Under the floor · the basement nobody visits

Crawl space encapsulation: the moisture barrier, the vents, the pipes, and what it costs

A crawl space is a basement nobody visits. That is its whole problem: the bare ground breathes moisture into it all year, the pipes run through it, and whatever goes wrong there — standing water, a frozen line, mould on the joists — is found late. Encapsulation is the industry’s word for sealing it: a heavy liner over the ground and up the walls, the vents closed, and the space brought inside the house’s envelope.

This page sets out what Natural Resources Canada’s retrofit guide says about crawl spaces — the minimum moisture barrier, when to vent and when not to, the frozen-pipe trap — what one Ontario company publishes as its prices, and the insurance problem of water that arrives slowly in a space nobody checks.

What the quote calls each part

Two ways to deal with a crawl space

NRCan’s choice is between insulating the walls, which brings the crawl space inside the heated house, and insulating the floor above it, which leaves the crawl space outside. It recommends the walls: there is less material, it is usually easier than working overhead in a tight space, and it spares you from insulating every pipe and duct that runs through. Encapsulation, as the industry sells it, is the wall route plus a heavier ground liner and sealed vents — NRCan’s heated crawl space under a commercial name.

The guide’s first instruction comes before either: fix any water leaks and remove sources of water infiltration. A liner over standing water is a liner over standing water. Where water collects, drainage and a pump are part of the job — see sump pumps — and the outside of the foundation is where it is stopped, on the eavestrough and foundation crack pages. And one rule is absolute: never vent a clothes dryer into a crawl space.

The moisture barrier

The detail in NRCan’s instruction is what separates a liner that works from plastic on the ground: overlapped and taped seams, fastened to the walls and to the posts it cannot cover, sealed to any wall barrier, weighed down where it billows, and protected with a mat where people walk. It also says not to cover it with sand or gravel. NRCan notes the downside too: a barrier can hide moisture or mould on the far side, which is why the space still needs looking at.

Four mil is a minimum, not a recommendation. Companies often quote heavier reinforced liners; the federal guide does not say a heavier one is required, and a quote should say what it is installing and why.

Vents: open, closed, or seasonal

The venting advice depends on which kind of crawl space you are building, and it is easy to get backwards. A heated crawl space — walls insulated, encapsulated — should not be open to outside air, because warm, humid summer air condenses on the cooler surfaces inside. NRCan’s alternatives are whole-house ventilation or a dehumidifier. A partially heated crawl space — floor and walls insulated — gets vents at a ratio of 1 to 500, open in spring and plugged and insulated in winter, with a monthly check of humidity and condition. Health Canada’s indoor humidity target of 30 to 50 per cent is a reasonable yardstick for what that check is looking for.

The pipes, and the frost

This is where a crawl space turns into a water damage claim. Insulate the floor above and leave the space cold, and the supply lines under the floor sit outside the heated house. Insurance draws exactly that line: IBC says standard policies cover damage from frozen and burst pipes provided the pipes are in the heated portion of the home.

A crawl space brought inside the envelope puts its pipes inside that heated portion. One left cold puts them on the wrong side of it. NRCan’s fixes for a cold one are heat trace on the pipes or a small thermostat-controlled heater. The frozen pipes page covers the rest of the cover question.

What it costs, where anyone publishes it

No public body publishes a crawl space price, and this site does not invent an average. One Ontario company publishes a detailed range, reproduced here as theirs.

The ladder in that table maps onto NRCan’s guidance. A barrier alone is the minimum the federal guide asks for where there is none. Wall insulation is its preferred way of heating the space. The full system adds what a wet crawl space needs — drainage and a pump — and what a sealed one needs — a dehumidifier. Ask which rung a quote is on, what thickness of liner it uses, whether the vents will be sealed, and what happens to the water that is there now.

Insurance and the space nobody checks

A crawl space produces exactly the kinds of loss GIO lists as typical exclusions: slow leaks and seepage, mould, and wear and tear. Water that rises through the ground is the subject of an optional cover that not every insurer sells.

The practical defence is the one NRCan builds into its guidance: look. A monthly check of a partially heated crawl space, as NRCan asks, finds a slow drip under a toilet flange or a weeping joint before it is a season old — and a leak found the week it starts is a very different conversation with an insurer from one that has run for months. The ground water coverage page covers the endorsement.

Questions people actually ask

Where to read next

• Frozen pipes
• Foundation cracks
• Sump pumps
• The other end of the house: attic mould
• Ground water coverage