The skin of the house
The building envelope is everything that separates inside from outside: the roof, walls, floor, windows and doors.
A good envelope does three jobs:
- Slows heat moving through it. That is insulation.
- Stops air leaking through it. That is air sealing.
- Manages water, from rain outside and from vapor inside.
Each job needs its own layer or material. One product rarely does all three well.
Text description
A simplified cross-section of a small house showing the control layers as continuous loops, listed in order of importance. 1, water: the outer skin of roofing, siding and the weather barrier behind it sheds rain. 2, air: an air barrier inside, sealed at every seam and gap. 3, vapor: whether you need a vapor retarder, and where it goes, depends on climate zone, so it is not drawn. 4, heat: a band of insulation unbroken around the whole house, roof, walls and floor. Near the left eave, the air barrier has a gap, labeled: a break in the air layer, ask how it is handled.
Insulation and R-value
Insulation slows heat as it moves through a material. Heat always flows from warm to cold: out in winter, in during summer.
How well a material resists heat flow is its R-value. A higher R-value slows heat more. Doubling the R-value roughly halves the heat that flows through.
Text description
Three blocks of insulation side by side, with the warm inside on the left of each and the cold outside on the right. The first is R-15 and lets a set amount of heat through, shown as four arrows. The second is R-30, twice as much, and lets about half as much heat through, shown as two arrows. The third is R-60, four times as much, and lets about a quarter through, shown as one arrow. A note says the wall area and temperature difference are the same in all three.
- Colder climate zones call for higher R-values. The energy code and DOE list recommended values by zone and by part of the house.
- Attics are the easiest place to add a lot of insulation. For an uninsulated attic, DOE suggests about R-30 to R-49 in the warmest zone, R-49 to R-60 in zones 2 and 3, and R-60 in zones 4 to 8.
- Wood studs and steel conduct heat faster than insulation. Heat leaking through framing is called thermal bridging. Steel framing and containers suffer from it most.
Air sealing
Air leakage is outside air getting in, and inside air getting out, through gaps and cracks. Leaks hide around windows, doors, pipes, wires, attic hatches and where walls meet floors and roofs.
Air leaks matter for two reasons:
- Heat: leaking air carries heat straight out. In a drafty house, it can be a large share of the heat loss.
- Moisture: air leaking into walls and attics carries water vapor with it. It can carry far more water than vapor seeping through solid materials.
An air barrier is a continuous layer that stops air. It can be drywall, sheathing or a membrane, as long as every seam and gap is sealed.
A blower door test measures how leaky a house is. A fan pulls air out and measures how fast it leaks back in. Since the 2012 edition, the IECC has required this test for new homes. Many places have adopted it.
Moisture
Water reaches walls in three main ways:
- Rain and snow from outside. Siding sheds most of it. A weather-resistive barrier behind the siding catches what gets past, and a gap lets it drain.
- Water vapor from inside: cooking, showers, breathing. It moves with leaking air, and slowly through materials.
- Ground moisture from below, through foundations and slabs.
When warm, moist air reaches a cold surface inside a wall, the vapor can turn to liquid. Wet walls lose insulating value, grow mold and rot.
Text description
A cutaway of an insulated wall in winter, outside on the left and inside on the right. From outside in, the layers are siding, a drainage gap and weather barrier, sheathing, insulation and drywall. It is cold outside and warm and moist inside. Warm, moist indoor air leaks through a gap in the drywall, such as around an outlet box, and moves up through the insulation to the cold back of the sheathing. There the vapor turns to liquid water. Wet insulation loses insulating value, and mold and rot can follow. Short arrows show that a little vapor also seeps slowly through the solid drywall, carrying much less water than the leak.
A vapor retarder slows vapor moving through materials. Where it goes, and whether you need one, depends on climate zone. The wrong choice can trap moisture. This is a question for your builder and the local code.
Label the wall
Match each layer of this wall section to its name, from outside to inside.
Text description (gives the answers)
A cutaway of an insulated wood-framed wall, from outside on the left to inside on the right. Siding or cladding (1) sheds rain. Behind it, a drainage gap and weather-resistive barrier (2) let water that gets past the siding drain away. Sheathing (3) stiffens the wall. Insulation (4) fills the cavity between the studs, which are the framing (5). On the inside, an air barrier and vapor control layer (6) stops warm, moist indoor air from leaking into the wall, and drywall (7) is the interior finish. Rain hits the outside face; warm moist air pushes from the inside.
Envelope before heater
A heater replaces the heat the envelope loses. A better envelope means a smaller heater, less fuel and lower bills, for the life of the house.
What you can do yourself
Some envelope work is low-risk and well suited to a careful beginner:
- Caulking gaps around window and door trim.
- Weatherstripping doors and windows.
- Sealing gaps around pipes and wires where they pass through walls, floors and the attic.
- Adding insulation to an open, easy-to-reach attic, with care around wiring and recessed lights.
Wear eye protection, gloves and a dust mask when handling insulation. Leave anything involving wiring, chimneys or flues to a professional, and keep insulation clear of flues and chimneys.
Check
A cabin has thick insulation in the walls but is still cold and drafty. What is the most likely missing piece?
Summary
- The envelope is the roof, walls, floor, windows and doors.
- It has three jobs: slow heat, stop air, manage water.
- R-value measures resistance to heat flow. Colder zones need more.
- Air leaks carry both heat and moisture. An air barrier must be continuous.
- Moisture control depends on climate zone. Get local advice on vapor retarders.
- Fix the envelope before you size the heater.
Next, you will use windows and the sun to heat the house for free.
Go deeper: the four control layers
Building scientists often describe a wall as four control layers, in order of importance: water (rain), air, vapor and heat. Each layer should be continuous around the whole house, without gaps at corners, floors or the roof. A useful check on any wall or roof drawing is to trace each layer with a different color pen. Wherever a line breaks, ask your designer how that gap is handled. The Building America Solution Center has many worked details by climate zone.