- Why Insulate Your House?
- How Insulation Works
Which Kind of Insulation is Best?
- What Is an R-Value?
- Reading the Label
- Insulation Product Types
Insulating a New House
- Where and How Much
- Air Sealing
- Moisture Control and Ventilation
- Installation Issues
- Design Options
- Crawlspaces and Slabs
- Advanced Wall Framing
- Metal Framing
- Insulating Concrete Forms
- Massive Walls
- Structural Insulated Panels
- External Insulation Finish System
- Attic Ventilation or a Cathedralized Attic
Adding Insulation to an Existing House
- Where and How Much
- How Much Insulation Do I Already Have?
- Air Sealing
- Moisture Control and Ventilation
- Insulation Installation, the Retrofit Challenge
- Basement Walls
- Floors and Crawlspaces
Resources and Links
About This Fact Sheet
Which Kind Of Insulation Is Best?
Based on our email, this is one of the most popular questions homeowners ask before
buying insulation. The answer is that the 'best' type of insulation depends on:
Whenever you compare insulation products, it is critical that you base
your comparison on equal R-values.
- how much insulation is needed,
- the accessibility of the insulation location,
- the space available for the insulation,
- local availability and price of insulation, and
- other considerations unique to each purchaser.
What Is an R-Value?
Insulation is rated in terms of thermal resistance, called R-value, which indicates
the resistance to heat flow. The higher the R-value, the greater the insulating
effectiveness. The R-value of thermal insulation depends on the type of material, its
thickness, and its density. In calculating the R-value of a multi-layered installation, the
R-values of the individual layers are added.
The effectiveness of an insulated ceiling, wall or floor depends on how and where the
insulation is installed.
- Insulation which is compressed will not give you its full rated R-value. This can happen
if you add denser insulation on top of lighter insulation in an attic. It also happens if you
place batts rated for one thickness into a thinner cavity, such as placing R-19 insulation
rated for 6 1/4 inches into a 5 1/2 inch wall cavity.
- Insulation placed between joists, rafters, and studs does not
retard heat flow through those joists or studs. This heat flow is called thermal bridging.
So, the overall R-value of a wall or ceiling will be somewhat different from the R-value
of the insulation itself. That is why it is important that attic insulation cover the tops of
the joists and that is also why we often recommend the use of insulative sheathing on walls.
The short-circuiting through metal framing is much greater than that through wood-framed
walls; sometimes the insulated metal wall's overall R-value can be as low as half the
Reading the Label
No matter what kind of insulation you buy, check the information on the product label to
make sure that the product is suitable for the intended application. To protect consumers,
the Federal Trade Commission has very clear rules about the R-value label that must be placed
on all residential insulation products, whether they are installed by professionals, or whether
they are purchased at a local supply store. These labels include a clearly stated R-value and
information about health, safety, and fire-hazard issues. Take time to read the label BEFORE
installing the insulation. Insist that any contractor installing insulation provide the product
labels from EACH package (which will also tell you how many packages were used). Many special
products have been developed to give higher R-values with less thickness. On the other hand,
some materials require a greater initial thickness to offset eventual settling or to ensure
that you get the rated R-value under a range of temperature conditions.
Insulation Product Types
Some types of insulation require professional installation, and others you can install yourself.
You should consider the several forms of insulation available, their R-values, and the thickness
needed. The type of insulation you use will be determined by the nature of the spaces in the
house that you plan to insulate. For example, since you cannot conveniently "pour" insulation
into an overhead space, blankets, spray-foam, board products, or reflective systems are used
between the joists of an unfinished basement ceiling. The most economical way to fill closed
cavities in finished walls is with blown-in insulation applied with pneumatic equipment or with
sprayed-in-place foam insulation.
The different forms of insulation can be used together. For example, you can add batt or roll
insulation over loose-fill insulation, or vice-versa. Usually, material of higher density (weight
per unit volume) should not be placed on top of lower density insulation that is easily compressed.
Doing so will reduce the thickness of the material underneath and thereby lower its R-value.
There is one exception to this general rule: When attic temperatures drop below 0°F, some
low-density, fiberglass, loose-fill insulation installations may allow air to circulate between
the top of your ceiling and the attic, decreasing the effectiveness of the insulation. You can
eliminate this air circulation by covering the low-density, loose-fill insulation with a blanket
insulation product or with a higher density loose-fill insulation.
Blankets, in the form of batts or rolls, are flexible products made from mineral fibers,
including fiberglass or rock wool. They are available in widths suited to standard spacings of
wall studs and attic or floor joists. They must be hand-cut and trimmed to fit wherever the joist
spacing is non-standard (such as near windows, doors, or corners), or where there are obstructions
in the walls (such as wires, electrical outlet boxes, or pipes). Batts can be installed by
homeowners or professionals. They are available with or without vapor-retarder facings. Batts
with a special flame-resistant facing are available in various widths for basement walls where
the insulation will be left exposed.
Blown-in loose-fill insulation includes cellulose, fiberglass, or rock wool in the form
of loose fibers or fiber pellets that are blown using pneumatic equipment, usually by
professional installers. This form of insulation can be used in wall cavities. It is also
appropriate for unfinished attic floors, for irregularly
shaped areas, and for filling in around obstructions.
In the open wall cavities of a new house, cellulose and fiberglass fibers can also
be sprayed after mixing the fibers with an adhesive or foam to make them resistant to settling.
Foam insulation can be applied by a professional using special equipment to
meter, mix, and spray the foam into place. Polyisocyanurate and
polyurethane foam insulation can be produced in two forms: open-cell and closed-cell. In general, open-celled foam allows water vapor to move through
the material more easily than closed-cell foam. However, open-celled foams usually have a lower
R-value for a given thickness compared to closed-cell foams. So, some of the closed-cell foams
are able to provide a greater R-value where space is limited.
Rigid insulation is made from fibrous materials or plastic foams and is produced in
board-like forms and molded pipe coverings. These provide full coverage with few heat loss
paths and are often able to provide a greater R-value where space is limited. Such boards may
be faced with a reflective foil that reduces heat flow when next to an air space. Rigid
insulation is often used for foundations and as an insulative wall sheathing.
Previous Section - Introduction
Reflective insulation systems are fabricated from aluminum foils with a variety of
backings such as kraft paper, plastic film, polyethylene bubbles, or cardboard. The resistance
to heat flow depends on the heat flow direction, and this type of insulation is most effective
in reducing downward heat flow. Reflective systems are typically located between roof rafters,
floor joists, or wall studs. If a single reflective surface is used alone and faces an open space,
such as an attic, it is called a radiant barrier.
Radiant barriers are installed in buildings to
reduce summer heat gain and winter heat loss. In new buildings, you can select foil-faced wood products
for your roof sheathing (installed with the foil facing down into the attic)
or other locations to provide the radiant barrier as an integral part of the
structure. For existing buildings, the radiant barrier is typically fastened across the bottom of joists,
as shown in this drawing. All radiant barriers must have a low emittance (0.1 or less) and high reflectance (0.9
Next Section - Insulating a New House