Weather Barrier Myths and Facts: From Airtightness to R-Value

Three Misconceptions about the building envelope

Airtightness, insulation and water management are priorities for building professionals. But myths about all three can get in the way of creating residential homes and commercial structures that are more durable, more energy- efficient and less costly to operate.

In this article, DuPont  Building Science experts take on three common myths – and provide building envelope facts for contractors, builders, envelope consultants, and specifiers.

Myth 1: A Weather Barrier Could Make a House “Too Tight”

Modern energy codes and standards continue to emphasize the importance of energy efficiency.  With this in mind, a consistent rule of thumb holds true: “build tight, ventilate right.”  Simply put, mechanical ventilation is a more important consideration for today’s homes built with sophisticated energy saving features.  The primary way to establish the optimal degree of airtightness is through the building envelope.

Fact: Tyvek® Puts Builders in Control

DuPont™ Tyvek® weather barriers, and DuPont™ Flashing Systems, help builders control air flow to help achieve both energy efficiency and comfort. In fact, blower door testing on homes wrapped with a DuPont™ Tyvek® WRB show natural air exchange rates per hour that are well within acceptable guidelines per ASHRAE Standard 62.

Myth 2: A Vapor Barrier is the Best Water Barrier

In addition to airtightness, preventing water intrusion is another key function of the building envelope.  Building professionals often focus on managing moisture with a vapor barrier, but in some cases barriers can actually create moisture problems.

Firstly, a vapor barrier manages water vapor diffusion, the smallest moisture source in buildings.  Secondly, a vapor barrier is generally required on the inside of the envelope, and rain (the major moisture source for above grade walls) comes from the outside.  Lastly, if moisture does find its way inside a wall system, through a leak, a burst pipe or exposure during construction, a vapor barrier (especially if located at the wrong side of the wall) would prevent drying. That can create ideal conditions for mold, rot and corrosion.

Fact: Tyvek® Helps Walls Dry Faster

The unique material science behind Tyvek® allows it to be both water tight and vapor permeable.  It’s a breathable, non-perforated, product with microscopic pores that allow moisture vapor to pass through it.

So while it’s highly effective at preventing bulk water intrusion, it’s also permeable, or breathable, to allow any water that does find its way into the wall system to find its way back out again as water vapor.  This ability to allow unwanted moisture to escape can allow walls to dry more quickly, to protect against water damage from within. That’s an important added layer of protection vapor barriers can’t provide.

Myth 3: Insulation Eliminates the Need for a Weather Barrier

Maximizing the R-value of insulation begins with a clear understanding of its performance dynamics.  The installed R-value of insulation is only achieved when the air trapped inside the wall cavity remains dry and free of movement.  This means that even a relatively light wind of 5 mph can sap up to 40% of the insulation’s original installed R-value if it penetrates cracks, crevices and small openings in the structure.

Moisture can rob insulation of R-value, as well. Regardless of thickness, wet insulation retains less than 40% of its effective R-value**. By helping to protect against both bulk water intrusion and air-transported moisture in walls, Tyvek® also helps protect the insulation inside them.

Fact: Tyvek® Can Help Protect R-Value 
Tyvek® weather barriers are designed to prevent air and water from intruding into the wall system.  By keeping the wall insulation dry and free of drafts, it enables the insulation to perform to its full installed R-value.  Keeping air and moisture out helps keep warmth and comfort inside, where it belongs, and helps ensure insulation actually delivers the R-value and improved energy efficiency you’ve paid for.