Always use unfaced fiber glass insulation with no vapor retarder when adding another layer to existing
blanket or loose-fill insulation. If your attic has existing insulation but no vapor retarder, you will need
one square foot of free vent area for each 150 square feet of attic floor area. (To add ventilation, consider
a combination of soffit and roof vents.)
My home has about six inches of fiber glass insulation in the attic, and I'm planning to add
more. Should I use faced insulation?
No. Use only unfaced fiber glass insulation with no vapor retarder. A facing acts as a vapor retarder, which
helps reduce the amount of moisture entering a wall, ceiling, or floor. In
heating climates, your original layer of insulation should already have a vapor retarder facing the "warm-in-winter"
side (living area) of your home.
If you add a second vapor retarder with another layer of faced insulation, any
moisture that does get through the first layer may condense on the second. This might cause water stains on the
ceiling and could lead to severe structural damage.
Install the second layer of unfaced insulation perpendicular to the first, covering the wood ceiling joists to
reduce heat loss through the wood. (NOTE: Apply the second layer of insulation perpendicular to the first
layer only when the joist cavity is full. Otherwise, apply the second layer right on top of the first layer,
between the joists.)
How do I add additional layers of attic insulation to a joist cavity that is only half full?
If joist cavity is only half full and the recommended insulation level in your area is R-38, fill the joist cavity flush with
the top with the appropriate thickness of fiber glass insulation. Then lay a second layer of unfaced insulation
perpendicular to the existing layer to give a total of 12" (or an R-value of 38) in the joist cavity. Covering the
attic joists with the second layer will reduce heat loss through the wood.
What are radiant barriers? Are radiant barriers an effective way to upgrade my home's
insulation performance?
Radiant barriers recognized by energy codes come in three basic types: single layer film material, multilayer or
bubble film material, and single films applied directly to the underside of roof (plywood/OSB) sheathings.
Radiant barriers are not insulation, and by definition, have no R-value. However, there are some radiant barrier
products that have entrapped air spaces (bubble pack or multilayer films) where an R-value may be available for the product.
In these situations, the product is operating as an insulation as well but the winter R-value (heat flow up) is very small.
Often the manufacturer of these product types will list the summer R-value (heat flow down) because it's so much higher
than the winter time (heat flow up) value.
Testing has shown that it is more cost effective to add insulation than a radiant barrier.
Additionally, the actual reduction in heat flow achieved through properly installed radiant barriers is
substantially less than that claimed by some manufacturers. In cold weather, radiant barriers may
reduce beneficial heat gains from the sun; because of this, they are mainly sold in areas with warmer climates.
If I install a vapor retarder in my attic, do I still need ventilation?
Yes. Even with a good vapor retarder, proper ventilation is considered essential to prevent damaging
condensation. Eave vents openings at the roof overhang combined with a ridge vent, roof vent, or gable vents,
are effective ways to create a positive movement of air in and out of the attic. As a general rule, when a vapor
barrier is used, 1 sq. ft. of free vent area per 300 sq. ft. above floor area is recommended. When no
vapor retarder is used, 1 sq. ft. of free vent area for every 150 sq. ft. of attic floor is recommended.
In the winter, I notice a wet spot on my ceiling where it meets the inside of the exterior wall.
My attic area above this room is insulated. What might be causing this moisture problem?
Check to see if your attic insulation completely covers your ceiling area. Attic insulation should extend out
over the wall top plate, but not over the eave. You may have a cold spot caused by a lack of insulation over
this area, or you may have a ventilation problem or "ice damming".
Insulation should extend out over the exterior wall, right up to the roof line, but should not cover the eave
vents. Install eave baffles wherever there are eave vents to assure air flow. For additional ventilation, install
roof vents.
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