What is the link between airtightness and MVHR?

Today I would like to share with you some considerations in the regards of airtight buildings and what are the consequences when high level of airtightness are not accompanied by the use of mechanical ventilation systems. As a Passive house designer I find this topic very crucial but controversial, and even though I am passionate about designing airtight buildings because of their great benefits, I think that often the link between airtightness and ventilation is not very clear, therefore designing airtight can result in even worse performing buildings than the leaky ones.

Recently I have come across a few projects of new domestic buildings where the design air permeability of the envelope was quite good, around 3 m3/hm2 @50 Pa, better than average houses. I am talking of the design stage where the property is not been built yet, so this is an assumed airtightness level according to the materials specified to the building contractor. This is certainly a brilliant way to reduce air leakages through the building envelope and guarantee that i20150928085420-airtight2the internal air we are heating through the heating systems does not escape by any means but it keeps us warm and comfortable. However, the design of the property did not include a mechanical ventilation system.

What happens in the long term, if we keep this warm air all enclosed in the house without any provision for ventilation? Well, this very performing envelope is going to turn into the reason of degradation of the fabric and after all the money spent in choosing the best materials for the building components, we are going to face the nightmare of any house owner: increased humidity and condensation.

It’s necessary to highlight, however, that a very airtight building can perform well also without the use of a MVHR, therefore relying on the natural ventilation. Yes it is! Certainly if we keep the windows open in the wintertime with an external perceived temperature of 2 degrees in the best sunny day in Oxford, in this case we could ventilate the house naturally, but does this really work out? Basically, it’s a matter of comfort and not wasting the heat we deserve after having done a great job wiping out any air leakages and having chosen high efficient services and so on. And probably also a matter of consistency with the design approach for a sustainable and green building. Of course natural vantilation strategies can be also a solution when they are properly planned and integrated in the very early stages of the design process.

Besides, the mechanical ventilation makes sure the IAQ (indoor air quality) is maintained with decent level of CO2 concentrations (in fact high level of Carbone dioxide make us feel drowsy and tired) and controlled humidity. The risk of high quantity of VOC (Volatile Organic Compounds), toxic particles that are contained in the carpets, adhesives and paints, is also heavily reduced by the system.

Certainly having a MHVR implies some costs to run the fans, however these costs are more than offset by the savings achieved in the reduction of the uncontrolled infiltration, as many Passive houses case studies have already proved.

As a result, I think that overall it is necessary to have consistency in the design approach based on the building physics rules and in the comfort levels we are aiming to achieve (or we must achieve!). As a rule of thumb, when the air permeability is equal or less than 3 m3/hm2 a mechanical ventilation system should be consider, whatever its efficiency is. Whether with figures higher than 3 m3/hm2 a good and well thought natural ventilation design can work.

IAQ cannot be underestimated as it should be always a priority for any designer and much efforts must be put into the process of making houses comfortable and safe for the human beings.