Case file — Burke, Fairfax County, VA. One of the 25 jobs in our 250-home series. New here? Start with our open letter.

At a glance

Humidity and water are two different problems

Most crawl space work is about humidity: damp air, condensation, the slow transfer of moisture up out of bare soil. A vapor barrier laid on the dirt handles that well.

This crawl space had a different problem. Water was arriving as water — running in, sitting on the floor, going nowhere.

That distinction changes everything about what you install. Lay a sealed vapor barrier over a floor that actually collects water and you have not solved anything. You have built a lid on a puddle. The water still comes in, it now has nowhere to evaporate from, and you have made it invisible so nobody notices until the plastic is floating.

What went in first

A dimple board across the floor — about 210 square feet.

Dimple board is a sheet of high-density plastic moulded into a grid of raised bumps. Laid bump-side down, it holds a continuous air gap between the soil and whatever sits on top of it. Water that arrives at the floor runs through that gap to the low point instead of spreading out and standing.

It is not glamorous and it is not what most crawl space quotes include. It is also the difference between an encapsulation that works and an encapsulation that traps the problem underneath itself.

Then the encapsulation, on top

With drainage handled, the sealed system went in over it: a 20 mil Class A fire rated barrier across the floor, carried up to a mechanically fastened insulated wall liner, every seam sealed with waterproofing tape.

The order matters more than the materials. Drainage first, barrier second. Done the other way around — or with the drainage layer left out to make a number look better — the same materials produce a worse result than doing nothing.

Rim joists, about 60 linear feet, insulated into each pocket.

A condensate pump, to move collected water out rather than letting it find its own way back into the ground under the house.

A small crawl space is not a small job

This one is about 210 square feet. That is small — many of the crawl spaces we work in are three or four times the size.

It still took two days, and it still needed a drainage layer that a larger, drier crawl space would not have needed at all. Square footage tells you how much material to order. It does not tell you what the space is doing.

That is why we price crawl spaces after we have been under them, and why a number quoted over the phone from a floor plan is a guess wearing a suit.

How to tell which one you have

If you can get under your house safely, look at the ground, not the air.

Damp soil, a musty smell, condensation on the ducts — that is a humidity problem, and a properly sealed and taped barrier plus a dehumidifier is usually the answer.

Standing water, mud, a visible path where water runs in after a storm, or a previous vapor barrier with water sitting on top of or underneath it — that is a drainage problem. It needs somewhere for the water to go before anything gets sealed.

Anyone quoting you the same system for both has not looked.