Can Blown-In Insulation Be Added to Walls With Existing Fiberglass Insulation?

Typically fiberglass insulation goes in walls and ceilings and cellulose in attics.

Blown-in cellulose insulation consists of shredded newspaper and a variety of chemicals to lower its flammability. Fiberglass batt insulation is made up of tiny fibers of glass. Both fibreglass and blown-in cellulose insulation will insulate your home well but over time their insulating capacity will decrease. Contractors often blow in cellulose insulation over fiberglass batting in attics and even in walls but they have to consider some things before doing so. This might not be a job you'll want to do yourself.

How Insulation Works

.Insulation, whether blown-in cellulose or fibreglass batting, works by trapping air inside the material. The combination of air pockets and solids (insulation) slows the flow of heat through the space, keeping it in your home rather than allowing it to flow out through walls and cracks. The greater the ability to trap air means the higher the R-value (insulating capacity) of the insulation.

For this reason insulation has its maximum insulating value when it is not compressed. Over time, cellulose insulation naturally compresses and loses some of its R-value. It is common to see blown-in insulation added into attics on top of old fiberblass batts but this happens when the insulation level is low and the cellulose increases the overall R-value of the insulation.

Performance

The insulating performance of cellulose and fibreglass insulation depends on whether the insulation is installed properly and in the right spaces. Fiberglass insulation must be cut around electrical boxes and pipes and to add more, you have to remove the drywall, which is inconvenient. Cellulose will conform around electrical boxes and behind walls. If you overfill the wall with blown-in cellulose, you may decrease air flow between the insulation and the walls which might decrease insulating values.

Moisture

Cellulose can cause corrosion on some metals but it will insulate the entire wall cavity and flow around outside studs where fibreglass batting cannot reach unless it is intentionally placed there, which is not usual. Cellulose can take anywhere from 72 hours to 1 year to fully dry. Contractors usually recommend that the wall not be sealed up until the cellulose has time to dry. Consider though, when putting cellulose over fiberglass, that fiberglass insulation can absorb moisture that doesn't drain properly. If the cellulose is not drying, this can lead to issues such as mold, mildew and a lower insulating capacity in your fiberglass insulation.

Do You Need to Add Insulation

An experienced contractor should install cellulose insulation, whether in your walls or attics. It needs special equipment, such as a blower, and the contractor can evaluate your current insulation's performance before adding more. When blowing insulation into a wall, you can't see where it is going which could cause problems later.

Too much insulation isn't a good thing either because it doesn't allow air to flow properly. Heat loss may not be the result of lowered R-value of your insulation either and a professional contractor would be able to determine this for you. Poorly installed or improperly sealed windows and doors, for example, can also cause heat loss. You may not need to add any insulation to your walls.

References

• Trained Eye Home Inspection Ltd.: Fiberglass Over Cellulose in Attic
• Green Energy Efficient Homes: Attic Ceiling Insulation
• Superseal Construction Products: Cellulose Insulation vs Fiberglass Insulation