eli5: home insulation question

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If home insulation is so great (and no argument that it is), why is my attic 90% air above about a foot of insulation? Why isn’t the insulation 2 or 3 or even 8 feet deep? It works to keep heat out just as much as it keeps heat in, right?

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Do you mean that the insulation is on the floor of the attic? If so then the attic itself is not insulated but the rest of the house is. Heat rises

You could stuff the loft space with insulation. But lifts are often used for storage, and there are benefits to allowing air to move in the roof structure; if there is any kind of leak in your roofing materials having sure free moving allowsthe roof structure to dry.

Insulation is great, but there is diminishing returns. And insulation is not free. So at some point it is not worth adding more insulation. The heat is not going through the insulation you have so why spend money on more? Especially if you have areas with little or no insulation most of the heat will go through there. It is a waste of money to buy more insulation in your attic if most of the heat transfer is in the joint between the roof and walls.

Insulation is installed according to a minimum depth that depends on your location. For instance I am in zone 2 and requires 8 inches of R-30 insulation. I installed 12 inches because I had extra. You can add any extra you want.

https://images.app.goo.gl/tNPEe8ZxwAfqWW4z7

You can’t fill the entire attic with insulation because you need to have airflow. Without adequate airflow you get mold. You also need to have room to move around to deal with things like rodents or other infestations. In many places the furnace is in the attic, or the air handler for the AC/heat pump, as well as all the ducts for the HVAC system, and you need to have easy access to all of those.