Placing the fiberglass insulation between the attic floor joists effectively insulates the lower portion of your home and avoids wasting energy on heating an unused attic space. Use this method if you don't plan to convert your attic into living space.
Fitting the Insulation Snugly
Begin by unrolling the insulation, paper face down, toward the heated area of the house, between the joists, starting in a corner. Use a stick to tuck it into the comer, but be careful not to compress the fiberglass. If there are soffit vents, leave a space at the eaves for air circulation.
Your greatest concern should be to leave no cracks in the insulation, because these could allow heat from the rooms below to infiltrate the attic. Therefore, be very careful about the way you install the insulation around any obstacles in the joist space. These include plumbing, piping, heating ducts, chimney stacks, or bridging. Cut the insulation to fit snugly around the object.
Insulating Around Heat Sources
Unfaced fiberglass must be used when working around heat sources like a chimney, flue, or heating duct. The paper facing on most insulation is flammable, so a two-inch air space between the chimney and the insulation is recommended. With prefabricated flues and chimneys, check the manufacturer's recommendation.
You can cover all electrical junction boxes (but not electrical fixture boxes), because they do not give off heat. Again, do not distort or compress the fiberglass. Leave about three inches around recessed lighting fixtures for air to circulate and to keep the fixture cool. Wrap pipes separately to cut off air passage around them and stuff scraps of fiberglass into small, hard-to-cover areas.
Adding a Second Layer
If one layer of fiberglass batting between floor joists does not meet the value you need, a second layer of fiberglass insulation can be added on top of and at right angles to the joists.
There is less thermal loss with this method because the joists are covered as well. Your concern here is to avoid trapping moisture between the two layers by having installed a vapor barrier between them. So, if possible, use unfaced insulation for this layer. If not, install the second layer with the paper face down and puncture the paper barrier on this second layer.
Since you have already taken care of any penetrating problems, your next concern with this second layer is that the batting fit good and snug, side by side, and end-to-end.
Start the second layer butted against the bottom of the rafters, beginning in a corner. Continue to install it end-to-end until you get to the center of the floor or near the stairwell. Then, begin again at the opposite side and install to the center again to avoid walking on and compressing the insulation over the joists. Install insulation on the door to your attic as well.
Adding Loose-fill Insulation
Another method is to pour or blow in loose-fill or cellulose insulation up to the joists for an even surface. Then, unfaced or punctured batt insulation can be installed perpendicular to the joist system as before. Use a trouble light to help you see that hard-to-reach places are being adequately filled with cellulose. Fill the joist spaces past the top of the wood framing to achieve a higher R-value. As you work back into corners and around soffit vents, make sure not to block ventilation with insulation. A long straight board will help even out the cellulose. Drag it along the joists to push loose piles of insulation into the spaces between the joists.
After completing your insulation, you may find that your skin itches from fiberglass irritation. Vinegar makes an effective rinse when you bathe or shower after working with fiberglass. It almost always eliminates the itching, which comes from the small particles of glass left on the skin.
Once your insulation is all in place and you've cleaned yourself off, take a moment to step back and appreciate your accomplishment. Over eight different articles, you've properly insulated—and ventilated—your attic. Congratulations!