Insulation
Insulation is the material added to a building structure when the building materials themselves don't provide the desirable amount of resistance to heat transfer. The amount of insulation that can be added is usually limited to the available space between the framing material, and is typically the most significant factor in determining how well a wall insulates, although foam board can be added to either side of walls and the exterior of roofs. Since the framing material itself is at best a mediocre insulator, framing act as a thermal bridge leaking heat, and reducing the overall "R" value of the wall. Advanced framing is a method of eliminating unnecessary framing pieces, and framing in such a way that every part of the wall can be insulated. The use of light-gauge steel framing to replace wood creates a problem because it conducts heat so well, and so must always be used with an exterior layer of foam board insulation to stop the thermal bridging of the steel (see the condensing potential section). In all cases light-gauge steel framed walls have a lower overall "R" value to a wood framed wall.
In other building methods, such as SIPs, ICFs, strawbale, rammed earth or adobe, the building structure also acts as the insulation (for more information on these see the Materials section). The high mass systems like adobe or rammed earth should either include additional insulation or be limited to climates that high thermal mass walls work well in.
Insulation material comes in three primary categories, each of which has its own particular application:
- Fill: Material that you use to fill a cavity between structural materials (e.g. between wood or steel framing). Examples of these are loose fill fiberglass, cellulose, and spray in foam. The R-value of loose fill varies by the density it is filled to. Up to a point, higher density yields a higher R-value (more smaller air pockets), but beyond that the R-value starts to go down.
- Batts: Material that holds itself together in a blanket like fashion, and is attached to structural framing. Batts often contain a backing or other form of vapor barrier in addition to the insulation. See the condensing potential section for a complete discussion on vapor movement. Batts come in various combinations of thickness and widths, the majority of which are designed around standard framing lumber and standard framing spacings (ie 3-1/2" by 15", or 5-1/2" by 23").
- Boards: A rigid material (typically polystyrene) that comes in sheets like plywood. Typical thickness are between 1/2 and 3 inches.
There are many kinds of insulating materials, each of which has its own set of advantages and disadvantages, and none of which are the perfect solution. Not all insulation materials are appropriate for all situations. There is much information out there provided by manufactures of the various products, each listing their own benefits and the disadvantages of their competitors. As could be expected they are mostly very biased. A sampling of some web sites is included.
A quick note on "R" values of materials: Everywhere you look, you'll find somewhat different "R" values listed for the various materials. A materials "R" value will differ based on how it is manufactured and how it is installed and possibly other conditions as well. The numbers used here are typical, and should be used for relative comparison purposes only.
Fiberglass: In its familiar form, glass fibers are spun together and formed into batts with a glue and then typically also attached to a vapor barrier backing. This glue is a skin irritant, making this form of insulation unpleasant to handle. Also available are various blown in versions, which use a non-irritating glue.1
Fiberglass in is a good insulator, between R3.2 and R4.2 for every inch installed, depending on the density its installed at (unfortunately this results in different R-value being reported every place you look, its best to rely only on what the manufacturer says, and even then their R value will only be true if the product is installed exactly the way they say). One controversy with fiberglass is its potential significant degradation in R value (up to 60% lower) at colder temperatures (ie bigger differences in temperature between inside and out), due to voids which end up having convection currents in them.2
The BIBS (for blown in batt system) products report the highest R-value per inch, and are somewhat popular with green builders.3 In this system, some kind of netting (fabric) is stapled across the studs, and then the fiberglass is blown into the cavity.
There is some concern that fiberglass fibers break down with age and create tiny sharp filaments causing a disease called silicosis. Virtually all cases of silicosis are job related. Although there is some real risk, that risk is very small because insulation is contained in the wall. Much information is available on the web, for example by entering "silicosis fiberglass" into any search engine.
Manufacturers are beginning to include some recycled glass content in their fiberglass products, but currently old fiberglass insulation is difficult to recycle due to containing many contaminants. The raw material (sand) is readily available, and although glass manufacture requires a relatively large amount of energy, a long product life and recycling could mitigate that impact.
Mineral Wool: A fibrous product made out of various mineral byproducts (molten rock or slag waste from steel production), and all having similar properties to fiberglass. Mineral wool was common used on the 1940's and 50's, and is having a revival of popularity due to its recycled content and otherwise reputation as a benign product (in addition, it is fireproof). The R-value of mineral wool is generally a little higher than the equivalent fiberglass batts, but essentially the same as denser blown in fiberglass or dense batts. Functionally, the two products are very similar.
Cellulose: Typically made of shredded recycled newspapers soaked in a solution borates (related to borax cleaning powder) to increase its fire resistance and insect resistance. Cellulose is either blown into a cavity dry, or sprayed on wet (when using wet spray, make sure the insulation dries completely!), and has a similar "R" value to fiberglass (its generally considered to be just a bit higher, but in both cases it depends on the density its blown in at).
When installed densely, cellulose is a decent air barrier (although no substitute for an actual air barrier), and will hold its R value over a wide temperature range, but if not installed densely enough, voids will form in walls, and blown in attics will perform at much lower rates than advertised, lowering the effective R-value significantly. Voids have been especially problematic in retrofits.
Any liquid water (condensation or whatever) in the wall may leach out the borates over time leaving the insulation vulnerable, but liquid water, or for that matter high moisture content will cause the wood framing to mold first. Although borates are considered safe, some people are sensitive, so anyone who has chemical sensitivities needs to find out is they are susceptible before using cellulose. Due to it lower amount of processing, Cellulose is popular among the natural building crowd.Note 3
Cellulose is a 100% recycled product, but it not recyclable due to contaminants and the breakdown of fibers. It is however completely biodegrading, and the original product is a renewable resource. Its manufacture is a relatively low energy process, although until manufactures eliminate chlorine bleaching, there are dioxins produced.
Cotton: Made of waste cotton (typically industrial waste, not post-consumer), cotton insulation is a product that has limited availability and currently a significantly higher cost. Because it is a plant material containing cellulose, its properties and problems are like those for Cellulose.
Polystyrene: Polystyrene is a plastic (known mostly by the brand name "Styrofoam") is made from petroleum that is "blown" with some kind of gas (i.e. filled with lots of bubbles) and formed into boards. Historically the gas used was either a CFC of HCFC, materials that are greenhouse gases and contribute to ozone depletion. More recently, manufactures have switched over to using Pentane or Carbon Dioxide gas instead, which are more benign. There are at least three different kinds of polystyrene: molded expanded (MEPS), expanded (EPS), and extruded (XPS). The R-value of the EPS variety varies from 3.5 to 4.5 per inch, while the XPS variety is around R5/inch. The older CFC/HCFC blown materials had a higher R-value per inch, but over time that R-value sank, so over time the difference is less.
Although Polystyrene isn't as flammable as cellulose, it will burn, and produces toxic smoke when it does. Like cellulose, all board insulation is currently required to contain flame retardants. Polystyrene is also susceptible to bug damage in some regions.
Currently Polystyrene is the main component of SIP and ICF building systems (see Materials), and due to being available in a form with high compressive strength, is the only insulation for under concrete slabs and below grade foundations. It is also sometimes used on roofs, walls and in other places where its board form is more convenient--especially because it can be a vapor barrier as well.
Environmentally, Polystyrene leaves much to be desired. Its made from a non-renewable resource. Although technically possible to recycle (its a thermoplastic, meaning it can melted and reformed) hardly anyone will even take clean Styrofoam packing material, so dirty old insulation is even less likely. Because it is made from the polymerization of a simple gas, its technically possible to make it out of an alternative renewable material, but as long as there is abundant cheap oil there will be little motivation to do so. In addition, the flame retardants in being used are considered hazardous to health, and this is probably the main reason green builders have a love-hate relationship with foam board.
Polyurethane/PolyIsocyanurate(polyiso): Like Polystyrene, these are plastics blown with a gas, but promising a higher "R" value: up to R7.5 per inch for Polyiso. Since each product has a somewhat different manufacturing process, one must examine the specific product to determine is actual properties and environmental impacts. Most manufactures have phased out the use of CFCs and HCFCs, resulting in a somewhat lower "fresh" R-value of around R6/inch. In all cases, its probably best to assume that the "aged" R-value is closer to R-5/inch.
Polyurethane can be "foamed in place" in an existing wall cavity. Icycene brand polyurethane is marketing itself as a more environmentally friendly foam, and has been used on a number of "Healthy House" demonstration projects sponsored by the American Lung Association.
Both of these products are thermosetting, meaning that one made they can't be melted and reformed, so they are not easily recycled.
Air Crete: This unique product is cement with a lot of air in it, installed as a foam. Its "R" value is similar to fiberglass, but it has all the environmental properties of cement (and therefore does not have problems with silicosis). Its raw materials are abundant, but it takes a lot of energy to make it. Its is recyclable, but there is currently not a strong market for cement products.
Resources
Home Energy guide to insulation (Nov 2000) - Home energy magazine
www.eeba.com - the energy efficient building association, publishers of the "Builders Guild" series of construction manuals.
www.buildinggreen.com - web site of Environmental Building News, an authoritative, independent reviewer of Green Building. Alas, much of the information is only available if you subscribe to the on-line version of GreenSpec.
Polystyrene Insulation: does it belong in a green building? Environmental Building News, Aug 2009
www.owencorning.com - Makers of "pink" fiberglass as well as Miralfex, a glue free fiberglass
www.certainteed.com - Makers of Insulsafe 4, a "glue free" fiberglass loose fill.
www.dow.com - Makers of Styrofoam brand Polystyrene insulation
www.icycene.com - Makers of Icycene brand Polyurethane foam insulation
www.pima.org - the PolyIsocyanurate manufactures of America
www.polyurethane.org - the Polyurethane industry
www.airkrete.com - Air Krete's website
www.bondedlogic.com - a maker of cotton insulation
Notes
1: whether there is actually glue or not in BIBS insulation isn't really clear to me, but either way it feels like the fake cotton they stuff vitamin bottles with.
2: Whether this degradation in R value is only due to being installed poorly, or also occurs in low-density installations is not clear. Best to install it densely and make sure there are no voids, and then the problem won't occur.
3: insulation choice among green builders can be like a religious discussion. All products have their pluses & minus, and so I'm specifically avoiding taking a stand, other than "insulation is good".