Why Does Old Insulation Need to be Removed from the Attic Before Installing Spray Foam?
The roof deck should be insulated for several reasons – protecting the roof, giving your attic a space that is conditioned, and ensuring there are no drafts in your attic. When the roof deck is insulated, the air moves into the attic, hits the roof where it cools down, and then falls back down. This means your attic is now a conditioned space, which means you heat and cool that area. You need to have air moving freely in and out of that area.
If there is a layer of fiberglass or cellulose insulation on the attic floor, as the air from your living space heats up and rises, it will want to move into your attic. You want to let that air get into that space without the hindrance of the old insulation.
Airflow circulation is crucial in your house because your furnace, AC unit, and mechanical ventilation move air throughout your home. You want that air to move freely because of the cold air intakes in your home that takes in the cooler air inside your house, running it through the furnace where it warms the air back up and blows it back out.
If the air isn’t moving freely through your home and if it isn’t circulating efficiently, those cold air returns have to work harder to suck air through your house. This means that the vents that are blowing hot air have to force the air harder.
It also makes your appliances work harder. They will need more power to suck the air and blow it back into the house. Your appliances will be working harder, thus wearing themselves out faster. They will be working more minutes and hours, so you will be paying more to run them.
This lends itself to one of the main reasons you want to remove the old insulation from the floor of the attic when insulating the roof deck with spray foam. The old insulation hinders the airflow throughout the entire house and attic.
Another reason you will want to remove the insulation on the attic floor is that if the airflow is hindered, there will be a difference in temperature from the attic to the rest of the house. For example, in the summer, if it is 85 degrees outside, it is around 100 degrees in the attic, and the house is about 70 degrees. That is about a 30-degree difference between your living space and your attic.
Any time there is a temperature difference, there is a potential for condensation – the higher the temperature difference, the higher the probability of condensation because you have a bigger window for a dew point to fall into. This also has to do with the humidity in your home.
The old insulation on the floor of the attic will almost always be cellulose or fiberglass, which are both fibrous materials that are both proven to retain and hold moisture and foster and invites condensation along with those moisture issues. This condensation can then move into the drywall of your ceiling and the trusses inside the attic floor. The big issue is this creates the perfect condition for mold to grow not only in the insulation, but in your ceiling and other areas of your attic. If this mold is allowed to form and continue to grow, the cold air returns will pull that contaminated air throughout your home. Here is how that happens – the hot air rises and will stop at the drywall because the old insulation hinders it. As the air moves across the now contaminated ceiling, it will move to the cold air intake and get recirculated throughout the house.
The bottom line is the old insulation needs to be removed because it hinders the airflow into the attic, it can promote mold growth, and it is dirty.