Tightening Up the Envelope: Controlling Crawlspace Moisture

By Chet Zebroski

So, this is the first post in the Home area. I thought to share how we’re remodeling our home, bringing it up to date aesthetically and performance-wise.

We own a small ranch home in Marin County, just outside of San Francisco. We bought it because of it’s location and potential for improvements. The house sits on a keypoint in the hills facing East and water accumulation is a seasonal issue on our property.

One of the largest issues faced by designers in meeting California’s Energy 2020 goals are poorly performing existing buildings. We understand that conservation is one of the most powerful tools we have to meet that goal. Using our home as a source to share techniques and lessons learned seems logical.

We started with the building envelope, making sure insulation was in place and further tightening up the walls by sealing around outlets and other penetrations on the exterior skin. We discovered that we have R-19 batt insulation in the attic and R-11 blown-in cellulose insulation in the walls.

Then, we quickly turned our attention to the underfloor crawl space, addressing the water percolating up in our subfloor crawlspace. Water accumulation in a crawl space can cause many structural and health issues if not resolved. For this property, we knew we had to somehow alleviate that potential. Consider that when these wet conditions are active, vapor pressure will force the moisture into the house through any means available increasing humidity inside bringing along with it potential health issues.

We hired Foster Pump for the consultation, solution, and installation. After reviewing the situation, they recommended creating channels around the inside perimeter of the foundation and midpoints guiding the seeping water to a sump pump. The solution would include a continuous pool cover as the vapor barrier , covering the entire crawlspace, in case water accumulates above the channels. If so, the barrier will rise with it. When the water reaches the sump pump, a float triggers the action and water is directed to the curb/gutter via a 1.5″ diameter pipe.

They also recommended a continuous sub-drain around the exterior perimeter of the house, but that was just too expensive for us. Plus, we had concerns that even with the continuous exterior sub-drain, water would still percolate at the midpoints in the crawlspace. So, we decided to place the interior work first to discover how it would perform.

I have a few pictures to share of this primary performance remedy below. The application of the channels, vapor barrier, and pump made a radical improvement on interior humidity levels and reduced the amount of condensation that accumulated on our single pane aluminum windows to near zero. It is doubtful that we will be adding a perimeter drain system because the water intrusion seems to be under control.