A Guide to Proper Attic Insulation

This guide to proper attic insulation includes tips on vent areas, gaps, right side up, puckers, loose or compressed insulation, animal nests, mold, mildew and rusty nails.


Although installing insulation in an unheated attic is usually a fairly straightforward job, a variety of pitfalls await the unwary.


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Thinking about putting some (or some more) insulation in your attic? This guide to proper attic insulation can help. 

A Guide to Proper Attic Insulation

Although installing insulation in an unheated attic is usually a fairly straightforward job, a variety of pitfalls await the unwary. So, with cold weather approaching in most parts of the country, we present this Chamber of Horrors of attic insulation mistakes, maladies, and oversights. Look it over, and then pay a critical visit to your own attic—if you dare.

Vent Areas Blocked in the Eaves 

Don't push batt or blanket insulation past the top plate at the end of joist runs . . . or fold it back and up between the rafters . . . or pour fill insulation into such areas. If you do, you'll obstruct the flow of air from soft-it vents (or, in older homes, from gaps between the outer wall and the roof). In fact, if your roof's pitch is steep enough to allow access to the eaves, you can install a slanted-board baffle at the end of each joist run to prevent insulation from clogging the area.

Holes and Gaps in the Attic Floor 

Fill in the extra space around openings where pipes, ducts, and wires enter the attic floor, using unfaced fiberglass or caulk. Caulk nail and drill holes, as well.

Recessed Lighting Fixtures Covered With Insulation 

Recessed lighting fixtures that are covered with insulation become extremely hot and present a serious fire hazard. So maintain a minimum clearance of three inches between recessed fixtures and any kind of insulation. Be particularly careful when using poured or blown-in cellulose, which over time can drift onto recessed fixtures. Yes, open spaces do allow some heat to enter the attic, but the cost of the wasted heat is nothing compared to that of a fire. Furthermore, there is an alternative: Replace the fixtures with flush-mounted lighting.

Vapor Barrier Placed Wrong Side Up 

Install foil- or paper-faced insulation with the facing side down. If the facing is up, moisture will become trapped in your insulation and turn it into an ineffective mess. If you use loose-fill insulation, you may need to install a vapor barrier of polyethylene sheeting between each joist run before you pour the material, or to coat the interior ceiling below with a vapor-retardant paint.

Faced Insulation Used for a Second Layer 

If you decide to add a second layer of insulation, use only loose-fill or unfaced fiberglass. Placing a vapor barrier on top of the original insulation will trap moisture and render the first layer ineffective. If you can buy only faced bans or blankets in your area, peel the backing off before installing the new insulation over the old.

Possible Overinsulation 

If you already have at least six inches of insulating material in your attic, you'd probably be wise to take other energy-saving measures (caulking and weather-stripping doors and windows, installing insulation around heating ducts and in unheated crawl spaces, etc.) before investing in a second layer of attic insulation. Many energy analysts say the first four to six inches of attic insulation are ten times more cost-effective than the second four to six inches; in other words, for every fuel dollar you saved when you installed the first layer, you'll save only about a dime by adding another layer.

Insulation Compressed and Stuffed Into Tight Areas 

It's the air trapped in insulation, rather than the material itself, that resists the passage of heat. When laying batts or blankets in place, push down on them only lightly—just enough to get a snug fit between joists. In areas where there is cross bridging, or where the space between two joists is unusually narrow, take the time to cut the insulation to the size needed; don't just stuff it into the space. If your attic has a lot of cross bridging or hard-to-fit spaces, fill insulation may be a better choice than bans or blankets.

Gaps and Puckers in Batt/Blanket Insulation 

Be sure to install batts or blankets with care, leaving no open space on either side or at the point where two sections butt together. Before laying the insulation, look for rips and tears in the facing and repair them with tape.

Loose Fill Insulation Near Attic Fan 

Poured or blown insulation is subject to shifting and will almost surely drift out of place when used near an attic fan, creating bare or sparsely insulated floor areas. Use batts or blankets in such areas instead.

Animal Nests 

If you hear critters scurrying about in your attic, they've probably set up housekeeping somewhere in your insulation—and may very well have cleared sizable, heat-leaking openings in the material. Evict the offenders, repair the damage, and try to find and block or screen off all entryways.

Mold and Mildew, Rusty Roofing Nails 

These symptoms suggest that your attic isn't adequately ventilated—a condition that causes insulation to become wet (and therefore worthless) and, ultimately, leads to wood rot in roof decking and framing. Replace any matted insulation, check to be sure all vents are clear, and install additional roof or gable vents if necessary.

Most experts recommend a minimum of one square foot of free vent area for every 300 square feet of attic floor space. Ideally, 50% of the venting is positioned low at the eaves, and the remaining 50% higher, at the roof, ridge, or gable ends. If you have air-conditioning or live in a humid region, or if you've elected not to use a vapor barrier between your ceiling and attic insulation, you'll need twice as much venting—a minimum of one square foot for every 150 square feet of attic floor.

Uninsulated Attic Hatch 

Before you cast an approving eye over your new insulation job and climb out of the attic, be sure to cover the hatch itself with rigid foam board insulation or a leftover fiberglass bats, cut to size and glued in place. It's also a good idea to put adhesive-backed foam weather stripping around the inside of the molding that the trap door rests on when it's closed, or to simply seal around the door with duct tape.

Installer Tired, Impatient, Uncomfortable 

Insulating an attic can be hot, unpleasant work in cramped quarters, but since the care with which you install your insulation is as important as the insulation itself, it pays not to rush through the job. To make things easier, insulate only on cool, cloudy days. Fear gloves, loose-fitting clothing, a dust mask, and if you wear contact lenses-safety goggles. Take frequent breaks. And when you finish for the day, take a hot shower followed by a cold rinse, to flush insulation fibers from your skin and pores.

For an overview of the various kinds of home insulation and their applications, see "Know Your Insulation" on pages 54 through 56 of MOTHER EARTH NEWS NO. 78. An excellent and sensible manual on cost conscious home energy conservation is Stop Burning Your Money: The Intelligent Homeowner's Guide to Household Energy Savings, by John Rothchild (Random. House; $15.50).  

Is Your Attic Well Insulated? Here's an Easy Test

Researchers at the Center for Energy and Environmental Studies at Princeton University have devised a simple way for you to determine whether excessive amounts of heat are leaking into your attic.  

Climb up into your attic on a cold winter evening and leave a thermometer there for an hour or so. Then note the reading and compare it to [1] the outside temperature and [2] the temperature downstairs in your living quarters. If your attic is more than 10 degrees Fahrenheit warmer than the outdoor temperature, or if it's closer to the temperature inside than outside, you can figure something's amiss.  

That doesn't necessarily mean, however, that you don't have enough insulation in your attic—any of the other problems outlined in this article could be the source of the heat loss. Check for installation flaws and heat leaks first—then consider adding new insulation.