for Cold Climate Housing and much more

Last Updated: , Created: Thursday, October 14th, 1999


Any insulation will work in an attic.

It is easy to insulate an open attic, so expensive specialized insulations are not necessary -- anyway, it is supposed to be dry up there. My preference is loose-fill cellulose to the top of the joists (if your ceiling is sturdy enough to hold the weight) with fiberglass batts on top, laid across the beams.

The loose-fill cellulose will easily and effectively fill all the difficult little spaces that often get left out with batts (under and around wires, under old wooden cross supports, and all the corners) and block air movement more than all other inexpensive insulations, increasing the effectiveness of the air barrier. It is easy to level off using the top of the joists. Because this material tends to settle a bit (and we don't want an air space between two insulating materials) I would use a "raised rake" (a board with inch-high feet set wide enough apart to ride on the joists) that will leave the cellulose slightly higher than the joists; it should settle a couple of months later. Adding more cellulose than that to an attic is heavy and difficult to spread. Exposed loose fill can blow around in the attic drafts of 5 mph or more, a wind speed not uncommon in attics.

Glass fiber or mineral wool batts can be easily rolled out (if you can get the roll into the attic) or lay out the batt type. They should be placed tightly together, across the joists, with the joints between the batts lying over the cellulose so the wood is well insulated and does not provide a thermal short circuit. The batts protect the loose fill from flowing and can be piled to any height with no weight problem. Use open-faced batts with no paper backing or slit holes in the paper to avoid condensation.

In general contractors will not use two different insulations -- it is just too much trouble. And with modern truss roofs, there are so many obstructions over the ceiling joists that batts often leave large gaps as they go around the vertical members of the trusses. With these modern houses, blown in insulation to the full depth required is probably the best way to go. But it is critical that the installer walk all over the attic during the installation. If he just shoots from a vent hole or from an access hole, there will be very damaging empty uninsulated spaces behind every 2x3 he shoots at.

Blown-in or poured rock wool is almost as dense as cellulose, fills corners well and although more costly than cellulose it is still inexpensive. It is more readily available than cellulose simply because contractors make a larger profit with it.

Blown-in or poured glass fiber had been modified in the early 1980's to make it more dense in an effort to correct a problem of cold air currents being able to dive down from the attic into the insulation -- negating all insulating effect for as deep as the wind could blow. This wind significantly reduced the effective insulation. Also it is very easy to fluff the blown-in glass fiber insulation too much while spraying -- filling the attic but not giving full insulating value. Attics with these problems have been saved by applying a few inches of a more dense material over the old glass fiber. Today's glass fiber, properly sprayed, will work in an attic but the contractor's skill is extra important with this product.


Keywords: Insulation, Attic

Article 846