Caulk and Moisture

Scott Gibson, Contributing Editor

I am a painting contractor in San Francisco and I work on a lot of older homes with wood siding. My question is how does wood siding vent vapor from the inside of the house and, more important, where exactly should one caulk or not caulk?

There's no single answer to your first question because building methods have varied so widely over the years.

Many of the building materials we take for granted are relatively recent developments. Our building (and painting) forbears didn't have the luxury of housewrap, adhesives, vapor barriers, spray-foam insulation and, of course, elastomeric caulk.

All of these affect the movement of moisture through the outside skin of the house.

Trapped moisture causes rot. In older homes, let's say those that were built 100 years ago, moisture could move easily between inside and outside. Buildings weren't as well sealed as they are today, and as a result moisture wasn't as likely to become trapped inside the walls. Remember grandpa's endless stories about how tough winters used to be in a drafty old house?

Water vapor could pass through most materials, and the minor chinks and gaps in outside walls provided some air movement that helped the process along.

The dynamics of a house changed considerably once we began adding thick layers of insulation, central heat, air conditioning and non-permeable building materials like polyethylene sheeting.

These products and mechanical systems make houses more comfortable. But there also is a greater risk that moisture will become trapped inside the walls, leading to mold and the possibility of decay.

How moisture vents to the outside depends on what building materials are used and how they're installed. An effective air and vapor barrier will keep most of the moisture that originates inside the building from migrating outward. Moisture that's driven into or below the siding should dry to the outside.

A sloppy job of installing air and vapor barriers means that more moisture will be pushed outward in winter, from the hot side to the cold side of the wall. And the problem is worse when nothing is done to control moisture inside, failing to use bathroom fans, for instance, or venting a clothes dryer in the basement.

Paint that pops off the side of a house long before it should may be a sign that there's lots of moisture trying to get out. These problems are beyond the scope of a new coat of paint.

Caulk is an ally but not the first line of defense. As good as modern caulks are we shouldn't rely on them as much as we sometimes do. They're great for sealing minor gaps and cracks that would otherwise allow water to get in. However, they shouldn't take the place of flashing, roof underlayment, and other more permanent weather barriers.

I would use caulk to seal gaps between siding and window and door casings (but not as a replacement for head flashing) and to seal butt joints between adjacent pieces of exterior trim or beveled wood siding. I'd use caulk to close minor gaps in trim, not only to keep water out but also to close voids that I don't want to look at.

But keep caulk out of weep holes you may find at the bottom of walls or in some windows that are designed to let trapped water escape.

And it should not be used as a permanent repair for roofing or siding. For that, get a carpenter.

About the Author
An accomplished woodworker and carpenter, Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He also is former managing editor of the Kennebec Journal, a daily newspaper in Maine.


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