At the dawn of our current interest in building science, sheets of polyethylene were routinely stapled to interior framing before drywall was installed. The idea was to block the flow of water vapor into exterior walls. (Some builders tried to make their polyethylene seams airtight, so that the poly would do double duty — acting as an air barrier as well as a vapor barrier.)
Installing a vapor barrier (or more properly a vapor retarder) was considered cutting-edge.
But in time, builders began to see flaws in their approach. For one thing, it was virtually impossible not to damage or puncture the barrier during installation or after it was applied. The plastic sheeting was usually full of holes by the time the house was complete. As it turns out, these holes did not affect polyethylene’s performance as a vapor retarder — a ripped, torn vapor retarder works very well. However, these holes undermined the poly’s performance as an air barrier.
Since polyethylene reduces the ability of a wall to dry to the interior during the summer months, some builders began to worry the vapor barriers trapped moisture inside walls where it could do a lot of damage.
Thus began a muddying of the waters: when and where are vapor barriers a good idea?
Sandra Heiser wades into this debate with a Q&A post that’s the subject of this week’s Q&A Spotlight.
Leave the poly in, or take it out?
Heiser writes about a house near Buffalo, N.Y., that is ready for drywall. She describes the exterior wall assembly as follows: vinyl siding, 1 in. of rigid polyisocyanurate foam, 1 in. of sprayed closed-cell foam, fiberglass batts, and, finally, clear plastic.
“I’m thinking [the plastic] has to go,” she writes. “What are our options? Is it possible to vent the plastic with strategic slits in the plastic?”
“No, wrong,” replies Dan Kolbert, a builder in Portland, Maine. “There’s always moisture somewhere, and it has to escape either inward or outward or it will rot things out (sometimes with remarkable speed).”
“A typical 2,200 SF house, with 17,600 board feet of lumber, will release 300 gallons of water just from the lumber (never mind the concrete and other water-based materials like paint) in the first heating season as the wood acclimates from 19% moisture content (KD standard) to the typical 10% in a heated house,” Riversong says.
Slitting the vapor barrier will only allow more air movement into the walls and ceilings, but it won’t do much to prevent condensation, Riversong adds. No house in the lower 48 states should require an interior vapor barrier. Codes only require a 1-perm vapor retarder, such as a vapor retarding primer.
Does exterior foam help or hinder?
A related issue is the layer of rigid foam insulation on the exterior of the building. Polyiso foam is a vapor retarder, meaning the described wall assembly has a vapor barrier on both the inside and the outside.
If water gets in, how will it get out? And will water vapor condense on the inside of the sheathing?
Riversong is no fan of exterior foam and suggests a good rule of thumb is to keep the outer skin of the house at least five times as vapor permeable as the inside. “This means no exterior foam or ZIP wall or self-adhering flashings,” he says.
“The more we slow down heat flux through a building assembly, the more we inhibit its ability to dry,” he adds.
“Most of what passes for ‘smart’ building today has significant unintended consequences. Using relatively impermeable foam to insulate our homes will ultimately be understood to have caused more problems than it was intended to solve.”
Sorry, writes Holladay, I disagree.
Holladay recommends omitting the interior polyethylene, noting that condensation occurs when it’s cold, rot when it’s warm. Condensation forming as frost on the back side of the sheathing won’t damage the house because temperatures are too low. As long as the moisture can evaporate to the interior when the weather turns warm, the house will usually be fine.
“That said,” he adds, “it’s better to design walls that don’t allow condensation to occur. The thicker the exterior foam the better…there is accumulating evidence that exterior foam helps protect wall sheathing from moisture problems.
“Needless to say, walls need to be able to dry out, which is precisely why I advised Sandra to get rid of the interior poly.”
Beware of bulk moisture problems
Water vapor is but one threat to the house.
“The main concern for Sandra now and in the future is bulk water leaks from the exterior that gets past exterior foam board to OSB [sheathing],” says an anonymous poster.
“The OSB will then rot easily because any water getting in her foam sandwich will not evaporate fast enough to avoid decay starting. Once decay starts it takes very little moisture to keep the process going. Window and door flashing must be done very well. “
Leaks, ice dams, mechanical systems leakage, wind pressure, stack-effect pressures and water intrusion via capillary action all are potential sources of water, Riversong says, all the more reason to avoid trapping moisture with well intentioned but ultimately misguided wall assemblies.
Quoting building scientist Joseph Lstiburek, he adds: “Things get wet from the inside, the outside, and they start out wet.”
“When the rate of wetting exceeds the rate of drying, accumulation occurs.”
“When the quantity of accumulated moisture exceeds the storage capacity of the material, problems occur.”
“Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior.”
“The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow.”
“As such, energy conservation has the potential to destroy more buildings than architects.”
We asked GBA advisor Peter Yost for an expert opinion:
You are right to be concerned when you have two vapor impermeable (or nearly so) layers on opposite sides of the assembly.
1. Get rid of the poly
Given how you will be “warming” the air and vapor permeable components of the assembly, you won’t have trouble avoiding dewpoint temperatures during the coldest months of the year, unless you are running very high (above 50% interior relative humidity) during that same period. And remember that vapor pressure or vapor diffusion is a field effect; if you have 90% coverage of a plane, you have a 90% effective vapor barrier or retarder.
2. Think about the assembly, not the parts
Don’t worry about the vapor permeability of just one designated layer. Instead, think about the vapor permeability of all layers. And be just as focused on drying potential as you are about wetting. Take a look at my blog on vapor profiles as a way of addressing assemblies in this manner.