Blown in insulation - Quantity ?

[shooon]

Getting ready to get materials to do blown in insulation in the attic and was wondering if I could get some help figuring out the quantity?

I've asked the local hardware store how much I'll need based on attic dimension and I'm coming up with a vastly different number than they gave me. I'm thinking maybe they didn't take into account roof slope into their equation.

They quoted me for 75 bags.

Dimensions:
24' x 31'4" with a 4'4" rise. I've got about 7'6" of cardboard baffles installed between each of the trusses to keep the blown in out of the soffit. Aiming for 17.9" depth (16" final settled depth) to give me R60

The insulation states 11.2 Sq.Ft coverage per bag for R60 (16.35 Cu.Ft / bag)



I was thinking the most accurate way might be to calculate the volume of the attic (trapezoid) and then apply the 16.35 Cu.Ft / bag amount to figure out quantity rather than going off the sq. ft of the attic ... ?

Insulation Spec:
http://www.weathershield.ca/wp-content/uploads/2014/08/weathershield_coverage_chart-REV-2014.pdf

 

[kTHREE]

Figure out what you need, add 30% more, then return any extra.
Is returning some not an option?

 

[shooon]

Figure out what you need, add 30% more, then return any extra.
Is returning some not an option?

I will be paying to get the stuff delivered and was trying to avoid a second delivery charge if I end up with double the quantity I actually need. Just trying to confirm my estimate is close as well so I can get 30% extra and not 100% extra! Although I suppose I could just dump the extra in my attic in the house.

 

[tjdux]

Well first we convert everything to inches.

24x31'4" = 288x376"

~18 inches insulation height.

288 x 376 x 18 = 1,949,184 inches cubed volume

1,949,184.000 ÷ 1728 = 1128 cubic feet of insulation needed without factoring in the amount removed by roof slope. (1728 is amount of cubic inches in 1 cubic foot)

1128 ÷ 16.35 = 69 bags.

Now to calculate the slope volume.

Since OP didnt give much info on the roof slope were going to have to make assumptions and use some trigonometry. Note that all triangles will be 90 degree triangles.

Im assuming when he says 4'4" rise he is saying thats the internal peak height....

That makes the entire roof a triangle that is 4'4" high by 12' long. (assuming again the gable end is 24')

In inches we have

52 squared + 144 squared =
23,440.00000
Square root that to finding the hypotenuse of the triangle is
153 inches = 12.75 feet total roof slope.

Using a triangle calulator with trig functuons we can easily find that the angle from the base to the hyootenuse is 19.55 degress.

So next we redraw our triangle with 18 inches height and a hypotenuse angle of 18.55 degrees. To calculate area we need the remaining 2 sides length.

Now the calculator will do all this for us but may as well continue the math lesson. Sarcastic cheering all around.

A triangle with that height and angle will create sides 18' high x 49.7 long x 59.2 hypotenuse.

To calculate area is A=h×b÷2

That equals 448 square inches. To get the volume we muliply by length of building in inches (376)

448×376=168,448 cubic inches.

Convert to cubic feet = 97.5 cubic feet.

97.5 cubic feet is the volume the sloped edges of the roof compose of 18 inches high. In the first calculation we found the area of the attic in a rectanular fashion that includes volume that doesnt exhist due to the angled edges in the trapizoodal shape. The 97.5 cubic feet is the amount we need to subtract from the original figure.

97.5 ÷ 16.35 = 5.9 bags of insulation.

We found we need 69.5 (70) bags to start with and we can remove 5.9 (6) so in a perfect world and if all our math is corret (hopefully) you only should need 64 bags total.

In my opinion, even though I've never done blown in insulation, it never hurts to have some extra just in case so 75 maybe a little high but not crazy.

Second point most places will take back the bags you dont use and assuming thats the case It's better to have more on hand than run out in the middle.

Also just curiois OP you said your math came up different that the hardware store, what number of bags did you come up with?

Also final note if you google triangle calulator you can use the same things i did for this somewhat complex math stuffs.

Good luck.

Sent from my SM-G900V using Tapatalk


Edit... Someone above said its common to have approx 30% extra so to figure that into the maths here we have.

64×1.3= 83 bags. So that actualky mak3s 75 look pretty solid.

 

[tjdux]

I will be paying to get the stuff delivered and was trying to avoid a second delivery charge if I end up with double the quantity I actually need. Just trying to confirm my estimate is close as well so I can get 30% extra and not 100% extra! Although I suppose I could just dump the extra in my attic in the house.

Could you load thr stuff up and return it yourself if there's extra? How far we talking to need a delivery truck for return?

And yeah if your home attic doesnt have any or coukd use/handle more then its certianly not waste.

Sent from my SM-G900V using Tapatalk

 

[shooon]

Well first we convert everything to inches.

...Im assuming when he says 4'4" rise he is saying thats the internal peak height....

That makes the entire roof a triangle that is 4'4" high by 12' long. (assuming again the gable end is 24')

Perfect!

Now the calculator will do all this for us but may as well continue the math lesson. Sarcastic cheering all around.

...Also just curiois OP you said your math came up different that the hardware store, what number of bags did you come up with?

Also final note if you google triangle calulator you can use the same things i did for this somewhat complex math stuffs.

I tried to calculate the volume of a trapezoid and came up with ~440 Cu.Ft. or ~27 bags. I went with 8' height for the trapezoid since that's roughly the length the baffles extend up the slope. :tard:

https://www.easycalculation.com/area/volume-of-trapezoidal-prism.php


Thanks so much for taking the time to help me sort this out!

 

[tjdux]

Perfect!



[emoji38]



I tried to calculate the volume of a trapezoid and came up with ~440 Cu.Ft. or ~27 bags. I went with 8' height for the trapezoid since that's roughly the length the baffles extend up the slope. :tard:

https://www.easycalculation.com/area/volume-of-trapezoidal-prism.php


Thanks so much for taking the time to help me sort this out!

No problem.

Sent from my SM-G900V using Tapatalk

 

[Scott H in Wheaton]

744sq ft / 11.2 sq ft per bag = 66 bags

its insulation, not rocket surgery. don't overthink it. You measure to the nearest foot and round up and don't worry about the slope. any extra can be mounded in the middle

 

[j p smith]

Well first we convert everything to inches.

24x31'4" = 288x376"

~18 inches insulation height.

288 x 376 x 18 = 1,949,184 inches cubed volume

1,949,184.000 ÷ 1728 = 1128 cubic feet of insulation needed without factoring in the amount removed by roof slope. (1728 is amount of cubic inches in 1 cubic foot)

1128 ÷ 16.35 = 69 bags.

Now to calculate the slope volume.

Since OP didnt give much info on the roof slope were going to have to make assumptions and use some trigonometry. Note that all triangles will be 90 degree triangles.

Im assuming when he says 4'4" rise he is saying thats the internal peak height....

That makes the entire roof a triangle that is 4'4" high by 12' long. (assuming again the gable end is 24')

In inches we have

52 squared + 144 squared =
23,440.00000
Square root that to finding the hypotenuse of the triangle is
153 inches = 12.75 feet total roof slope.

Using a triangle calulator with trig functuons we can easily find that the angle from the base to the hyootenuse is 19.55 degress.

So next we redraw our triangle with 18 inches height and a hypotenuse angle of 18.55 degrees. To calculate area we need the remaining 2 sides length.

Now the calculator will do all this for us but may as well continue the math lesson. Sarcastic cheering all around.

A triangle with that height and angle will create sides 18' high x 49.7 long x 59.2 hypotenuse.

To calculate area is A=h×b÷2

That equals 448 square inches. To get the volume we muliply by length of building in inches (376)

448×376=168,448 cubic inches.

Convert to cubic feet = 97.5 cubic feet.

97.5 cubic feet is the volume the sloped edges of the roof compose of 18 inches high. In the first calculation we found the area of the attic in a rectanular fashion that includes volume that doesnt exhist due to the angled edges in the trapizoodal shape. The 97.5 cubic feet is the amount we need to subtract from the original figure.

97.5 ÷ 16.35 = 5.9 bags of insulation.

We found we need 69.5 (70) bags to start with and we can remove 5.9 (6) so in a perfect world and if all our math is corret (hopefully) you only should need 64 bags total.

In my opinion, even though I've never done blown in insulation, it never hurts to have some extra just in case so 75 maybe a little high but not crazy.

Second point most places will take back the bags you dont use and assuming thats the case It's better to have more on hand than run out in the middle.

Also just curiois OP you said your math came up different that the hardware store, what number of bags did you come up with?

Also final note if you google triangle calulator you can use the same things i did for this somewhat complex math stuffs.

Good luck.

Sent from my SM-G900V using Tapatalk


Edit... Someone above said its common to have approx 30% extra so to figure that into the maths here we have.

64×1.3= 83 bags. So that actualky mak3s 75 look pretty solid.

Wow, thanks for doing that for the Op, but it still gave me a headache trying to follow along

 

[Ehcrain]

Take any extra bags back with the machine. Cover any exposed skin with baby powder before starting to avoid some itching when you are done.

Sent from my SM-G950U using Tapatalk

 

[Dragfluid]

E=mc2

 

[Scott H in Wheaton]

Take any extra bags back with the machine. Cover any exposed skin with baby powder before starting to avoid some itching when you are done.

Sent from my SM-G950U using Tapatalk

If he is looking at 16-18" and R60 when he's done, that isn't fiberglass. Must be cellulose. It's gonna be DUSTY!!!!!

 

[johnnyradiant]

24 X 31.3 X 1.5 / 16.35 means I'd show up with no less than 75 bags, maybe 80 - 85 depending on mobility in the attic and humbleness about how accurately I could keep it all at 18", a mask, a spray painters hoodie, googles, coveralls with tape for the ankles and wrists, and a bunch of water bottles. Maybe a hammer to bend over any prominent head piercing nails in key areas if need be, and/or maybe a hard hat. A few scraps of plywood for a catwalk if there isn't any and lots of light 'cause once you get blowing it can get real dark and cloudy in there. Whoever feeds the hopper will find the guy blowing isn't the only one getting dusty, just not as bad cause it won't be as confined an area. I'd also do everything in my power to schedule this job not in the summer, especially if you have already compromised air quality from all our fires.

I haven't fired up my Kendal in years but I still have some fun memories blowing that stuff in attics. Not enough memories to want to fire it up again but a few none the less.