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The living room of my '20s house has a cathedral ceiling (a trapezoidal ceiling). Most of it is accessible from the attic and insulated with fiberglass batts. However, on the right side (and a tiny bit on the left) is covered by a roof and there is no insulation. I can see the lack of insulation in this part clearly with the thermal camera.

This is a view from the attic and the arrow pointing at the section where part of the cathedral ceiling is covered by roof:

Attic

And this is a thermal image from the living room:

Thermal

There is a narrow cavity between roof and ceiling (maybe 3 inches thick). First I thought I could cut a piece of foam-board insulation and stuff it through but this is very hard as there nails and other obstructions coming from the roof. Using rock wool or fiberglass batts is even less realistic.

Hence I got the idea getting just a bag of blown-in cellulose insulation and manually throwing into this cavity. I do not want to rent a blower because it's expensive and it's really just this small section on the picture.

I am in climate zone 3 (California, Bay Area). There is some humidity in winter (with ocean breezes and days not long enough for the sun to dry) but I wouldn't say "humid" compared to the Southeast or other places.

Is there any concern with my plan?

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There's probably no concern with this plan. Moisture can still migrate fairly freely (as it does in a house of this era). You stand to gain at least a bit of thermal insulation. Do your best to shred and fluff the cellulose, as it performs best when lofted.

I would seriously consider blowing a layer over your fiberglass while you're at it, though. I can see framing showing between the batts, and what you have is likely barely adequate anyway, by modern standards. This is a fairly easy job and would show significant improvement. Energy efficiency is a game of averages, after all.

By the way, many stores that sell the insulation loan you the equipment for no additional cost (only a refundable deposit). You just need a stout assistant to feed the hopper. (Protip: set it beside the vehicle or trailer carrying the insulation to reduce lifting.) Protect your lungs from irritation with a basic mask fit well.

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  • Thank you! Would you mind commenting on: (1) No concerns in terms of weight? It is a lath and plaster ceiling. I would assume not, but just in case. (2) On the additional layer, how would I then enter/use the attic for maintenance? I can only step on the trusses and now it's clear where I can step (=between the batts). Also, how high would you suggest to blow an additional layer, if any? Commented 17 hours ago
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    I can't comment on weight, knowing almost nothing about your structure, except to say that if it hasn't been a problem this far it probably won't be. I'm not sure what maintenance you expect to need to perform in your attic. Unless you have HVAC equipment up there that requires servicing, it should be a once every 5 years rare occurrence. In that case you still step on framing, but you might have to go by feel. There are charts all over the internet with recommended attic insulation our values for every climate zone. You want to look and make your decision. Commented 13 hours ago

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