revised January 18, 2023
Note: This page is an ever-evolving work in progress. It’s meant to be a helpful resource so we welcome feedback on if it’s accomplishing that goal, or on ways we can make it better.
There are a few key principles that can guide you to the answer to almost any building envelope question. Think of these principles as a roadmap—an algorithm—that will always guide you to the right destination.
- It’s about water.
- Gravity works—water flows downhill.
- Water can also be carried by the air.
- Keep the condensing surface warm.
- Maintain continuity.
- Match the expected service life of adjacent materials.
- Start with the code and manufacturer’s instructions.
- Beyond code, think about risk.
- Don’t make stuff up.
1. It’s about water.
The building envelope has a lot on its plate, from reducing energy use to aesthetics. The most important job of the building envelope, however, is to prevent water from getting to where it is not supposed to be.
It’s pretty rare that someone gets sued because a building is drafty or ugly. But people are getting sued all day long over water leakage that causes mold, rot, and corrosion. Mold, rot, and corrosion are big problems that cost big money to fix.
Keep water from getting where it is not supposed to be and your buildings will last longer and cost less to maintain. And you’ll be less likely to end up on the wrong end of lawsuit.
2. Gravity works—water flows downhill.
It’s not hard to figure out how water will move. It goes downwards. Use this knowledge when designing and building.
Use water-shedding laps.
When one material laps onto another, the higher one should lap over the lower one—like the shingles on a roof.
This seems obvious when talking about shingles, yet often materials like membrane underlayments are installed in such a way as to “buck” water. Even when materials are well-adhered or sealed, a lap that bucks water is more likely to deteriorate over time. When the lap finally fails water will have an entry point, as opposed to a properly shingled lap which does not require any type of adhesive or sealant to shed water.
Provide slope to drain.
Slope to drain means that the surface that water is flowing on is pitched, or sloped, towards a drain, roof edge, flashing edge, or other location where you want the water to go. Makes sense, right? Unless you’re building a swimming pool, make everything slope to drain.
How much slope? The more the merrier. Generally at least 1/4 inch per foot is a good rule of thumb, but more is better. The reason is that slope to drain makes deficiencies in the water-control materials less likely to cause an actual problem. A pinhole in the roof membrane probably won’t leak much if the water flows by quickly. That same pinhole under a few inches of standing water will leak profusely.
3. Water can also be carried by the air.
You’ve probably heard about air barriers. They’re used to—you guess it—control airflow. But, I bet you didn’t know that the main reason we care about controlling airflow is not about drafts, energy efficiency, or air quality (though those are all important). No, the main reason to control airflow is to control the movement of water!
The air can carry a lot of water. Mixed up with the oxygen, nitrogen, and other gasses in the air can be a significant quantity of H2O, i.e. water in the gas (aka vapor) form. Water vapor is generally not a problem until it condenses somewhere and turns into liquid water.
So the issue with airflow is that it can carry moisture-laden air to a location where the water vapor condenses into liquid water on a moisture-sensitive surface and then we’ve got problems.
To keep it simple, remember two things:
- build airtight
- keep moisture-laden air away from cold surfaces
4. Keep the condensing surface warm.
The condensing surface is where condensation forms. What’s condensation? Condensation is the process of converting water vapor (water in the gas form) into liquid water.
Condensation happens when water vapor encounters a surface that is below the dew point. Don’t worry about what the dew point is. Do remember that for any set of environmental conditions there is a magic number and when moist air touches a surface that is colder than that magic number, boom—condensation.
So we just need to keep any potential condensing surfaces warmer than the magic number and we’re good! We can do this in a couple of ways, including:
- Warm up the potential condensing surface.
- Prevent the moist air from reaching the potential condensing surface.
You might notice that option 2 is really just a different version of option 1. By keeping the moist air from reaching the potential condensing surface, you’re really just putting another potential condensing surface in the way (and keeping that one warmer or you’re in the same sinking boat).
5. Maintain continuity.
In the building envelope world it’s all about systems and assemblies rather than individual components. The best materials in the world are useless if they are not properly connected to one another to form a continuous assembly.
The vast majority of building envelope problems occur due to discontinuities in otherwise well-performing systems. It’s these connections between individual materials that make or break a system.
Think in terms of function not material.
Tyvek is a material that can serve multiple functions. Tyvek (the material) can be an air barrier (function) or a water-resistive barrier (function).
The distinction is much more than just semantics. By referring to different components within our building envelope assemblies by their function rather than simply the material they happen to be, it helps both the designer and the builder. Referring to components by their function is a tool for thinking. It will point you in the right direction even if you don’t know the answer at the start.
We typically talk about four main functions related to the building envelope. These four functions are to control:
- liquid water intrusion
- air flow
- heat flow
- water vapor diffusion
The four functions are listed in order of importance, with controlling liquid water intrusion being the most important and controlling water vapor diffusion being the least. Don’t worry about items lower on the list until you’re got the ones above nailed down.
Don’t lift the red pen.
Once you’ve got the concept of function rather than materials in mind, the next step is to maintain continuity of those functions throughout the entire building envelope. The entire building envelope means “all six sides of the building” (the walls, roofs, and bottom level floors). Maintaining continuity means there are no breaks in the systems meant to perform each function (e.g. the system of components meant to control liquid water infiltration).
A good way to confirm that the systems are continuous, or to identify any discontinuities, during the design process is to trace each system from one side of a drawn detail to the other. Put your pen (analog or digital) on top of the line representing the component with the function to control liquid water infiltration (e.g. Tyvek). Follow that line until it stops. When it stops, see if the drawing shows how to it is joined to the next material (e.g. a window). If you need to pick up your pen in order to get from the Tyvek to the window, then there is a discontinuity that needs to be addressed.
Repeat this process for every detail and every function (liquid water, air flow, heat flow, diffusion). You can also use the same process at a macro level to identify if you have enough details. Trace from one end of a wall section to another. If there is not a detail representing every condition that the pen traces over, it means there are missing details.
6. Match the expected service life of adjacent materials.
Soldered copper pipes and garden hoses both transport water from one place to another. Both can be snaked through tight spaces, split into multiple branches, and terminated with various fittings.
But you wouldn’t want to plumb your house with garden hoses because the hoses wouldn’t last as long as the other parts of your walls. You’d have to tear up perfectly good drywall to get access to repair or replaced failed hoses. We use soldered copper pipes for in-wall plumbing because the piping is intended to last the life of the structure.
The same concept applies to building envelope components. Wherever possible, match the service life expectations of adjacent materials. This is especially important when materials are covered and will be inaccessible.
A good example of this is in-wall flashing materials. Think about how long a brick veneer cladding is supposed to last—decades right? Any flashing that you plan to install in that wall should be just as durable. This is why stainless steel and copper through-wall flashings are typically better than PVC and other membranes.
7. Start with the code and manufacturer’s instructions.
When in doubt, check the building code. Actually, always check the building code. For most situations the code will tell you how much insulation to use, where to put it, how to determine the roof wind uplift requirements, what performance criteria you need from your windows, and all kinds of other important stuff.
Manufacturer’s installation instructions provide clear guidance on exactly how to use their products. The installation instructions will also tell you which accessory products you’ll need, and how to install those materials as well.
8. Beyond the code, think about risk.
The building code is more or less black and white. It says what it says and it’s the law to comply. Yes, there may be some room for interpretation but the idea is it represents hard and fast requirements (for the cheapest/worst building you can legally build).
But when you get beyond the code the idea of the “right” or “wrong” way to build something gets blurred. So it’s important to start thinking in terms of risk rather than right and wrong. Every choice should be a balance of risk against other factors like cost, schedule, and aesthetic preferences. The key is to properly characterize the factors and identify the decision maker so that person or group can make an informed choice.
A problem is that we often don’t have a good way to evaluate future risk. In my experience, when faced with a choice between high likelihood of future success paired with certain pain now (higher cost, schedule impact) vs. lower (but some) chance of future success without the present day pain—people most often choose to roll the dice. This is especially true when there is no hard data to compare (e.g. $100 now saves $1,000 down the road) and it’s just a matter of qualitative statements like “Option A is lower risk than Option B.”
So we need to make do with imperfect information. The important part to keep in mind is that decisions should be made based on as clear an understanding of risk as can be obtained with the information available, and those decisions (along with any assumptions) should be well documented.
9. Don’t make stuff up.
A lot of building envelope problems arise because good people make decisions with good intentions but without referencing appropriate background information. In other words, they make stuff up.
Don’t make stuff up.
There is a wealth of great information available to help inform any building envelope design decision. The second part of this page is devoted to highlighting some of these resources.
In 2023 technical information about how to build good buildings is a commodity. Much of it is freely available on the internet and anyone can find it.
Most firms like mine don’t have a secret vault of information. We use the same freely-available information to solve problems that we’re sharing with you below.
Pro Tip: Incorporate relevant resources described below by reference into your project specifications.
Material manufacturers all publish detailed drawings, specifications, and installation instructions for their products. Many also provide other design resources such as calculators.
After the building code, material manufacturers’ websites should be your first stop when researching a building envelope question. Always consult relevant material manufacturers’ websites for information during investigation and design.
Various U. S. government departments and organizations publish building science and building envelope-related information online that is free to access and is a reliable source of good information.
Whole Building Design Guide
The National Institute of Building Sciences (NIBS) under guidance from the past Federal Envelope Advisory Committee developed this comprehensive guide for exterior envelope design and construction for institutional / office buildings.
Building America Solutions Center
The Building America Solution Center provides access to expert information on hundreds of high-performance construction topics, including air sealing and insulation, HVAC components, windows, indoor air quality, and much more.
Cool Roof Calculator
Estimates Cooling and Heating Savings for Flat Roofs with Non-Black Surfaces
Secretary of the Interior Standards
The purpose of the Secretary of the Interior’s Standards for the Treatment of Historic Properties…is to provide guidance to historic building owners and building managers, preservation consultants, architects, contractors, and project reviewers prior to beginning work.
Building Science Corporation
Building Science Corporation has published an extensive collection of white papers on their website covering most building science first principles topics in great detail. As a starting point, read these 5 articles:
- BSD-018: The Building Enclosure
- BSI-001: The Perfect Wall
- BSI-039: Five Things
- BSD-013: Rain Control in Buildings
- BSD-014: Air Flow Control in Buildings
Our blog is meant to be a friendly, informal place to learn about building better.
For example, learn more about how water moves through buildings via airflow and diffusion, and the difference between air barriers and vapor barriers here: How Water Moves: Airflow and Diffusion
Industry Advocacy Organizations
Nearly every material, or class of materials, used for construction seems to have its own advocacy organization to promote its use. As part of these advocacy efforts these organizations publish a range of helpful technical content that is generally reliable and recognized within the industry as accepted standards.
We’ve included several organizations below, but this list is certainly not exhaustive. If you’re working with a material, chances are it has an advocacy organization—so do a Google search and then check their website for helpful info.
The Brick Industry Association (BIA)
Technical Notes on Brick Construction are FREE bulletins that contain design, detailing, and construction information based on the latest technical developments in brick masonry. Drawings, photographs, tables, and charts illustrate appropriate topics.
Cast Stone Institute (CSI)
The Cast Stone Institute (CSI) was formed in 1927 by a group of visionary Cast Stone producers who identified the need to have a common voice for the Cast Stone industry. Today our mission remains, not only to be the authoritative spokesperson for Cast Stone, but also to provide expert counsel to the architectural and engineering communities.
Indiana Limestone Institute (ILI)
The Indiana Limestone Institute is a resource for architects, contractors, building owners and others seeking accurate, unbiased information about the use of Indiana Limestone in construction.
International Masonry Institute (IMI)
The Masonry Detailing Series (MDS) is an exhaustive collection of illustrative construction details and diagrams produced by International Masonry Institute (IMI) for architects and engineers to use as a design resource. This compilation includes hundreds of details for brick, block, and stone masonry systems, as well as details for ceramic tile, marble, terrazzo, plaster, rainscreen systems, terra cotta, AAC, and masonry restoration.
National Concrete Masonry Association (NCMA)
ICPI-NCMA unites, supports, and represents the producers and suppliers of concrete masonry systems – including concrete masonry, segmental concrete pavements, manufactured stone veneer, segmental retaining walls, and other hardscape systems.
Copper Development Association (CDA)
Copper in Architecture – Design Handbook is a comprehensive resource presenting as much information about copper’s properties, existing technology and application to the educational design and construction field as presently exists.
National Roofing Contractors Association (NRCA)
NRCA is one of the construction industry’s most respected trade associations and the voice of roofing professionals and leading authority in the roofing industry for information, education, technology and advocacy.
Roof Wind Designer is intended to provide users with an easy-to-use means for determining roof systems’ design wind loads for many commonly encountered building types that are subject to building code compliance.
Single Ply Roofing Industry (SPRI)
…the leading authority in single-ply roofing. Our network deals exclusively with thermosets, thermoplastics, and modified bitumens. Here, you’ll find easy access to online publications and documents that range from industry standards, generic technical guidelines for design and application to general information about roof maintenance and emergency repairs.
This Wind Design Calculator is used to calculate the roof edge design pressure. All versions of International Building Code since 2003 have required per Section 1504.5 that metal edge systems, except gutters, be tested per ANSI/SPRI ES-1 or ANSI/SPRI/FM 4435/ES-1 to resist wind loads in accordance with Chapter 16.
Asphalt Roofing Manufacturers Association (ARMA)
The Asphalt Roofing Manufacturers Association (ARMA) is a trade association representing North America’s asphalt roofing manufacturing companies and their raw material suppliers.
Slate Roofing Contractors Association (SRCA)
Dedicated to the Preservation of Traditional Roofing Trades and Practices
- Roofing Slate Installation Guidelines
- Roofing Slate Specifications
- Roofing Slate Repair, Restoration and Maintenance Guidelines
Air Barrier Association of America (ABAA)
We are the National Voice of the Air Barrier Industry in America. We do Air Barriers right for energy efficiency, better buildings and healthier communities.
Western Red Cedar Lumber Association (WRCLA)
Founded in 1954 and known as “the voice of the cedar industry”-the WRCLA operates customer service programs throughout Canada and the United States to support its members’ cedar products with information, education and quality standards.