These are the terms that come up most when we talk about how a home breathes and how that affects the air inside it.

Terms

ACH Air Changes per Hour, or ACH, is a unit of measure for how many times the entire volume of air in a home is replaced by an equal volume of outside air in one hour. It covers both the air changes you control on purpose, like a mechanical ventilation system, and the unintentional ones caused by gaps and cracks in the building envelope. A blower door test is the tool used to measure it, usually expressed at a reference pressure of 50 pascals and written as ACH50.

Air Barrier An air barrier is the material in a home's envelope that stops air from moving through it, things like plywood, thermo ply, or foam board. House wrap only counts as an air barrier if it's sealed at the perimeter and the seams. Air barriers sit outside the home's sheathing and under the cladding and water plane, and they're a different thing from a vapor barrier or a moisture barrier, though some products do double duty.

Blower Door Test A blower door test is the industry standard way to measure how much air leaks out of a home. A powerful, calibrated fan gets sealed into an exterior doorway, along with a pressure measurement tool called a manometer, and every other door and window in the house gets closed. The fan either pressurizes or depressurizes the house, and the resulting pressure difference between inside and outside lets a technician calculate the home's air leakage rate at a specified pressure, usually 50 pascals, compared to the home's volume.

Building Envelope The building envelope, sometimes called a home's shell or thermal envelope, is the boundary that separates the conditioned space you heat and cool from the unconditioned space and the outdoors, made up mainly of the floors, walls, and ceilings. When a home's insulation and air barriers work together the way they should, the envelope is effective. When they don't, the envelope fails, and that's usually behind high energy bills, uncomfortable rooms, moisture problems, and air quality issues.

ERV An Energy Recovery Ventilator, or ERV, brings fresh air into a home without wasting the energy already used to heat or cool the house. Blowers pull fresh air in and push stale air out through a heat exchanger, which transfers heat, and some of the moisture, from the outgoing air to the incoming air in winter, and reverses that in summer. Because it moves both heat and water vapor, an ERV also helps keep a home's humidity levels in check. That water vapor transfer is what sets an ERV apart from an HRV, which only transfers heat.

HRV A Heat Recovery Ventilator, or HRV, brings fresh outside air into a home while it exhausts stale indoor air, running both streams through a heat exchanger so the outgoing air's heat gets transferred to the incoming air instead of being lost. In winter that means the warm air leaving the house preheats the cold air coming in, and in summer the exchange runs the other way, with the cool air leaving the house chilling the warmer air coming in. Today's HRVs recover 50 to 90 percent of the energy that would otherwise be lost heating or cooling a home's air.

HVAC HVAC stands for heating, ventilation, and air conditioning, the umbrella term for the systems that heat, cool, and move air through a home.

MERV Minimum Efficiency Reporting Value, or MERV, is a scale from 1 to 16 that rates how effectively an air filter captures particles, with higher numbers capturing a greater percentage of particles on each pass. A MERV 16 filter captures more than 95 percent of particles across the full range the scale measures, from 0.3 to 10 micrometers. Most residential filters sold in stores run below MERV 10, while a MERV rating of 13 or higher is generally considered enhanced filtration and is what hospitals and clean rooms typically use. The tradeoff with a higher MERV filter is that it clogs faster, since it's catching more.

R Value R value measures a material's resistance to heat flow by conduction and is mainly used to label insulation products. It's the inverse of U value: divide 1 by the U value to get the R value. Some in the building industry treat R value as the single most important indicator of a home's energy efficiency, but it only accounts for conduction, one of three ways heat moves through a building, the other two being convection and radiation. As much as 25 to 40 percent of a typical home's energy use comes from air infiltration, which R value alone doesn't capture.

VOC VOC stands for Volatile Organic Compound, a substance often considered a pollutant that's detrimental to indoor air quality. New home materials release VOCs and other toxic substances as they cure, which is why some builders build in flush out time before homeowners move in. Building products labeled Low VOC give off little to no Volatile Organic Compounds once they're installed in a home.

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