rates
Frequently Asked Questions
What is on‑site airtightness design advice?
Practical guidance to help maintain a continuous airtight layer before testing.
When is the best time to bring in airtightness design advice?
Before test day — ideally while junctions are still accessible.
Why do individual plots fail airtightness tests?
Hidden service penetrations, meter cupboard gaps, loft hatch sealing, and boxing that hides rather than seals leaks.
Does design advice reduce retests across the site?
Yes — especially when fixes are applied across multiple similar plots.
Can ATSPACE support after a failure too?
Yes. We can diagnose dominant leakage routes and develop a fast remedial plan.
