Build tight, ventilate right

“A house needs to breathe.” We hear it often, usually from someone who has lived in an older home and remembers it fondly. The instinct is half right. People need fresh air, and walls need to be able to dry. But neither should depend on air finding its way through gaps. Our approach is the opposite of leaving a house leaky: build it tight, then bring in fresh air deliberately.

Why airtightness matters

Air leaks are one of the least visible problems in a house and one of the most consequential. On a January night, warm indoor air escaping through gaps in walls and ceilings carries moisture with it, and that moisture can condense on cold sheathing and framing inside the wall, where no one sees it until the damage is done. The same leaks pull cold air in low in the house, which is why some rooms feel drafty no matter where the thermostat is set, and they bring dust and pollen in with it. The consensus across the building science community, from Building Science Corporation’s research to years of coverage in Green Building Advisor, is that leaky houses tend to have more moisture and durability problems, not fewer.

An air barrier only works if it is continuous: foundation, walls and roof, with every seam, corner and opening tied together. That continuity is the hard part. Studies of typical houses have found that a handful of large openings, behind tubs and showers, around chimneys, at soffits and where an attached garage meets the house, account for a large share of the leakage. Those are the places we pay closest attention to.

Do houses need to breathe?

The phrase mixes up three different things. Walls need to dry when they get wet, which depends on how materials let moisture move, not on air leaks. People need fresh air, which is a matter of ventilation. And air leakage is simply air moving through gaps in amounts no one chose, mostly when it is coldest or windiest outside.

A leaky house is not well ventilated. It is ventilated at random: too much air on a bitter, windy night, and too little on a mild, still day in May.

Older homes deserve one note. Solid masonry walls rely on the masonry to release moisture, so on renovations the repair materials, lime-based mortars for example, need to let vapor pass. That is about vapor moving through the wall, not about leaving the house drafty.

How tight is tight?

Airtightness is measured with a blower door test. A calibrated fan set in an exterior doorway pulls air out of the house, and the result is expressed as air changes per hour at a standard pressure, written ACH50. Michigan’s current energy code, based on the 2015 model code with state amendments, allows up to 4 ACH50. Michigan’s move to the 2021 codes was put on hold by the Court of Claims in July 2025 while a lawsuit proceeds, so we confirm current requirements with the local building department on every project. A carefully built custom home can test well below either number.

The number matters less than the work behind it. As Joe Lstiburek of Building Science Corporation has argued, reaching a good result is mostly a matter of sealing the big holes carefully. Chasing an arbitrary target is less important than building a continuous air barrier and then ventilating the house properly.

Ventilate on purpose

Once a house is tight, fresh air has to be supplied by design. The International Residential Code, on which Michigan’s residential code is based, requires whole-house mechanical ventilation in homes tight enough to need it (Section R303.4), with the rate set by floor area and number of bedrooms.

There are several ways to provide it. The simplest uses quiet bath exhaust fans that run continuously. Balanced systems bring in and exhaust equal amounts of air, and the best of them, heat recovery or energy recovery ventilators, pass heat between the outgoing and incoming air so winter ventilation does not mean a cold draft or a large heating bill. In a very tight house, balanced ventilation also keeps the house from drawing replacement air from places it should not come from, such as an attached garage.

Kitchens, fireplaces and combustion

A tight house changes how other equipment behaves. A professional-style range hood can remove several hundred cubic feet of air per minute, and that air has to come back in somewhere. That is why the residential code calls for makeup air on hoods rated above 400 cubic feet per minute. Fireplaces and gas appliances should be sealed-combustion units that draw their air from outdoors, so a running hood can never pull exhaust back into the house. These are decisions for the design phase, not something to sort out after the walls are closed.

The result

A tight, deliberately ventilated house is more comfortable, quieter, cleaner and longer-lasting, and it costs less to heat and cool. It also gives the heating and cooling system a predictable house to work with, which is the subject of a coming entry on sizing equipment to the house as it is actually built.

Further reading: Green Building Advisor, Buildings Don’t Need to Breathe; International Code Council, Significant Changes to the IRC: Section R303; Insulation Institute, 2015 Michigan Residential Energy Code summary; Mondaq, Court pauses new energy-efficient building codes; Building Science Corporation, BSI-053: Just Right and Airtight.

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The Journal shares general information about home design and construction for educational purposes. It is not professional advice for any specific property and does not replace guidance from a licensed architect, engineer or your local building department. Codes, products and site conditions vary, and recommendations change over time. Views expressed are those of the author. Reading this article does not create a client relationship with Mosher Dolan; any work we perform is governed solely by a written contract. Full Journal disclaimer.