Reply to Best way to insulate 100 year old unfinished attic ceiling?

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[quote=Anonymous][quote=Anonymous][quote=Anonymous]I (OP here) really appreciate the discussion.  From what I saw in the article and read online, it is still very hard to know what to do.

We have an 18x10 space in a semi detached house.  100 years old.  No vents.  Typical DC semi detached - brick, front slope of roof covered in terracotta tile, rest of roof is flat with a 10 year old membrane on it.

We have a HVAC unit that provides AC to our upstairs located in the attic.  The attic is totally unfinished with no signs of moisture.  There is no insulation and two 27x38 inch windows facing south.

There is electrical for one light and the HVAC.  We don't have knob and tube up there.

ALSO:  When we redid our bathroom last year, the ceiling went up to the flat part of our roof where there is no crawlspace.  This was loaded with old insulation that looked to be like finely shredded paper.  There were a lot of moths in there and moth larvae on the exposed rafters. The contractor removed the insulation.  We think it might have been original to the house as there were generally very few updates throughout.

Is blown in cellulose the way to go?  Put it in the rafters, put the drywall over it and move on?  Or is this a disaster?
[/quote]

Your HVAC unit in the attic is exhausting hot air in the summer. Your attic will need to breathe. It sounds like your attic is part of the strategy to vent to the exterior. Ergo, your attic is acting as part of the exterior. If you insulate the attic roof, you are trapping the hot air in the attic, that is, hot humid air in the DC summers. I am imagining the cold ceiling surface below will be the sweat layer. Think of what a cold glass with ice does in the hot humid air - it sweats at the point of contact to that hot humid air which is the glass surface.

Why do you want to insulate the attic? To insulate the conditioned indoor air within the house?
If so, it sounds like the insulation should go on the attic floor where the conditioned indoor air is below and hot attic/exterior air above. BUT, I'm a little worried about needing a vapor barrier. Depending on your climate zone, you might need a vapor barrier.

https://buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation[/quote]

This is gibberish. The HVAC doesn't exhaust hot air into the attic. The issue isn't heat, it's moisture. The linked article is good but it doesn't sound like you got it. [/quote]

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