The Humidity Problem Nobody Warned Me About
Years of monitoring electrified homes convinced one building-science practitioner of something rarely discussed: dehumidification capability deserves equal or greater attention than heating performance. What follows is what the data kept showing.
The Setup
In a series of electrification projects starting in the mid-2010s, indoor temperatures held set point reliably — even on the coldest nights. Humidity control was a completely different story.
Indoor air quality monitors placed throughout these homes revealed far less control over humidity than expected. One older house provided the clearest lessons.
An Older Home, Tightened Up
The project involved a small early-20th-century house that had been insulated and air-sealed thoroughly — attic, walls, and basement. The heat load after upgrades was low enough for a right-sized variable-speed heat pump.
The basement was damp. Moisture came through the walls and up through the slab. Spray foam on the walls helped, but floor moisture continued. An energy-recovery ventilator was also installed for fresh air.
The expectation was that the combination of a tight envelope, a properly sized heat pump, and mechanical ventilation would cover the main bases. It did not.
Humidity Stayed High
Summer targets of roughly 40–50% relative humidity were not being met. First-floor humidity regularly stayed above 60%. Understanding why starts with recognising the three humidity zones:
The basement ran drier because a standalone dehumidifier was operating there — but without a return path, the dry air stayed isolated. The upper floors were essentially their own humidity zones.
Three Fixes That Made a Difference
What the Data Kept Showing
Tightening older homes stops the moisture that previously escaped through the stack effect. In houses without a slab vapour barrier, ground moisture continues to enter. Once the envelope is sealed, that moisture has nowhere to go unless actively managed.
Oversized systems with weak dehumidification are already contributing to problems in humid climates. Equipment optimized more for cooling efficiency than for sensible heat ratio has reduced dehumidification performance across much of the market.
Average dew points have risen in many regions, and nighttime temperatures — a major driver of humidity — are rising faster than daytime temperatures. Fully communicating equipment that can run a cold coil at low capacity is increasingly important.
The Core Lesson
Dehumidification capability deserves equal or greater attention than cold-weather heating performance when specifying and sizing air conditioning systems.
Most clients will not ask about it. Most installers will not raise it. But the homes that struggle with humidity — mould, rot, persistently damp air — are often the ones where the dehumidification question was never asked at the point of installation.
In humid climates it is worth discussing whole-home dehumidification on every quote. Even if the client declines, the conversation should happen.
Concerned About Humidity In Your Home?
We can advise on the right system setup — including dehumidification capability — for your property. Get in touch for a no-obligation conversation.
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