A well-sealed building envelope is not complete without a designed ventilation strategy. The envelope does its job — it resists heat loss, blocks wind, manages moisture — and in return it creates an interior that requires deliberate fresh air supply. In cold dry climates, that supply must be managed carefully: too little and air quality suffers; too much or wrong equipment and you exhaust the indoor humidity that a dry Colorado winter barely sustains.
The Problem with Natural Infiltration as Ventilation
Older, leaky buildings ventilate by accident through gaps at windows, doors, penetrations, and imperfect construction joints. This uncontrolled infiltration is unpredictable — it responds to wind pressure and stack effect rather than occupant need. On a windy night, the house over-ventilates and the heating load spikes. On a calm day, it under-ventilates and moisture concentrates around cooking and bathing areas.
When we design for a tight envelope — blower door targets at or below 1.5 ACH50 — we eliminate that uncontrolled exchange and replace it with a controlled one. The ventilation strategy is designed, measured, and adjustable.
ERV Selection for Cold Dry Climates
In Colorado, the outdoor air is already dry. Outdoor relative humidity in winter can drop below 15 percent. An interior occupied by people, plants, and cooking will have humidity in the 30 to 45 percent range — a significant moisture gradient from interior to exterior.
An HRV (heat recovery ventilator) transfers heat from exhaust air to incoming fresh air but passes moisture in both directions independently. In practice in Colorado, an HRV exhausts interior humidity with the outgoing air. Over days of continuous operation, the interior dries out faster than occupancy can replenish it.
An ERV (energy recovery ventilator) transfers both heat and moisture. The incoming cold dry air picks up moisture from the outgoing warm humid air before entering the house. The result is better humidity retention and the same heat recovery efficiency (75 to 85 percent in quality equipment).
ERV is our standard specification for sealed envelopes in cold dry climates. The cost premium over HRV is modest and the humidity management benefit is meaningful.
Integrating Ventilation with Passive Solar Design
A passive solar house has a deliberate thermal zonation: the south-facing spaces are solar collection zones, the north-facing spaces are buffered, and the thermal mass is concentrated near the south glazing. The ventilation strategy should respect this zonation.
Supply air at cold outdoor temperatures feels like a draft if delivered too close to occupants. We typically introduce supply air:
- At the perimeter near windows, where it mixes with room air before reaching the occupied zone
- At ceiling level in rooms with adequate ceiling height (over 3 meters), using the mixing effect of the supply jet
- Through a short duct run that passes through the warm zone of the house before discharge, allowing the supply air to be tempered passively
Return air is collected from bathrooms, kitchen, and utility spaces — the high-humidity, high-odor zones. This keeps the ERV core managing the dirtiest air and producing the cleanest supply.
Moisture Balance as a Design Target
In a cold dry climate, the ventilation strategy has two moisture targets that pull in opposite directions:
- Exhaust enough moisture to prevent condensation on cold surfaces (windows, especially)
- Retain enough moisture to keep interior relative humidity above 25 to 30 percent for occupant comfort and wood protection
The balance point is specific to each house — it depends on occupancy, cooking habits, plant load, and envelope airtightness. We calculate a moisture budget in design development: estimated interior moisture generation versus estimated losses through ventilation and envelope diffusion. This informs the ERV sizing and the acceptable ventilation rate range.
If the calculation shows that the house will be too dry even with an ERV, we consider localized humidification at the air handler rather than whole-house humidification. Targeted is more controllable than blanket.
Controls and Demand-Controlled Ventilation
Continuous ventilation at the minimum rate is the conservative baseline. Demand-controlled ventilation — where CO2 or humidity sensors modulate the ventilation rate in real time — can reduce energy use by 20 to 40 percent versus continuous operation while maintaining air quality above occupancy thresholds.
For author residential projects in Colorado where the home may be unoccupied for weeks at a time (vacation use, seasonal residents), demand-controlled ventilation is particularly valuable. The house breathes minimally when empty and ramps up quickly when occupied.
We specify a simple CO2-based control rather than complex building automation in most residential projects. The sensor is cheap, reliable, and easy to maintain.
Próximos pasos
The ventilation strategy for your Colorado residence should be designed alongside the envelope, not added after the structure is decided. The equipment selection, duct routing, and controls are all simpler when resolved in the design phase.