What it is

Old houses leaked like a screen door. Air sneaked in around windows, outlets, and the rim joist, so the house "breathed" whether you wanted it to or not — wasteful, but it diluted indoor pollutants by accident. Modern construction is sealed tight for energy efficiency, which is great for the power bill but means the building no longer breathes on its own. Without a deliberate way to bring in outdoor air, indoor contaminants — CO2 from people, moisture from cooking and showers, VOCs from furniture and finishes, odors — just build up.

Ventilation is the intentional, controlled exchange of stale indoor air for fresh outdoor air. In a tight house it's not optional; it's the only thing standing between the occupants and a stuffy, high-CO2, high-humidity box.

How it works

There are three basic strategies, and the difference is which direction you push air:

  • Exhaust-only: a fan (often a quiet, continuous bath fan) pulls stale air out. The house goes slightly negative, so fresh air leaks in through whatever small gaps remain. Cheap and simple, but in a really tight house there aren't enough gaps to make up the air, and negative pressure can backdraft combustion appliances or pull in radon/soil gas.
  • Supply-only: a fan pushes filtered outdoor air in, pressurizing the house slightly so stale air leaks out. You get to filter and (somewhat) control the incoming air, and positive pressure keeps soil gas out — but you're dumping unconditioned outdoor air inside, which adds heating/cooling and humidity load.
  • Balanced: equal amounts pushed in and pulled out, so the house stays neutral pressure. The premium version uses an ERV or HRV — a box with two airstreams (incoming fresh, outgoing stale) passing through a core that transfers heat (HRV) or heat and moisture (ERV) between them. You recover most of the energy you'd otherwise throw away with the exhaust air.

HRV vs ERV: an HRV transfers only heat — good for cold, dry climates where you want to keep the moisture out of the recovered stream. An ERV transfers heat and moisture — good for humid climates (keeps humid outdoor air from dumping its moisture inside in summer) and for very cold climates where you want to retain some indoor humidity in winter. Climate picks the box.

In the field

When you're spec'ing or troubleshooting ventilation:

  1. Find out how tight the house is. A blower-door number (ACH50) tells you whether the house can rely on infiltration at all. Tight homes (low ACH50) need mechanical ventilation, period.
  2. Size the ventilation rate to the home's floor area and occupancy using the residential ventilation standard's method — bigger/more-occupied homes need more airflow.
  3. Pick a strategy that fits the climate and the combustion situation. If there are atmospheric (naturally drafted) gas appliances, avoid strong negative pressure — exhaust-only can backdraft them and spill CO. Balanced or supply is safer there.
  4. Commission the airflow. Measure actual CFM at the ERV/HRV ports or exhaust grilles — don't assume the rating. Balance the two streams on an ERV/HRV so it's actually neutral.
  5. Check the ERV/HRV core and filters. A plugged core or filter kills the airflow and the recovery. These need maintenance like anything else.
  6. Mind the controls. Continuous low-speed ventilation usually beats big intermittent blasts for steady IAQ; many systems boost during cooking/showering.

Normal values & targets

  • Whole-house ventilation rate (residential standard concept): roughly 0.03 CFM per square foot of floor area plus 7.5 CFM per occupant (occupant count often estimated as bedrooms + 1). That's the ballpark continuous fresh-air rate a tight house targets.
  • Air changes per hour (ACH): healthy whole-house ventilation often lands around 0.3 ACH of outdoor air as a rough target for general IAQ. (Distinct from ACH50, the blower-door leakage metric measured at 50 Pa.)
  • CO2 as a proxy: indoor CO2 staying under about 1,000–1,100 ppm generally indicates adequate fresh air for occupancy; persistently higher means under-ventilated.
  • ERV/HRV recovery: good cores recover roughly 60–85% of the energy (sensible, and latent for ERVs) that would otherwise leave with the exhaust.
  • Spot ventilation: kitchens and baths still need their own local exhaust (bath fans commonly 50–80+ CFM, range hoods 100+ CFM) vented to the outdoors — not into an attic.

Common faults & what they mean

  • Stuffy, high-humidity, or odor-heavy tight house: under-ventilated. Either no mechanical ventilation or it's undersized/not running.
  • Backdrafting / CO spillage at a water heater or furnace: exhaust-only ventilation (or big exhaust fans) pulling the house too negative around atmospheric combustion appliances. Dangerous — rebalance toward neutral/positive.
  • High winter energy bills from ventilation: supply- or exhaust-only dumping/pulling unconditioned air with no recovery. An ERV/HRV would recapture most of it.
  • ERV/HRV "not working": plugged filter or core, frosted core (cold-climate HRVs need a defrost strategy), or unbalanced/disconnected ducting.
  • Mold or window sweat despite ventilation: wrong device for the climate (HRV where an ERV belonged, or vice versa) or moisture sources overwhelming the rate.

Tech tips & gotchas

  • Tighter the house, more critical the mechanical ventilation. You can't lean on "the house leaks enough" in modern construction — measure it.
  • Don't create negative pressure around atmospheric combustion. Exhaust-only plus a naturally drafted furnace/water heater is a CO risk. Balanced ventilation sidesteps it.
  • Vent bath/kitchen exhaust to the OUTSIDE. Dumping moist air into an attic grows mold and rots sheathing — a classic callback.
  • Climate decides HRV vs ERV. Humid summers → ERV. Cold, dry → HRV (with defrost). Mixed → usually ERV.
  • Commission the actual airflow. A balanced unit that isn't balanced (or runs at half its rated CFM through a dirty filter) isn't doing the job. Measure.
  • Filter the incoming fresh air. Outdoor air carries pollen and particulate; a filter on the supply/ERV intake protects IAQ and the core.

Safety / code notes

  • Residential whole-house and local ventilation rates follow ASHRAE 62.2 (referenced by the IRC/IMC). Size to its method.
  • Combustion safety: avoid depressurizing spaces with atmospheric combustion appliances — relates to combustion-air and venting provisions in the IFGC/IMC. Worst-case depressurization testing is the check.
  • Exhaust terminations and clearances to openings/intakes follow IMC provisions; never terminate exhaust into attics, crawlspaces, or soffits.