HOW HOME’S EXTERIOR ENVELOPE AFFECTS THERMAL COMFORT AND ENERGY COSTS
The exterior walls of our houses are more than just the walls that define our home space, give it structural support and present a face to the street. They serve as the outer shell to help maintain the indoor environment (together with the mechanical conditioning systems) and facilitate the homes’ climate control as well mold, mildew, and water infiltration control. They affect the temperature we feel in the house and our comfort in it. Since in order to feel comfortable we often need to adjust the home temperature with heating or cooling systems (HVAC), the exterior envelope affects also our energy costs.
Generally, although changes with each individual, our ideal comfort zone lies between 68-78 F (20- 25.5 C) at 45% relative humidity. Heat flows naturally from a warmer to a cooler space. In winter, the heat moves directly from all heated living spaces to the outdoors and to adjacent unheated attics, garages, and basements – wherever there is a difference in temperature. During the summer, heat moves from outdoors to the house interior. To maintain comfort, the heat lost in winter must be replaced by the heating system and the heat gained in summer must be removed by the air conditioner. Insulating ceilings, walls and floors decreases the heating or cooling needed by providing an effective resistance to the flow of heat.
Many of the older homes have walls, floors and roofs that do not contain enough insulation, or none at all, and the existing materials are not sufficient to control the temperature differences between the exterior and interior, especially not in the New England weather. In addition there may be loose joints, cracks and old materials which allow air leakage.
New construction homes have to comply with strict energy code requirements regarding insulation, control of air infiltration, vapor infiltration, etc and will perform better. However, there are options to improve existing homes’ exterior envelope insulation to achieve better comfort and lower energy bills.
SO WHY INSULATE?
- to achieve better comfort
- to achieve long term cost saving on energy bills (saves homeowners 20% to 50% on utility bills)
- to help fight global warming
- to increase the fire resistance of the exterior wall
The amount of energy you conserve will depend on several factors: your local climate; the size, shape, and construction of your house; the living habits of your family; the type and efficiency of the heating and cooling systems; and the fuel you use. Once the energy savings have paid for the installation cost, energy conserved is money saved – and saving energy will be even more important as utility rates go up.
So, if you’re a home owner who’s looking to build your new home from scratch, or invest in updating your existing home envelope’s insulation, this article is intended for you. It will provide a quick summary of residential exterior envelope construction, differentiate commonly used materials, define and describe some of the terms used by construction professionals, and give a quick overview of insulation- where, why and how.
WHAT MAKES UP AN EXTERIOR WALL?
There are many types of residential exterior envelopes. Typically, exterior walls have one face to the outside and one face inside the home (conditioned space). Not all exterior walls are load bearing, but in most cases they will serve some structural function like supporting the roof.
Exterior walls come in different types (like wood, Structural Insulated Panels…) and are assembled using various construction methods:
is probably the most commonly used wall system and can be constructed on site or in a factory (i.e. modular). This wall type is comprised of several components:
- A single 2x sole plate is attached to the sub-floor or foundation to which wooden studs are attached by toe-nailing
- Wooden 2x studs, attached to the sole plate are spaced 16” on center (standard framing), or 24” on center (advanced framing).
- On top of the studs a double or single 2x top plate ties the studs together
The exterior side of the wall typically is a multilayered system comprising of:
- A layer of either plywood* or OSB* which acts as bracing and helps resist moisture moving into the house,
- A layer of House Wrap* which acts a protective moisture barrier* to the house.
- A layer of exterior finish material which can be a wide range of materials like wood, cementitious fiberboard, vinyl siding, metal siding, stucco, etc.
- The cavity of an exterior wooden stud wall is typically insulated with batt insulation, blown in, or spray foam insulation.
- The interior side of the wall typically is sheathed with gypsum board, but it also can be sheathed with plywood, or OSB before the layer of gypsum board* is applied.
This exterior wall type uses an extruded steel channel shaped stud in the place of wooden studs. The main difference between wooden stud walls and steel stud walls, other than the material, is the Sole Plate and Top Plate connections which are both metal channels and use Metal Self Tapping Screws. The other components that make up the wall assembly are the same as wooden stud walls.
(SIPs) Are prefabricated wall panel systems. The panels usually consist of plywood, OSB, or drywall glued and sandwiched around a core consisting of EPS (expanded polystyrene), polyurethane, polyisocyanurate, compressed wheat straw, or epoxy. They provide the structural support, rather than the studs used in traditional framing. These need to be prepared in the factory, but save substantially on framing time.
This exterior wall type is comprised of several components:
- A double 2x sole plate is attached to the foundation or slab to which the SIP panel will be glued and nailed
- 2x splines are installed in one side of the panel to create a male connection for the corresponding panel’s female pocket connection that allows the panels to be glued and nailed together to form a continuous wall system
- A 2x top plate is installed at the top of the panel that the roof panel will tie into or the second floor subfloor will tie into.
- The exterior side of the panel is either OSB or plywood
- The interior cavity of the wall panel is EPS or equivalent
- The interior side of the panel is OSB or plywood to which gypsum board is attached
This type of Structurally Insulated Panels (SIPs) has metal skin on both the exterior and interior faces of the panel with an EPS foam core. Instead of using a 2x sole plate and top plate, a metal track is used to connect the panels to the subfloor or foundation and the top of the panel is also capped with a metal track.
Most exterior walls contain some version of the layers described above. The major difference in wall construction is in the insulation characteristics, and there lies the ability to improve the home envelope and the comfort and savings with it.
Other types of exterior walls used :
Brick and concrete block are the most common types of masonry in use in industrialized nations and may be either weight-bearing or a veneer.
A masonry veneer wall consists of masonry units, usually clay-based bricks, installed on one or both sides of a structurally independent wall usually constructed of wood or masonry. This bricks masonry is primarily decorative, not structural. The brick veneer is generally connected to the structural wall by brick ties (metal strips that are attached to the structural wall, as well as the mortar joints of the brick veneer). There is typically an air gap between the brick veneer and the structural wall. As clay-based brick is usually not completely waterproof, the structural wall will often have a water-resistant surface (usually tar paper) and weep holes can be left at the base of the brick veneer to drain moisture that accumulates inside the air gap. Interior insulation in the form of fiberglass batts between wooden wall studs or rigid insulation boards covered with plaster or drywall are used with this type of wall. In most climates this insulation is much more effective on the exterior of the wall, allowing the building interior to take advantage of the thermal mass of the masonry.
Blocks of cinder concrete, ordinary concrete blocks, or hollow tile are generically known as Concrete Masonry Units (CMUs). They usually are much larger than ordinary bricks and so are much faster to lay for a wall of a given size. Furthermore, cinder and concrete blocks typically have much lower water absorption rates than brick. They often are used as the structural core for veneered brick masonry. Such blocks often receive a stucco surface for decoration. A CMU wall can be reinforced by filling the block voids with concrete with or without steel rebar.
Insulating concrete forms
(ICFs) are stay-in-place formwork made from insulating materials to build energy-efficient, cast-in-place, reinforced concrete walls. These are good for new construction- for residential mostly in basement walls.
SO HOW TO INSULATE?
There are many materials used today as insulation layers in exterior walls. For new construction, the variety is wide and open. However, many of our existing homes are the ones we look to improve. Factors affecting the type and amount of insulation to use in a building include:
- Ease of installation
- Durability – resistance to degradation from compression, moisture, decomposition, etc.
- Ease of replacement at end of life
- Cost effectiveness
- Environmental impact and sustainability
We’ll discuss the characteristics, advantages and disadvantages of the common insulation materials:
Open cell foam spray- spray polyurethane foam (SPF)
- liquids mixed in field then sprayed
- sticks to surface
- uses water as the blowing agents
- air sealer- expands to fill in cavities
- thermal insulator
- in-organic. No mold or mildew
- low density 0.5 – 1.0 pound per cubic foot
- does not contain any CFCs, HCFCs, urea-formaldehyde, asbestos, or any cellulosic material
- R-value of approximately 3.2 – 4.5 per inch of depth
- cost effective
- excellent sound attenuation- STC ratings from 49 to 52
- suitable for all building types
- chars and flakes when exposed to fire, it will burn but will not propagate a flame.
- allow water vapor to permeate through it
- not designed for contact with bulk water
- no inherent structural capability
- foam insulation often uses hazardous chemicals with high human toxicity
Closed cell foam spray (medium density foam spray)
- liquids mixed in field then sprayed
- air sealer- expands to fill in cavities
- 1.7 to 2.5 pounds per cubic foot
- more rigid and stronger than low-density SPF
- non-ozone depleting blowing agents
- R-values available up to R-7.4
- does not require a separate air barrier since it is also an effective air sealing foam
- class II vapor retarder meaning it has very low permeance. no need for a separate vapor retarder
- four-in-one application, providing an exterior thermal insulation, an air barrier, a vapor retarder, and a drain plane all in a single application
- can add some rigidity and structural strength to a framed assembly
- suitable for all building types
- chars and flakes when exposed to fire, it will burn but will not propagate a flame.
- Foam insulation often uses hazardous chemicals with high human toxicity, such as isocyanates, benzene and toluene
There are other types of spray foam like cementitious foam, Polyisocyanurate, Phenolic injection foam and Polystyrene based foams. These are less common.
- organic compound
- made of 75%-85% recycled content (waste newspaper) treated with chemicals, such as boric acid to retard the spread of fire
- comes in dry cellulose( loose fill for existing homes), wet spray applied (for new construction), stabilized and low dust
- R-value of 3.8 per inch
- very good at fitting around items in walls like pipes and wiring
- performing 20-30% better at reducing energy used for heating than fiberglass
- less expensive the spray foam
- Cellulose is approximately three times denser then fiberglass, providing a slight improvement in sound reduction.
- The borates in cellulose insulation provide added control against mold
- The borate treatment also gives cellulose the highest (Class I) fire safety rating. (Many cellulose companies use a blend of ammonium sulfate and borate).
- A vapor barrier may not be needed with cellulose insulation
- doesn’t use environmentally harmful blowing agent
- does not produce significant gaseous emissions
- The embodied energy of cellulose insulation is the lowest of the popular insulation types
- No formaldehyde-based binders
- not proven that the borates in the cellulose provide pest control properties
- Cellulose contains some small particles which can be blown into the house through inadequate seals around fixtures or holes
- There is slight concernover the off gassing of ink from the newspapers but the material is sealed behind walls
Rigid panels (board insulation)
Rigid panel insulation is made from fibrous materials (fiberglass, rock and slag wool) or from plastic foam.
Fiber glass batting and blankets (glass wool)
- Made from molten glass, usually with 20% to 30% recycled industrial waste and post-consumer content, a maximum amount of 50% recycled content
- Batts are precut, whereas blankets are available in continuous rolls.
- most common residential insulation, used as batts pressed between studs
- sound transmission class STC of 36-39, depending on stud and screw spacing
- Nonflammable, except for the facing (if present). Sometimes, the manufacturer modifies the facing so that it is fire-resistant. Some fiberglass is unfaced, some is paper-faced with a thin layer of asphalt, and some is foil-faced
- Foil-faced batts are vapor barriers
- traditional batt insulation is often compromised by the presence of mechanical and electrical lines, boxes, etc. Insulation is often compressed, cut away, or pushed aside in those cases.
- Potential cancer risk stated by NIOSH (national institute of occupational safety and health)
- Can cause irritation
- Batts can be challenging and unpleasant to hang under floors between joists
- Gaps between batts (bypasses) can become sites of air infiltration or condensation
- Fiberglass is energy intensive in manufacture
- A type of building exterior wall cladding system that provides exterior walls with an insulated finished surface and waterproofing in an integrated composite material system
- A lightweight synthetic wall cladding that includes foam plastic insulation and thin synthetic coatings. Sometimes looks like stucco, but is not related to it.
Other materials that are used in exterior wall insulation but are less common are:
- slag wool- iron ore blast furnace slag
- rock wool- basalt, diabase
- cotton batts (blue jeans)
- wood fiber
- sheep wool insulation
- natural fiber (cork, cotton, recycled tissue/clothes, hemp, flax, coco, wool, lightweight wood fiber, cellulose, seaweed)
Is the reduction of heat transfer (the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence.
Building envelope (building enclosure)
Is the physical separator between the interior and the exterior environments of a building.
- Maximum: 78 F/ 25.5 C (summer)
- Minimum: 68 F/ 20 C (winter)
- Relative Humidity (RH): 30% – 60% RH
- Ideal condition is temperatures between 68-78 F (20-26 C) and 45% RH year round
A manufactured wood panel made from thin sheets of wood veneer. Plywood layers (called veneers or plys) are glued together, with adjacent plies having their wood grain rotated relative to adjacent layers up to 90 degrees.
Oriented strand board is an engineered wood particle board formed by layering strands (flakes) of wood in specific orientations.
Expanded polystyrene insulation is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. It is used for disposable trays, plates, bowls and cups. Other uses include molded sheets for building insulation and packing material.
Also called drywall, plasterboard, wallboard, or gyprock- is a panel made of gypsum plaster pressed between two thick sheets of paper. It is used to make interior walls and ceilings.
All synthetic materials that have become replacement materials for asphalt-treated paper, or asphalt saturated felt applied to a house shell. Housewrap functions as a weather-resistant barrier, preventing rain from getting into the wall assembly while allowing water vapor to pass to the exterior. House wrap may also serve as an air barrier if it is sealed carefully at seams.
Moisture or water vapor moves into building cavities in three ways:
- With air currents,
- By diffusion through materials,
- By heat transfer.
Of these three, air movement accounts for more than 98% of all water vapor movement in building cavities. A vapor retarder and an air barrier serve to reduce this problem, but are not necessarily interchangeable.
Any material for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability. Normally available as coatings or membranes.
How, where, and whether a vapor barrier should be used depends on the climate. For building in most parts of North America, where winter heating conditions predominate, vapor barrier are placed toward the interior, heated side of insulation in the assembly. In humid regions where warm-weather cooling predominates within buildings, the vapor barrier should be located toward the exterior side of insulation. In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapor barrier may not be necessary at all.
It is often better to have a vapor-open building assembly, meaning that walls and roofs should be designed to dry: either to the inside, the outside, or both, so the ventilation of water vapor should be taken into consideration. A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation. This is because no vapor barrier is perfect, and because water may get into the structure, typically from rain. In general, the better the vapor barrier and the drier the conditions, the less venting is required.
In areas below foundation level (subgrade areas), particularly those formed in concrete, vapor retarder placement can be problematic, as moisture infiltration from capillary action can exceed water vapor movement outward through framed and insulated walls.
Inside a steel building, water vapor will condense whenever it comes into contact with a surface that is below the dew point temperature. Visible condensation on windowpanes and purlins that results in dripping can be somewhat mitigated with ventilation; however insulation is the preferred method of condensation prevention.
Vapor barrier retards the migration of water vapor, not intended to retard the migration of air. This is the function of air barriers. Air is mixed with water vapor. When air moves from location to location due to an air pressure difference, the vapor moves with it. This is a type of migration of water vapor. In the strictest sense air barriers are also vapor barriers when they control the transport of moisture-laden air.
Air barriers control air leakage into and out of the building envelope. Air barrier products may take several forms:
- Mechanically-attached membranes, also known as housewraps, usually a polyethylene-fiber or spun-bonded polyolefin, such as Tyvek is a generally accepted moisture barrier and an air barrier.
- Self-adhered membranes, which are typically also a water resistant barrier and a vapor barrier
- Fluid-applied membranes, such as heavy-bodied paints or coatings including polymeric based and asphaltic based materials
- Closed-cell medium density spray-applied polyurethane foam, which typically provides insulation as well
- Some open-cell spray-applied polyurethane foam that are of high density
- Boardstock, which includes 12 mm plywood or OSB, 25 mm extruded polystyrene, etc.
Is the exterior weather-facing surface of an exterior wall detail that stands off from the moisture-resistant surface of the structural backup wall. The rainscreen is the first interruption between conditions that exist on the outside of a walled building and conditions that are required on the inside of a walled building.
In a rainscreen the air gap allows the circulation of air across the air vapor barrier. This helps remove condensation and helps direct water away from the dry insulation where it otherwise might cause problems such as mold formation and water leakage. The Air Vapor barrier prevents water molecules from entering the insulated cavity but allows the passage of air.