Your house is an island

Crawl spaces can be one of the biggest contributors of moisture in the attic space. Crawl spaces with exceptionally high moisture levels can overwhelm even crawl spaces that have “technically” adequate ventilation. Moisture as vapor (a gas) will move from the crawl space, through the living space, and end up in the attic where it can condense on the cold roof sheathing. This moisture vapor can overwhelm an attic that also has “technically” adequate ventilation.

On a recent inspection, I had such a crawl space. Much of the crawl space had flooded to such an extent that the vapor barrier was either floating or covered with water.

The technically adequate ventilation was unable to deal with this much water. The wooden form-ties had rotted away long ago and evidence of water intrusion was obvious.

The high humidity was finding its way through the living space and condensing on the underside of the cold roof surface. Staining and mold or mold-like fungal growth was evident on much of the roof sheathing.

While if there had been adequate ventilation of the attic space, the condition might not have been quite as bad, I have seen attics with technically perfect ventilation that appeared worse than the following picture.  It is more related to the number of pathways that the moisture can find to travel.

Condensation was such that water was actually dripping out from between the boards at the stained areas of the soffits at the exterior.

Homes with this condition can also see lots of condensation on single pane windows or metal window frames as the moisture moves through the living space. To add insult to injury, if exhaust fans are not used or are not functional, the moisture conditions in the attic will be exacerbated.

So what is the cause of this problem?

Some will say that it is the moisture in the crawl space–but is it really?

The real cause of the problem is inadequate, non-functional or non-existent drainage around the exterior of the home and/or improper termination of roof drains. When a home is built, the exterior drainage should not be just an afterthought–it is perhaps one of the most important design considerations. All too often it is not even considered, or just plain ignored.

Too much water in the crawl space is merely a symptom of the problem. Some approaches to repairs will attempt to address the symptoms–and this is where the snake-oil salesmen show up at the door.

Attempts to deal with the problem from the interior are almost always going to be less than ideal–whether it is interior foundation sealing approaches, or interior drainage and sump pump installations. While under some circumstances these approaches may be all that is possible, by far the best approach is to install proper drainage around the exterior of the home such that water can never approach the crawl space (or basement for that matter).

While you may be able to reduce the amount of water inside the crawl space to the point that it is not affecting the attic, it can still be impacting the soils under the house footings leading to structural issues.

A good French drain that both collects surface run-off and has a drain at the bottom below the depth of the house footing will essentially leave the house crawl space an “island” higher than the perimeter drainage. This is particularly effective for homes that back up against steep hillsides with high water tables. Somehow the house must be isolated from all this water.

Of course installing proper drains to collect the roof water–independent of the ground water drains–is essential as well.

Many houses are built such that this “island” approach is not possible–these are typically houses that should never have been built where they are in the first place–but that topic is best left for another rant.

By Charles Buell, Real Estate Inspections in Seattle

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