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I'm preparing to insulate my attic and need to know how much is necessary and how much weight the ceiling can support.

I live in San Diego (mild/warm climate). The house was built in 1969. Currently there's no insulation in the attic. I've airsealed, insulated the ducts, put in rafter vents.

Joist spacing is 24". Gypum board thickness is 5/8". I assume the ceiling, like the walls, is nailed not screwed in.

My plan was to blow in 15" of cellulose to get me around R-49. My concern is the weight of the insulation could cause the ceiling (original, as far as I can tell) to bow. The table for the insulation shows the "Minimum Weight" for R-49 is 1.591 lbs/sqft:

  • R-13 0.34 lbs/sqft
  • R-19 0.519 lbs/sqft
  • R-22 0.613 lbs/sqft
  • R-25 0.710 lbs/sqft
  • R-30 0.878 lbs/sqft
  • R-38 1.164 lbs/sqft
  • R-49 1.591 lbs/sqft
  • R-60 2.059 lbs/sqft

For the weight gypsum can support I found this link which quotes weight limits:

1.3 lbs/sqft for 1/2" sheetrock w/ 24" framing

2.2 lbs/sqft for 5/8" sheetrock w/ 24" framing

How much insulation could my ceiling support w/o risk of bowing?

Is R-49 even warranted in this mild climate?

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    I'm guessing R49 is vast overkill, especially if the rest of the house is a heat sieve (e.g. no storm windows, no insulation in walls, etc). – Daniel Griscom May 17 '16 at 2:31
  • R49 is beyond the minimum required by code but within the DoE recommended range of R30-R60. I'm tempted to go this high because I want to do this only once and the cost between R30 or R38 and R49 isn't that great. – TheMightyQuinn May 19 '16 at 18:49
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5/8" drywall on 24-inch centers is standard practice across the industry. Unless you have some weird early-version panels with poor sag resistance, 15" of cellulose will not cause a problem.

I'm with Daniel Griscom, however. R-49 is in the ballpark of what we install in new homes here in frigid Minnesota (regularly seeing -30F in the winter). It's almost comically excessive in your case. You should look for something in the R-20 to R-30 range as your sweet spot (bang for the buck, ideal payoff, whatever). If you're feeling chilly when it gets way down in the 60s like it does there, go nuts. Just don't expect to see the value in your "heating" bill. :)

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