Why tight and new homes need a planned way to breathe

Older houses get a steady, uncontrolled trickle of outdoor air through gaps around windows, doors, and framing. Tighter homes leak much less, which saves energy, but it also means fresh air has to come in on purpose.

Less leakage means less accidental fresh air

Air sealing, better windows, and newer energy codes all cut down on leakage. The EPA describes three ways outdoor air gets into a home: infiltration through cracks and gaps, natural ventilation through open windows and doors, and mechanical ventilation from fans. Tightening a house shrinks the first one.

That has a side effect. The EPA notes that homes designed and built to minimize air leakage may have higher indoor pollutant levels, because pollutants can build up when too little outdoor air comes in. Moisture builds up too: the Building America Solution Center points out that newer, tighter homes tend to hold more water vapor from cooking, bathing, and laundry than older, leakier ones.

None of this makes a tight house a bad house. It means the old accidental ventilation is gone, and something planned needs to take its place.

Build tight, ventilate right

Building Science Corporation sums up the idea as "build tight and ventilate right." The goal is a home that leaks very little, paired with a ventilation system that brings in a known amount of outdoor air where people need it.

Codes have moved the same way. The International Residential Code (2012, 2015, and 2018 editions) requires whole-house mechanical ventilation in relatively airtight homes. ASHRAE Standard 62.2 gives a method for calculating a minimum whole-house ventilation rate based on floor area and number of bedrooms. Which code edition applies, and with what local changes, depends on where you live, so your building department or installer is the place to confirm.

If your home was built before these requirements, or was air sealed later, it may have no planned ventilation at all beyond bathroom and kitchen fans.

Three ways to ventilate on purpose

Exhaust-only. A bathroom or other fan runs on a schedule to move air out, and replacement air leaks in through gaps. It's simple and low cost. The Building America Solution Center notes it can work well in cold, dry climates, but Building Science Corporation warns that in tight houses the suction can pull air in from attached garages or the soil under the house.

Supply-only. A fan brings filtered outdoor air in, often through the furnace or air handler ducts, and stale air leaks out. The Building America Solution Center describes it as a good strategy in some humid-climate homes and a poor one in cold, dry climates where indoor air is more humid than outdoor air. Colorado's Front Range falls in that cold, dry category.

Balanced. Two fans move roughly equal amounts of air in and out, usually through an HRV or ERV that recovers heat. It can deliver fresh air to specific rooms, including bedrooms. The tradeoffs are a higher cost, more maintenance, and a more involved install.

Signs a tight home may be under-ventilated

These are clues, not a diagnosis. Each one can have other causes.

• Bedrooms that feel stuffy or stale by morning, especially with the door closed.
• Condensation on the inside of windows in cold weather. The EPA notes that condensation can be a sign of high humidity.
• Cooking smells or bathroom moisture that hang around long after you're done.

If you notice any of these, measure before you buy anything.

Why bedroom CO₂ is a cheap first check

You breathe out CO₂ all night. In a closed bedroom where little outdoor air reaches the room, the level climbs; with more fresh air, it stays lower. That makes an inexpensive CO₂ monitor a practical way to see how much outdoor air a room gets relative to the people sleeping in it.

Know what it can't tell you. ASHRAE says indoor CO₂ is not an overall measure of indoor air quality, though it can be a useful tool if you understand its limits. A CO₂ reading says nothing direct about radon, smoke, or chemicals from furnishings. ASHRAE also notes that the widely quoted 1,000 ppm figure is not a limit its ventilation standard sets. For bedrooms, though, there's now a research-based goal: a 2025 review of sleep studies for ASHRAE recommends 800 ppm or less, on average, while you sleep, and found sleep measurably disturbed from about 1,000. Holding that takes about 17 cfm of outdoor air per sleeper, at least twice what many home ventilation standards provide, which is one reason tight, code-compliant homes can still miss it with the bedroom door shut. Why 800.

A few nights of readings, followed by one free change such as sleeping with the door open, tells you whether a room's fresh air is the issue before you spend money. That's the order HouseExhale follows, and a planned ventilation system only comes up if the free changes aren't enough.