Build tight, ventilate right
In lesson 9 you sealed the air leaks to save heat. But those leaks were also how stale air got out and fresh air got in.
Ventilation is swapping indoor air for outdoor air on purpose. A leaky house ventilates by accident, too much on windy winter days and too little on calm ones. A tight house needs ventilation by design.
Building scientists sum it up as: build tight, ventilate right.
What builds up indoors
Indoor air collects things you cannot always see or smell:
- Moisture from breathing, cooking, showers, drying laundry and houseplants. A family can add many pints of water to the air each day.
- Carbon dioxide from breathing, which makes a room feel stuffy.
- Pollutants from cooking, cleaning products, new furniture and finishes.
- Combustion gases from stoves and heaters, including carbon monoxide (CO).
- In some areas, radon, a radioactive gas from the soil. A simple test kit can measure it.
Moisture is the one that damages the house itself.
Humidity and mold
Relative humidity is how much water vapor the air holds, compared with the most it could hold at that temperature. It is given as a percent.
EPA advises keeping indoor humidity below 60 percent, and ideally between about 30 and 50 percent.
Text description
A horizontal scale of indoor relative humidity from 0 to 100 percent. Below 30 percent is marked drier than ideal. From about 30 to 50 percent is marked ideal. From 50 to 60 percent is still acceptable. A heavy line at 60 percent marks the EPA advice to keep humidity below 60 percent. Above 60 percent, condensation and mold are more likely. A marker at 70 percent shows a winter-morning reading that is too damp.
When humidity is high, water condenses on the coldest surfaces: windows, outside corners, closets on outside walls. Mold is a fungus that grows on damp surfaces. It can stain, rot materials and affect health, especially for people with allergies or asthma.
A small humidity meter, called a hygrometer, costs little and tells you where you stand.
Kinds of ventilation
- Spot ventilation: fans that pull air out where moisture and smells are made, such as the bathroom and over the stove. Ducted to the outside, not into the attic.
- Exhaust ventilation: a fan pulls air out of the house, and fresh air leaks in elsewhere.
- Supply ventilation: a fan pushes fresh air in, and stale air leaks out.
- Balanced ventilation: one fan brings air in and another sends the same amount out.
- Heat-recovery ventilator (HRV) or energy-recovery ventilator (ERV): a balanced system that passes warmth (and, for an ERV, some moisture) from outgoing air to incoming air.
Text description
Three small house sections side by side. Exhaust ventilation: a fan in the wall pulls stale air out, and fresh outdoor air leaks in through gaps wherever it can. Supply ventilation: a fan pushes fresh air in, and stale air leaks out through gaps. Balanced ventilation: one fan brings fresh air in and another sends the same amount of stale air out. An HRV or ERV, shown separately, is a balanced system that also passes heat between the two airstreams.
Text description
A schematic of a heat-recovery ventilator, outside on the left and inside on the right. Stale, warm air from the kitchen and bathroom enters the unit from the inside and crosses a core. Fresh, cold outdoor air enters from the outside and crosses the same core in a separate path. In the core, heat passes through thin walls from the warm stale air to the cold fresh air, but the two airstreams do not mix. The fresh air goes on to the rooms, warmed. The stale air goes outside as exhaust, cooled. An ERV works the same way and also passes some moisture. The unit has fans and filters, so it uses some electricity and the filters need cleaning.
Diagnose it: the damp cabin
“The cabin is damp. Windows drip every winter morning and black spots are growing in a closet on an outside wall.”
Clues
- The cabin was air sealed last fall and is much less drafty now.
What is the most likely cause?
Tight houses and combustion
Simple steps that help
Low-risk things you can do now:
- Run the bath fan during showers and for a while after. Check that it vents outside.
- Use a stove hood that vents outside when cooking.
- Dry laundry outdoors or in a dryer vented outside.
- Keep a hygrometer in the main room and one in a bedroom.
- Do not block vents or cover air inlets.
- Test for radon if your area has it.
Check
You have just air sealed a small house with a wood stove. What should come next?
Summary
- Ventilation swaps indoor air for outdoor air on purpose. Tight houses need it by design.
- Keep relative humidity below 60 percent, ideally about 30 to 50 percent.
- High humidity and cold surfaces lead to condensation and mold.
- Options run from spot fans to HRVs and ERVs.
- Tight houses and combustion appliances need safety checks for backdrafting and CO alarms.
Next, you will learn how codes, permits and inspections work, and how to work with them.
Go deeper: how much fresh air
Ventilation rates in US codes and standards are usually set by house size and the number of bedrooms. The idea is to give each person a steady supply of fresh air without wasting heat. Exact rates belong to your designer and the local code. A useful planning habit is to treat ventilation as part of the heating budget: every cubic foot of fresh air in winter has to be heated, which is the main reason HRVs and ERVs exist.