Building Enclosure Basics

Basic building science principles we use when consulting for clients on Building Enclosures:

Following are some building science truisms we often find ourselves repeating. While they are basic concepts, and relatively easy to comprehend, their application can involve nuance and complexity, and we often need to remind building design professionals (and even ourselves) to take a step back from the details involved and pay attention to the building science basics that apply to the situation in question.

Building science touches on a lot of issues building designers care about. For example, building science topics include controlling drainage to keep the structure from getting wet, and designing good enclosures to control the flows of heat, air, and moisture. Other issues include controlling indoor air quality, noise, and both the quantity and quality of lighting. There’s a lot to this building science thing and it takes time to gain the experience and knowledge base to apply it. Attention to the following basic building science principles greatly reduces the risks inherent in building and assures a durable structure.

Balance wetting and drying

Designing for long-term durability involves balancing the structure’s capacity for carrying water (without sustaining damage) with the forces that are wetting and drying it. What matters is both the quantities of water involved and the rates of wetting and drying. We want to slow the rate of wetting by providing control layers, and provide as much drying capacity as is reasonable to reduce the structure’s time of exposure to wet conditions and reduce the chance that structural or environmental damage will occur.

One wetting source that often gets overlooked is the wetting that can occur during the construction process. The contractor must understand the importance of keeping materials dry or properly drying them out should they get wet during construction. Testing helps to assure the materials have reached an ideal moisture content before they are incorporated into the structure. The long-term wetting (and drying) sources are influenced by the control layers.

Proper placement of the 4 Control Layers between the inside and the outside

Water Control Layer
Air Control Layer
Vapor Control Layer
Thermal Control Layer


  1. Water, because liquid (also known as bulk) water does the most damage to the structure and the interior environment.
  2. Air, because it can bring water with it. Uncontrolled air movement causes the next largest level of damage. Also, it’s really bad for your thermal (energy) efficiency to allow air to move through the building enclosure without controlling or tempering it.
  3. Vapor, water in its gaseous state can cause damage by raising the moisture content of sensitive materials or condensing on cold surfaces and becoming liquid water (see 1 above).
  4. Thermal, energy consultants, energy codes, and the Department of Energy all confirm the importance of thermal insulation in reducing energy usage. Keep in mind when working with the thermal control layer that the building has 6 sides and orientation and layer continuity is important. Thermal energy can aid in drying, but using thermal energy for drying is usually the least efficient way to solve the problem of uncontrolled water.

To compare typical quantities from different modes of water intrusion; moisture intrusion quantities from vapor diffusion would be measured in teaspoons per year, intrusion from air leakage in cups per week, and intrusion from liquid water entry in gallons per hour.

Once water intrusion starts causing damage, it’s critical to get the source or mode diagnosed and stopped before undertaking repairs to the damage it has caused. (Or you’ll end up repairing the same kind of damage over and over.)

To confirm the effectiveness of the water control layer it’s important to test for water leaks. Water testing is important at vulnerable transition points in the building enclosure like parapets and windows.

To confirm the effectiveness of the air control layer we recommend testing for air leaks. Air testing is done on both a whole-building level and on individual assemblies and components.

To verify the level of vapor control needed, consult a hygrothermal model.

To check the continuity of a control layer do a “pencil test”. To do this test, trace the control layer on each detail from end to end without lifting your pencil from the paper. We also recommend running this test on the water and air control layers.

The 4 Ds of Building Enclosure Design





Some would add a fifth D, distance, as in, provide a gap between your vulnerable building materials and the ground.

The focus here is on the first priority, water control. If you can solve water problems everyone involved with the building will be happier.

Deflection involves directing water away from the building. It’s the “first line of defense” against water intrusion. Examples of deflection include: eaves, gutters, rainscreen cladding, overburden or ballast, drain rock next to foundation walls, recessed windows, and shutters that cover vulnerable openings. We also want to be able to deflect ultraviolet radiation, wind, and fire.

Drainage involves dealing with the water that makes it past the deflection layer and wets the outer surface of the building enclosure. Again, we’re directing the water away from the building. Good drainage is (relatively) easy – all you need to do is provide a clear path for water to travel on with a good slope allowing gravity to pull it to where it’s intended to go. Drainage uses gravity to move liquid water and direct it to a location where it won’t harm the building. Roofs with slopes less than 1/8 inch per foot will pond water in the valleys. Ponding water leads to premature deterioration of the roofing. Unfortunately, in standard construction, ponding occurs so regularly that it is often widely visible in satellite photographs. Ponding can also occur on copings and flashings. To avoid ponding on flashings provide a minimum slope of 1 inch per foot, and as a safety factor always include a backup water control layer behind or under the flashing. For more on the slope to drain at low-slope roofs – see John Guill’s article elsewhere on the DTR website.

Drying is the other side of the water equation. Drying occurs naturally when water migrates from a material with a high concentration of water (or humidity) to one with a lower concentration. If you can move some dry air past your wet material it will pick up that moisture and dry out the material. Weep holes and vents installed at the top and bottom of a section of a brick veneer wall to create a stack effect similar to the one in a traditional ventilated attic. As new air floods the ventilated cavity from the bottom, older, wetter air is exhausted through the top. This helps the brick masonry and the face of the building enclosure dry out more quickly lessening the chance of damage to the mortar during any freeze-thaw cycle. Single-wythe Concrete Masonry Unit (CMU) walls have real trouble recovering from wetness because the CMU is often coated on both sides with a paint that resists moisture vapor diffusion. Once water gets in, it can’t get out quickly and causes the coatings on both sides to blister and fail. This means that small leaks over time become BIG problems. The other input that helps with drying is thermal energy. South-facing walls and walls with little to no insulation dry out more quickly than north-facing or highly insulated walls.

Durability is the final D that can help your structure last a long time. You can add durability by choosing appropriate materials (for example pressure-treated wood, or brick) where the exposure might be greater. In addition to durability when exposed to water, consideration should also be given to physical (impact) durability, and potential chemical interactions (e.g. cooking grease or adjacent control layers). Chemical interactions are often hard to predict. Here’s a good “rule of thumb”: If two flexible or liquid-applied materials touch, confirm that they won’t interact.

The Perfect Assembly

Coined by Building Scientist Joe Lstiburek in his Building Science Insight 001, the “Perfect Wall” refers to the collection of materials used that are assembled to create the “perfect” exterior to interior environmental separator. To create a perfect assembly, locate the control layers on the outside of the structural elements of the building. Locating them on the outside of the structural components allows the control layers to protect the structure from the negative effects of weather. Expansion, contraction, corrosion, and decay, are negative exterior climatic forces that are influenced by moisture and temperature. The structure needs protection from water in its various forms, from going through temperature extremes, and from ultraviolet radiation. Arranged from outside to inside, the assembly includes Cladding (deflection layer), over the 4 Control Layers (Thermal, Water, Air, Vapor), over the structure (wood, steel, or concrete live and dead load resisting elements). Sometimes multiple control functions (e.g. water, and air control) can be achieved using a single material (e.g. liquid-applied air/water barrier membrane). The drainage layer is located directly over the water control layer and (if ventilated) can aid in drying. The physics of the forces acting on walls, roofs, and slabs are pretty much the same, so the assemblies for walls, roofs, and slabs are installed in the same sequence from inside to out and are just oriented differently.

Putting it all together

Take care of the water and you’ve solved the highest priority building enclosure design question. Pay attention to protection or deflection strategies so you’re not stress testing the control layers. Continuity is critical. Check your design for continuity. A small leak is still a leak. If you’re not testing you’re guessing about building performance. So test for conformance with the expected performance. It’s better to correct any non-conforming work during construction than after an un-planned weather event that occurs after the building is occupied. There are basic building science concepts and strategies that can be applied to assure a positive outcome for any building project.


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