i have roof insulation but if i add more to the required depth the bottom layer will get compressed. Will it still insulate?
Yes. But, remember this; it is the trapped air that causes the insulation to work properly. If you squeeze the air out of it, it is less effecient- will not conserve energy as well. I would not compress it at all.
Yes, but not as well. the air trapped inside it works as an insulator as much as the fill itself. The weight of insulation on top of it will take away some insulating properties, but it's replaced by the insulating properties of the new insulation, so it's a net gain.
Theoretically yes, the insulation is more dense at the base because of the additional weight, but unless you physically press it, it is normal. Use blown-in cellulose, it is cheaper and more efficient than fiberglass batts! You cannot dead-press insulation until you get in the R-50 to 60 range (depends on the manufacturer), they will all give you a maximum depth measurement on the packaging.
Compressed insulation is LESS effective than uncompressed insulation. It will still insulate more than not having it. If you are adding enough to actually compress the bottom layer, you have probably passed the point where additional insulation actually helps.
Simple - make your bottom layer 1" styrofoam sheets- it has a higher R value per depth (1 inch of styrofoam is equal to 4 inches of fiberglass ) and it will not take-up much space and it will serve as a vapor barrier - and it will not compress when covered by fiberglass insulation. (So you can add as much R value as you want - as much as you can afford - with styrofoam sheets because they take-up such small depths in your rafters --- Also, humidity, as on rainy days or sunless winter days, decreases the R value of fiberglass -- weather conditions do not affect the R value of styrofoam sheets.
