Most clients order a blower door test once, and order it late: when the house is nearly finished. At that point the test is no longer a diagnosis, it is a verdict. Whatever number comes out of the instrument goes into the report, and you live with it.
The test worth paying for is the one nobody talks about: the mid-construction test, run while the airtight layer is still exposed and can still be repaired. The acceptance test only confirms what you achieved six months earlier. If you can pay for one test only, pay for the first.
When is a blower door test actually worth the money?
Airtightness is the one property of the building envelope that cannot be calculated from the drawings. I can work out a wall U-value at my desk and model a thermal bridge, but tightness is created on site, out of thousands of tapes, junctions and penetrations made by three different people in three different weeks. That is why the value of the test is decided by its timing, not by which company you pick.
The ideal moment can be described precisely. The airtight layer (a membrane or vapour retarder in timber frame, the internal plaster coat in masonry) is complete and continuous, windows and doors are installed and sealed, and penetrations for wiring, water and mechanical ventilation with heat recovery are sealed. Plasterboard, screeds and linings are not yet in place. At this moment a leak is found with a hand and fixed with tape the same day, for the price of materials and a few hours of labour.
| Aspect | Mid-construction test | Final (acceptance) test |
|---|---|---|
| Stage of works | Airtight layer complete, finishes not yet installed | Building complete, before handover or certification |
| Purpose | Find leaks and repair them | Document the final figure |
| What you can do with the result | Repair the same day with tape, sealant and grommets | Almost nothing without breaking into finished surfaces |
| Cost of fixing one leak | Materials and hours of labour | Opening up, repair, redecoration, retest |
| Who benefits | Contractor, site supervision, designer | Client, bank, certifier, subsidy programme |
| Retesting after repairs | Normal and desirable | Expensive and unwelcome |
| If it is skipped | Leaks get covered up and are never found | No evidence of the value achieved |
What does the test physically do, and what number comes out?
In a blower door test, a panel carrying a calibrated fan is sealed into an external door frame; it extracts air from the building (depressurisation) or pushes air into it (pressurisation), and the instrument records the pressure difference and the airflow needed to hold it. A series of pressure points is measured and a leakage curve fitted to them; the value at 50 Pa is only a reference point, high enough to swamp the effect of wind. The test is run in both directions and the report carries the mean, because some leaks press themselves shut under pressurisation and open up under depressurisation, so a single-direction measurement systematically flatters or penalises a building. The measuring conditions are set out in the standard EN ISO 9972.
The airflow at 50 Pa yields two different numbers. The n50 value is that airflow in m3/h divided by the internal volume of the building, expressed in 1/h, that is, air changes per hour. The q50 value is the same airflow divided by the envelope area, in m3/(h.m2). Codes and certification schemes ask for the first; the second says more about the workmanship.
Why does identical work score different n50 on two houses?
Because n50 divides the measured leakage by volume, and volume has nothing to do with the quality of the joints. A sprawling bungalow with long facades has considerably more envelope area for the same internal volume: more metres of joint, corners and penetrations. If it was built exactly as well as a compact two-storey house, it gets a worse n50, even though it is not worse work. An illustrative model with deliberately identical workmanship and only the geometry changing:
| House type | Internal volume | Envelope area | q50 | Resulting n50 |
|---|---|---|---|---|
| Compact two-storey house | 350 m3 | 300 m2 | 1.0 m3/(h.m2) | 0.86 1/h |
| Sprawling bungalow | 350 m3 | 450 m2 | 1.0 m3/(h.m2) | 1.29 1/h |
| Larger compact house | 700 m3 | 480 m2 | 1.0 m3/(h.m2) | 0.69 1/h |
The same care over every detail, and an n50 spread from 0.69 to 1.29 1/h. Compactness of form therefore helps you meet an n50 target as reliably as tape does, and an articulated shape has to be paid for with better execution of the details. The passive house limit of n50 no greater than 0.6 1/h at 50 Pa is meanwhile fixed. When comparing two contractors, ask for q50; otherwise you are comparing the shapes of houses, not the work.
What does the test not tell you?
It does not tell you where the air is escaping. n50 is a single number for the whole house: it says how much, not where. Locating leaks is a separate job, done during the depressurised phase, when air is drawn inward through every hole. The tools are surprisingly simple:
- A hand held to the junction. A cold stream on the back of your hand in the corner between wall and roof is found faster than by any instrument.
- A smoke pen. Makes the direction and intensity of the flow visible where you already know something is wrong.
- A thermal camera. Under depressurisation and with a sufficient temperature difference it draws the cold tongues of incoming air along the joints. In summer, with a two-degree difference, it shows nothing.
Here is the second reason the mid-construction test is worth more: hunting for leaks only makes sense where you can repair them immediately. On a finished building, the list of locations found is mostly a list of disappointments.
How is a building prepared for the test?
A badly prepared test does not measure the building, it measures the preparation: over-sealing hides the real leaks, inadequate preparation needlessly worsens the number. What gets closed is only what is a deliberate opening for ventilation or extraction: ventilation terminals, the cooker hood, the tumble dryer outlet, the chimney and the combustion air supply to a stove. Traps are not taped over, just filled with water.
What is left exactly as built is everything that forms part of the envelope: windows and doors closed in their normal operating position, window installation joints, roller shutters and their boxes, loft hatches, electrical boxes and service penetrations, in other words precisely the places the test exists to expose. Internal doors, by contrast, are opened so the whole measured volume is pressure-connected. The report has to state what was sealed.
Where does air usually leak in Slovak construction practice?
The list is short and the same year after year. Leaks are not random: they sit where two trades or two forms of construction meet and the detail belongs to nobody.
| Location | Why there | Fix during construction | Fix after completion |
|---|---|---|---|
| Service penetrations (electrical, water, ductwork) | The layer is pierced by a different trade from the one that installed it, and nobody makes it good | Proprietary grommet or sealant, minutes of work | Open up the ceiling void or chase and trace the run |
| Wall-to-roof junction | Two materials and two contractors meet; the detail is in neither contract | Taped along its full length to the detail on the drawing | Break into the ceiling around the whole perimeter |
| Roller-shutter boxes | The box is a cavity open to the outside and tightness is improvised on site | Airtight boxes, or sealing before the lining goes on | Effectively unsolvable without removing the shutter |
| Loft hatches and access panels | Bought as an accessory, with no tightness requirement attached | A hatch with a perimeter gasket and clamping catch | Replace the hatch, one of the few cheap repairs |
| Window installation joint | Expanding foam is not an airtight layer; tape or a plaster connection is | Tapes and connection profiles around the whole frame | Strip out reveals, sills and linings |
| Sockets and switches on external walls | The box vents the wall cavity or the insulation behind it | Airtight boxes, or a continuous layer in front of them | Swap the boxes, and dust through the whole room |
| Soil vent pipe through the roof | The collar is missing or merely glued to the sheet metal | A proprietary roof collar fitted during installation | Working on the roof, with a risk of damaging the covering |
What do you do if it does not pass?
First of all: a mid-construction test that fails is not a disaster, it is exactly what you paid for. Had it come out perfect first time, you would have learned nothing new. The procedure that works is a boring one:
- Leave the fan running under depressurisation and walk the whole house with a hand, smoke, a notepad and a camera.
- Write the locations down and assign them to trades: not "the contractor will fix the leaks", but named places for named people, with a date.
- Repair. Most of the work is tape and sealant, one to two days.
- Retest while the equipment is still on site.
If the second measurement still misses the target and the gap is large, the problem is usually not the sealing but the concept: the airtight layer is not continuous and somewhere it does not meet itself. That is a conversation with the designer rather than the installer. In a deep renovation this is more common than in a new build, because the original construction was never designed to carry an airtight layer.
Who commissions the test, who pays, and where does it sit in the contract?
This works best as follows: the client commissions and pays for the test independently of the contractor, while the contract sets out the target value, two test dates and the contractor's obligation to allow the test and remedy the leaks found at their own cost.
Three things belong in the contract in one sentence each: the target value with both figure and method (n50, optionally q50, both directions), the date of the first test defined by construction stage rather than calendar, and who pays for the retest after repairs, the most frequent source of argument.
When is the test mandatory and when is it your choice?
It is mandatory whenever an airtightness figure forms part of a claim somebody is going to verify: passive house certification requires it as evidence the criterion is met, and subsidy programmes require proof of the parameters the money is paid against. An assumed value can be entered into the calculation behind an energy performance certificate, but a measured figure is always the stronger argument.
The precise requirements shift with regulation; Slovak construction law has been governed since 1 April 2025 by the new building act, no. 25/2025 Z. z., so verify the thresholds as at the date you submit. If somebody asks you for a value, you need a test report; if nobody does, the test is still a sensible investment, you are just making it for yourself.
And if you take one thing from this, let it be the timing: book the measurement for the moment when the airtight layer is complete and still exposed. The rest is paperwork. The wider context is covered in our main article The Complete Guide to Passive Houses in Slovakia.