Blower Door Test vs EPC: Measured Airtightness vs a Rating
Blower door test vs EPC (Energy Performance Certificate).
Reviewed by George Sora, Certified Passive House DesignerUpdated June 2026Quick answer & key takeaways
5 min read- Bottom line: A blower door test measures the real airtightness of a building; an EPC is a simplified rating that assumes a default air-leakage value rather than measuring it.
- When Blower door is enough: You need real airtightness data
- When EPC is the better choice: You legally need a certificate to sell or let
- When you need both: You need the certificate and want to genuinely improve performance
- Biggest misconception: “A good EPC means an airtight, efficient home.” — An EPC is a rating from assumptions; it does not measure airtightness. A good band can still hide significant air leakage.
- Retrofit IQ’s approach: We measure airtightness directly rather than accepting an EPC's default assumption, because a tested figure — and the leakage paths behind it — is what actually informs sealing, modelling and heat-pump sizing.
Quick answer
A blower door test measures the real airtightness of a building; an EPC is a simplified rating that assumes a default air-leakage value rather than measuring it. They are not comparable: the blower door produces hard data used in heat-loss modelling, heat pump sizing and Passive House work, while an EPC gives a broad-brush rating from assumptions. If you want to know how your home actually performs, the EPC will not tell you — the measurement will.
At a glance
| Attribute | Blower door test | EPC (Energy Performance Certificate) |
|---|---|---|
| What it does | Measures real air leakage | Rates energy efficiency from assumptions |
| Airtightness | Measured (ach@50 / q50) | Assumed default |
| Method | Calibrated fan + gauge | RdSAP data entry + software |
| Accuracy vs reality | High | Indicative only |
| Used for | Modelling, heat pump sizing, Passive House, Part L | Compliance, marketing, EPC band |
| Repeatable | Yes | Depends on assessor inputs |
| Reveals causes | Yes, with tracing | No |
What is Blower door test?
A calibrated fan pressurises or depressurises the building to a reference pressure (usually 50 Pa) and measures the actual air leakage as an air-change rate (ach@50) or permeability (q50). It is a direct, repeatable measurement of how airtight the building really is.
What is EPC (Energy Performance Certificate)?
A standardised energy rating produced with RdSAP, largely from visual data and default assumptions. It rates a home A–G for compliance and marketing purposes but does not measure airtightness, heat loss or the real causes of poor performance.
What each method measures — and what it doesn’t
Blower door
- The total air leakage of the envelope at 50 Pa
- Air permeability (q50) or air-change rate (ach@50)
- A repeatable baseline for before/after works
- With tracing, the location of leaks
- The overall energy rating or running cost band
- Insulation levels (airtightness and insulation are independent)
EPC
- An estimated efficiency band (A–G)
- Indicative running costs and CO₂ on standard assumptions
- Recommended generic improvement measures
- Actual air leakage — it is assumed, not tested
- Real heat loss or thermal bridging
- The specific causes of cold, draughts or damp
The building science
Air leakage is one of the largest and most uncertain components of a building's heat loss. Uncontrolled infiltration carries warm air out in winter and moist air into cold construction, where it can condense. A blower door standardises the pressure at 50 Pa and measures the resulting flow, so the airtightness figure is real, repeatable and directly usable in a heat-loss calculation.
An EPC takes a different route. RdSAP — the reduced Standard Assessment Procedure — is built for consistency across millions of homes, not precision for one. It fills gaps with default assumptions, including a default airtightness value, because measuring every home is impractical at that scale. The output is a rating band, useful for compliance and comparison, but it is not a measurement of how your specific home behaves.
This is why EPCs and energy bills so often disagree, and why an EPC cannot reliably size a heat pump or prioritise a retrofit. The infiltration assumption alone can be wildly off in a draughty older house, and the EPC has no mechanism to discover that — it never tested anything.
For any decision that depends on real performance — heat pump sizing, deep retrofit, Passive House or EnerPHit, or simply understanding why a home is cold — the blower door is the tool that produces trustworthy numbers. The EPC remains useful for what it is: a standardised rating, not a diagnostic.
Key differences
- The blower door measures; the EPC assumes.
- Airtightness is tested by the blower door and defaulted by the EPC.
- Blower door data feeds modelling and sizing; EPC bands do not.
- An EPC cannot tell you why a home performs poorly; a measured test can.
Common misconceptions
Myth: A good EPC means an airtight, efficient home.
An EPC is a rating from assumptions; it does not measure airtightness. A good band can still hide significant air leakage.
Myth: An EPC is accurate enough to size a heat pump.
It is not. Heat pump sizing needs measured heat loss and airtightness, not RdSAP defaults.
Myth: A blower door test gives me an energy rating.
It gives an airtightness figure, which is one input to a rating or model — not the rating itself.
Real-world situations
Planning a heat pump and want it sized correctly
A blower door test plus a measured heat-loss survey; the EPC is not accurate enough for sizing.
Selling or letting and need a certificate
An EPC is the required document; commission a blower door only if you want to understand or improve real performance.
Home feels draughty despite a decent EPC
A blower door with smoke tracing to measure and locate the leakage the EPC assumed away.
Deep retrofit or Passive House/EnerPHit
Blower door testing is essential — the EPC plays no part in verifying airtightness.
Which do you actually need?
When Blower door is enough
- You need real airtightness data
- You are sizing a heat pump or modelling heat loss
- You are pursuing Passive House, EnerPHit or Part L verification
When EPC is the better choice
- You legally need a certificate to sell or let
- You want a broad efficiency band for marketing
- You only need a standardised comparison figure
When you need both
- You need the certificate and want to genuinely improve performance
- You are retrofitting and must satisfy compliance too
What Retrofit IQ checks on site
We measure airtightness directly rather than accepting an EPC's default assumption, because a tested figure — and the leakage paths behind it — is what actually informs sealing, modelling and heat-pump sizing.
- Calibrated blower door test to ATTMA/BS EN ISO 9972 methodology
- Air-change rate (ach@50) and permeability (q50) reported clearly
- Smoke tracing and thermal imaging to locate the leakage paths
- A baseline figure for verifying improvement after sealing works
- Interpretation of the result for heat-loss modelling and heat pump sizing
- Plain-English explanation of what the number means for your home
What a Certified Passive House Designer recommends
An EPC and a blower door test are not alternatives — they exist for different reasons. If you need a certificate to sell or let, get the EPC; if you need to know how your home actually performs, measure it. The trouble starts when an EPC is treated as a performance survey and decisions are made on assumptions it was never able to test.
For anything that matters financially — a heat pump, a deep retrofit, a Passive House standard — I want a measured airtightness figure and a real heat-loss assessment. That is the difference between designing for the building you have and hoping the building matches the spreadsheet.
— George Sora, Certified Passive House Designer, Founder, RetrofitIQ
Reviewed using current building physics principles and Passive House methodology.
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Read comparisonFrequently asked questions
Does an EPC measure airtightness?+
No. An EPC uses RdSAP, which assumes a default airtightness value rather than testing it. Only a blower door test measures the real air leakage of your home.
Is a blower door test better than an EPC?+
They do different jobs. A blower door measures real performance; an EPC is a standardised rating for compliance and marketing. For performance decisions, the test is far more useful.
Why doesn't my EPC match my energy bills?+
Because RdSAP uses default assumptions, including for airtightness, so the rating is indicative rather than a precise prediction of your actual use.
Can I use my EPC to size a heat pump?+
Not reliably. Heat pump sizing needs measured heat loss and airtightness; an EPC's assumptions can lead to incorrect sizing.
What does ach@50 mean?+
Air changes per hour at 50 pascals — how many times the home's air volume leaks out per hour at that test pressure. Lower is more airtight.
Is a blower door test destructive?+
No. The fan seals temporarily into a doorway and leaves no marks; the test is entirely non-destructive.
Do I legally need a blower door test?+
It is required for new dwellings under Part L and for Passive House/EnerPHit certification, but not for an existing home unless you want the data.
Will the test tell me where my home leaks?+
The test measures how much leaks; combined with smoke tracing and thermal imaging it also shows exactly where.
Can a leaky home still have a good EPC?+
Yes — because the EPC assumes airtightness rather than measuring it, a draughty home can still score reasonably.
How long does a blower door test take?+
Set-up and a single-point test take roughly one to two hours for a typical home; tracing adds time.
Does wind affect the result?+
Strong, gusty wind reduces accuracy because it adds pressure to the building. We note conditions and retest if needed.
Is the EPC useless then?+
No — it is fine for its purpose as a standardised rating. It simply is not a measurement of how your specific home performs.
Do you provide a report?+
Yes — a clear report with the airtightness figures, the leakage paths found and what they mean for modelling and improvement.
Who carries out the test?+
A Certified Passive House Designer, so the result is interpreted in the context of real building performance.
Need professional advice?
A comparison like this helps you understand the theory, but every property behaves differently. The only reliable way to establish the real cause in your home — rather than guessing — is professional building performance diagnostics. At RetrofitIQ we verify buildings using the appropriate combination of investigations:
- Thermal imaging
- Blower door testing
- Moisture investigation
- Building physics assessment
- Passive House methodology