Should You Use Self-Adhered or Mechanically Attached Air Barriers?
In today’s green building environment, truly continuous air barriers are king.
However, an air barrier can become compromised by holes, gaps or other defects. The problems associated with an impaired air barrier are amplified when the forces associated with unequal air pressure intercede.
Here are some reasons why you should opt for self-adhered air barriers rather than mechanically-attached alternatives.
Rising building code requirements
Today’s stringent building codes in both commercial and residential building are enough to encourage architects, designers and builders to take a close look at the efficacy of traditional, mechanically-fastened building wraps. In the current construction environment, there are numerous reasons to use a self-adhered vs mechanically-attached barrier.
The integrity of the building envelope is more important than ever. Code requirements, energy savings targets, and mold-resistant construction all focus attention on the type of air and moisture barrier employed, and how it is affixed to the exterior wall system. Rot and mold are very real threats at temperatures exceeding 40-degrees F and humidity exceeding approximately 30-40 per cent.
Recommend reading…A Builder’s First Time Using Peel and Stick House Wrap
Air infiltration and exfiltration
Traditional building wraps are attached to sheathing with staples which create holes that threaten the integrity of the air barrier. Additional holes occur as certain forms of cladding are nailed into place. Following many building wrap installations, positive air pressure can allow conditioned air to escape through the numerous exit points.
Without an additional sealing around the staples and cladding fasteners, the building wrap has thousands of unsealed holes through which air will escape. Also, the bottom edge of the building wrap often remains unsealed in installations, and seams are either not taped or improperly taped. Rips and tears also compromise the integrity of the building envelope. The goal of a green building with optimized air flow control and maximum energy savings is then left unrealized.
Recommend reading…The Future of House Wraps
Because hammer tackers do not always insert fasteners completely flush to the surface, it is often necessary to go back and hammer all those fasteners flush with the building wrap. Although cap staple and cap nail systems are a partial solution to air migration issues, cap staples/nails still do not create an airtight seal.
If a building wrap is used as an air barrier, cap staples or nails are not going to be airtight, unless all penetrations are taped afterward. It should also be noted that these efforts and materials can add to a contractor’s expense if they were not anticipated.
Most building wrap systems typically have a complete series of installation instructions that protect against open exit points. However, the final steps, such as taping over staples for example, are frequently disregarded. A focus on time and expense savings coupled with a lack of understanding of how the entire air/moisture barrier system works can lead to installation jobs that ignore the final sealing steps. Manufacturers would consider incomplete installations to be improper installation of their product.
Building scientist Dr. John Straube explains the importance of recognizing that an air barrier is a complete system (and why your team probably needs to talk about it):
Billowing building wrap is a problem
Building wrap that billows from either interior or exterior air flow can be damaged from the fasteners. Air will always migrate from areas of greater pressure to those with lower pressure. Just as air flows from high pressure areas to low pressure systems in the earth’s atmosphere, air migrates through a porous wall system based on air pressure differences.
When interior air pressure is greater than exterior air pressure, or vice versa, air attempts to migrate, causing a balloon-like expansion of the very thin building wrap. When fasteners like staplers are used, the cycling of the billowing can wear on the membrane and enlarge holes, creating larger penetrations hidden behind installed cladding.
Air intrusion through mechanically-fastened building wraps is such a concern that some in the industry do not think of them as air barriers.
“Don’t think of building wrap as an air barrier. It’s there to be the drainage plane,” explains Energy Vanguard.
Indeed, the use of such building wrap presents a series of challenges to the integrity of the building envelope.
Lab vs. Field Environments
Although laboratory testing of building wrap has occurred, this often takes place under ideal conditions which are not easily replicated or representative of conditions in the field. For example, during some installations, loose fasteners are removed and replaced, but the holes they create are not adequately sealed. Also, siding installers may use box cutters to flatten multiple layers of building wrap bunched at the corners of a structure’s exterior. In the field, extreme weather during the installation phase may lead to rips, tears and enlarged holes in the building wrap.
Recommended reading…Air Barrier Details and Materials Considerations for Designers & Specifiers
Advantages of self-adhering air barriers
By contrast, self-adhering air barrier membranes eliminate the need for thousands of staples puncturing the building envelope. The holes created by other mechanical fasteners that penetrate during cladding installation are no longer a concern with a self-adhered product working with compatible accessories for the fine details.
Self-adhering membranes, like DELTA®-VENT SA, maximize air tightness, create watertight seals, and protect the building enclosure from wind-driven rain. DELTA®-VENT SA is light-weight and tear-resistant. It is constructed to withstand the rigors of jobsites, as well as tough wind and weather. The self-adhesive edge lap is a built-in security for greater energy efficiency. Its one of the most durable types of barriers available and provides long-term effectiveness.