Insulation slows heat transmission to the buildings floors, walls and ceiling or roof. Heat transmission is the average homes leading cause of winter heat loss. Both single-family homes lost three to six times as much heat through transmission as through air leakage.
When you say that insulation and resists heat transmission, we mean it resists conduction, convection, and radiation heat flow through a buildings component. In the case of a uninstall wall cavity, convection and radiation dominate heat transmission through the wall’s empty cavity. Insulation, installed in the wall cavity, forces the heat conduct from fiber to fiber and through the insulation’s tiny air pockets- a slower heat- transmission process than convection and radiation.
Insulation performs the following thermal functions:
- Conserves energy by slowing heat transmission.
- Enhances comfort by reducing temperature variations within the conditioned space.
- Reduces the size of heating and cooling equipment needed by a building in direct proportion to R value.
- Prevents wintertime condensation by improving low interior surface temperatures.
Insulation may also offer the following non-thermal benefits:
- Adds structural strength.
- Reduces noise and vibration.
- Impedes air leakage and water vapor transmission.
- Improves the building’s fire resistance.
Conductivity of Building Materials
Aluminum is the most conductive common building material. It’s thermal conductivity is more than five times greater than steel, 1000 times greater than wood, and 10,000 times greater than air. One square foot of steel, the next most conductive building material, conducts as much heat and 50 ft.² of concrete, 200 ft.² of wood, or 1000 ft.² of glass wool, assuming all are of the same thickness.
Insulation is installed in building cavities, attached to a building’s structural frame, or attached to the building shell’s exterior surfaces.
Insulation slows heat transmission in two important ways:
1. By forcing the heat to conduct through air or some other gas. Gases are generally poor heat conductors.
2. By reducing heat radiation and air convection within cavities where it is installed.
Insulating materials are not as continuous or dense as other building materials which are heat conductors- they harbor millions of tiny air pockets within their fibers of bubbles ( in plastic foam insulation). Heat transmission proceeds slowly through insulation, having to cross this myriad of slow conducting their pockets.
If air from inside or outside the building leaks into a insulated cavity, the effectiveness of the insulation is further reduced. This reduction typically varies from 15 to 50%. Air can even flow-through fibrous insulating materials such as loosely install fiberglass. Installation’s installed density an important issue, especially in cold climates.
Wind also affects insulation performance. Wind convects heat away from the surfaces of a building. If voids and edge gaps exist, wind can push outdoor air through building cavities around the insulation or push air through insulation. These effects increase heat transmission.
Absorbed water decreases the R-value of insulation. Water fills the insulation’s air spaces, and conducts heat far better than air. Water and ice also can damage insulation. Wet insulation can help corrode metals and supply water to insects and microorganisms that rot organic building materials.
Air leakage is the most potent moisture carrying mechanism affecting condensation in building cavities. Vapor diffusion is water vapor traveling through permeable materials like drywall and masonry. Low-R building materials combined with water-absorbent building materials create the largest potential moisture problems.
Types of insulation:
Insulation materials are made of mineral organic materials that trap air. Plastic foams also use other gases that conduct heat more slowly than air. Mineral insulation include: mineral and glass fibers, vermiculite, and perlite. Organic insulating materials include plastic foams and cellulose.
Insulation comes in various product types: flexible materials, such as batts and blankets; rigid materials, such as foam-board; and fiberboard; sprayed on materials, such as polyurethane; and loose fill insulation, such as cellulose. Batts are narrow blankets sized to fit between wall studs, floor joists, and ceiling joists.
Fiberglass is the most popular insulation material; it is manufactured in batts, blankets, loose fill and rigid boards. Cellulose insulation is also popular for residential buildings it is manufactured as a loose- fill insulation from wood fiber on recycled paper. Plastic foam insulation is manufactured in 4-by-8 foot sheets in thicknesses from 1/4 inch to 4 inches.
Cellulose and fiberglass loose-fill insulation are good air sealers for inaccessible building cavities providing air-leakage pathways. Cellulose is superior to fiberglass because it packs tighter and has smaller fibers that are driven into small gaps during installation. However, cellulose can absorb water from leaks and high humidity. Technicians can seal areas where they can’t crawl or reach by using fill tubes to blow tightly packed insulation into the cavities.
Loose-fill insulation has particular importance to energy retrofits because of its ability to fill spaces inside closed cavities, such as walls. Sprayed insulations are often used to retrofit masonry walls, especially those with irregular surfaces.
It’s important to know each insulation materials temperature, toxicity, fire and moisture characteristics. Foam insulation can be damaged by high temperature and sunlight. Fiberglass irritates skin and lungs. Cellulose absorbs water in humid conditions. Kraft paper batt facing is flammable. And foam insulation produces toxic smoke when burned.
Fiberglass batts and blankets
Fiberglass batts and blankets are the most common and widely available, American insulation products. Mineral wool have a small market in the U.S. but it is common in Canada and popular in Europe.
Batts are most commonly installed into building cavities during construction. Batts a commonly sized to fit between framing members that are spaced on 16 inch or 24 inch centers. As a retrofit, batts are applied most often to ceilings with an attic and below the floor when there is a crawlspace.
It blows quickly and easily, achieving a high density in wall. Cellulose usually contains a lot of small fibers that tend to pack into cracks and crevices of closed building cavities, retarding airflow through these cavities. This characteristic is of cellulose is used extensively for air sealing older homes. Cellulose has better resistance to air convection than fiberglass act is commonly installed density.
Blown fiberglass is manufactured in two types: chopped-up batt waste and virgin short fibers. The batt waste type as long fibers and binder, so its R-value per inch is slightly lower than the virgin fibers, which are thinner and shorter. The shorter thinner fibers create smaller and more numerous air spaces.
Fiberglass for blowing is packed in compressed 24-to-40 pound bales. The compressed fiberglass requires a blowing machine with an agitator that tears it up into small pieces that travel fluidly through the blower hose.
It is easy to over-fluff fiberglass in attics, leading to low-densities and excessive air permeability. Cellulose insulation is superior to fiberglass at resisting convection as attic insulation because it blows at higher density.