Until recently, most of the houses were so leaky that enough cold dry air seeped in to meet the needs of the occupants and ensured moisture free home. This is about to change. According to the Energy Efficiency Directive 2012/27/EU, from 2021 new constructions should approach the Nearly Zero Energy Building standards (nZEB).
This catchy phrase “Build it tight and vent it right” describes two of the basic principles of the nZEB standard (which is following the strategy of the Passive House). Airtightness and mechanical ventilation are factors that determine the energy performance of a building, offering comfortable, healthy living conditions and making conventional heating – air conditioning systems obsolete.
Build it tight
Air needs an opening or hole to flow through and a driving force to move it. Air leakage occurs when outdoor air enters and indoor air escapes your house uncontrollably and unintended through cracks and openings. Air leakages from indoors to outdoors are known as exfiltration and leakages from outdoors to indoors are known as infiltration. Uncontrolled and unintended air-exchange often cause several building performance problems. Airflow across the building envelop is driven by wind pressures, stack effect, mechanical air handling equipment like fans and furnaces.
Building more air-tight dwelling envelope is having a positive impact on indoor air quality and in home energy costs. In particular, controlling airflow through the building envelope or between units of occupancy is a cost-effective way to reduce heating and cooling costs, improve durability, increase thermal comfort, and create a healthier indoor environment.
A continuous, strong, stiff, durable and air impermeable barrier will stop uncontrolled air flow which may carry water vapor, and become trapped, condense, and create microbial growth issues.
Vent it right
A proper air sealing makes mechanical ventilation essential in a high performance home. Increased airtightness must be matched by an appropriate ventilation system to dilute pollutants, provide fresh unpolluted air, and control cold weather humidity levels. For example, cold, dry winter air would need to be warmed as well as humidified, while hot and humid air would need to be cooled and dehumidified during the warmer months.
The incoming fresh air and the outgoing exhaust air should be balanced. When our homes are airtight, we have the option to bring in the right amount of fresh air, through intentional ventilation, rather than unintentional infiltration (air coming in through uncontrolled gaps or cracks). We can also control exhaust ventilation such as bath fans, range vents, and other exhaust when needed (ie. radon control fans).
Choose a balanced Ventilation Systems
Heat Recovery Ventilators (HRV) or Energy Recovery Ventilators (ERV)
A balanced ventilation system injects fresh outdoor air into the home at the same rate as exhausting stale indoor air. The system has 2 fans, one for providing outside air into the building and another one for exhausting the interior air, resulting in roughly balanced airflows. These systems do not significantly affect the pressure of the interior space, if they are designed and installed properly.
In most balanced ventilation systems, heat—and sometimes moisture—are exchanged between the two airstreams, reducing the heating and cooling loads caused by outside air. These systems are known as HRVs (heat recovery ventilators) and ERVs (energy or enthalpy recovery ventilators). A HRVs exchanges heat between the airstreams, while a ERVs exchanges heat as well as moisture so your house won’t be as dry in the winter.
More specific, fresh air coming into the home through an insulated duct that passes through the heat exchanger core where it gets heated up by the outgoing stale air. After it is heated, the fresh air is ducted throughout the home. Stale air is pulled from the home and enters the other side of the core where its heat is given to the fresh air, before it is exhausted from the building via ductwork. In the winter months the incoming cold fresh air passes through the exchange core where is warmed by the outgoing stale air from the warm house. On the flip side during hot weather incoming fresh air is cooled by the exhaust air if the house air is cooler than outside air. It’s important to note that there isn’t any mixing of air. The air from one side cannot mix with the air on the other side. Airborne pollutants being flushed from the house cannot cross contaminate the incoming fresh air. It should be noted that with ERVs there is a possibility the shared moisture could contained some pollutants. Both ERV and HRV units include air filters to keep debris and particles from clogging the core. The filters also clean the fresh air delivered to the home helping reduce impurities (microorganisms, including germ microbes and chemicals) from the air and keeping the home more dust free. They should be checked regularly to keep the system running at peak efficiency.
In a typical HRV or ERV set up the tempered fresh air from the exchanged core is delivered directly to living areas and bedrooms through dedicated supply ducts. The outgoing stale air containing moisture, odors and particles is collected from the bathrooms, kitchen and sometimes utility or laundry room. The return air is ducted to the exchange core where it tempers the incoming fresh air before being exhausted out to the house.
At its most basic, airtightness aims to eliminate any unintended gaps or cracks in the external fabric of the building, while ventilation focuses on replacing the stale indoor air with fresh outdoor air to meet the occupant’s needs. Today, experts agree that energy costs can be reduced by as much as 30% by sealing the leaks in a home and ensuring that the insulation cannot be bypassed by hot or cold air.