A plain language insulation glossary for homeowners

Plain definitions

How to use this glossary

Terms are grouped by topic and not by letter, because most people look up a word while trying to understand a bigger idea. If you hear two or three unfamiliar words in one sentence, they probably belong to the same group, and reading that group will make the whole sentence clearer.

The definitions are written for homeowners and condo owners in and around Hollywood, Florida. They explain what a word means and why it matters in a hot, humid climate with salt air, strong sun and hurricane season. They do not give numbers, because the right numbers depend on your home and are for a trained installer and your city to discuss.

If a word is missing, ask the installer to explain it in plain language. A good installer will be glad you asked, and the question often opens a more useful conversation than the one you planned to have.

Keep in mind that many terms are used loosely. Two installers may use the same word to mean slightly different things, and a brochure may use a third meaning. When a word matters to your decision, ask the person to describe it using your own house as the example. A definition tied to your attic or your wall is worth more than a perfect one from a book.

Heat flow terms

Heat flow is the movement of warmth from a hotter place to a cooler one. In a South Florida summer, that means from the hot outdoors, the sunbaked roof and the attic toward your cooler living space. Insulation slows that movement, but it cannot stop it.

Conduction is heat moving through a solid thing, like warmth travelling through a wall or a metal fastener. Convection is heat carried by moving air or liquid, like warm air rising through an attic hatch. Radiation is heat travelling as invisible energy across an open space, like the heat you feel from a hot roof deck above your head.

Each type of heat flow calls for a different kind of answer. Slowing conduction needs a material that resists heat passing through. Slowing convection needs sealed gaps. Slowing radiation needs a reflective surface facing an air space. Our guide on how insulation works explains the three in more depth.

A related word is thermal mass, the ability of a heavy material like concrete block to absorb heat and release it slowly. In a South Florida house with block walls, that means the wall can stay warm into the evening after the sun has set. Thermal mass is not the same as insulation, and the two interact in ways that make the plan for a block home different from one for a wood frame home.

R value and the idea of resistance

R value is a measure of how well a material resists heat flow. A higher value means more resistance for a given material. It is a way of comparing materials fairly, but it is only part of the story, because gaps, compression, moisture and air movement can all reduce how well a material performs in a real home.

The value depends on the material and on how thick it is. It is not a promise about comfort or energy use, and it is not the same as a bill. We do not give R values on this site, because the right amount for your home depends on the building and on your city. Ask your installer to explain the choice in words.

Thermal resistance is the general name for this property, and thermal conductivity is its opposite: how easily a material lets heat pass. A good insulator has low conductivity and high resistance. Our page on understanding R value goes through the idea in plain terms.

You may also hear the word U factor, which is the reverse way of saying the same thing. R value measures resistance, while U factor measures how much heat passes through. A low U factor is good. You will see U factor most often on windows and doors. The point is the same: the less heat that passes through a part of the building, the steadier the room tends to feel.

Air and moisture terms

Air sealing means closing gaps and cracks where air leaks through the building shell, using suitable sealing materials. It is different from insulation. Insulation slows heat flow through a material, while air sealing stops air from moving through gaps. In a humid climate, sealing also keeps wet outdoor air from entering walls and attics.

Infiltration is air coming into the house from outdoors, and exfiltration is air leaking out. Both waste conditioned air and can carry moisture. Stack effect is the tendency for warm air to rise and escape at the top of a house, pulling replacement air in at the bottom. Attics and tall rooms make it stronger.

Humidity is the amount of moisture in the air. Relative humidity compares that amount with how much the air could hold at a given temperature. Condensation is moisture that forms when warm humid air meets a cool surface, like a cold duct or a chilled window. Dew point is the temperature at which that happens. Our page on humidity and insulation in Florida explains why these terms matter here.

Two more terms come up often. A building envelope, or shell, is everything that separates the inside of your home from the outside: roof, walls, floor, windows and doors. The conditioned space is the part of the home that you cool or heat on purpose. Good planning is about the boundary between the two, and about making that boundary continuous, with no weak points where heat and humid air slip through.

• Infiltration is outdoor air leaking into the home through gaps.
• Exfiltration is conditioned indoor air leaking out through gaps.
• Condensation is water that forms when humid air touches a cool surface.
• Dew point is the temperature at which air can no longer hold its moisture.

Vapor and moisture control terms

A vapor retarder is a layer that slows the movement of water vapor through a wall or ceiling. Where it belongs depends on the climate and on the building, and in a hot humid region the answer can differ from cooler places. This is a topic for a trained installer and your building department, not a do it yourself guess.

Permeable means a material lets vapor pass through it. Impermeable means it blocks vapor. Semi permeable sits in between. These words matter because a wall needs a way to dry out if it ever gets wet, and the wrong combination of layers can trap moisture inside.

A drying path is a route by which moisture can leave a wall or roof assembly. Good planning keeps at least one drying path open. Our comparison of vapor retarder versus no vapor retarder explains the debate in plain language.

The words moisture barrier, vapor barrier and vapor retarder are sometimes used as if they meant the same thing, but they do not always. A barrier suggests something that blocks, while a retarder slows. Housewrap and sheathing membranes belong to a related group of weather resistive layers on the outside of a wall. Ask the installer exactly which layer they are talking about and where it sits.

Material terms

Batts and rolls are blankets of fibrous material cut to fit between framing members. Loose fill, also called blown in, is fluffy material delivered through a hose and settled over an attic floor or into a closed cavity. Spray foam is a liquid that expands as it cures and sticks to the surface it is applied to.

Fiberglass is made of fine glass fibers. Cellulose is made of recycled paper treated for protection. Mineral wool is made of spun rock or slag. Each has its own feel, handling and moisture behavior. Rigid board is a stiff panel, often used where a thin, firm layer is needed.

Open cell foam is softer and lets some vapor pass. Closed cell foam is denser and blocks more vapor. A radiant barrier is a reflective sheet or coating that reflects radiant heat. Our guides on open cell versus closed cell foam and radiant barrier myths and facts explain these in more detail.

Loose fill has its own vocabulary. Blowing machines push material through a hose, and installers talk about coverage, meaning how evenly the material spreads over an area, and depth, meaning how thick it is. Settled depth is how thick it ends up after it has relaxed. We do not give numbers here, but you can ask an installer to explain how they check coverage and how they mark depth so you can see it.

Attic and roof terms

The attic is the space between the ceiling of the top floor and the underside of the roof. A vented attic has openings that let outdoor air move through it. An unvented attic, sometimes called a sealed or conditioned attic, has insulation at the roof line instead and no intentional vents.

Soffit vents are openings under the roof overhang that let air in. A ridge vent runs along the peak and lets air out. Baffles, also called rafter vents, are channels that keep insulation from blocking the soffit openings. The roof deck is the surface the roofing sits on.

Ventilation is the intentional movement of air through a space, and it is a different thing from air leakage. Our guide on attic ventilation basics explains how vents work, and ventilated versus sealed attic compares the two approaches.

Other words you may hear in the attic include knee wall, the short wall beside a sloped ceiling, and attic hatch or scuttle, the opening used to get in. A gable is the triangular end of a roof, often fitted with a vent. A hip roof slopes on all four sides. A plenum is a main duct box on an air handler. Learning a few of these makes a visit easier to follow.

Building and framing terms

A stud is a vertical framing member in a wall, and a joist is a horizontal member that supports a floor or ceiling. A rafter is a sloped member that supports the roof. A cavity is the empty space between framing members, and insulation often goes there.

Thermal bridging happens when a solid, heat conducting piece, such as a stud or a metal fastener, carries heat across the insulation. Because framing is spread through the wall, bridging can reduce how well the wall performs. Continuous insulation is a layer that covers the framing to reduce bridging.

Concrete block, or CMU, is common in South Florida homes. Furring is a framework added to a wall to hold interior finish or insulation. A slab is the concrete foundation floor. A crawl space is a shallow space under a raised floor. Our page on concrete block homes and insulation explains how block walls are treated.

Sound terms

Sound transmission is the passage of sound through walls, floors and ceilings. Airborne sound travels through the air, like voices and television. Impact sound travels through the structure, like footsteps and dropped objects. Insulation helps with airborne sound more than with impact sound.

Mass, decoupling, absorption and sealing are the four basic tools of sound control. Mass is heavier material. Decoupling separates the two sides of a wall. Absorption puts soft material in the cavity. Sealing closes gaps. A quiet wall usually uses more than one.

Our guide on soundproofing basics and the service page on soundproofing explain these in practice. If noise is your main concern, tell the installer which rooms and what kind of sound bothers you.

Condition and problem terms

Settling is the gradual loss of fluffiness in loose fill or batts over time, which can leave thin spots. Compression is material squeezed flatter than it should be, which reduces how well it works. A void is a gap in coverage where there is little or no material, often found around obstructions like pipes and boxes.

Wicking is moisture moving along or through a material, like a candle wick drawing in wax. Mold is a growth that can follow lasting dampness. Mildew is a similar surface growth. Efflorescence is a white powdery deposit on masonry that points to moisture moving through it. Staining on a ceiling or wall often marks a past or present leak.

Off gassing is the release of odor or vapors from a new material as it cures or ages. Friable describes a material that crumbles easily into dust. If a material in an older home is suspected to be a hazardous type, do not touch it. A trained professional can arrange proper testing. Our problem page on sagging or compressed insulation shows how these terms appear in real homes.

Terms from estimates and paperwork

A scope of work describes what will be done, where and with what. An exclusion is something the work does not include. A change order is a written change to the agreed work. A site visit or inspection is a look at the home by the installer before the work. A product data sheet is the manufacturer's information about a product.

A permit is permission from the city to do certain work, and an inspection is a check by the city. Requirements vary by city, so ask your city building department. HOA stands for homeowners association and a condo association governs a condominium building. Ask yours about rules for work in shared spaces.

Our page on reading an insulation estimate goes through these words as they appear on paper, and questions to ask an insulation contractor suggests what to ask about each. When you are ready, call and talk it over with a person.

Questions and answers