R50-60 code for attic too heavy for 24 inch OC ceiling joists ?
user-7177985
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I would like to know if my calculations for dense, loose-fill insulation are reasonable and is the new R50-60 code too heavy for 24″ OC ceiling joists ?
850 sq.ft attic with original 1997 loose-fill cellulose R20. Then in 2004 it was topped with R20 loose-fill fiberglass. Now it’s February 2024 and the new code is R50-60 so I need to top up.
This past month ( mild winter ) I have shifted a lot of the insulation to access top plates, brick rain screen, bathroom fan ducting, etc. Consequently all this movement has created a dense mixture of cellulose impregnated fiberglass. After some light, jello jiggling tamping technique to help prevent any air infiltration, I am left with 10 inches. ( perhaps the loose-fill fiberglass was fluffed up when installed in 2004 )
If I calculate this slightly more dense 10 ” base layer to be R3.5 per inch, I need to top up with one of the following :
6 inches cellulose
-or-
9 inches fiberglass
I like cellulose but another 6 inches will add more weight than 9 inches of fiberglass and my ceiling joists are 24″ OC.
Original insulation density and weight:
Installed 1997 Cellulose is 2 lb. / ft3 settled to 6″ over 850 sq.ft. is a weight of 850 lb
Installed 2004 Fiberglass is 1 lb / ft3 at 8″ over 850 sq.ft is a weight of 566 lb
Total weight currently installed is 1416 lb.
Insulation Code Top Up Required :
6″ Cellulose over 850 sq.ft weighs 850 lb.
-or-
9″ Fiberglass over 850 sq.ft weighs 637 lb.
Alternatives ?
Start over with 3 inches / R18 of closed cell spray foam which weighs 2 lb / ft3 , total weight 425 lb.
Start over with 2 inch XPS rigid board R10 which weighs 105 lb for 850 sq.ft.
( purchase price of $3200 not really competitive )
Thank you
Replies
I'm a little confused about you saying you have to meet the new code requirements. Typically new code requirements are not retroactive to existing installations, unless the installation is unsafe or the property is undergoing renovation costing more than 50 % of the assessed value of the property
having said that, upgrading your insulation is normally not a bad idea, however, if your house is warm and your bills not terrible, then maybe the added cost and aggravation isn't worth the added insulation.
just my thoughts
Agreed.
I cannot achieve the new R50 - R60 standard however, I can make an improvement which is limited to the extra weight of more insulation.
Presently my 850 sf ceiling has 850 lb cellulose and 566 lb fiberglass = 1416 lb.
My 5/8 inch drywall on 24 OC joists can hold 2.2lb per square foot. Therefore 850 sf ceiling can withstand 1870 lb. This leaves room for an additional 454 lb.
An extra 3" of cellulose will be 425 lb. R-11
An extra 6" of fiberglass will be 425 lb. and R-15
I was hoping someone knows how to evaluate the R-value of my existing insulation before I add more.
Has my original R-20 cellulose base with R-20 fiberglass top lost some effectiveness since I have been working extensively in the attic shoveling insulation out of the way ? Presently at 10" the mixture is a little more dense and cellulose will infiltrate the spaces in fiberglass.
Is this a subject manufacturers ( cellulose and fiberglass ) avoid addressing because there is no business benefit ?
On a separate note, has anyone improved the carrying load of their ceiling by adding a fine mesh across the top of joists so the weight of more insulation can be applied to meet the new R50-60 standard ?
Thank you everyone.
my other thought
was the fiberglass installed as batts/rolls and were the battsrolls installed at 90 degrees to the framing? that should distribute the weight at little more evenly across the structure of the framing(?)
typically the cellulose insulation is just sprayed in and "mounded" up (dense pack is done a little differently ) to get the proper depth, when adding fiberglass, it's usually batts or rolls applies over the framing and the cellulose in the bays.
If that's the case, I'm thinking the capacity of the drywall is only for the cellulose and the framing is carrying the other load.
good luck
Both layers are loose-fill.
Original 1997 install was loose-fill R-20 cellulose. Then in 2004 loose-fill fiberglass added another R-20. Total untouched insulation R-40
Working in the attic has disturbed these layers and I don't believe I have R-40. Everywhere I read " make an allowance " for this and add the appropriate top up. I cannot find any charts or conversations about evaluating the R-Value of this 10" mix and condition of insulation.
If I still have R-40 then I can add 6" loose-fill fiberglass R-15 and meet the new standard R-50 - 60 while not exceeding the weight my ceiling can hold.
If the R-Value is lower I can adjust how much I top up and not exceed the weight load.
Alternatively, I can sweep back the insulation above the 2x4 joist and apply netting and sweep back the insulation. Therefore taking weight off the ceiling. Now I can top up with much more insulation.
Are there ways to evaluate my existing insulation R-Value ?
Is applying netting across the joists a proven method for off loading insulation weight from a ceiling screwed to 24 OC joists ?
Thank you.
.