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Working Down Under

If you’ve never used duct mastic, one of the first things you’ll learn is that it’s messy. It’s slightly less viscous than drywall compound and usually manages to get on everything. Although it took a while, Scott and I eventually mastered the clean-hand/dirty-hand technique, in which one hand is used to apply the mastic, while the other is kept clean. Gloves are a must. We used plastic disposable ones, putting on several at a time so that a quick peel would restore a dirty hand to clean. We kept scissors and a utility knife handy, as well as a supply of rags for drying and cleaning the liner, and used a dishpan as a drag-along tool container.

We started in one corner of the crawlspace and worked our way back toward the crawlspace entrance, sealing every seam and all the joints around every pier.

In some places the tape was still partially adhered, in others it came away easily. We cut away the loose tape and left it in place where it was relatively snug. Where water had entered through the seams, dirt was left behind, so it was easy for us to identify vulnerable areas. But creating a good, tight overlap over an uneven dirt floor can be a challenge — especially if you’re lying on the material you’re trying to align. The pieces tend to wrinkle, but the wrinkles at the laps don’t politely align. We found that we could use the sticky drywall mesh tape to hold the sheets together while the mastic set up.

We applied a line of mastic about 3 inches wide and 1/8-inch thick, making sure we completely covered the mesh tape. We worked the piers together, with a person on each side, to make sure we sealed every joint.

In all, we used five gallons of duct mastic (at about $17 per gallon) and two rolls of fiberglass seam tape. The job took us about 16 hours.

The Results

Once I realized I had a serious water problem, I began measuring relative humidity in the crawlspace. During the time when water was on top of the liner, the RH was 70 percent. After I vacuumed and mopped out the water, the RH dropped to 55 percent within a few hours.

Next, before making the repairs, I sealed off the vents and began constantly running a dehumidifier in the crawlspace, which further lowered the RH to 45 percent during the winter. After we made the repairs, the RH stabilized at around 53 percent at 64°F without the dehumidifier; with the dehumidifier it drops to 33 percent. The other thing I’ve observed is that before the repairs, the humidity level in the house would rise with the crawlspace humidity. That’s no longer the case; now, the house humidity levels tend to follow the outdoor weather conditions.

I’ve removed the original fiberglass batt insulation from the floor joists and plan to spray-foam the perimeter walls and band joists in the near future, at a cost of $2,175.

Patty McDaniel is president of Boardwalk Builders in Rehoboth Beach, Del. Scott Gaston rebuilds boats and houses in Lewes, Del.