Air Tightness FAQs

1. What is airtightness?
Airtightness is the control of airflow in a building. It could be represented as how well the various components of the building have been put together. In an airtight building air leaks do not occur at ceiling or wall junctions, plaster board joints, doors, windows, service entry points etc. Air is not getting in or out at any point in the building except where it is designed to go in or out.

Draughts or leaks create an unpleasant environment. They are also expensive in that heat is lost. The result is that the heating system has to operate at a greater capacity to compensate for the losses. Airtightness is absolutely essential in maximising the effectiveness of thermal insulation, ensuring vast savings over a lifetime.

In an airtight house the movement of air is controlled by a heat recovery ventilation system. If air movement is not controlled air quality can be unpredictable. Airtightness could also be referred to as the lack of draughts in a house. In winter in a draughty building air may come in at 3Deg C. A comfortable indoor temperature is 21Deg C. The householder has to pay to heat that air. A good level of air tightness will mean that only the right volume of air will have to be heated. Similarly an air tight building will not let warm air escape.

2. Why should air tightness be looked at in the context of HRV installation and operation?
Air tightness is a measure of the flow of air in and out of a building. Improving air tightness in a dwelling will reduce air leakage – the uncontrolled flow of air through gaps and cracks in the fabric of a building. The target must be “build tight, ventilate right”. This means that there should be no breaks or gaps in the envelope of the building fabric. There is complete control over ventilation within an air tight building when using a HRV system. Having an airtight house does not mean there is insufficient air flow, it means that the air flow can be controlled.

The better the air tightness level the more effective the HRV system will be. More heat will be recovered. By recovering more heat the load on the heating system will be reduced to give lower heating costs.

3. Where are the most common air leakage paths?

Air Tightne

  1. Under floor ventilator grilles
  2. Gaps in and around suspended timber floors
  3. Leaky windows or doors
  4. Pathway through floor/ceiling voids into cavity walls and then to outside
  5. Gaps around windows
  6. Gaps at the ceiling-to wall joints at the caves
  7. Open chimney
  8. Gaps around attic hatches
  9. Service penetrations in ceilings
  10. Vents penetrating the ceiling/roof
  11. Bathroom wall vent or extract fan
  12. Gaps around the bathroom waste pipes
  13. Kitchen wall vent or extractor fan
  14. Gaps around kitchen waste pipes
  15. Gaps around wall-to-floor joints (especially with timber frame)
  16. Gaps in and around electrical fittings in hollow walls


4. How can a good level of airtightness be achieved?
Initially airtightness should be considered as one of the most important deliverables of a house building project. The simpler the shape of the building the better it will be from an air tightness point of view. All parties, trades etc. must be aware at all times of the requirement to deliver an airtight house.

(i) A target airtightness level should be agreed early on. It should be included in the overall specification.

(ii) Construction should be carried out to ensure that the envelope is continuous throughout the building.

(iii) The sequence of work should be such that there will be minimum disruption to membranes etc.

(iv) The number of penetrations should be minimised. Where they have to be made they should be well sealed.

(v) Responsibility for airtightness must be clearly assigned to one individual.

(vi) Measure levels achieved. “What gets measured gets done”

5. How should the airtightness of an existing house be approached?
Air tightness testing highlights areas of heat loss. Infiltration of cold air from the outside should be minimised by reducing unintentional air paths as far as is practicable by

(i) Fitting draught stripping in the frames of the openable elements of windows, doors and roof lights, etc.

(ii) Sealing around loft hatches.

(iii) Ensuring that boxing for concealed services is sealed at floor and ceiling levels.

(iv) Sealing piped services where they penetrate or project into hollow constructions, cavities, or voids.

(v) Sealing around underfloor ventilator grilles, gaps in and around suspended timber floors, extractor fans, etc.

Open chimneys are a major source of air leakage.

6. How do you find out how air tight a house is?
The level of airtightness in a building is measured by doing a blower door test. A large fan is installed within a temporary door frame in an external door opening and inducing a pressure differential. The test usually involves a combination of depressurising and pressuring the house to a pressure difference of 50 Pascal’s (50Pa). (A pressure difference of 50 Pa can be compared with the equivalent pressure induced by a wind speed of approximately 22mph on a building). The pressure imbalance between inside and outside of the building forces air through all holes and penetrations in the building. The resulting pressure differential between inside and outside the building is measured. By measuring the airflow through the fan and its effect on the air pressure in the building, the air tightness of the entire building envelope can be measured. It is non destructive.

7. How are the results of an air tightness test presented?
Two approaches can be be taken.

(i) Air Changes per hour at 50 Pascals pressure difference is called the n50.
The n50 is expressed in air changes per hour (ac/h). It tells us the number of times the complete volume of air in the house is changed in an hour. It is the relationship between the total volume of air in m³ and how leaks in the building envelope allow the air to go / out at 50 Pascals air pressure difference.

(ii) Air permeability rate per hour at 50 Pascals pressure difference is called the q50. The q50 is the air permeability rate and is expressed as m³/hr/m². It is the amount of air that escapes from a building in an hour, divided by the external envelope in square metres. It is an expression of the amount of air leaking from a house in relation to it’s total exposed surface area (i.e. the ground floor, all exposed ceilings and all external walls) at 50 Pascals air pressure difference.

Irish Building Regulations refer to the air permeability rate. The q50 test result is divided by 20 and inputted into the DEAP calculation to improve the BER and prove compliance.

Passive house calculations require air changes per hour (n50) data.

Standardised conversion between the air permeability rate (q50) and the air change rate (n50) values is not possible because they do not have a direct relationship with one another. Different measurement and testing protocols are used.

8. If you achieve an air tightness level of 5 m³/hr/m or better can you use natural ventilation (holes in the walls)?
Yes but you must increase the background ventilation by 40% of clear area. That is you must drill 40% more holes or increase the diameter of the existing holes proportionately.
Section 1.2.2.1 and Appendix 1 of Technical Guidance Document F – Ventilation 2009 refers.

9. What level of airtightness should be achieved so that a HRV system will be beneficial?
The revised Building Regulations (Technical Guidance Document – L Conservation of Fuel and Energy – Dwellings 2011 ) now require that houses will need to achieve a result of below 7 m³/hr/m². (This means that when there is difference of 50 Pascals between the inside and outside of the building 7 m³ of air will leak through every square meter of the external walls each and every hour). We suggest that all buildings, particularly new builds, should attempt to meet 2 – 3 m³/hr/m² or better. With a level of 5 m³/hr/m² the HRV system will add to comfort levels and reduce heating costs.

10. What is the best time to install an air tightness membrane if one is being installed?
In most instances it is best installed after all trades have completed their first fix and just before slabbing is done. This greatly reduces the possibility of it being damaged and should ensure that its integrity is not compromised.

11. What can be done to improve air tightness and the effectiveness of insulation at first floor level from a heat recovery point of view?
The ProAir HRV system works best when all of it is within the thermal envelope of the building. The ProAir D & J ducting is very suitable for this purpose. See here (Photos 1 and 2) for an example where the duct has been installed under the air tightness membrane. The process here outlines a method of installing D & J ducts under joists to achieve maximum thermal efficiency.