The Building Envelope
The Envelope and Structure Provide Shelter from Wind, Water, Heat, and Cold
Three parts united in keeping the outdoors out and indoor comfort in
The roof, walls, and foundation enclose the house, and the choices we make on these three parts of the structure might be the most important decisions we make. All have to accomplish the same things: stop air, slow the movement of heat, shed water, and dry out so that the building won't support mold growth and rot. The priorities of roof, walls, and foundations, however, are very different.
They all see the same elements — wind, water, sun, hail — but in different proportions.
Rain can run up roofs too.
Water is every house's worst enemy
Roofs and walls usually have redundant water barriers. For example, a concrete tile roof has a layer of #30 asphalt felt to stop any water that gets through the tiles, just as a brick-veneered wall has a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. to stop any water that gets past the bricks. A basement wall is coated with asphalt on the exterior to repel water, but the system depends on a good footing drain to keep the basement dry if the water table rises.
In winter, heat is good; in summer, it's bad
When you're paying money to heat a home, it's important to keep that heat in; and when you pay to cool a house, it's important to keep heat out.
Insulation in roofs and walls can stop radiant heat and reduce the transfer of conductive heat. Windows can also play a huge role in heat management, and they use a variety of technologies to keep heat in during winter and out during summer.
Air barriers are crucial: Leaks happen at corners, junctions, and holes
Where the foundation meets the floor framing, the floor meets walls, and where the walls meet the roof are trouble spots for air leaks.
Other problem areas include interior kitchen soffits, plumbing chases, cantilevered floors, and enclosures around metal fireplaces.
Careful detailing at these spots with gaskets, sealants, and spray foams can ensure a continuous air barrier.
Design a house to dry out (or in) when it gets wet
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Realistically, a little moisture intrusion is inevitable, so it's a good idea to design a house so that it can dry. Walls that can dry to either the outside or inside are good, but walls that can dry to both the outside and inside are even better. Dry design can be accomplished by thoughtful material and design choices.
ABOUT ENCLOSURE AND STRUCTURE
The building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., or shell, is the part of a house that you can draw a line around: the roof, the walls, and the floor. The enclosure begins in the ground with the foundation and floor. It extends out of the ground as aboveground walls, and it's capped with a roof. Each part of the enclosure faces different challenges, but together they must achieve the same goals of stopping or slowing the flow of air, water, and heat while still allowing the inevitable intrusion of water a way to dry out. The parts of the house where this balance is always the trickiest are where the roof meets the walls and the floor meets the foundation. Further complicating matters is the fact that these components don't always line up — either literally or figuratively.
Green homes are simple
On conventional blueprints, it should be easy to draw a line around the part of a house that sheds water, but it's not so easy to do so with the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. or thermal barrier (simple-shaped houses make this easier). When these lines aren't clear, there's a potential weak spot in the design. When the lines are uncertain, there's a certain weak spot.
Green design looks for ways to combine an air barrier with an insulation layer. Materials that can accomplish a couple of the goals of an enclosure simplify the overall design.
Roofs are bombarded by heat, rain, and hail. Chimneys, skylights, and penetrations from plumbing and ventilation equipment are trouble spots for leaks, so it's important to get the flashing and weatherization details right.
Walls contend with wind and rain. Most walls receive very little direct sun each day: South walls get the most; north walls, the least. Because they're vertical, they shed water quickly. Wind-blown rain is more of a problem for walls than just rain. The weak links are the holes: doors; windows; and decks. During a rainstorm, the pressure difference between inside and outside can actually suck water into small leaks around windows and doors, which can become a steady stream.
Foundations see very small temperature swings because they're buried underground where it's never warmer than 70°F and rarely colder than 32°F. But unless you live in the desert, the ground is always wet, and that water is always pushing its way in. Water in the soil will wick all the way up to the roof framing if you let it. CapillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. breaks such as brush-on damp-proofing, sill sealer, and rigid insulation block this process.
MORE ABOUT ENCLOSURE AND STRUCTURE
A well-designed air barrier requires cooperation from everyone on the job site. Often, the task of training tradespeople falls on the shoulders of the builder, who must be sure that everyone working on the building understands what an air barrier is.
Some products — for example, drywall, plastic, or housewrap — are considered air barriers. But when building scientists talk about a home's air barrier, they're not talking about a single material; they're talking about a collection of materials that reaches from the basement slab, around the entire exterior of the building's thermal envelope, and across the finished ceiling.
It's difficult for a builder to construct an effective air barrier if the designer doesn't know where it's located and how it will be built. It's common for some professionals to have a weak understanding of air barriers. Where required, a builder may need to educate the designer, or vice versa, about air-barrier issues.
An air barrier is the sum of many parts
An air barrier consists of materials assembled and joined together to prevent air leakage between the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. and unconditioned space — or the inside and out.
A typical air barrier incorporates more than a dozen materials, including some or all of the following: poured concrete; sill seal; wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ; housewrap; contractor's tape; caulk; spray foam; gaskets; window glass; drywall; polyethylene; and weatherstripping. If any of these materials are leaky, the home's air barrier may be compromised.
More information on air barriers can be found in the GBAGreenBuildingAdvisor.com Encyclopedia article on the topic: Air Barriers.
Measure how tight a house is
A high-quality air barrier is an essential component of an energy-efficient home. Before a new home is occupied, the barrier's tightness should always be verified with a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas..
Blower-door testing is usually scheduled after plumbing and electrical rough-in work is completed but before drywall is hung. Whether the test occurs before or after insulating depends upon the builder's preference and the type of insulation being installed.
Before the blower-door contractor arrives, a responsible worker should inspect the entire house for air-barrier problems, paying particular attention to mud sills, rim joists, rough openings, wiring and plumbing penetrations, soffits, fireplaces, and access hatches. The inspection will involve every floor, from the basement to the attic. Several tubes of caulk and cans of spray foam should be on hand to repair any discovered defects.
For more information on blower-door testing, see "Blower Door Basics."
Handy products can help with air tightness
By now, most builders are familiar with the use of spray foam to seal penetrations in a home's air barrier. But there are other less familiar products that can also improve a home's airtightness.
Airtight electrical boxes. Every manufacturer has its own approach to improving the airtightness of electrical boxes, but most types include a flange that seals against the drywall, as well as a system for sealing the holes at the back of the box, where the wires enter.
Rubber gaskets. Although spray foam is commonly used to prevent air leakage through gaps that are too wide to caulk, some builders have been disappointed with the results of this technique. One such builder is Hans Porschitz, a building systems associate at Bensonwood Homes in Walpole, N.H. According to Porschitz, his crew has had inconsistent results when using spray foam to seal SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. seams, especially if the foam is installed in cold weather.
Porschitz has found that EPDM (synthetic rubber) gaskets perform better than spray foam. "The flexibility of EPDM gaskets is far superior to what you can get with either one-component or two-component foam," says Porschitz.
Rule #1: you can't stop heat, but you can slow it down.
Heat always moves from hot areas to cold areas. In summer, exterior heat will flow toward the cooler interior of a home. In winter, interior heat will flow toward the exterior. The role of insulation is to slow this heat flow. In general, thicker insulation is more effective than thinner insulation.
Many energy consultants have proposed the following rule of thumb: The R-value of insulation installed in a green building should be about twice the code minimum. This is, of course, a guide to planning rather than a hard-and-fast rule.
More insulation is better, to a point
Doubling the thickness of a layer of insulation will double the insulation's R-value, cutting heat loss in half. Each time that the insulation layer is doubled in thickness, this rule applies. But the energy saved per year by doubling insulation from R-10 to R-20 will be considerably more than the energy saved by doubling insulation from R-20 to R-40 because of the law of diminishing returns.
It's best to insulate outside the box
The most common types of insulation used in residential construction are fiberglass batts, cellulose, spray polyurethane foam, and rigid insulation. Insulation materials are usually sold with a label indicating their R-value. The higher the R-value, the more effective the insulation. To achieve the R-value on the label, however, the insulation must be installed without compression or voids.
Although residential wall insulation is traditionally installed in stud cavities, the best place to locate wall insulation is outside of the frame. This reduces the thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. effect that studs have in a wall — each piece of framing is a thermal bridge through the insulation. These thermal bridges seriously degrade the performance of the wall.
The thermal bridge effect can be partially addressed by using rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. — usually 1 or 2 inches of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. or polyisocyanurate. Even better are wall designs that place all of the insulation — 6 to 10 inches of rigid foam — outside of the framing.
When insulation is outside of the frame, framing materials stay warm and dry. When stud bays are not filled with insulation, the work of electricians and plumbers is greatly simplified.
Houses that have foam sheathing should not include an interior polyethylene vapor retarder.
Wet houses invite bugs, mold, and rot
Moisture comes from the sky, the air, and the ground. When houses get too wet, they attract carpenter ants, termites, mold, and decay fungi, which can destroy a house, make people sick, or both. The materials and techniques used in building a green house have to keep water from getting in as well as promote drying of surfaces that do get wet. This begins at the roof and includes all parts of the enclosure.
Steep roofs and deep overhangs move water away . A steep roof moves water a lot more quickly than a shallow one, and deeper overhangs carry water away from the foundation while sheltering windows and walls. Gutters can gather all of the roof water and move it away from the foundation to a drywell, rain garden, or other landscaped area, where is can be absorbed.
Wind-driven water can get behind the siding or windows, so including a couple of back-up strategies are a good idea.
- Provide an air space behind siding so that water can drain out and the siding can dry.
- Sill-pan flashing in windows and doors catches wind-driven water leaks and directs the water out to the drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. behind the siding.
Soil distributes moisture. Stormwater is directed away from a foundation by sloped grade and carried off by a footing drain, but a lot of moisture is held in and distributed throughout the soil. This soil moisture can wick into a foundation through capillarity.
Capillarity can move water to the top of a tree, so it shouldn't be surprising that it can transport water from the footing to the roof of a house. Without capillary breaks, water is drawn into a foundation through the footing or the wall, and will continues further up to drier concrete, where it can get into the framing.
When moisture moves aboveground into the framing, it may encounter cooler surfaces and condense into liquid. If the wall can't dry out, the liquid can accumulate and support mold and rot.
Insulating the exterior of a foundation can block moisture and keep the concrete warm. A warm inside surface means that humidity won't condense into liquid and can cause mold growth. Rigid foam is a good choice under slabs. Rigid or spray foam installed inside the foundation walls stops capillarity and condensation because it prevents warm air from reaching the cool concrete.
Drying direction is complicated by new materials
When you design or build a roof or wall assembly, it's important to think about how the assembly will dry, but new materials make this difficult to assess.
Consider wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. : In the last 50 years, we've gone from using solid lumber to plywood to OSB. These three products alone have different moisture-absorption tolerances and drying potentials. Their resistance to mold and fungi also differ.
To make things more complicated, there are now rigid foam sheathings, wax-impregnated OSB sheathings, membrane-coated OSB sheathings, and foam-backed cement-board sheathings.
Foundation walls cannot dry to the outside
Because foundations are buried in the ground, they'll always be damp, so it's important to keep them from absorbing moisture. Brush-on damp-proofing — or, even better, dimple mats — help keep basement walls dry. Basement walls should be backfilled with coarse granular material to interrupt capillarity and to help ground- and stormwater flow toward the footing drains.
There are two approaches to preventing the inside of a foundation wall from acting as a condensing surface for warm, interior moisture. The first is to keep the concrete warm by installing exterior insulation. The second is to prevent interior air or water vapor from reaching the concrete by installing a layer of rigid foam with sealed seams on the inside of the foundation wall.
Let siding dry
The majority of the leaks in the walls above the foundation occurs at holes in the siding and at bad flashing details. Once water gets in, it's critical that it gets out. Outside the wall, back-vented siding allows water to drain away after it gets behind the siding. Housewrap can allow moisture vapor to pass from the inside out while stopping exterior liquid.
Plastic vapor barriers can keep walls wet
Plastic vapor barriers are often used inside the wall to keep walls dry, but in fact, they can trap more moisture than they keep out. Most building scientists agree that plastic vapor barriers are unnecessary and counterproductive in all but the coldest climates, such as very cold northern states and most of Canada. The permeability of so-called smart vapor retarders (such as kraft paper, MemBrain, or even asphalt felt paper) can change depending on the humidity in their surroundings. They're impermeable in dry conditions but permeable when wet. This allows a wet wall to dry out. Another option is to use a vapor-retarding paint and insulation (such as cellulose) that can absorb and release small amounts of moisture.
For more information on vapor barriers, see:
- Vapor Retarders and Vapor Barriers
- Forget Vapor Diffusion — Stop the Air Leaks!
- Do I Need a Vapor Retarder?
Roofs are trickier because they're sloped, so sometimes a leak inside isn't anywhere near where it came in from outside. It's easier to locate a roof leak in a house that has a ventilated, unconditioned attic than in a house with a conditioned attic or insulated cathedral ceilings. The sooner a roof leak is noticed, the less likely it is that the roof sheathing will have extensive rot.
- Don Mannes/Fine Homebuilding #172
- Dan Morrison
- Daniel Morrison
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