The Result
A new‑build Passive House achieved a first‑time pass on its blower‑door test, meeting the 0.6 n50 Passivhaus limit without remedial scramble or retest delays. ATSPACE supported the project with early‑stage checks, a pre‑finishes walkthrough, and a final readiness process that kept the airtightness line continuous and protected from late trade damage.
Project Snapshot
Service: Passive House air leakage testing
Client: Main contractor delivering certified Passivhaus dwellings
Site: The Mews at Stonely Park, Plot 4, 2 Maple Court, Stamford PE9 3FN
Building type: Semi‑detached pair (case study focused on Plot 4)
Construction: Timber frame, airtight membrane, taped junctions, service zone, MVHR
Programme stage: Pre‑completion
Performance target: n50 ≤ 0.6
ATSPACE delivery: Pre‑plaster walkthrough, mid‑build check, final blower‑door test + reporting
Engineers: ATSPACE accredited airtightness engineer + coordinator (no names listed)
Why First‑Time Passes Are Harder on Passivhaus
A standard new build might tolerate a close pass. Passivhaus does not.
A first‑time Passivhaus airtightness pass requires three things:
1. The airtightness line must be deliberate
Passivhaus homes have a defined airtightness layer. If any trade treats it as optional, performance suffers.
2. Penetrations must be controlled
Every penetration needs a method, a product, and an owner.
3. Quality must be repeatable
Good airtightness is not one “hero plot” — it’s consistent detail applied every time.
The contractor wanted a repeatable method so later plots would not drift.
What ATSPACE Was Asked To Do
- Support the site team with checks that reduced failure risk
- Confirm readiness before the certification test
- Deliver the final blower‑door test and certification‑ready reporting
- Standardise “what good looks like” for future plots
What ATSPACE Did That Made the Pass Predictable
Step 1. Pre‑plaster walkthrough to lock in the airtightness line
The biggest gains happen before finishes cover junctions.
We checked:
- membrane continuity
- service penetrations
- floor‑to‑wall transitions
- window/door preparation for taped returns
- ceiling & loft penetrations
- plant/ventilation penetrations
Issues were fixed early while access was open.
Step 2. Mid‑build check to stop drift
Airtightness often starts strong and then deteriorates.
We confirmed:
- penetrations sealed consistently
- tapes applied to correct substrates
- service zone rules followed
- design changes hadn’t created new leak routes
This stopped repeat defects before they spread.
Step 3. Final readiness check before testing
We confirmed:
- no late penetrations
- loft hatch and access panels sealing correctly
- plant room penetrations closed out
- window/door seals complete
- thresholds continuous
- service void interfaces complete behind kitchens/utility zones
Step 4. Final blower‑door test and reporting
We delivered the final blower‑door test under controlled conditions and provided certification‑ready evidence.
The Problems We Prevented Before They Became Retests
Risk 1. Window return tape continuity
Long junction lines — even a small break can affect n50.
Risk 2. MVHR and duct penetrations
Ventilation penetrations must be airtight, not just tidy.
Risk 3. Service zone discipline
Rule reinforced: if a service crosses the airtight layer, it must be sealed immediately.
Risk 4. Loft hatch and ceiling penetrations
Compression + continuity confirmed ahead of test day.
Outcome
Plot 4 achieved the 0.6 n50 target on the first attempt.
Benefits included:
- no retest delays
- no disruptive rework behind finishes
- a repeatable airtightness checklist
- clearer trade‑level ownership
- stronger confidence heading into Passivhaus certification
What This Proves
Passivhaus airtightness success is built early.
If you wait until the final test to find leaks — you pay for it in rework and programme disruption.
A simple staged airtightness process makes first‑time passes normal.
CTA
If you want to hit 0.6 n50 first time on your Passive House project, ATSPACE can support with:
- staged airtightness checks
- pre‑plaster walkthroughs
- readiness checks before testing
- final blower‑door testing + certification reporting
Frequently Asked Questions
Do we need more than one airtightness test on a Passivhaus project?
Not always — but staged checks reduce the risk of a failed final test.
What are the biggest Passivhaus leakage risks?
Penetrations, window returns, thresholds, plant rooms, ceiling interfaces, and connected voids.
Can you help standardise airtightness across multiple plots?
Yes — repeatable details and simple checklists drive consistency.
