One of the main technical criteria of a passive house involves ensuring the air tightness of the enclosures that form the envelope.
Imperfections in the building envelope, in the form of holes or cracks, can cause a large number of problems, particularly during the coldest periods of the year. The flows of air from the interior to the exterior through these holes or cracks have a high risk of causing condensations in the construction.
Infiltrations of cold air also produce a feeling of loss of comfort for users. These infiltrations of cold air also increase the difference in temperature between the different floors of the building.
Air tightness in passive houses
Due to the fact that, in most climates, a passive house requires mechanical support for the continuous supply of outdoor air, an excellent sealing of the building envelope is required. If the envelope is not sufficiently tight, the air flow will not follow the planned routes and the heat recovery will not work correctly, resulting in a higher energy consumption.
In very mild climates, it is possible to build a passive house without heat recovery systems. In this case, if there is no ventilation system, the sealing is no longer that important. On the other hand, those buildings which are highly airtight but which do not have any ventilation systems are very likely to have poor air quality and excessive humidity. Good sealing is achieved by an appropriate design. It is important that a single airtight layer covers the entire building.
The sealing can be checked by means of a pressurization test, known as "Blower Door test", which consists of forced pressurization and depression by the action of a fan placed in an exterior door or window, which creates a difference of pressure of 50 Pa.
The result obtained in the pressurization test for the outer envelope of the building, according to EN 13829, must be lower than 0.6 air changes per hour (sealing value 50 Pa) in severe winter climates (central Europe), and less than 1.0 / h in regions with a milder climate, where the night time temperatures are above 0ºC, as for example some areas in Spain.
The closed cell projected polyurethane insulation (CCC4) and its contribution to the airtight barrier
When projecting closed-cell polyurethane (CCC4) on any surface of the building envelope, it acts as an airtight barrier, as it has an internal structure where closed cells represent up to 90%. This characteristic means that projected polyurethane, in addition to providing thermal insulation, can be a sealant for cracks and fissures, which allows to end with unwanted air renewals, avoids thermal bridges thanks to its continuous application and protects the envelope of the building.
To sum up, we can say that the closed cell projected polyurethane (CCC4) is the most suitable type of polyurethane product for passive houses.
Photo by Aislamientos Rodríguez
Why are closed cells, and in particular those with projected polyurethane, so important in the structure of any insulating system?
An open cell insulation allows the air to fill the cells, so that the foam is less insulating and moisture permeable and it may require a vapour barrier in cold climates. In contrast, the projected polyurethane with closed cell structure (CCC4) creates a rigid foam that resists water and does not need additional vapour barrier.
Beyond the principle of action of the passive houses on the infiltrations in the envelopes in terms of interior comfort, the projected polyurethane acts in favour of their occupants’ health, since these sealings prevent the entry of dust, pollen and other allergens. In addition, they do not contain formaldehyde or volatile organic chemicals.