Airtight Homes & Ventilation: Why a Tight House Has to Be Given a Way to Breathe
Sealing the leaks is the easy part. The moment a house stops leaking, the fresh air it used to get by accident has to be put back on purpose — and that air is a load.
When you make a house airtight, you stop the uncontrolled air leakage that used to wander in through gaps and cracks. That cuts the infiltration part of the load — but it does not cut the load in half, and it does not mean the house needs no outside air. A tight house still has to bring in fresh air, now through mechanical ventilation, and that ventilation air is its own heating, cooling, and (in humid climates) dehumidification load. A proper load calculation accounts for it.
- Air leakage (infiltration) is one slice of the load — conduction, solar, and internal gains don’t change when you seal the house.
- A tight house has almost no natural air exchange, so it needs mechanical ventilation for healthy indoor air.
- The building-science rule is “build tight, ventilate right” — seal the envelope, then add controlled fresh air.
- ASHRAE 62.2 sets the fresh-air rate: roughly 0.03 cfm per square foot of floor area plus 7.5 cfm per occupant.
- An ERV recovers heat and moisture from the air it exhausts, so the incoming fresh air costs far less to condition — which matters most in hot, humid climates.
Air leakage is a load you never chose
A leaky house breathes whether you want it to or not. Hot, humid outdoor air pushes in through every gap, seam, and penetration, and the air conditioning has to cool it and wring the moisture out of it. That uncontrolled exchange is the infiltration load, and on a leaky house it can be large — it is one of the heat-gain paths we measure in any heat gain and heat loss calculation.
Sealing the envelope is the right move: it turns that random, weather-driven leakage into something you control. But sealing changes only the leakage. It does nothing to the heat conducting through the walls and roof, the sun pouring through the windows, or the heat people and appliances add inside. Those loads are still there.
Sealing the house does not cut the load in half
This is where a lot of equipment gets sized wrong. The reasoning goes: “the house is tight now, so cut the tonnage.” But infiltration was only ever one component. Trim it to near zero and the conduction, solar, and internal gains are untouched — so the load drops by that one slice, not by half.
And here is the part that gets forgotten: once the house is sealed, you have to introduce outdoor air on purpose, because the leakage that used to supply it is gone. That mechanical ventilation brings in a known volume of outdoor air every hour, and that air carries its own load — sensible heat in summer and winter, and a real moisture load in a humid climate. So tightening the house doesn’t make the ventilation load disappear; it replaces uncontrolled leakage with controlled, measurable ventilation. The right answer isn’t “cut the equipment in half” — it’s “calculate the real load, including the air you now have to bring in.”
Build tight, ventilate right
The building-science community settled this years ago, in a phrase long championed by Joseph Lstiburek of Building Science Corporation: build tight, ventilate right. Seal the envelope as tight as you reasonably can — then deliberately supply the fresh air the occupants need. The two go together. A tight house without ventilation traps moisture, cooking byproducts, and everyday indoor pollutants, which is how you end up with stale air, condensation, and mold. A tight house with the right ventilation is comfortable, durable, and efficient.
The mistake isn’t building tight. The mistake is building tight and then assuming the house will sort out its own fresh air. It won’t. Tightness and ventilation are two halves of the same decision.
How much fresh air? What ASHRAE 62.2 calls for
The residential standard for whole-house ventilation is ASHRAE 62.2. The baseline rate works out to about 0.03 cfm for every square foot of conditioned floor area, plus 7.5 cfm per occupant (counted as the number of bedrooms plus one). For a typical 2,000-square-foot, three-bedroom home, that is roughly:
| Component | Calculation | Result |
|---|---|---|
| Floor area | 0.03 × 2,000 sq ft | 60 cfm |
| Occupants | 7.5 × (3 bedrooms + 1) | 30 cfm |
| Whole-house fresh air | 60 + 30 | 90 cfm |
Ninety cfm of outdoor air, brought in continuously, is a steady load the system has to carry. In a hot, humid climate that 90 cfm is also a steady stream of moisture — which is exactly why how you bring the air in matters as much as how much.
ERV vs. HRV: bringing air in without paying full price for it
You can satisfy the ventilation requirement with a simple exhaust fan, but that just dumps conditioned air out and pulls raw outdoor air in — you pay full price to condition every bit of it. A balanced system with energy recovery does better. It runs the outgoing stale air and the incoming fresh air past each other through a core, so the air you’re throwing out pre-conditions the air you’re bringing in.
There are two kinds. An HRV (heat recovery ventilator) transfers heat only. An ERV (energy recovery ventilator) transfers heat and moisture. In a hot, humid climate the ERV is usually the better choice, because it strips much of the humidity out of the incoming air before it ever enters the house — so your fresh-air requirement doesn’t turn into a humidity problem. That connection is why ventilation and a hot, humid climate load calculation have to be worked together.
Why all of this belongs in the load calculation
Ventilation air isn’t a footnote to the load — it’s part of it. The outdoor air a tight house brings in adds sensible load year-round and latent (moisture) load in summer, and that has to be in the numbers before any equipment is selected. Leave it out and you undersize. Assume the old leaky-house infiltration and you oversize. Either way the house suffers.
We calculate the real load for the house as it will actually run — envelope sealed, ventilation included, and the recovery a balanced ERV or HRV provides accounted for. Those numbers then drive the right equipment selection. We work nationwide and residentially, with select light commercial such as small offices and recreation centers.
Frequently asked questions
Does making a house airtight cut the HVAC load in half?
No. Air leakage is only one part of the load. Sealing the house reduces the infiltration portion, but conduction, solar, and internal gains are unchanged, and a tight house then needs mechanical ventilation that adds a load of its own.
Why does an airtight house need mechanical ventilation?
Because sealing the envelope removes the accidental air leakage that used to supply fresh air. Without controlled ventilation, a tight house traps moisture and indoor pollutants, leading to stale air, condensation, and mold.
What does "build tight, ventilate right" mean?
It is the building-science principle that you should seal the envelope as tight as is reasonable and then deliberately supply fresh air with a ventilation system, rather than relying on leaks for air exchange.
How much ventilation does a home need?
ASHRAE 62.2 sets the rate at roughly 0.03 cfm per square foot of conditioned floor area plus 7.5 cfm per occupant. For a 2,000-square-foot, three-bedroom home that is about 90 cfm of continuous fresh air.
What is the difference between an ERV and an HRV?
An HRV transfers only heat between the outgoing and incoming air streams. An ERV transfers both heat and moisture, which makes it the better choice in humid climates because it limits the humidity the fresh air brings into the house.
Do you account for ventilation in the load calculation?
Yes. We include the mechanical ventilation air as part of the sensible and latent load and account for the recovery an ERV or HRV provides, so the equipment is sized for how the house will actually run.
Get a load calculation that includes the air your house has to breathe
Envelope sealed, ventilation accounted for, equipment sized to the real load — not a leaky-house guess and not a tonnage cut in half. We work with builders, contractors, and homeowners nationwide.
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