The Two Numbers That Make a High-Performance Home, Part 2: Ventilation
Last Friday I argued that a genuinely high-performance home really comes down to two numbers: how airtight the house is, and how well it's ventilated. We spent that whole piece on the first one. The short version, if you missed it: airtightness decides how much of your heating you keep, whether the walls stay dry, and — the part people find backwards — how much say you have over the air your family breathes. Seal the envelope, and air stops sneaking in through random gaps. Now you decide what comes in.
I ended on a caveat, and it's the reason there's a Part 2. A tight house isn't automatically a healthy one. Seal it up with nothing bringing fresh air in on purpose, and you've traded one problem for another. Airtightness makes good indoor air possible. Ventilation is what delivers it.
So that's today: what to do with a tight house so it breathes the way it should.
Why a tight house needs mechanical ventilation — and a leaky one isn't ventilated on purpose either
Here's the trade at the center of all this. A leaky house ventilates by accident — air pushes in and out through gaps, all the time, whether you want it or not. It's "ventilated" the way a tent is ventilated. You get fresh air, sure, but you also get whatever's outside, you get it unfiltered, and you pay to condition every bit of it. A tight house stops the accidents. Which means once you seal it, you have to put the air exchange back on purpose.
That word — on purpose — is the whole idea. Mechanical ventilation is just a fan system that brings outdoor air in and pushes stale indoor air out at a steady, controlled rate. Not when the wind decides. When you decide.
And the air genuinely needs replacing. People, cooking, showers, and finishes all load up indoor air with moisture, COâ‚‚, and VOCs. ASHRAE Standard 62.2, which is widely used in residential ventilation design, sets a whole-house target of roughly 0.03 cfm per square foot of floor area, plus 7.5 cfm per occupant (counted as bedrooms plus one). For a typical home that lands in the range of 60 to 90 cfm, continuously. That's the floor for "enough fresh air." The question is never whether to ventilate a tight house. It's how.
Three ways to do it — and they're not equal
There are three common approaches, and the gap between the cheapest and the best is the whole story.
Exhaust-only. The budget option: run bath fans or a single exhaust fan to push stale air out, and let "make-up" air leak back in through the envelope. The problem is obvious the moment you've read Part 1. This depressurizes the house and pulls replacement air in through every crack — which is exactly the uncontrolled, unfiltered, unconditioned infiltration airtightness was supposed to stop. On a tight house it's worse, not better: the fan can't easily pull make-up air through a sealed envelope, so it starts hunting for it in the worst possible places, like backdrafting a water heater or pulling soil gas up through the slab. Exhaust-only undoes the thing you paid for.
Supply-only. The mirror image: push filtered outdoor air in, let stale air find its way out. Better than exhaust-only in some ways — at least the incoming air is filtered and you're pressurizing rather than depressurizing — but you're still conditioning 100% of the incoming air with no recovery, and in a cold climate you're pushing cold air into the house and paying to heat all of it.
Balanced, with heat recovery. Often the best choice for high-performance homes. A balanced system runs two airstreams at the same rate — fresh air in, stale air out — through a single unit: an HRV (heat recovery ventilator) or an ERV (energy recovery ventilator). The two airstreams pass next to each other inside a core and trade heat without mixing. In winter, the outgoing warm air pre-warms the incoming cold air. In summer, it works in reverse.
That last one is where airtightness and ventilation finally click together as a pair. You can only run a balanced system properly because the house is tight. The two numbers aren't two separate decisions. They're one system.
HRV or ERV — and why it matters here in Colorado
People mix these up constantly, so here's the clean version. An HRV transfers heat only. An ERV transfers heat and moisture. That moisture transfer is the entire difference, and which one you want depends on your climate.
In a cold, dry Colorado winter, the air inside your house is more humid than the air outside — from showers, cooking, and just breathing. An HRV recovers the heat from that outgoing air but dumps the humidity outside, which can leave the house uncomfortably dry. An ERV recovers much of that moisture and keeps it in the house, where in our climate you usually want it. That's why, for most of the high-performance homes we model in Colorado, an ERV is the better fit. Not a rule — there are exceptions, and the PHPP model settles it project by project — but it's the right starting assumption here.
Heat recovery is the number that makes the bill work
Here's the figure that makes balanced ventilation more than a comfort upgrade. A good unit recovers most of the heat that would otherwise go straight out the exhaust. Passive House component criteria commonly require at least 75% heat recovery efficiency, depending on the certification path and test conditions — and the better units run higher.
Sit with that for a second. You're exchanging the entire volume of air in the house several times a day — and keeping three-quarters or more of the heat you already paid for. Without recovery, continuous ventilation in a Colorado winter is a steady leak of conditioned air you're choosing to open. With it, you get the full fresh-air exchange and almost none of the energy penalty. That's the move that lets a tight, well-ventilated house be both healthier and cheaper to run, instead of trading one for the other.
It's also why the order matters. Recovery only works if the house is tight enough that the air actually goes through the unit instead of around it through leaks. Build tight, then ventilate right — in that order.
Where I'll push on the industry
Two honest frustrations, because this is where good intentions go sideways on real projects.
First: ventilation is the system most likely to be value-engineered out, and it's almost always a mistake. When a budget gets tight, the balanced ERV is an easy target — "we'll just run the bath fans harder." That decision quietly cancels the airtightness investment sitting right next to it on the same plan set. If you're going to build tight, the ventilation isn't the optional half. It's the half that makes the first half safe.
Second: a ventilation system that's specified well and installed badly performs like neither. Ducts pinched into joist bays, an ERV stuck in an unconditioned attic, balancing dampers nobody ever set — I've seen all of it. A balanced system only stays balanced if someone commissions it: measures the airflows, sets them, and confirms the recovery is actually happening. On our projects that's not an optional add-on. It's the difference between a system that performs as modeled and an expensive box humming in the attic doing half its job.
The takeaway
Airtightness and ventilation are not two items on a checklist. They're one idea split across two numbers. Sealing the house gives you control over the air; balanced ventilation with heat recovery is how you spend that control well — bringing in fresh, filtered air on purpose, and keeping the heat you already paid for.
Get them both right and you get a house that's quiet, comfortable, dry inside the walls, cheap to run, and full of air you actually chose. Get one without the other and you've half-built something. Build tight, ventilate right. They only work as a pair.
Builders and architects — when a budget gets squeezed, where does the ventilation system land on your cut list, honest answer? And for those building tight: HRV or ERV, and what made the call? I'd especially like to hear the commissioning detail — the airflow you measured that didn't match the spec, and what you did about it.
#PassiveHouse #BuildingScience #Ventilation #ERV #HRV #IndoorAirQuality #HeatRecovery #ASHRAE622 #ColoradoConstruction #HighPerformanceBuilding
Sources
ASHRAE — Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings: whole-dwelling mechanical ventilation rate = 0.03 cfm/ft² of conditioned floor area + 7.5 cfm per occupant (occupants counted as number of bedrooms + 1). https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2 — plain-language corroboration: Green Building Advisor, "How Much Fresh Air Does Your Home Need?" https://www.greenbuildingadvisor.com/article/how-much-fresh-air-does-your-home-need
Passive House Institute (PHI) — Certification criteria for ventilation units with heat recovery: minimum 75% heat recovery efficiency at a maximum electricity demand of 0.45 Wh/m³ for PHI certification. https://passipedia.org/planning/building_services/ventilation — see also Passipedia, "Comparison of energy performance of ventilation systems," https://passipedia.org/planning/building_services/ventilation/comparison_of_energy_performance_of_ventilation_systems_using_passive_vs._active_heat_recovery
