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Question DetailsAsked on 10/19/2017

how do calculate the amount of insulaton I need for my walls

my walls are 2x4 , 182 feet long and 20 feet high

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1 Answer


Depends on type of insulation - and need to include wastage.

With board insulation (not ideal for between-studs, fine outside them) then the amount would just be 182x20 feet pluys maybe a bit of wastage - or about 3640 SF of the thickness you need. IF you did decide to go between-studs, assuming you have accurate cutting ability (hot wire cutting table or table saw) so there is very little wastage, then your quantity would be about 91% of that or about 3310SF if going to DIY and cut pieces to fit exactly (meaning at least some crosswise to maximize material use) - using only long vertical strips you would get about 43" width out of 48" wide sheets (if you can get them locally) with about 5" wastage on most sheets, so more like the same 3640 SF that outside insulation covering would use.

For batt insulation - using correct width for the stud spacing, typically about 2-3% wastage for larger areas, so about 94% (the approximate 15" batting space between studs divided by total wall space) so 0.94 x 182 x 20 feet (actually a bit less than 20' probably because of blocking and upper and lower plates - so say about 0.94 x 182 x 19.7' = 3370SF - maybe 3470 SF worth of 3.5 inch roll batt, counting for a bit of wastage. For instance, Owens Corning roll batt insulation for 2x4 walls (assuming standard 16" o.c. stud spacing) comes in 92" length (for 8' walls) and also 15" wide by 39.2' long - so with the 39.2' roll you would get two 20' stud bays filled per roll - or about 53.3SF of wall area, so would take around 65 rolls in that case.

Of course, if you pick your spot, normally you can return unopened CLEAN rolls within typically 7-28 days depending on store - but bgenerally not a big thing to make a final run for a few more rolls as you see how yuour usage is going, because you always have odd-width stud bays at doors and corners and such, so what you estimate will never be exactly what you calculate.

Blown-in insulation varies a lot by manufacturer and also on whether it is being blown into open bays or through holes in a wall that is already drywalled - typically the bags give a yield range in the instructions in SF of wall coverage per bag. For instance, Ownes Corning blown-in fiberglass typically yields (depending on R-13 or R-15 insulation value) about 65-75 SF of wall per 28.5# bag - so in that case you would be looking at about 182 x 19.7' = 3585 SF, or about 48-55 bags depending on which R value you are after.

You can find calculators online at manufacturer websites and Lowes/Home Depot and such - be sure to pick correct R-value and cavity thickness. You can also typically get free use of the blowing machine for a day or two if you buy a certain minimum order of bags of insulation from them - bear in mind it takes two people minimuim - one to feed the contents of the bags into the machine, the other person doing the application.

Answered 3 years ago by LCD

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