Building envelope science: heat, air and moisture

The building envelope, also called the building enclosure, is everything that separates the inside of a building from the outside: foundations, walls, windows, doors and roofs. It has to keep out rain and groundwater, control the flow of heat, stop uncontrolled air leakage, manage water vapor, resist wind, fire and sound, and still look good for decades. Most envelope failures that construction managers deal with, such as leaks, mold, rot, condensation, ice damming, cold rooms, high energy bills and cladding damage, come from a misunderstanding of how heat, air and moisture move. This lesson builds the physics behind Lessons 7 to 9. It explains the four control layers, thermal resistance and U-factors with worked calculations in RSI and R-value, air leakage and airtightness testing, water vapor and dew point, condensation risk in walls, rain control strategies and the energy codes that set envelope requirements.

The four control layers

Building scientists describe a good envelope as having four continuous control layers, in rough order of importance:

• Water control layer: keeps rain and groundwater out. It includes cladding, the water-resistive barrier behind it, flashings, roofing membranes and below-grade waterproofing.
• Air control layer: stops air flowing through the envelope. It is made of materials that are airtight and continuous across joints, penetrations and transitions, such as sheathing membranes, taped sheathing, sealed drywall, spray foam or roofing membranes.
• Vapor control layer: slows the diffusion of water vapor through materials. It is placed where it will prevent vapor reaching cold surfaces where it could condense, usually on the warm side of the insulation in cold climates.
• Thermal control layer: insulation, which slows heat flow. It should be continuous, with minimal thermal bridges.

A single material can serve more than one role; for example, a self-adhered membrane on sheathing can be the water, air and vapor control layer at once. The critical idea is continuity: each layer must be traceable around the entire building, through every transition such as wall to window, wall to roof and wall to foundation, without gaps. A useful test is the pen test: on a building section, you should be able to trace each control layer around the building without lifting the pen.

Heat flow

Heat flows from warm to cold by three mechanisms: conduction through solid materials, convection by moving air or fluids, and radiation between surfaces. Insulation works mainly by trapping still air or gas in small cells, slowing conduction and convection. The resistance of a material to heat flow is its thermal resistance, which depends on its conductivity and thickness.

In the United States, thermal resistance is expressed as R-value in ft²·°F·h/Btu. Energy codes (the IECC and ASHRAE 90.1) express requirements as minimum R-values for insulation or maximum U-factors in Btu/(h·ft²·°F). Metric RSI values are used internationally; one RSI equals about 5.678 R.

For layers in series, where heat flows through one layer after another, the total resistance is the sum of the individual resistances. The U-factor, or thermal transmittance, is the reciprocal of total resistance, and heat flow through an assembly equals U times area times temperature difference:

Thermal bridges and effective values

The clear-wall calculation above overstates performance because studs, plates, slab edges, balconies, cladding supports and window frames conduct heat around the insulation. Wood studs reduce the effective value of a cavity-insulated wall by roughly 10 to 20 %, and steel studs by around 50 %. Calculating the effective value uses parallel paths for wood framing, weighting each path by its area fraction, and specialized methods or published tables for steel framing. Slab edges, balconies and shelf angles are linear thermal bridges that can dominate heat loss in high-rise buildings; newer codes and guides account for them with linear transmittance values (psi values). Continuous exterior insulation and thermally broken attachments are the main tools for reducing bridging.

Air leakage

Air leakage moves heat and moisture far more than conduction or diffusion through the same area. Air moves through the envelope whenever there is a hole and a pressure difference. Pressure differences come from wind, from the stack effect (warm indoor air rises and pushes out at the top of a building in winter, drawing cold air in at the bottom), and from mechanical systems such as exhaust fans and unbalanced ventilation. In cold climates, warm moist indoor air leaking outward through the envelope can deposit large amounts of water as condensation or frost on cold sheathing, a major cause of mold and rot in walls and attics.

The air barrier system must be continuous, structurally supported to resist wind and stack pressures, durable and sealed at all joints and penetrations. Typical weak points include rim joists, top plates, window and door perimeters, electrical boxes on exterior walls, plumbing and duct penetrations, attic hatches and transitions between walls and roofs. Airtightness is measured with a blower door test, which depressurizes or pressurizes the building with a calibrated fan and measures the airflow needed to hold a test pressure, usually 50 Pa. Results are expressed as air changes per hour at 50 Pa (ACH50) or as airflow per unit of envelope area.

Water vapor, relative humidity and dew point

Air always contains some water vapor. Relative humidity (RH) is the amount of vapor in the air compared with the maximum it could hold at that temperature. Warm air can hold much more vapor than cold air, so when moist air cools, its RH rises; at the dew point temperature, RH reaches 100 % and condensation begins. Any surface colder than the dew point of the air touching it will collect condensation, such as a cold window pane, a cold water pipe or cold sheathing inside a wall.

Vapor moves through materials by diffusion, from areas of high vapor pressure to low, usually from warm inside to cold outside in winter, and can reverse in summer in humid climates. Materials are rated by their vapor permeance: low-permeance materials such as polyethylene and foil are vapor barriers, while gypsum board, plywood and many membranes are more permeable. Diffusion moves far less moisture than air leakage, which is why airtightness matters more than a perfect vapor barrier, but both must be designed for the climate.

The IRC and IBC classify vapor retarders as Class I (impermeable, such as polyethylene), Class II (semi-impermeable, such as kraft facing) and Class III (semi-permeable, such as latex paint), with requirements by climate zone. Class I or II retarders are required on the interior side in colder zones (4C and 5 to 8), and permitted alternatives depend on the amount of continuous exterior insulation. Hot-humid zones avoid interior polyethylene because vapor drives inward in summer.

This temperature-gradient method, sometimes called a dew point analysis, is a simple screening tool. For complex assemblies, designers use hygrothermal simulation software such as WUFI, which models heat, moisture storage, rain wetting and drying over time.

Rain control

Rain is the largest source of moisture problems in walls. Water needs three things to get through an envelope: water on the surface, an opening, and a force to drive it in, such as gravity, wind pressure, capillarity, surface tension or momentum. Strategies, often summarized as the four Ds, are:

• Deflection: roof overhangs, drip edges, sloped sills and flashings shed water away.
• Drainage: a drainage gap behind cladding and a water-resistive barrier carry water that gets past the cladding back out through weep holes and flashings.
• Drying: ventilation behind cladding and vapor-open materials let wet materials dry.
• Durability: materials that tolerate wetting, such as treated wood or masonry, are used where some wetting is unavoidable.

Wall systems are classified as face-sealed (relying on a perfect outer seal, which rarely lasts), concealed barrier, drained or rainscreen. A pressure-equalized rainscreen uses vented, compartmentalized air spaces so that wind pressure on the cladding is matched in the cavity, removing the force that drives water inward. Lesson 7 applies these ideas to cladding systems.

Energy codes

In the United States, envelope requirements come from the International Energy Conservation Code (IECC; the 2024 edition is the latest) and, for commercial buildings, the IECC or ASHRAE Standard 90.1 (2022 edition). States and cities adopt energy codes on their own schedules, and some, such as California with Title 24, write their own. Requirements vary by climate zone from 0 to 8, and include insulation R-values or U-factors, air leakage limits and blower door testing for houses.