There are solutions to squeaking, even if carpeting covers your hardwood floor, or there is no access to the floorboards from below.
If you have hardwood floors, it is likely that, you will someday have squeaks. Although squeaks can be caused by improperly fastening the floor to the subfloor, more often squeaks are not caused by defects in the wood or poor workmanship. Most squeaks occur as a result of the normal cycle of seasonal contraction and expansion. From dry winters to humid summers, and back to dry winters, this seasonal change causes fine cracks to develop during drier conditions and wood to swell during humid conditions. When the wood swells it causes the boards to rub together, resulting in squeaks.
There are several options for silencing squeaks:
The easiest solution is to apply lubrication. Identify the squeaky floor boards and then apply a generous amount of powdered soapstone, talcum powder, or powdered graphite between the boards. Place a cloth or towel over the area and work the powder in by stepping on the towel. One or two additional applications may be necessary.
Another solution is to drive glazier points, those small triangular pieces of metal used in the installation of window glass, into the spaces between the boards. Use a putty knife to wedge them in place.
If your floor has a wax coating (not urethane or a varnish), then work liquid wax in between the boards.
It may also help to drive finishing nails through the floor into the subfloor. Start by pre-drilling pilot holes to prevent the wood from splitting. Use a gauge bit that is about the same or slightly smaller than the nail and start the pilot hole no closer than 1/2 inch to the edge of the floorboard. Angle the drill slightly toward the center of the board. Make sure that you don't drill into the subfloor, or the nail won't hold as well. After the pre-drilling is complete, put downward pressure on the floorboard as you drive the nail into the pilot hole. Use a nail set to countersink the head of the nail, and fill the hole with wood filler to match the color of the floor.
You can also approach the problem from below:
Sometimes the squeak is not located in the floor boards but rather in the subfloor. One method for solving this problem is to carefully pound a shim into the gap between the subfloor and the joist in the area of the squeak. The best shims for this purpose are exterior wood siding shingles because their taper is more gradual than the pre-made shims found in most stores. You may want to apply construction adhesive to the seam between the joist and subfloor as well as the shim before hammering it in place.
Another technique is to apply construction adhesive to the seams where the joist meets the subfloor. Then, drive a screw on an angle through the side of the joist into but not through the subfloor. To get a snug fit, after you have applied the adhesive but before you have driven the screw, ask someone to stand on that section of the floor in the room above, then drive the screw.
If you suspect the squeak is coming from the subfloor and not the floor boards, you may want to try these ideas. Install a bridge, a small square piece of 3/4" plywood to join two pieces of subfloor. Apply construction adhesive to the one side of the plywood before placing it against the subfloor, and then drive screws through the block into but not through the subfloor. A block involves installing a short piece of 2x3 or 2x4 along side of a joist. Apply construction adhesive to one edge and one side of the block and position the adhesive side of the block against the side of the joist, with the adhesive edge against the bottom of the subfloor. Then, with someone standing on the floor above, drive two screws through the block into, but not through, the subfloor and into the side of the joist.
Although you may not be able to prevent squeaks entirely, there are steps you can take to reduce the likelihood that they will occur. By maintaining a consistent humidity level in your home you will reduce the impact of seasonal change. A simple way of accomplishing this is by installing a humidifier. Maintaining consistent humidity level year 'round will not only help your floors, but will be healthier for your family, too.