An airtight house is one where air does not pass through gaps in the envelope, but comes and goes where you decide. It is checked with a Blower Door test: a fan in the outside door creates a pressure difference and an instrument measures how much air passes through the building. It is achieved with a separate layer on the room side, in which every overlap, connection and penetration is sealed.
This article explains what exactly is measured, what the result means and how the layer is built. How it works together with the insulation and the boards is described on the System page.
Why airtightness matters
Insulation keeps heat in only if air does not pass through it. Without a tight layer on the inside, warm, humid air from the rooms gets through the gaps into the construction — exactly where the insulation is coldest. Better airtightness has another benefit besides lower energy use: it protects the components of the envelope from damage.
Wood fibre boards insulate, but because of their porous structure they do not count as airtight materials under DIN 4108-7. So airtightness is not expected from the insulation but from a separate layer: a vapour barrier or boards with taped joints.
How the Blower Door test works
- Preparation. Outside openings are closed and interior doors are opened.
- Fan. The fan is sealed into an outside opening, usually the front door.
- Underpressure. The fan extracts air until the difference between inside and outside reaches 50 Pa (equal to a 5 mm water column).
- Finding the gaps. During the underpressure phase, the places where air comes in are found by hand, with a fog machine or with a thermal camera.
- Overpressure. The measurement is repeated with the air flowing the other way and the result is calculated.
The method is described in EN ISO 9972. The older EN 13829 has been withdrawn but is still cited in some documents; the two define the interior volume differently, so when comparing results, check which one was used.
What n50 means
The result is n50: how many times per hour the air in the building is exchanged at a pressure difference of 50 Pa. The calculation is the fan flow (m³/h) divided by the interior volume (m³). The lower the number, the tighter the house. The last column of the table is only an example — a house with an interior volume of 300 m³.
| Requirement | n50, h⁻¹, up to | Air at 50 Pa for 300 m³, m³/h |
|---|---|---|
| Germany, GEG § 26: building without a ventilation system | 3.0 | 900 |
| Germany, GEG § 26: building with a ventilation system | 1.5 | 450 |
| Passivhaus certificate (voluntary) | 0.6 | 180 |
These are the German limits and the Passivhaus criterion. We do not cite a Bulgarian standard here; for a building in Bulgaria, what counts is the requirement in its design.
When to do the test
The test is done once the airtight layer is finished. It is most convenient to measure while the layer is still visible and within reach, before the lining closes it: it is easier to seal a gap now than to hunt for it later. Waiting times for sealants matter too: the sealant on a roll AIRSTOP DIMAROLL has none, so the test can be done straight away.
When a roof is renovated from outside and the plaster stays on the inside, the test is also one way of proving that there is a working airtight layer on the inside. The other is an airtight, vapour-permeable membrane outside, directly over the insulation.
Where the gaps usually are and how they are sealed
Most leaks are at window connections, cable penetrations and roof penetrations, as well as at vapour barrier connections on pitched roofs. The tape is not an extra; it is part of the layer.
| Where | How it is sealed | Products |
|---|---|---|
| Vapour barrier overlaps | Taped with an adhesive tape | AIRSTOP FLEX, AIRSTOP KB |
| Joints between boards (ESB, OSB, MDF) | Taped with a tape for boards; wood-coloured if they stay visible | AIRSTOP FLEX, TIMBERFLEX |
| Connections to walls, floors and ceilings | A stretchy tape or a sealant adhesive; on a dusty surface, primer first | AIRSTOP ELASTO, AIRSTOP SPRINT, OMEGA UNI Primer |
| Pipes through the layer | A grommet for one pipe or for a bundle of up to 6 pipes | AIRSTOP GD2, AIRSTOP Multi 6 |
| Cables and ceiling spots | A grommet for a bundle of up to 6 cables; an airtight box for a spot where there is no service cavity | AIRSTOP Multi 6, AIRSTOP SPOTI |
| Windows and doors | A window tape inside and a vapour-permeable tape outside | ISOWINDOW FEBA SOFT ES, ISOWINDOW UVAU ES |
All tapes and grommets are in the sections Adhesive tapes and sealants and Grommets and penetrations.
The airtight layer in a timber house
- Vapour barrier. A membrane on the room side with taped overlaps, for example AIRSTOP DIVA with a variable Sd value. See Vapour barriers.
- Boards. ELKA esb Plus with taped joints and connections can be both the structural and the airtight layer.
- Services in front. Cables and pipes run in a batten frame in front of the layer so that they do not pierce it. See The service cavity lining on the inside.
The airtight layer is not closed to vapour: it stops air flow, and vapour is managed through the choice of material. The rule is airtight inside, open outside.
Common mistakes
- Expecting airtightness from the insulation itself — wood fibre boards are not airtight.
- Taping overlaps and joints with packing tape or general building tape instead of a system tape.
- Running cables and pipes straight through the membrane, without a grommet.
- Testing after the lining is closed, when the gaps can no longer be reached.
Who will install it?
Your builder: we give them the manufacturer’s instructions for the tapes and grommets and help over the phone. No builder yet? Write to us and we will help.
Questions
What is a Blower Door test?
A measurement of how much air leaks through the building envelope. A fan is sealed into an outside door and creates a pressure difference of 50 Pa between inside and outside, first as underpressure and then as overpressure. The result is the n50 value: how many times per hour the air in the building is exchanged at that pressure.
What n50 value is good for a house?
In Germany the Buildings Energy Act (GEG, § 26) sets a limit of 3.0 h⁻¹ without a ventilation system and 1.5 h⁻¹ with one. The voluntary Passivhaus certificate requires no more than 0.6 h⁻¹. The lower the value, the tighter the building. For a building in Bulgaria, what counts is the requirement in its design.
When should the Blower Door test be done?
After the airtight layer is finished. It is best to test while that layer is still accessible, before the interior lining closes it: at that point gaps are found and sealed most easily.
How much does a Blower Door test cost?
We do not give a price because we have not found a published price for Bulgaria. Ask a firm that does the measurement for a quote, stating the volume of the building and the stage of construction.
Are wood fibre boards airtight?
No. Because of their porous structure they do not count as airtight materials under DIN 4108-7. Airtightness comes from a separate layer on the room side: a vapour barrier or boards with taped joints.