Decades of research on how buildings function explain why synthetic housewrap long ago replaced polyethylene as a building material.
Walking through a historic section of Portland, Maine, this week, I noticed this grand old house, wrapped in polyethylene (sheet plastic). Why, I wondered, would anyone in the 21st Century still use this product in this way?
It's not clear whether the owners of this 100-year-old building in Portland, Maine, intend to shingle over the plastic wrap, or whether the wrap serves some other purpose such as insect treatment. If the answer is the former, it's a recipe for water woes.
It can't be price. Housewrap such as DuPont Tyvek costs about .50 per sq. ft. Even for a large house, that's a pretty minimal outlay to wrap your house in a high-tech product that allows water vapor to pass through from inside the home to outdoors, versus suffocating your house and trapping humidity from showers, cooking, breathing and basement drying inside walls.
Before the advent of homewrap, most homes in the Northeast had a simple paper barrier on the exterior or what's now called felt paper, an asphalt-impregnated paper that's primarily intended to keep rain from entering roofs or walls.
Here are 3 reasons why you should not wrap your house in plastic:
1: Water Gets Trapped in Walls
As moist air from inside the warm house enters the colder wall cavities in the fall and winter, it hits cold surfaces and condenses, and water runs down the framing, or interior of the drywall.
What if you inherit a home that's improperly sealed tight in this way? All is not lost. All it means is that you're now the permanent caretaker of the home's humidity balance. In this scenario, your best option is to install a combination of HVAC tools that will allow you to monitor humidity in real time. This includes installing a smart thermostat that will automatically adjust/reduce humidity, such as the one shown in this video. In addition, you will need to maintain a constant supply of fresh outdoor air with an ERV or HRV to replace polluted air trapped indoors.
The wrong type of building wrap can allow the amount of moisture to build up quickly inside wall cavities, creating an ideal environment for mold, mildew and termites.
2. Plastic Degrades When Exposed to Repeated Heat
It’s a little known aspect of plastic’s chemical makeup that it breaks down rapidly when heated and cooled repeatedly. When the Smithsonian studied astronaut spacesuits from the 1960s, for example, they were shocked to learn that the buttons on the spacesuits had nearly melted away. This is especially true of plastic sheeting installed under siding on south-facing walls. It’s likely to become brittle in just a few years.
3: Plastic Is a Lone Wolf Product
To create a complete vapor barrier, seams in polyethylene have to be carefully taped (as do housewraps), but unlike housewrap, plastic does not come with special tapes and flashing that make sealing around penetrations, pipes and windows easy.
Matt Power is an expert on building science. He has covered construction practices for nearly 30 years, winning dozens of awards for editorial excellence. As Green Builder’s Editor-in-Chief for nearly 10 years, Power continues to urge builders and homeowners to new levels of performance and innovation.
Should I put plastic sheeting on my interior walls?
Well, "should" is too strong a term. You can use plastic if you put polyethylene between your drywall and the wall studs. This way, you should get a tighter wall without trapping moisture inside the exterior walls. If you are framing out a below-grade basement, however, do not use plastic as a vapor barrier. Concrete tends to wick moisture, and if you cover insulated cavities with plastic they can become wet and optimal for mold growth.