If the floor is being built from scratch, it will be much easier to insulate, but this will also work if the floor is wooden and existing. There are only a few tools needed, and it should take about 4 hours to complete, depending on the manpower available.
Gather up a hammer, strong twine or string, large nails, and then go buy that pink insulation that comes in rolls. All items can easily be obtained at any large hardware store. The insulation is the only expensive item, but it’s worth the cost when compared to the electrical and heating bills reduction that will occur.
If the wooden floor to be insulated already exists, it will be necessary to go under the house into the crawlspace. If the floor does not have an outside opening, then a trap door will need to be cut into the floor. Cut the opening at least 3 feet square with a jigsaw. Use a stud finder, or a hammer and an ear to the floor to be sure the cut is in between the floor joists.
Be sure to use chalk or pencil to mark the opening, and a ruler or T-square to keep the lines in check. It is important not to cut the opening across more than two-floor joists, but remember, try not to cause issues with sturdiness in the floor. A good place to cut is in a corner, so the house will remain solid. Some homes have trap doors in the bedroom closet, so that may also be a crawlspace entry option, depending upon how far the floor to be insulated is located from the room with the trap door.
When working under an existing floor, make sure to wear old clothing and eye protection during fiberglass insulation installation. Fiberglass is just that-finely-spun glass that can cut skin easily and can fly into the eyes. A flashlight or droplight will also be invaluable. If the floor space is new, still wear eye protection when using fiberglass, but the clothing worn is unimportant.
Measure out a pattern on the floor joists in a zigzag pattern, diagonally, making a mark about 18 inches across, back and forth between two joists from one end of the room to the other. Using a yardstick will help keep the marks consistent when marking from one joist to the other. Make the marks inside the joists, from the top of the joist to the bottom.
This will help to line up the nails on an existing floor and enables the job to be performed by only one person. Once the marks have been made, take the hammer and place a nail in each of the nail marks on the joists, being sure that the nail heads are on the bottom of the floor joists. Next, take the twine or heavy-duty string and tie it to one of the beginning nails. Now stretch the twine diagonally to the 2nd nail and wrap it one time around the nail’s head.
Then stretch the twine diagonally on to the 3rd nail, which should be attached to the original joist where the twine was knotted. Keep alternating diagonally between the two-floor joists until the opposite end of the floor is reached. When standing to look at the completed row of twine, a zigzag pattern should be showing. Complete the entire room in the same manner.
Once the floor has all the twine installed, it is time to put in the insulation. Depending upon the width between floor joists, usually, 12-18 inches is the width of insulation that should be purchased and installed. Put on work gloves and eye protection before working with fiberglass insulation.
Roll out the insulation, paper side up, to determine the length necessary for one row. Cut the insulation even with the length of the row and set the other insulation aside. Now it is a simple matter of laying the insulation on top of the zigzagged twine. It should fit snugly between the floor joists in one easy rolling movement. Continue this process until the floor is completely insulated.
If this is a new floor, now the plywood flooring can be measured to fit, but not yet attached to the floor joists. After laying down the plywood, walk around the room; if any squeaks are noticed in the flooring, mark a big X on the spots and remove the plywood flooring one piece at a time. As the plywood is removed, measure where the X should go and mark the floor joist in those spots.
Using wood shims, available at any large hardware store, place the shims with a paneling nail at the spots where the floor creaked. Then put the plywood back down and walk around the room again. When all creaks have been eliminated, the flooring can be hammered with nails into place. Now padding and carpeting can be installed for further insulation purposes, or tile can be placed on top of the plywood.
If this is an existing floor, when the insulation is completely installed, either merely replace the trap door that was cut out earlier in the project, or secure it with two hinges on either side and nail a loop of twine on the opposing side for a handle.
Regardless of the type of final flooring installed, the insulation underneath the floor will last at least 10 years, assuming there are no entry spots for rodents into the crawl space. This is an inexpensive and easy do-it-yourself project that should not take more than 4 hours to complete.