Q&A / 

Water Behind House Wrap

DEAR TIM: I have a house wrap installed on my house over plywood sheathing. No clapboards are in place yet. I'm getting moisture behind the house wrap that is of concern to me. The product was installed at about 55 F, last October. The moisture appears in varying temperatures and also whether or not exposed to direct sunlight. The inside of the house is unfinished, with just insulation only in between the wall studs. No drywall is installed. I don't want my sheathing to rot away after I install the clapboards, so what can I do? What should have been done before winter set in? Steven Z., Jeffersonville, VT

DEAR STEVEN: I hope you’re sitting down. I’ve got some sobering news for you. Based on the photograph you sent to me, and it’s a perfect example of a picture is worth a thousand words, you’ve got to redo lots of work on the outside of your home once the weather warms up. The water you see behind the house wrap has at least four sources from just the one photo you provided.

This house is missing two things. The exterior siding and the interior vapor barrier and drywall. Photo Credit: Steven Zajchowski

This house is missing two things. The exterior siding and the interior vapor barrier and drywall. Photo Credit: Steven Zajchowski

Let’s start with the exterior and then move indoors. I’d like to go back in time to share how carpenters and builders of old protected wood frame houses from rot. They used time-tested technology.

In the late 1800’s tar paper was invented and builders quickly saw an advantage in using it under wood siding on home. Caulks from over 100 years ago that were used to try to seal water infiltration were crude and didn’t offer the long-term flexibility of modern caulk. Water could easily leak behind wood siding clapboards where they touched up against window and door trim and frames.

Carpenters would nail asphalt-saturated felt paper onto the wood sheathing of a home before installing the siding. Long sheets were installed in rows 3 feet high. They would start at the bottom of the house making sure the felt paper overlapped the foundation. Then each successive piece of felt paper would overlap the one below it.

Vertical seams also were overlapped to ensure water that got behind the wood siding would drain down the paper never touching the wood sheathing behind the paper. You can watch a video I made showing how felt paper was installed here:

Your house wrap was not installed this way. There are numerous places where the wrap is not overlapped correctly. What’s more, the vertical seams have not been taped shut. Blowing rain can easily get behind the house wrap since you don’t have your siding installed.

I also see where the nailing fins of the windows do not overlap the house wrap. This is another place water can get behind the weather barrier. Modern rubberized asphalt flashing tapes are available that allow you to seal window and door nailing fins to the house wraps. It must be done working from the bottom up just like the carpenters did 100 or more years ago.

Just think about how the clapboard siding works. Each piece overlaps the one below just like shingles on a roof. Gravity then pulls the water down to the ground. You need to do the exact same thing with house wraps, flashing tapes and metal flashings that are part of a wall system. You must maintain the overlap in the correct direction at all times.

Another possible source of the water behind the house wrap is interior humidity that has a direct path to the exterior. You indicated you moved into the home, yet it’s not finished. You live in a cold climate and the interior relative humidity is almost always higher in the inside of your home than it its outside.

The water vapor inside your home originates from cooking, showering, a possible running humidifier, indoor plants, washing dishes and clothes, breathing, etc. These things and activities all add water vapor to the inside air. This water vapor is trying to get outdoors and needs to be stopped with a cross-laminated vapor barrier that’s installed on all exterior walls over the insulated walls. It’s then covered with drywall.

If you don’t have this vapor barrier in place, the water vapor passes through the insulation, the plywood or oriented strand board and then can collect on the back side of the house wrap. Most house wraps are designed to allow water vapor to pass through them, but it’s possible for liquid water to collect on the back side if more water vapor is trying to get through than the house wrap will allow to pass at one time.

Interior humidity was not a real issue in older homes up where you live decades ago. Most older homes had no insulation and they were exceedingly drafty. Vast amounts of dry colder air would pass through the walls and lower the overall humidity in the house. There was so much air moving through the walls that even if condensation did somehow form inside a wall cavity, it rapidly evaporated. This is why older homes never rotted away like you see happening with modern homes.

I have numerous other videos here on my AsktheBuilder.com website that show you how to install house wraps and how to install the flashing tape around windows and doors. I’d invest the time and watch these so your house doesn’t rot away!

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