Intro - Materials - Safety - Prep - Attic Preparation - Ventilation - Rafters - Floors
Insulating the Attic Floor
Margin of Error: Fill all cavities and plugholes exactly
Placing the fiberglass insulation between the attic floor joists effectively insulates the lower portion of your home and avoids your using added energy to heat unused attic space. Use this method if you don't plan to convert your attic into living space.
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 ca
reful not to compress the fiberglass. If there are soffit vents, leave a space at the eaves for air circulation.
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The greatest concern comes with infiltration of the heat from the rooms below up through any cracks that may occur around the insulation. Therefore, be very careful about the way you install the insulation around any obstacles in the joist space. These include plumbing, piping, heater ducts, chimney stacks, or the bridging. Cut the insulation to fit snugly around the object.
Note: 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. A 2" 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 3" 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.
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 will have already taken care of any penetrating problems, your main concern with this second layer is that the batting fit good and snug, side by side and end to end. Start this 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 opening hatch door to your attic as well.
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. A trouble light is
needed to help you see that hard-to-reach places are being adequately filled with the cellulose. Blow in the cellulose to 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 eaves' vents, take care that you do not cover any ventilating areas.
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. I've found that vinegar makes an effective rinse when I bathe or shower after working with fiberglass. It almost eliminates the itching, which comes from the small particles of glass left on the skin.
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