My house attic is extremely under insulated. Its only about an r-11 right now. It gets very humid on the first floor in summer. I am planning on adding another r-19 blown in cellulose. I would do more but I can't afford it right now. Do you think I will noticed a difference by adding this?
Yes if there's proper roof venting.
Really need to go back and add your location to your profile for any useful advice.
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You need soffit and a ridge vent. Also baffles to hold back the insulation so it does not block the air flow to the ridge vents.