Crawl Space Encapsulation in Greater Victoria

What an encapsulation actually includes

A loose sheet of plastic laid over the dirt is not an encapsulation. It is the most common thing already sitting under a house when a homeowner calls about a musty smell, and it is why so many people believe the approach does not work.

The US Department of Energy publishes the reference assembly in its Building America Solution Center: a 6 mil polyethylene vapour retarder over the floor and up the perimeter walls, seams overlapped 6 inches at the code minimum with 12 inches recommended, and insulation on the walls rather than in the floor above, run down to grade and then a further 24 inches (610 mm).

Three details separate that from a plastic sheet. The barrier is continuous, floor to wall. It is mechanically fastened and taped rather than laid loose. And the insulation moves from the floor above to the perimeter wall, which is what brings the crawl space inside the heated envelope of the house instead of leaving it as an outdoor space with a lid on it.

Sealing the vents changes which code rules apply

This is the part the national how-to articles leave out, and it is the part that decides whether an encapsulation passes inspection in British Columbia. Section 9.18 of the BC Building Code divides crawl spaces into heated and unheated, and the split does not depend on a furnace being down there. A crawl space counts as heated when it is used as a hot air plenum, when it holds heating ducts that are not sealed and insulated, or when it is not separated from the heated space above in accordance with Section 9.25.

Encapsulating a vented crawl space moves it across that line deliberately. Under Section 9.18 an unheated crawl space vented by natural means needs not less than 0.1 m2 of unobstructed vent area for every 50 m2 of floor area, distributed on opposite sides of the building. Close those vents and the ventilation requirement is no longer a vent ratio at all: a heated crawl space is ventilated in accordance with Section 9.32, and insulation, an air barrier system and a vapour barrier become required in its walls under Section 9.25.

Assembled from BC Building Code, Division B, Section 9.18 Crawl Spaces, free-access text published by BC Publications. Read it alongside Section 9.25 for the wall assembly and Article 9.14.2.1, which is where Section 9.18 sends you for foundation drainage.

The practical consequence is the one worth arguing with a contractor about. An encapsulation quote that seals the vents but leaves the walls uninsulated and the ground cover unsealed at the perimeter has not produced a heated crawl space under the Code. It has produced an unvented unheated one, which is the assembly that traps moisture instead of removing it.

How thick the liner should be

The Code number is a floor, not a specification. Section 9.18 sets 0.10 mm polyethylene as the minimum ground cover in an unheated crawl space and 0.15 mm in a heated one, which is roughly 4 mil and 6 mil. Liners sold for encapsulation work are commonly 10 mil to 20 mil, because sheet at the code minimum tears the first time anyone crawls across it.

That choice is not free, and it is not large either. This Old House prices liner by gauge: about $0.05 to $0.20 per square foot at 4 mil, and $0.40 to $0.60 per square foot at 20 mil. Materials run 30% to 50% of an encapsulation total.

Material choice is also the owner’s call, not the inspector’s. In decision #1373 the BC Building Code Appeal Board heard from a building official who refused polyethylene ground cover as a matter of standing policy and required a concrete skim coat instead. The Board determined that polyethylene ground cover is acceptable in a crawl space, and that the choice among the materials the Code permits belongs to the owner or the builder, not the building official. If you are told a liner will not pass, that decision is the one to have in hand.

Humidity is the number that tells you it worked

A sealed crawl space is judged on what the air inside it does after the liner goes down, and there is a published target. The US EPA advises keeping indoor humidity below 60 percent and ideally between 30 and 50 percent relative humidity, and notes that materials dried within 24 to 48 hours of a leak will in most cases not grow mould.

That is why an encapsulation in this climate normally includes a dehumidifier or a supply of conditioned air, and not the liner alone. A sealed space with no moisture removal has nowhere to send the water vapour that was already in the soil and the framing. A cheap hygrometer left on a joist for a week is the honest test of whether the work did what it was sold to do.

What published data says encapsulation costs

We do not publish a price for our own work, because the number moves with square footage, headroom, standing water, and what is already growing under the floor. What can be published is what independent guides report the market charging, so a quote can be sanity checked before anyone visits.

Neither column is a Greater Victoria number and neither is a quote from us. Read them as the shape of the market: a liner-only job in the low five figures at most, and a drained, insulated, dehumidified space costing roughly twice a bare encapsulation.

The line items left off a cheap quote are the ones that decide whether the space stays dry. On top of the liner work, the same Ontario 2026 guide lists mould remediation at $500 to $4,000 and up, drainage with a sump pump at $1,000 to $3,000, and a crawl space rated dehumidifier at $1,200 to $2,500. A quote that is materially cheaper than its neighbours has usually dropped one of those three.

What to have ready before an inspection

The assessment goes faster, and the quote comes back tighter, when these are known before anyone opens the hatch.

• Roughly how many square feet the crawl space covers, and the headroom at its lowest point.
• Whether water pools down there after a storm, and where it comes in. Standing water is a drainage problem before it is a liner problem, which usually points at perimeter drains.
• What the ground is now: bare soil, an old sheet of poly, a concrete skim coat.
• Whether the space is vented to the outside, and whether any ducts or heating equipment run through it. That is the heated or unheated question above.
• Any cracking in the foundation wall around the crawl space, which is a separate scope from the liner. See crack repair and, where a full basement is also involved, basement waterproofing and our guide to basement waterproofing cost in Victoria.