Insulating Basement Walls

Basement Wall With Cinderblock

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A crucial step in transforming your basement into a comfortable space is controlling its temperature. Basements are already temperature-controlled—to a certain point—because the surrounding earth acts as a natural insulator. But you'll need more basement insulation than that to keep temperatures fully in check.

Several insulation methods are available both for pre and post-wall-framing situations. Any of these basement insulating techniques will keep you warm or cool, depending on the season. You'll also save on energy bills because your artificial heating or cooling needs are reduced.

Insulating Basements With Rigid Foam Sheets

What Are Rigid Foam Sheets?

Rigid foam sheets are large sheets of expanded polystyrene foam (XPS). Found in most home centers, sheets come as large as 4 feet by 8 feet. Typical thicknesses range from 1/2-inch to 2 inches, imparting R-values from R-2 on up to R-10. Most sheets are vertically scored. This allows you to snap off sections of foam board instead of cutting.

When to Use Rigid Foam Sheets

Rigid foam sheets are best for DIYers who are insulating basements prior to framing the walls. Rigid foam also works well when the concrete or block walls are smooth and flat. Rigid foam insulation is easy to use. In addition to scoring, it can be cut by an ordinary wood saw.

Rigid foam sheets are a good basement insulating choice when it is important to contain costs. The cost to insulate 32 square feet of basement walls with rigid foam at 1-inch thick will cost between $20 and $30.

How to Install Rigid Foam Sheets

Rigid foam sheets are installed either horizontally or vertically. Maintaining large sheets is valuable for keeping the insulation as seam-free as possible.

Add adhesive to the backs of the sheets, then nail them into place with nails fitted with washers or capped nails. Cover seams with tape. Fill large holes or cracks with low-expanding spray foam.

Insulating Basements With Spray Foam

What Is Spray Foam?

Spray foam insulation is a polyurethane foam that is shot into the cavities between basement wall studs with a hose and gun. The foam expands upon contact. After drying, excess foam can be trimmed away.

When to Use Spray Foam

Use spray foam insulation on basement walls when the walls studs have already been installed. Spray foam is best when you need continuous insulation with no gaps to permit air infiltration. It is an excellent solution for irregular walls and for walls with many cracks, seams, and joints.

How to Install Spray Foam

Spray foam is best installed by qualified technicians. Some complete do-it-yourself spray foam kits are available, but they only cover areas up to 200 square feet. Two-component self-install spray foam kits run about $2 per square foot for 1-inch foam.

Insulating Basements With Mineral Wool Sheets

What Is Mineral Wool?

Mineral wool is an insulating product made by spinning stone or silica under high heat and pressure. The result is a soft, fibrous mat similar to fiberglass batts. But mineral wool is also pressed into dense sheets that perform much like rigid foam insulation. These sheets resist moisture and are appropriate for basement use.

When to Use Mineral Wool Sheets

Use mineral wool sheets when you want to avoid using plastics such as XPS rigid foam or spray foam. Mineral wool is an all-natural product made from recycled substances. Unlike rigid foam's low melting point, mineral wool is nearly fireproof under normal household conditions, with a melting point of 2,150 degrees Fahrenheit. Mineral wool sheets 1-1/2 inches thick provide an R-value of R-6.

Mineral wool sheets are significantly more expensive than rigid foam sheets. Thirty-two square feet of mineral wool sheet insulation will run over $100. This makes mineral wool sheets three to five times more expensive than rigid foam.

How to Install Mineral Wool Sheets

Mineral wool can easily be cut with a kitchen knife or a saw. Mineral wool sheets install in large sections, like rigid foam insulation.

Typically, mineral wool sheets are nailed to the concrete basement wall with nails fitted with washers or nails pre-fitted with plastic or metal caps. Six to eight fasteners are required per board; the wall framing will mainly hold the wool sheets tight against the wall.

Insulation Methods to Avoid in Basements

Insulation typically used above-grade (that is, on upper floors) should not be used for insulating basements. Fiberglass and mineral wool rolls and batts, cellulose, and denim insulation are not appropriate. These materials are permeable and can quickly wick up moisture. In trapped, moist conditions, these insulation products can develop mold and mildew.

Also, this can rot out the wood framing studs and the top and bottom plates. The back of the drywall will also pick up the moisture and develop mold.