Just made your home tighter? Check the bedroom air

Air sealing, insulation and new windows do what they promise: fewer drafts and lower bills. They also cut some of the accidental fresh air the house used to get through its gaps. Most homes come out of a retrofit tighter but far from airtight. The room that's easiest to shortchange is a bedroom with the door shut, so that's the room to check.

What tightening changes

Leakage drops by about a fifth to a third. Lawrence Berkeley National Laboratory looked at more than 31,000 before-and-after blower-door tests from 18 states. The typical cut was about 20% in utility and private jobs and about 30% in single-family homes done through the federal Weatherization Assistance Program (WAP).

Air quality shifts a little. The largest US study measured 514 weatherized homes before and after the work. On average:

• radon went up slightly (about 0.4 pCi/L, more in homes that started higher);
• formaldehyde went up slightly;
• winter humidity rose about one percentage point;
• carbon monoxide didn't change.

Adding ventilation took most of that back. In a follow-up, a continuous exhaust fan sized to the ventilation standard (ASHRAE 62.2) cut radon by about 12% and humidity by almost two points. Later studies that added exhaust ventilation, sump lids and soil covers found no significant radon rise on the lowest floor people live on.

Bedroom CO₂ is rarely measured. That US study didn't publish CO₂ results. An Irish study of deep retrofits did look room by room, and found the living rooms improved while many bedrooms, with doors and windows shut at night, didn't get enough fresh air. Your monitor answers that question for your house.

Measure the bedrooms

Three ordinary nights in the main bedroom tell you where you stand against the 800 ppm goal. Keep the door, windows and fans as they were before the work, so the nights compare fairly. If you measured before the job, put those nights side by side with the new ones.

Most CO₂ monitors show humidity too. Write it down each morning: it's a second clue, covered below.

Ask whoever did the work for these

A good energy audit already produces most of this. Ask for it in writing:

1. The blower-door results, before and after. Two numbers each time: CFM50 and ACH50. They show how much tighter the house got.
2. Whether they checked ventilation. An auditor working to the BPI standard should determine the home's ventilation needs, often with an ASHRAE 62.2 calculation. Ask what it said, and whether anything was installed because of it.
3. The combustion-safety test results, if anything in the house burns gas, oil or wood: the depressurization test of the furnace and water heater area, spillage and carbon monoxide. Under the 2026 federal rebate rules, homeowners are to be told all test results.
4. The bath and kitchen fans' measured airflow, not the label. Installed fans often move less than they're rated for.
5. Any ventilation they added: what it is, how much air it moves, and how it's set to run.

If anything burns gas, oil or wood

A tighter house has fewer gaps to relieve the suction from bath fans, a range hood or a clothes dryer. A furnace or water heater that vents up a chimney by itself can then spill its exhaust back into the house. That's why programs test combustion safety before and after the work, and why a working CO alarm matters.

Replacing a furnace with a heat pump needs one more look. If the old gas water heater stays, it can be left alone on a chimney sized for two appliances (an "orphaned" water heater). In BPI’s 2012 test limits that’s the strictest case of all: it fails at a suction of only 2 pascals. Ask for a worst-case spillage test, and ask about a direct-vent or heat-pump water heater. Heat pumps themselves, ducted or ductless, heat and cool the air already inside; most don't bring in outdoor air.

What the programs require

It depends on who paid for the work:

• The federal Weatherization Assistance Program checks every home against ASHRAE 62.2 and installs ventilation where the calculation calls for it.
• Mass Save and NYSERDA's Comfort Home require mechanical ventilation when a home ends up below 70% of a minimum-leakage level (NYSERDA asks for a continuous-rated fan). NYSERDA's July 2026 manual still bases that level on a 1989 standard.
• The federal Home Energy Rebates now require insulation and air sealing before a heat pump upgrade in the HEEHR program, unless the home already meets the state's level (Program Notice 26-2, May 2026). Neither the notice nor the program's safety rules mention ventilation; the safety rules call for combustion testing where fuel-burning equipment is affected, and for telling the homeowner every test result.
• Colorado's Efficiency Works pays more for bigger leakage cuts and requires a combustion-safety test with air sealing.

None of these looks at a single bedroom. The 62.2 standard counts 7.5 cfm per person for the whole house, while a bedroom needs about 17 cfm per sleeper to stay at 800 ppm. A house can pass every check and still have a stuffy bedroom with the door shut. Rebates by state lists what's offered where you live.

Cheap fixes to try first

Try one at a time, for two or three nights each, and let the monitor say whether it helped. Free fixes covers more.

• Run a quiet bath fan all the time, on low. A continuous-rated fan on low (about 30 to 50 cfm) costs roughly $15 to $20 a year to run, according to Minnesota's Center for Energy and Environment. Don't run an old fan nonstop if it isn't rated for it, and keep its duct going outdoors, never into the attic.
• Give a closed bedroom a way for air to move. A transfer grille or jump duct to the hall lets air through with the door shut. The Building America rule of thumb is about 1 square inch of free area per cfm. Door shut? has more.

Know the limit of a bath fan. In Building America lab houses, an exhaust fan barely changed the air in closed bedrooms, while an ERV that supplied the bedrooms raised it a lot. Exhaust-only ventilation also pulls its replacement air from wherever it can: the attic, the garage, the soil, or a chimney. If you have fuel-burning appliances, have combustion safety checked before running a fan nonstop.

Watch the humidity too

CO₂ tracks how much outdoor air each person gets. Humidity tracks moisture: showers, cooking, the weather, and how long the air conditioner runs. The two can disagree, so read both.

• The EPA's range: below 60%, ideally 30 to 50%.
• Winter: water on the inside of windows is a sign the air is too humid for how cold it is outside. The colder it gets, the lower indoor humidity has to be to stop it.
• Summer: after insulation, an air conditioner runs less and so dries the air less. A dehumidifier, or its dry mode, can help.

When a fix isn't enough

If the bedroom still averages above 800 with a free fix in place, it needs a planned supply of fresh air. Do I need an ERV? sets out the choice, Why tight homes need a planned way to breathe explains the three kinds of ventilation, and the room planner works out what each room needs. If the house has no ducts, start with Fresh air without ducts.