The House as a System: Heat, Air, Moisture Interactions
Most real-world building problems come from coupled flows—heat drives moisture, air carries moisture, pressure moves air, and materials respond over time. This session builds the mental model we’ll use for the rest of the series.
Potential agenda
- Intro (2 min): “Failures are usually interactions, not products.”
- Desired outcome (6–8 min): What “stable” looks like: dry enough, warm enough, controlled enough
- Common failures (6–8 min): The usual coupled-flow failure patterns (with examples)
- Core concepts (6–8 min): A practical mental model for diagnosing causes
- Takeaways (2–3 min): A short checklist for thinking clearly
- Discussion (30–45 min): Guided conversation prompts embedded below
Desired outcome
A “good” enclosure isn’t one that’s perfect—it’s one that is predictable. We want assemblies that:
- Limit wetting and tolerate the wetting that still happens.
- Manage drying (in at least one reliable direction, in this climate).
- Control airflow/pressure so air doesn’t carry moisture into risky places.
- Control temperatures so surfaces don’t spend long periods below dew point.
- Stay robust against normal occupant behavior, minor defects, and aging.
Discussion prompt
- “Stable enough”: In your work, what does a “stable” house look like in year 1… and year 10?
Common failures
These are the patterns that show up when heat/air/moisture aren’t treated as a coupled system.
Coupled-flow failure patterns Short names you can use in conversation
- “Warm air finds cold surface”: exfiltration reaches a cold layer → condensation.
- “Pressure does the damage”: negative pressure pulls in soil/garage/moist air.
- “Drying path got blocked”: one upgrade removes the only reliable drying direction.
- “Small leak + time”: minor bulk-water leak becomes rot because drying is slow.
Examples (worked into failures)
Example — Exfiltration into a cold roof zone
Air leakage carries moisture to a cold surface; the roof “mysteriously” molds or frosts even with decent insulation.
Example — Basement/garage odor and humidity complaints
Pressure imbalances or return leaks pull contaminants/moisture into living space; symptoms look like “IAQ” but root cause is airflow.
Example — “We sealed it up and now it’s wet”
Air sealing is good—until it removes a drying mechanism without adding a controlled one (ventilation, dehumidification, or assembly changes).
Discussion prompt
- Coupled-flow story: What’s a project where a “good improvement” caused an unexpected problem?
Core concepts
Air moves moisture far faster than diffusion
When things are wet unexpectedly, look for air pathways and pressure drivers before blaming “vapor.”
Temperature controls risk
Cold surfaces are moisture magnets. Insulation and thermal bridges change the location of the risky surface.
Drying is a design feature
If you can’t articulate the drying path, you’re gambling. Aim for at least one robust, climate-appropriate drying direction.
Pressure is the invisible “mechanical system”
Stack effect, wind, ducts, and exhaust fans can dominate where air (and moisture) goes.
Takeaways
- Most failures are interactions: heat + air + moisture + time.
- Diagnose with order: bulk water → air/pressure → temperature → materials.
- Always ask: “What’s the drying path?” and “What’s driving pressure?”
- Next session (P06): we formalize this as the four control layers.
Further reading
Optional links will be added here (short, high-credibility readings that match the session focus).