How to Insulate Tongue & Groove Hardwood Planks on an Attic Floor

Measure the length and width of your attic and multiply the two figures to arrive at the square footage of your attic floor.

Buy enough fiber board underlayment to cover your attic floor. Fiber board underlayment offers better insulation than a foam product due to its thickness. Add 10 percent to the measurements you took of your attic when you place your order to allow for waste.

Allow your fiber board underlayment to acclimatize to the conditions in your attic for at least 72 hours before laying it. Like hardwood floor boards, fiber board underlayment contracts and expands as the temperature and moisture levels change in your home.

Prepare your subfloor. Vacuum the existing floor in your attic and then wash it over with warm water and detergent. Remove any tacks or nails with a claw hammer and use a sander or self-leveling compound to get it as level as possible. Use a bubble level to check how even your floor is.

Lay your fiber boards over your attic's subfloor. Use spacers to leave a one-inch expansion gap around the perimeter of your attic. Cut your boards to shape with a saw when you reach the end of a row. Wear a protective mask while you do so to avoid inhaling any fiber particles given off.

Lay polyurethane sheets on top of your fiber boards to protect your hardwood flooring from rising moisture. Then lay your flooring as per manufacturer's instructions.