Vented vs Sealed Crawlspace — Coastal BC Makes Sealing the Better Bet
Claim: in coastal BC’s wet climate, a sealed (encapsulated) crawlspace outperforms a traditionally vented crawlspace for moisture control — because the outdoor air brought in through vents is often warmer and more humid than the crawlspace air it is supposed to dry, raising rather than lowering relative humidity.
Mechanism
The traditional vented crawlspace logic was: bring outdoor air in through perimeter vents to dilute and remove moisture. This works in dry climates where outdoor air is drier than crawlspace air for most of the year.
It does not work reliably in coastal BC because:
- Metro Vancouver receives significant year-round precipitation and has high ambient outdoor humidity
- In summer, warm outdoor air holds substantial water vapour; when it enters the cooler crawlspace and cools down, relative humidity can rise above the outdoor level — the opposite of the intended effect1
- Building science research (Advanced Energy, 2002) showed vented crawlspaces in humid climates track or exceed outdoor humidity levels, while encapsulated crawlspaces maintain RH below 60% even in summer1
The modern sealed/conditioned design:
- Seals all perimeter vents permanently
- Installs rigid insulation on the interior of the foundation walls (not between floor joists — this moves the thermal boundary from the floor to the perimeter walls)
- Installs a continuous vapour barrier on the ground (minimum 6-mil; professional encapsulation typically 12–20 mil reinforced)
- Manages humidity with a dedicated crawlspace dehumidifier set to ≤55% RH
- Treats the crawlspace as a semi-conditioned buffer zone — not heated to living-space temperature, but not open to outdoor swings either
Decision routing (apply The Decision Lifecycle)
| Scenario | Decision | Why |
|---|---|---|
| Bare soil, no existing vents, no active moisture problems | Install ground vapour barrier (DIY) — don’t encapsulate yet | Cheapest, highest-impact first step; see if it stabilizes humidity |
| Vented crawlspace with persistent humidity problems despite good drainage | Encapsulation is warranted | Venting is the moisture source; cost 16,000 in Metro Vancouver |
| Vented crawlspace with standing water | Fix drainage first; then assess venting vs sealing | Encapsulation on a wet crawlspace creates a sealed moisture trap |
| Existing encapsulated crawlspace with failed dehumidifier | Replace dehumidifier immediately | A sealed crawlspace without humidity control is worse than a vented one |
This decision crosses the irreversible + >$500 threshold (you cannot un-seal a crawlspace trivially after sealing the vents and installing perimeter insulation) — it warrants getting 2–3 quotes and confirming the moisture source before committing.
Scope
This decision-rule applies to the vented vs. sealed design choice for the crawlspace as a whole. It does NOT cover:
- The choice of insulation R-value on the perimeter walls (see insulation (Home Systems))
- Drainage corrections upstream of any encapsulation decision (see foundation-drainage-waterproofing (Home Systems))
- Radon sub-slab depressurization (a crawlspace encapsulation can be combined with a radon vent pipe — see radon (Home Systems))
Sources
Idea Compass
North: Where this comes from
- crawlspace (Home Systems) — the parent component note
- Building science research (Advanced Energy 2002; Joe Lstiburek / Building Science Corporation) — empirical basis for the humidity data
East: Tensions / failure
- Crawlspace-Moisture-Migrates-Up-Into-Living-Space-Via-Stack-Effect (Home Systems) — the failure consequence when the humidity control choice is wrong
- A sealed crawlspace with no dehumidifier — the failure mode where sealing makes things worse
- BCBC still includes provisions for ventilated crawlspaces (BCBC 9.18)1 — code permits ventilation; building science suggests sealing is better in practice in wet climates
South: Where this leads
- insulation (Home Systems) — sealed design shifts insulation from floor joists to perimeter walls
- radon (Home Systems) — sealed crawlspace is the prerequisite for sub-slab depressurization radon mitigation
- vendor-roster (Home Systems) — crawlspace/moisture specialist needed for full encapsulation
West: What’s similar
- The Decision Lifecycle — encapsulation crosses the irreversible + >$500 threshold
- Attic venting vs. unvented conditioned attic — the same building-science debate, applied one floor up; hot-humid climate = seal it; dry climate = vent it
Footnotes
-
Energy Vanguard, a building-science education site — crawlspace building science; 2002 Advanced Energy research: encapsulated spaces maintained <60% RH, vented spaces tracked outdoor humidity; failure mechanism in humid climates — https://www.energyvanguard.com/knowledge/crawl-space-encapsulation/crawl-space-building-science/ ↩ ↩2 ↩3