THE ENVELOPE—PLEASE!

By Beverly Smirnis with special thanks to input by Allison A. Bailes III, PhD and Zero Energy Homes Designer, Consultant & Instructor Armando Cobo

Yes, please! We depend upon savvy builders to make use of the latest tools and surround themselves with a team of expert advisors to stay vigilant about new codes, techniques and products. 

Energy efficiency and moisture durability are top priorities in specifying the essential components of the building envelope. What material is best for sheathing?  Water-Resistive Barriers (WRBs) are required over wood structural panel wall sheathing, but which type is better?  Which type of insulation is preferable?  And what is the appropriate method for sealing openings and panel joints? What about HVAC choices?

Before you can answer these questions, it’s first important to have an understanding that true performance is evaluated on how all the various materials in the assembly work together.  Implementing best practices for constructing wall and roof assemblies is critical.

“Manufacturer’s reps may tell you that their product has a certain R-value. But when that product reaches the stated R-value only in a properly detailed assembly, those claims can be misleading,” says Allison A. Bailes III, PhD and author of the book “A House Needs to Beathe…Or Does It?” 

Second, you must know that not all products are the right product for any application and/or climate zone.  Fort Worth Texas-based Armando Cobo, a Zero Energy Homes Designer, Consultant & Instructor explains, “How I design a house in the South may have 80% or 90% of the same products as products used in other climate zones, but that 10%-20% difference in products is what makes or breaks a job, or bring you back for repair trips, if not lawsuits.”

Goal #1 Airtightness and Goal #2 Keeping it Dry

You may have heard it said that buildings need to “breathe” because mold problems are caused by increased air-tightness standards. But Bailes’ book title hints at the consensus of building science experts who insist that buildings don’t need to breathe. Instead, preventing mold and rot without sacrificing the energy performance means implementing sound moisture management principles.

First and foremost, reducing air leakage is the most important way to control the movement of water vapor.  Further limiting the risk of moisture and mold issues requires using control layers in walls and roofs. This ensures management of air movement, heat transfer, bulk water, and water vapor.

making the envelope airtight and watertight
Build it tight and implement control layers.

Various Types of Membranes Can Create a Dependable Watertight Seal

making the envelope airtight and watertight
The two places where flashing is most critical for moisture management are around windows and doors and where a sloped roof intersects with a wall.  DuPont Flashing Tape protects the heads and jambs of rectangular windows and doors from bulk water intrusion, air leakage and more.

Bulk water from improper flashing, bad roof design and failed gutters causes most water intrusion problems. But water vapor if still a factor to consider and control, says Bailes. Using a WRB to seal around joints, seams and penetrations provides essential protection against water and water vapor.

The WRB’s main objective is keeping humid air away from cool materials. So, in a hot, humid climate, your placement of the WRB is going to be the on the outside of the wall structure. In cold climates, it would be the opposite, with the WRB placed on the interior side of the wall structure to keep the warmer, humid air in.

Here in the South, your exterior installed WRB can also play an important role during the construction process, shielding the structure from severe weather prior to applying the finish cladding.  A good one should have enough enhanced UV resistance to allow for up to 120 days of exposure.

The current International Building Code also requires a home’s roofing system to include an underlayment.  The underlayment protects the roof deck and frame if water does manage to penetrate the roof cladding. But specifying a roof underlayment is one thing. Proper sealing and installation is needed to actually protect the home. To maintain their roofing products’ warranty, many manufacturers stipulate that underlayment must be installed to the manufacturer’s requirements for a proper horizontal and vertical lap.

Engineered Wood Products are a Proven Choice for Sheathing

Photos, courtesy of APA, The Engineered Wood Association

Wood structural panel sheathing provides a rigid base for use with high-density wall insulation.
making the envelope airtight and watertight
Construct homes that balance structural integrity, energy efficiency and cost by using advanced framing techniques.
 

The valuation of any material must consider everything from the raw materials used to the manufacturing process, transportation, construction, and end of life of the solutions.  Builders must also balance every product decision on availability, labor and cost.

 “In our Southern climate zones, it is more cost effective to frame a typical 2×6 at 24 inches on center with DP cellulose, OSB and outsulation than a double stud wall, a thermally-broken stud wall, a Larssen-truss wall, SIPS or ICFs. And it costs less to install Water Resistant Barrier (WRB) rolls than sprayed-applied WRBs,” says Cobo.

3/8-inch Performance Category or thicker plywood and OSB panels are code-recognized air barrier materials that can contribute to tighter, more energy-efficient homes.  And advanced wood framing methods allow builders to reduce the amount of wood and increase the amount of insulation by covering the outside of the house with continuous insulation. 

WRB Soulutions Designed To Multi-Task

The Zip® System by Huber is a structural sheathing panel with an integrated drainage plane.

Many commonly used drainage plane materials are the type that come in rolls.  Installed correctly, spunbond polyolefin and other synthetic house wraps and roof underlayments can provide a moisture and air barrier while allowing vapor permeability. These synthetic materials generally offer better protection than felt paper and are also more durable, and faster to install. And synthetic roof underlayments have better traction to provide a safer working surfaces.

Protecto Wall VP Water Resistive Barrier is a primer-free self-adhering, self-sealing, drainable, nonwoven, vapor permeable, microporous polyolefin laminate that provides 98.5% drainage efficiency. No mechanical fasteners are required for the installation.
making the envelope airtight and watertight
The complete TYPAR Weather Protection System includes TYPAR wraps, along with TYPAR flashings, construction tape, and sealant.
Tarco Leak Barrier Butyl is a high-performance, self-adhered waterproofing membrane It features a robust butyl adhesive and a specially engineered polyethylene film suitable for a broad temperature range.  The 20-mil thick membrane excels on both vertical and sloped surfaces.

Newer self-adhered membranes that allow some vapor permeability are now being used more in warm climate areas. They require no mechanical fasteners to apply, and their sticky backing helps to create a consistent seal between the sheathing or roof deck and the membrane.

Rigid foam sheathing can be used as both sheathing and a drainage plane, as long as seams are sealed to complete the water control layer.  Liquid-applied membrane is a less commonly used but is another effective air and water control layer.

Rainscreens Help Keep Southern Homes Dry and Cool

away from the building envelope by creating a gap between the cladding and the water-resistive barrier (WRB). When moisture penetrates the cladding, the air gap will prevent it from penetrating the building envelope.  A ventilating channel helps to keep the wall dry.

Rainscreen air gaps can also help keep houses cooler in Southern climates.  And it allows for an area of lower pressure differential compared to the exterior, reducing windblown rain forces.

To break a capillary water movement, the rainscreen gap should be a minimum of a 1/4 inch, but 3/8 inch is safer. The wetter and colder the climate zone, and the more moisture absorbing the cladding is, the wider the gap of the rainscreen should be.

There are two common types of rainscreens, plastic mesh mats and rainscreen strips. Mesh mats like Benjamin Obdyke’s Slicker Classic and Slicker Max are available in different gap thickness. Mortairvent by Advanced Building Products is a mesh mat well suited for wood shingle, stucco and thin veneers claddings. Rainscreen strips are usually made on the jobsite out of CDX plywood or OSB strips.  Or they can be purchased as 1x3s, 1x4s.  Manufactured rainscreen strips like Cor-A-Vent’s Sturdi-Batten or Roll-On Rainscreen by Advanced Rainscreen Furring are better suited for horizontal, vertical and panel siding.

Check with your code officials in fire-prone areas, as some wider rainscreens are restricted or not allowed due to a chimney effect. As a pest deterrent, you may need to install metal screens on top and bottom.  And always check for the best installation instructions with your cladding of choice.

Insulation Can Serve as One, Two or Three Control Layers

Energy codes specify insulation requirements according to climate zone and are starting to require exterior insulation as well. 

Air permeable insulation (fiberglass, cellulose, mineral wool) is a control layer for heat only.  It needs to be protected from air flowing through it by being next to an air barrier.  Spray foam and foam board are air impermeable.  In some cases, these can reduce air flow enough to qualify as an air barrier., effectively controlling both heat and air.

Open-cell spray foam is the only foam that is vapor permeable, so it may serve as the control layer for heat, air and water vapor.  Expanded Polysterene (EPS) can be semi-vapor permeable as long as it’s not too thick, Bailes advises. Closed-cell foam and some types of rigid foam classify as vapor retarders.

“Windows are the Achilles’ Heal of the Thermal Control Layer”

Windows are thermal bridges, but of course they also have many benefits, says Bailes.  The number of panes of glass, frame material, or the presence of low-emissivity coatings will determine the U-factor and Solar Heat Gain Coefficient (SHGC).  These are the ratings you need to evaluate to determine how well the window will control heat flow.

The U-factor is a key indicator of how well a window prevents heat loss.  The lower a window’s U-factor the better. The SHGC is a measurement of how much solar radiation passes through it.  SHGC range from a low of 0 to a high of 1, with the lower numbers indicating less solar heat transmitted. 

making the envelope airtight and watertight

Other Choices Matter

GAF’s new facility in Georgetown, Texas, will significantly expand the availability of Timberline Solar, which directly integrates solar technology into traditional roofing processes and materials.
Metal roofs coated with reflective paint or granules are one example of a cool roof.

Window orientation, roof overhangs, and other external shading devices on windows can help keep the heat out and cooling loads lower.  Overhangs are also an important component of water management.

Cladding and roof shingle choices can either help or hinder energy performance. Color choices matter, and not just for aesthetics. Cool walls and cool roofs reflect sunlight and absorb less heat.  GAF’s Timberline Solar and Tesla’s Solar Roof shingles can power your home with a fully integrated solar and energy storage system.

With over 200 days of sunshine per year in Texas, investing in solar panels promises immediate savings on utility bills. It generally takes six to nine years for Texans to see a return on their investment through monthly energy savings.  And various rebates and incentives can help recoup a better return on the upfront investment

Next in the Series:

Making Houses Less Prone to Damage in a Natural Disaster is Goal #3
Fewer Components, Less Waste is Goal #4
Right-Sizing the HVAC is Goal #5

Life On The Estate

Life On The Site

Life On The Set

Life With A Twist

About The Savvy List