Learn the Basics of Home Insulation

Updated: Mar. 22, 2024

You can save energy and reduce your carbon footprint by insulating your home. Get started with tips from an expert with years of building experience.

You wouldn’t think of going outside in freezing weather without the proper gear. But when you come home and switch into lighter clothing, what’s protecting you from the elements? That’s right … your home’s insulation.

It’s no secret home insulation keeps you comfortable while reducing your heating and cooling bills. That’s why it’s a must when building a new home or renovating an existing one. In most cases, installing insulation is a DIY job.

But Reuben Saltzman, CEO of the Minnesota-based home inspection service Structure Tech, says insulation by itself won’t work if you have gaps in your building envelope where air can pass through. “Air leaks are the cause of 70% of problems in attics, and are more important to address than insulation,” he says.

Once you’ve taken care of the gaps, it’s time to decide which type of insulation to use, how much you need and where to put it. Here’s a primer to get you headed in the right direction.

Types of Insulation

As counterintuitive as it seems, air is one of the best insulators there is, as long as it remains static and isolated. That’s why most insulation products are lightweight, porous and even fluffy. They hold small cells of air, separated from each other so heat can’t pass from one cell to another.

Here are six common insulation options you’ll find at building centers:

  • Fiberglass: Available in pre-cut batts, rolls or loose fill, fiberglass insulation is spun from ultra-thin glass fibers. Faced batts and rolls come covered with a fire-resistant Kraft paper backing, which acts as a moisture barrier; unfaced batts lack this backing. Batts are cut to fit standard stud spacing (16 inches) and joist/rafter spacing (24 inches).
  • Cellulose: Available only as loose fill, cellulose is manufactured from bits of recycled newsprint and other paper and treated with boric acid to resist insects and fires. You need special equipment to blow it, which is usually available to rent.
  • Mineral wool: Also known as rock wool, mineral wool comes only in unfaced batts. It’s fire- and mold-resistant, insulates as well as fiberglass and installs easier, but it’s heavier. It’s a good choice for basements, especially furnace rooms and subfloors.
  • Rigid foam: Made of polystyrene, this comes in four-by-eight-foot sheets of thicknesses ranging from one-half to two inches. A reflective foil coating on both sides prevents heat from radiating through. It’s most often used to insulate cement walls in basements.
  • Polyurethane spray foam: Inch-for-inch, this is the best insulator of all. It’s available in open-cell or denser closed-cell formulas and acts as its own moisture barrier. Installation requires special equipment, and it’s messy, so it’s best left to pros.
  • Pipe insulation: This consists of foam-rubber tubing with a split along its length that lets you easily fit the insulation around the pipes.

What to use where

  • Attics: You can use pretty much any type of insulation in an attic except for rigid foam, but Saltzman recommends blown cellulose. “It’s the most cost-effective for new construction in Minnesota,” he says. That’s no doubt true in other states as well.
  • Walls: Fiberglass batts are sized to fit exactly between wall studs, so installing them is easy. The paper backing of faced batts extends an inch beyond the insulation, making it easy to staple to stud faces. Saltzman, however, has two reasons for recommending the unfaced variety and covering it with a plastic moisture barrier: The moisture barrier will be tight and continuous, and you’re more likely to fill the entire cavity and leave no gaps.
  • Basements:┬áBasement insulation needs to be moisture-resistant, so mineral wool and rigid foam are the best choices. Use mineral wool to fill wall cavities, rigid foam to insulate bare concrete walls and pipe insulation to cover exposed water pipes.

How Much Insulation Do I Need?

The R-value of an insulation product measures its resistance to heat transfer, and it’s usually measured per inch. The R-value for cellulose is roughly 3.5 per inch, so you’ll need four inches of material to achieve a total R-value of 14.

The R-values you need for your walls and attic depend on your climate. Energy.gov publishes a chart that details recommended wall and attic R-values for every climate zone in the U.S.

It’s best to stick to these numbers and not overdo it. Adding more insulation than you need can lock in moisture by preventing air flow and do more harm than good.

Installation Tips

Seal gaps

Before you add any insulation, be sure to seal gaps with caulk or spray foam from a can. It’s especially important to seal gaps in attic floors, especially near walls and around lighting fixtures.

In the basement, be sure to seal the ends of floor joists where they meet the rim joists.

Cut batts to fit tightly

Batts are pre-cut to fit exactly between studs with standard spacing, but you may have to squeeze them into smaller spaces in corners and around windows and doors. The insulation works best if it fits without being compressed and stuffed. You can cut batts with a utility knife.

Install attic baffles

When you blow insulation into the attic, avoid covering the soffit vents or the attic won’t breathe, causing mold and moisture problems. Baffles are simple cardboard or plastic barriers stapled to the rafters above each vent to keep insulation away.

Don’t Cover Attic Insulation

You need a moisture barrier (typically plastic sheeting) between attic insulation and the ceiling drywall, but you should never cover the insulation from the top. If you do, it won’t breathe and condensation will form, causing it to clump and become moldy.

About the Expert

Reuben Saltzman is CEO of Structure Tech, a home inspection company serving Minneapolis/St. Paul, Minnesota and surrounding areas. He’s been remodeling homes for most of his life.