Insulation is the most important home design feature you’ll never see. The right choices will make sure your house is comfortable, your energy bills are low and your water-sensitive materials are protected from moisture. There is an ever-expanding buffet of types of insulation and everyone has their own names and opinions about each. Here’s a primer on what is what, and what goes where:
The fluffy pink stuff you are probably familiar with, which remains the low-cost go-to for basic thermal comfort. Found in stretch-wrapped plastic packaging often featuring the Pink Panther, fiberglass batt insulation comes in rolls designed to shove or lay into the space between your studs or joists. Some come with a paper backing with flaps to staple to the stud for support. The paper backing was once touted as a vapor retarding feature, however, lots of ambiguity with what that means along with geographic uncertainty as to which side of the wall should have vapor retarding properties to avoid condensation issues. In reality, it didn't do much either way as a barrier, but if you can, get the unfaced stuff in New Orleans.
Mineral Wool Batts
Like fiberglass, these come in rolls designed to stuff between the studs. Unlike fiberglass, it doesn't burn, it provides additional thermal resistance (R-value) per inch and has superior acoustic properties for a slight upcharge in price. Rockwool (formerly Roxul) was first to introduce this product into the market, but most of the fiberglass manufacturers offer a similar option.
Rigid Insulation Board
Foam panels, which generally come in three flavors:
white puffy compressed globes (EPS)
bright color - pink or blue - board (XPS), and
the creamy colored board with the foil-facing. (Polyiso)
They all have slightly different properties but the main thing is that they are all meant to be attached OUTSIDE your main structural enclosure. The bonus of this is they run uninterrupted by studs and pipes and outlets and all the other stuff you have in your wall that is NOT insulation. You can also walk on some of them making them good choices for flat roofs. They are generally associated with commercial construction, though that is starting to change. They are a great choice for building new, since a new building can be designed around a completely different insulation concept. You may encounter raised costs due to them being a less familiar install for some homebuilders and laborers. For existing houses, unless you're ripping the entire thing down to its skeleton, they are tough to justify, unless you’re looking for floor insulation under a raised structure.
The piles of fluff with no real structure, great for retrofits because, being made out of tiny pieces, they can be used to fill up any size space from a relatively small insertion points. Piles of this stuff usually sits above ceilings but can fill in wall cavities, and there are even netted products that allow it to hug sloping roof rafters. You can make your fluffy stuff out of lots of things: fiberglass, mineral wool, old newspaper and blue jeans.
Polyurethane spray foam is an increasingly popular choice for both new construction and retrofits because in addition to providing resistance to heat transfer, it also acts as an air seal. Where batts and blown-in allow air in and around their voids and installation gaps, which get worse over time due to sagging and settling, spray foam stays put, and doesn't allow any air at all to trespass through its depth which helps a lot with the interior air temperature. Also its thermal resistance per inch is at worst on par with batts and can be many times higher, depending on the product, which comes in two flavors, both of which are a bit environmentally dubious in terms of nasty chemicals involved:
Open cell - this type is cheaper, about as effective or slightly better per inch as batts at resisting heat transfer, and is impermeable to air.
Closed cell - more expensive, but has added bonuses: much more resistive to heat per inch, which makes it good for tight spaces like cathedral ceilings, impenetrable by moisture of any kind, and it strength when dry actually adds to the building's strength.
If you're looking to be cutting edge, you'll of course pay a premium, but there are options beyond the more commonly available methods listed above. Wood studs have been developed that actually have foam in the center, like an ice cream sandwich, that removes the problem of wood studs conducting heat past your insulation layer. There is even a rigid product that is actually made of mushrooms engineered to grow into a foam-like material that has great insulating properties.
There's many options in the fight to achieve thermal comfort while reducing our energy use, and it's important to know what's best for your particular situation as early as possible so that your project feels as good as it looks.