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Batt Insulation
Types of Batt Insulation | Fiberglass | Mineral Wool | Applications for Batt Insulation | Correct Installation | Wall Insulation | Attic Insulation | Floor InsulationQuick Facts
- Commonly made of fiberglass which is inexpensive and widely available
- Often contains formaldehyde which can off-gas and impact indoor air quality
- Proper installation is time consuming but is needed to achieve effective insulation value
- Incorrect batt installation can reduce effective R-value (insulation value) by 28%
Types of Batt Insulation
Like all insulation, batt insulation impedes heat flow by creating many tiny air pockets which resist temperature change. Batts or blankets of insulation are available in the following materials:
| Material | Raw material component | R-value per inch |
| fiberglass | glass | 2.6-4.3 |
| mineral wool | rock or mineral ore | 3.0-3.6 |
| cotton fiber | recycled cotton and denim | 3.5-3.8 |
These materials are similar in their insulation characteristics but differ in their composition and in where there are used. The insulation values of both are equally affected by improper installation methods which are quite common.
Fiberglass
Fiberglass insulation is made by creating fibers from molten glass which are then enrobed with organic (carbon based) binders which usually contain formaldehyde. Batts or blankets formed of the fibers are the most widely used form of residential insulation. Fiberglass batts come in a variety of colors and may have a paper, foil or vinyl backing which is meant to reduce air flow, act a a vapor barrier, and aid in installation.
Fiberglass fibers can irritate the skin eyes and throat, especially during installation but are now thought to be less harmful than asbestos fibers which are proven to contribute to lung cancer if inhaled. Health concerns surrounding fiberglass insulation primarily stem from the long term off gassing of formaldehyde which can accumulate in the home and worsen indoor air quality, aggravating asthma and allergies.
Manufacturers include:
Mineral Wool
Mineral wool, also called rock wool, is also sold in batts or blankets but is less common in the US. It is much more widely used in Canada and Europe. Mineral wool is manufactured similarly to fiberglass glass but by using molten rock (and/or blast furnace slag) instead of molten glass as a raw material. Like fiberglass, these batts are formed using adhesive binders which often contain formaldehyde. Mineral wool batts, which are generally yellow, have similar insulation qualities as fiberglass batts and are less flammable than fiberglass batts, generally due to the lack of paper or vinyl backing.
Like fiberglass, mineral wool fibers can also irritate the skin eyes and throat, especially during installation. Mineral wool which contains formaldehyde is also susceptible to long term off gassing of formaldehyde which can accumulate in the home and affect indoor air quality, possibly leading to health concerns for those with sensitive respiratory systems.
Manufacturers include:
Cotton Fiber
A more recent alternative to fiberglass and mineral batts are batts made of recycled or post-consumer cotton and denim fibers. They have the advantaged of being non-toxic and easy to handle (unlike fiberglass which can cause skin irritation) Cotton is naturally flammable and susceptible to mold growth and rodent infestation, though, so the fibers must be treated with the benign chemical boric acid (non-toxic boric acid (with a toxicity comparable to that of table salt) to address these concerns . However, even when treated with boric acid cotton fiber insulation may not be as protected from mold growth as other fibers such as fiberglass or cellulose (newspaper).
Manufacturers include:
Applications for Batt Insulation
Batt insulation is commonly used in new construction for insulating walls, attics and floors. While batts are inexpensive and widely available, they are difficult to install correctly since insulation must be installed without air gaps on all sides to be the most effective. This is because insulation works by creating pockets of air which resist temperature change. If air blows freely through the air pockets, they will lose their capacity to resist heat flow.
Correct Installation
To be installed correctly, batts should be cut to fit exactly in length and width to completely fill the cavities where they are installed. If they are too long they will bunch up; if they are too short there will be gaps at the top and bottom of the cavity which leads to air flow. Care should also be taken to make precise cuts around any pipes or electrical wiring or outlets. Such methods are time consuming to implement and are usually not followed, resulting in compressed batts and large gaps. Rigorous scientific testing at ORNL has shown that effective R-value (a measure of insulation value) of batt insulation is commonly decreased by 28% percent due to typical installation methods which create gaps and allow more air flow.
Wall Insulation
Batts are commonly used to insulate exterior walls during new construction. They are less practical for retrofits and add expense and waste because the whole wall must be opened to expose the cavity to be insulated. Other possibilities for adding insulation to existing walls include blown in fiberglass and cellulose insulation and pour-in expanding foam insulation, both of which tend to realize higher effective R-value. This is due to their ability to better eliminate gaps and achieve proper air sealing.
Attic Insulation
Batts are also typically used to insulate attics by simply laying them between joists. However, this insulation is largely ineffective if not installed properly and joists are still exposed (creating a thermal bridge where heat flow can easily bypass the insulation) or if the attic has not been properly air sealed. Alternatives include complementing existing batts with air sealing and blown in insulation on the attic floor or air sealing the entire attic (including removing air vents) and insulation the attic rafters with spray foam insulation.
Floor Insulation
Batts are also commonly used to insulate the underside of floors by securing the insulation between joists in the crawlspace area. It is quite difficult to achieve effective insulation value with this technique because the batts have a tendency to fall or be compressed by the cords or wires which are used to secure them to the underfloor. An alternative which is easier to install correctly and which achieves much better insulation is expanding foam which can be sprayed on directly. As with any insulation, it is vital to verify that the electrical wiring is compatible with the spray foam (active older knob and tube wiring may not be).