Airtight Construction
At 35%, the amount of heat loss in a typical home due to air leakage is almost double the heat loss through windows and doors. Significantly reducing this air leakage is therefore critical to meeting Passive House standards. It also ensures building air quality and durability by avoiding condensation, and improves occupant comfort by eliminating drafts. A continuous air barrier is a mandatory strategy for airtightness, but once air leakage is reduced, the need for fresh air is crucial. The figure below, courtesy of UltimateAir, shows the dangerous elements that can build up in a home once it has been made airtight…
Why is an air barrier specified rather than a vapor barrier?
An air barrier prevents air leakage, but allows vapor diffusion. A vapor barrier prevents both. The quantity of water vapor moving through exterior building components from air leakage is about 100-times greater than the amount of water vapor transported by diffusion. Any condensation in a building component resulting from vapor diffusion is therefore insignificant because the quantities of water are miniscule. This is true in all but the most severe of heating and cooling climates. Condensation from air leakage, however, can result in catastrophic quantities of water that lead to significant mold growth and moisture damage. The drawback of preventing vapor diffusion is that if water, for whatever reason, makes its way into the exterior building envelope, it has no way of escaping. An air barrier, in contrast to a vapor barrier, allows this water to escape during periods of high drying potential, and is therefore extremely important for maintaining building air quality and durability.
Is an air barrier used in The Naturally Passive House concept?
Yes, a continuous external air barrier is an integral part of The Naturally Passive House concept. Because straw and cellulose are very porous materials, condensation will not occur within these materials, but will take place on the first condensing surface (e.g. exterior sheathing). This surface forms the air barrier and is able to transport the condensed vapor during periods of high drying potential. If this first condensing surface was a vapor barrier, the condensed vapor would not escape and would be absorbed by the adjacent straw or cellulose, leading to mold growth and moisture damage.
Heat-Recovery Ventilation
The only effective way to reduce air leakage in a home while still ensuring a supply of fresh air is through ventilation, and the only way to reduce heat loss while supplying that fresh air is through "heat-recovery" ventilation. A heat recovery ventilator (HRV) exchanges stale indoor air for fresh outdoor air while recovering most of the heat from the exiting indoor air. An energy recovery ventilator (ERV) also exchanges stale indoor air for fresh outdoor air, but in addition to recovering most of the heat, it also recovers most of the humidity from the exiting indoor air. The diagram below, courtesy of RenewAire, shows the energy recovery concept…
As seen below, the supply of exiting stale air for the HRV or ERV comes from bathrooms, the laundry room, and kitchen — areas that typically generate humidity and/or odors. Incoming fresh air that passes through the HRV or ERV is supplied to living areas and bedrooms — areas that typically have high occupancy levels for extended periods of time.
How efficient are HRV and ERV units at recovering heat, and how much electricity do they use?
State-of-the-art HRV and ERV units have heat-exchange efficiencies of 75-95%, and typically use 40-250 watts of electricity. During operation, they recover as much as 15-times the energy in heat as the electrical energy required to operate the units.
Which is the preferred choice, a HRV or ERV, and why?
An ERV is a better choice in areas with higher humidity levels and colder winter temperatures. In milder climates, a HRV is usually sufficient. During the heating season, an ERV helps prevent over-drying of the interior air when exterior temperatures are very cold. During the cooling season, an ERV provides partial dehumidification during periods of high humidity.
What brands of ERV are the best choice?
The ERV unit with the highest heat-recovery efficiency is the RecoupAerator, manufactured by UltimateAir. This unit uses a rotating-wheel core. Another choice is the EV series of ERV units, manufactured by RenewAire, which use a fixed-plate core that requires less maintenance.