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calculating blown insulation


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#1 toomanytoys84 OFFLINE  

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Posted January 13, 2016 - 01:21 PM

Anyone done blown-in insulation in walls?  My parents did it when I was a kid, but I remember nothing about it but helping dump stuff in the hopper.

 

I am assuming that my 2 living room exterior walls, master bed room 1 exterior wall, 1 exterior wall in the laundry room and my kitchen 2 exterior walls have no insulation in them.  I am going to some exploratory looking and see what I see before but today is stupid slow and I have been browsing looking for information about blown-in insulation in my walls.  I am not going to worry about the kitchen as it is going to be gutted down to the bare studs 100%.

 

Now if my walls have a 3.5" cavity 16" wide, and 96" tall how many of those spaces could I fill with 1 40sqft bag?

 

I am not going to post my calculations because I'll look dumb..

 

We need a home remodel sub section!


Edited by toomanytoys84, January 13, 2016 - 01:41 PM.


#2 Little Irish Men OFFLINE  

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Posted January 13, 2016 - 02:26 PM

O k . That's just simple math, the wall is 96'' inch. oka  8' ft  and the space between the 2x4 is   14 & 3/4 inch  by 3 1/2 inch. The insulation comes 14-3/4'' wide all ready . and some ins come in pre cut pieces of 96'' inch . And the wrapper will tell you  how much is in that roll . So you with me so far ? ok,  good  You just count how many spaces between the 2x4 and there you go.....  No , No ,  you wanted  for blown in insulation ,length x height x width ,It will take about 2 sqft per cava tee .

the space between the 2x4

 

 

Patrick.   


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#3 toomanytoys84 OFFLINE  

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Posted January 13, 2016 - 02:33 PM

O k . That's just simple math, the wall is 96'' inch. oka  8' ft  and the space between the 2x4 is   14 & 3/4 inch  by 3 1/2 inch. The insulation comes 14-3/4'' wide all ready . and some ins come in pre cut pieces of 96'' inch . And the wrapper will tell you  how much is in that roll . So you with me so far ? ok,  good  You just count how many spaces between the 2x4 and there you go.....  No , No ,  you wanted  for blown in insulation ,length x height x width ,It will take about 2 sqft per cava tee .

the space between the 2x4

 

 

Patrick.   

 

Got ya, is that a rule of thumb or is there math behind that?

 

I was sitting figuring out cubic feet of volume in that space, and came up with basically 2.2 sq foot per cavity, and 10 cavity per bag roughly.

 

2.2252 sq feet per cavity to be exact..

 

Maybe I'm not as dumb as I thought


Edited by toomanytoys84, January 13, 2016 - 02:34 PM.


#4 Jazz OFFLINE  

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Posted January 13, 2016 - 02:40 PM

I did some years ago. One thing we had to watch was some houses had fire brakes(a 2x4 horizontal half way up wall)  that meant drilling hole top of wall and halfway up wall or wherever fire brake was...The stuff was dirt cheap however it does settle a couple inches over time.  Our big sell was the Urea formaldehyde insulation..that stuff was awesome, it would be oozing out the outlets! However a few years later claims that it caused health issues put end to that..


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#5 David Brown OFFLINE  

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Posted January 13, 2016 - 02:42 PM

Wait, am I missing something?  You're talking filling the cavities so that's cubic feet but the bag covers square feet?  Does it say so many square feet at a certain thickness?


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#6 toomanytoys84 OFFLINE  

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Posted January 13, 2016 - 02:52 PM

Wait, am I missing something?  You're talking filling the cavities so that's cubic feet but the bag covers square feet?  Does it say so many square feet at a certain thickness?

 

...that's what I thought was messed up.

 

It's a material that covered cubic, think if you are spraying it in your attic.  It's 20x30 and then you want it 4" deep.  It's friggin confusing!

 

The website for the manufacturer says this

 

 

R-13 4.29 3.86 56.0 17.9 0.340 61.8

 

 

That's r-13 value at 4.29" thick which is reasonable close the the 3.5"(was reading that would be R-11)  Says it covers 56" min per bag and 61.8 sq ft max to maintain 4.29"

 

So that being assumed one bag would fill 28 cavities.  40 sq feet would probably be assuming about 6" depth. 

 

I am also going to blow this into my attic.  I'll buy something like 40-50 bags, an 1.5" hole saw, drill a hole top and bottom and blow the crap in.  Then I'll temp repair the holes with the plugs until I do the new drywall. 

 

10 dollars a bag, 500 bucks, and a free machine rental for a day.  Or buy 20 bags, use those, return machine, next weekend 20 bags, return machine, then next weekend 20 bags, and return machine and 10 bags. 

 

You get a free machine rental for 1 day if you buy 20 bags.  I can use that to my advantage and break it up. 

 

I have SO MUCH work to do.  The garage conversion, insulation, a bathroom, and a kitchen.  I have not told the wife, but the kitchen is not going to get done this year.  I am scared to death of heating and cooling the garage.  The HVAC guy suggested a separate system for this.  But that will be another thread one day.


Edited by toomanytoys84, January 13, 2016 - 03:02 PM.


#7 JiminRI OFFLINE  

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Posted January 13, 2016 - 03:47 PM

My calculations are a little different. We are all assuming that each bay is 96 inches high by 3.5 inches deep. But if studs are 16 inches on center with 1.75 inch wide 2 by 4s, I get 14.25 inches wide per bay - which is 4788 cubic inches per bay (96 x 3.5 x 14.25).

There are 1728 cubic inches per cubic foot (12 x 12 x 12). So that is 2.77 cubic feet per bay (4788/1728).

 

I don't know how many cubic feet in a bag of insulation, but whatever it is, divide it by 2.77 and multiply by the number of bays.

 

If there is a firebreak you need about 87 less cubic inches per bay (14.25 x 3.5 x 1.75 = 87.3 using a 2 x 4 as a firebreak) so about 4701 cubic inches per bay.

That reduces the volume per bay to about 2.72 cubic feet - even a little less if there is wiring or plumbing in a bay.

 

You probably need to add 10% for loss, so I'd use an even 3 cubic feet per bay to be safe.

 

If the wall is actually 8 ft total height (rather than 8 ft studs with 2 x 4 top and bottom for a total height of 8 ft 3.5 inches), then the bay volume is a little less.

 

Without firebreak: 2.67 cubic feet

 

With firebreak: 2.63 cubic feet

 

JiminRI


Edited by JiminRI, January 13, 2016 - 03:53 PM.


#8 Bmerf OFFLINE  

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Posted January 13, 2016 - 03:52 PM

Not to start an argument, but like JiminRI, I came up with ~2.8 cubic feet per bay. If 40 cubic feet per bag then ~14 bays. I don't think so. Math is much easier with 40 square feet per bag per ~4". 40 sq. ft.= 8' x 5' or 8' x 60" = ~4 bays per bag. More realistic. That being said; I have never done this myself and didn't' read the bag.

 

I think it would change the calculation if you really packed it in there; this would hurt the R-value also. R-value is based on the inability to transfer heat though air. A solid object transfers heat easily from one side to the other; Where as a "fluffy" object or one that has many air pockets resists the exchange of heat. Pack it in solid and you loose the resistance to transfer heat.  

 

Old house may have full 2x4" walls (many did) This also would change the calculation also. I like the idea about buying a few bags and seeing how far they go. This is what I would do.


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#9 HANKG OFFLINE  

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Posted January 13, 2016 - 04:14 PM

The contractor you hire for the job knows exactly what it will take to insulate your home hire a reliable contractor with good (comefrom) my word, meaning reputable with reference's and you'll be fine.


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#10 Genem OFFLINE  

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Posted January 13, 2016 - 04:31 PM

There is no way that I would attempt to blow insulation into a wall that was going to be removed in the future. The insulation would all fall out , lost on the floor and all your expense and effort compounded because of the cleanup. In my last 30 years of remodeling house, I learned that doing a job only once is the least expensive. I would remove a section of wall, insulate with craft faced insulation, put new Sheetrock and finish one wall at a time so you can live in the house with having total disaster. Just my opinion. Gene
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#11 James Bosma OFFLINE  

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Posted January 13, 2016 - 04:58 PM

2.8 to 3 cubic feet per hollow, depends on how tight it packs. If walls do not have blocking I recommend doing in 2 sections. 4 feet and top. I know more holes and plugs but you will be able to pack the bottom a bit tighter so there is not as much settling. I have seen the top few feet no longer insulated after 5 to 7 years due to settling. If just putting holes in top I would recommend checking in 5 years to see how much it settles and top up if needed. After you drill the holes it is a good idea to use a plumb bob and line to drop down the hole to see it the is any crossers which was mentioned once already. Blowing the walls is alriight but remodeling later is messier as mentioned


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#12 toomanytoys84 OFFLINE  

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Posted January 13, 2016 - 06:41 PM

The contractor you hire for the job knows exactly what it will take to insulate your home hire a reliable contractor with good (comefrom) my word, meaning reputable with reference's and you'll be fine.

Not paying for a contractor. I'll succeed or screw it up on my own.

Not removing the walls later. Cover over with sheet rock over lathe and plaster
As I always do it that way.

So 3 on the high side per bay

Edited by toomanytoys84, January 13, 2016 - 08:31 PM.

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#13 shorty ONLINE  

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Posted January 13, 2016 - 06:47 PM

Blowing it in will definitely give a better insulation than batts. Fills the voids easier.
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#14 Little Irish Men OFFLINE  

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Posted January 13, 2016 - 07:25 PM

OK OK ,  going on the assumesion that the house was frame with 2x4  .. . . . . .  after ww II  THE 2x4 will measure 3 1/2 x 1 1/2 inch and if it was frame with a single plate on the bottom and a bubble on the top ..... The 2x4 it self will be 91 1/2 inch tall  and 14 1/2 inch  in between each other and that gives you ? ? ?  any one , anyone , bluer , bluer , 2 sq ft per cavity + or -  an inch or two . In estimating  or guessing you all ways add just a little more .  

 

 

Patrick . 


Edited by Little Irish Men, January 13, 2016 - 07:25 PM.

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#15 toomanytoys84 OFFLINE  

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Posted January 13, 2016 - 07:26 PM

House was built pre ww2. 1922.




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