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   Articles | How-Tos | Ask the Expert   
Insulation - Insulating an Unfinished Attic

Insulating an unfinished attic
Animation Enhanced


Your attic is the most critical area of your house to be adequately insulated. Since heat rises, a poorly insulated attic allows valuable heat from the living space of your house to escape during the winter. During the summer, heat trapped in your attic can reduce your ability to keep your house cool. In colder, Northern climates, an R-value of 49 is recommended for adequate attic insulation. In warmer climates, an R-value of 30 is recommended. Fiberglass insulation has an R-value of roughly 3.0 per inch of thickness. Cellulose is has an R-value of almost 4.0 per inch.

When insulating an attic, you should also consider how your attic is ventilated. Particularly in northern climates, your goal should be to insulate the living space of the house, while allowing the roof to remain the same temperature as the outside. This means you should allow a free flow of outside air from the eaves of your roof (in through soffit vents) all the way to the gable (out through gable vents, louver vents or other types of roof vents). In areas where insulation butts up against rafters or roof sheathing, use baffles to prevent the obstruction of free airflow.

If you currently have a floor in your attic, you need to pull up pieces of the floor to install the insulation. It will be easier to use a blower and loose-fill insulation to effectively fill the spaces between the joists. If you choose to go with blown-in insulation, you can usually get free use of a blower when you purchase a certain amount of insulation.


  •Beginner - 6 to 8 hours
• Intermediate - 4 to 6 hours
• Advanced - 3 to 5 hours
Make sure you wear a dust mask to prevent breathing in fiberglass and dust. You should also wear eye protection, a long sleeve shirt and gloves.
If the attic does not have a floor, lay planks across the ceiling joists to use as a platform to work from.
Keep all insulation away from ceiling electrical fixtures and fans.
Do not push insulation up against roof sheathing. This will trap warm air in the winter and can lead to icicle problems.
  If you install fiberglass insulation without a vapor barrier, use sheets of plastic stapled in place. Lay the plastic on top of the drywall and staple it to the ceiling joists.
  Get a helper to run the insulation blower machine for you while you handle the nozzle end of the hose.

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1.  Start by installing baffles in areas where you need to hold back insulation. Where ceiling joists intersect with roof rafters, insert a plastic or polystyrene baffle in each rafter bay. Staple the baffles in place. (Figure 1)
2.  Look for electrical fixtures that protrude into your ceiling. Since these fixtures can become hot when in use, you need to keep insulation a safe distance away. The best thing to do is to install baffles to hold back the insulation. Use a 1" x 4" or 1" x 6" to create a retaining wall on either side of the fixture. Keep the baffle about 3" away from the fixture (Figure 2)
3.  Next you need to make sure you have a vapor barrier in place. If you are installing insulation with a vapor barrier, make sure it faces the interior of your house. Another option for a vapor barrier is to take sheets of plastic and lay it between the ceiling joists. Use a staple gun to tack it to the sides of the joists. (Figure 3)
4.  To install fiberglass insulation, start by measuring out each section. Cut a piece to length and lay it in place. Make sure the insulation is tucked snugly into the space with no gaps next to the ceiling joists or baffles. Do not compact the insulation. This will defeat its insulating effect. (Figure 4)
5.  If you are not planning to put in an attic floor, you can lay a second layer of insulation perpendicular to the ceiling joists. (Figure 5) The second layer should not have a vapor barrier because moisture could potentially get trapped in between the two vapor barriers. This second layer will make it easier for you to obtain the recommended R-value for your climate.
6.  If you have an attic floor and need to blow cellulose insulation into the spaces, you will need to temporarily remove some of your flooring. Strategically select pieces that will allow you to access all of the spaces in between joists. Run the blower hose up into your attic. You will need a helper to control the blower. Blow the insulation into the spaces between the ceiling joists. (Figure 6) Make sure you do not fill areas near any electrical fixtures (as discussed above, baffles will help hold back the insulation). Replace the flooring pieces you removed.
7.  Cellulose (or loose-fill) insulation can also be installed in an attic without a floor. You probably will not need a blower since you can easily access all spaces in between joists. Pour the insulation into the spaces and spread it around evenly (Figure 7)

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Shopping List

Materials Needed  
Fiberglass insulation or
Cellulose insulation
Plastic sheets
Tools Needed
Tape measure
Utility knife
Dust mask
Straight edge
Insulation blower
Pry bar

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