Cellulose Attic Insulation Over Fiberglass

Roll or batt insulation.

Cellulose attic insulation over fiberglass.

Whether you live in a warm weather state as i do or in a cooler northern climate it s hard to stay comfortable and keep your energy bills in check if you don t have adequate insulation in the attic. Insulation works better when you add the same type to existing material. The trapped air molecules in the insulation are pulled up through the insulation into the colder attic air. Also cellulose is a dusty product whereas fiberglass blows in pretty clean.

When walls are already finished injecting loose fill cellulose insulation is one of the few ways of adding. The bottom line is that cellulose can burn but fiberglass will not support combustion. This problem has been successfully solved by installing fiberglass batt insulation over the top of loose fill or blown insulation. If fiberglass batts become compressed losing some of their air pockets then this insulation may be more affected by air flow than cellulose which performs well regardless of density.

Loose fill cellulose is relatively inexpensive yet still has an r value of about 3 5 per inch of thickness compared to fiberglass r value between r3 to r4 per inch. Do not tack down roll or batt insulation because compacting reduces its effectiveness or r value. Just piling material up in an attic doesn t always make for a better insulated space. To install fiberglass batts over existing insulation lay the batts perpendicular to the joists so they do not compress the insulation below.

This type of insulation typically fiberglass or cellulose insulation can be added over existing insulation provided it s installed perpendicular to joists. When used in an attic space however both types of insulation are ineffective at air flow across the attic floor such as a strong gust of wind. Assuming your current attic insulation is made from fiberglass and has a value of r 13 you d have to add roughly 10 inches of additional fiberglass to hit r 38. It will however fill in all the gaps that fiberglass batts leave along side of your ceiling joists if you see wood its no good and add the to your r factor cellulose is second best with spray 2 part high density spray foam as first in my book.

When there are fiberglass batts in your attic the cellulose will not draft block the top plates. Loose fill cellulose insulation can settle around and conform to most of the obstructions found in walls and attics. I ve been insulating for over 30 years and we have never added cellulose over fiberglass.