Insulation saves money, improves comfort and cools the planet.
Modern Building Science
It's important to understand modern building science before deciding how best to tighten the "building envelope" (foundation, walls and roof) of a conditioned structure.
For many years, the focus was on installing as much insulation (measured by R-factor) as we could fit into our attics. Modern building science has shifted the focus from R-factors to blocking air movement. Unless the building was built of an especially modern material, like stress skins, structural insulated panels (SIPs) or insulated concrete forms (ICFs), it's best to focus on blocking the building's stack effect -- the natural flow of air from warm to cool within a leaky building envelope -- as shown in this diagram.
Beyond weatherizing around doors and windows, it's best to begin by sealing the attic and the foundation and rim boards, where they intersect with the first floor's joists. Then, if the building still is especially leaky, consider insulating the walls. Besides addressing the stack effect, this approach is usually easier and requires the least mess. If building a new structure, insulating the attic, rim boards and walls is well worthwhile.
Similarly, emphasis used to be on laying insulation on the floor of the attic, plus installing roof vents high and low or at dormer end to let hot summer air out. Nowadays, if insulating with an air-blocking foam product in the attic, the foam can be sprayed on the underside of the roofing and the vents, removed or sealed, making the attic dry and more suitable for storage.
Regardless of our attention to detail, we can't see inside closed cavities, such as walls, finished attics, vaulted ceilings, interior soffits, etc. To determine how leaky your structure's envelope is and how best to tighten it, it's best to hire a building performance auditor to use an infiltrometer (or blower-door) and infrared (or thermographic) camera to identify hidden gaps, holes and voids. Ask for the photos so you can methodically address them.
There are many types of insulation. Choosing which one to use depends on various factors, including how the building was constructed, available headroom, costs and whether you prefer a DIY product. The most commonly used products in our area are spray foam, Fiberglas and cellulose. Learn more here.
Spray foam insulation can be applied on the attic floor or, better, the underside of the roof. Choose a product without cyanate compounds or cover with drywall.
Fiberglas comes in batts, as shown, or loose fill. It usually is made partly of old glass bottles and jars.
Cellulose is a loose fill product and blown onto attic floors and into walls. It's made of recycled newspapers, treated with boric acid as a flame retardant.