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3 Critical Design Considerations for Multifamily Building Envelopes

As the old saying goes, a chain is only as strong as its weakest link. A more perfect analogy cannot be found when considering the design of a building's envelope. Consider the impact of having top quality building products but a low quality "fit". Products that weren't designed to fit together can create thermal leakage, or even worse, the potential for water to find it's way in. For this reason, it's vitally important that the design of the building envelope is done with an understanding of how each building product performs in conjunction with the adjacent product.

1. Design with Energy in Mind

The envelope or exterior construction of all buildings directly impacts the energy performance and related energy costs. Therefore, conducting an analysis of a new or existing building envelope is one of the first and best steps a design team can take to understand how to achieve an energy-efficient and environmentally friendly building.

This is especially important in the renovation of existing buildings, particularly if the building is being converted from some other use (i.e. warehouse, school, etc.) into multifamily housing as has become very popular in many areas. This analysis can be done in-house by architectural firms using available computer modeling software or by working with manufacturers who have customized software to analyze the options of different systems in a given building envelope system. Either way, a baseline building can be modeled, while the relative changes can be compared using specific envelope designs, types, and options.

One common and critically important component of building envelopes is fenestration. Hence, it is not surprising that some window manufacturers have embraced its importance and seek to partner with design professionals on conducting a building envelope analysis. Doug Phelps, Director of Commercial Business Development for Pella EFCO Commercial Solutions, points out that they work with designers on a "whole building analysis, which is truly unique from the component approach. This way, we're able to analyze and run 'what-if' scenarios to compare potential envelope improvements in terms of energy savings, occupant comfort, and building aesthetics. Every project has specific goals, and we are able to support the project team with flexible, real-time analysis."

He also notes that this interest in full analysis came from occupants commenting that their spaces felt more comfortable after replacing windows. By adding the analysis of occupant comfort as part of the envelope analysis, it is also possible to quantify that improvement while showing projected energy savings for the building.

This process is available to any design team and supported by the national, independent energy engineering firm, The Weidt Group. According to David Eijadi, Principal in Charge of Energy for The Weidt Group, "The envelope analysis identifies potential problem areas and offers solutions to make buildings more energy-efficient and comfortable. In the process it leverages some of the best technology available in software for computing hourly energy simulations."

2. Water Barriers to Entry

One significant part of a building envelope is its ability to be sealed completely. Hence, building codes require barriers be incorporated into envelope assemblies to prevent unwanted air infiltration and water penetration. Water-resistive barriers (WRBs) are commonly used that are synthetic, non-woven building wrap products. The high-performance advantages of this modern solution include a barrier that is more breathable, more durable, and more easily sealed along the seams. Further, this type of synthetic WRB can also double as a continuous exterior air barrier, meeting the code requirements for both in a single layer.

Using a high-performance building wrap in a multifamily housing project takes advantage of several innovative characteristics.

  1. As an engineered product, it creates a weather barrier behind exterior cladding to protect the sheathing and reduce water intrusion into wall cavities. This is important since all exterior cladding will likely allow some degree of water intrusion at some point. It is also particularly important in rainscreen assemblies where water is expected to enter behind a cladding material and is allowed to drain away.
  2. As a vapor permeable or breathable product it will allow water or moisture to escape, thus allowing any damp or wet materials to dry in a relatively short amount of time. During this drying, WRBs can maintain their water resistance because they are constructed with pores that are large enough to allow moisture as a vapor to pass through but too small for water as a liquid to pass.
  3. As an air barrier, a WRB will be an energy-efficient means to stop air infiltration and exfiltration through walls.

When specifying WRBs or building wraps for multifamily housing projects, it is important to recognize that there are dozens of building wrap products available with wide variations in performance and cost. They can also vary in water resistance, drainage efficiency, water vapor transmission or breathability, ability to impede air flow, durability, tear resistance, cold weather flexibility, flammability, and smoke developed ratings.

Beyond that, however, it is important that the WRB can be sealed properly and fully along the seams without undue penetrations from staples or nails. Some manufacturers offer full sealing systems that avoid the use of nails and staples and instead use a compatible adhesive, sealant, or tape. Equally important are the details of how the WRB deals with openings in the wall such as doors and windows. Being able to flash and seal the WRB properly with window and door flashing materials will assure that water draining down the face of a drainable WRB will flow away properly and not behind other building elements into the wall.

3. Leverage the Manufacturer

It's nearly impossible to predict how the infinite number of product combinations will perform when the envelope comes together. Many manufacturers have in house engineers that will happily look through the plans and desired performance to determine the likelihood of meeting a certain standard. An even greater benefit comes from using multiple products from the same manufacturer. The primary benefit here is the known performance. Any manufacturer worth their salt will be able to give a definitive evaluation on how two of their products perform in conjunction. Along that same line, if you later find a defect, there is no blame game to play out.

The last advantage is familiarity with application. Most manufacturers have a list of sub-contractors who are familiar with the installation of their systems. It's in the manufacturers best interest to supply a sub that will perform the install efficiently and properly. After all, the manufacturer wants to see their product perform with a high level of success and installation is a key component of that success.

Be sure there’s no weakest link in your next multifamily construction project with our free guide to advances in multifamily construction.

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