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How do I calculate volume and then confirm coverage of dense-packed cellulose?

I have odd sized rafter, wall stud and floor joist bays and Larsen trusses to boot with no top plates. This poses a problem in calculating the quantity of cellulose I need to get the optimum R-value (assuming the best is 3.5 to 4.0 lbs/ft3) in my all season cottage.

Can anyone point me to a formula on-line that I can punch in my numbers and get the volume estimates for my different dimensions of cavities at different densities (R values)?

Are there recommended, reasonably priced, infrared cameras that I can use to check a contractors or my own work, as we go, when blowing into blind cavities, behind drywall, etc?

Asked by Oak Orchard
Posted Jan 14, 2013 12:48 PM ET
Edited Jan 14, 2013 1:26 PM ET


3 Answers

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I don't know of any online tools for volume (though I'd just use Excel), but you could also try core sampling. See for example: http://www.karg.com/pdf/Insulaton_density/Core_Sample_Kit_document.pdf

Answered by Cramer Silkworth
Posted Jan 14, 2013 1:24 PM ET


At the risk of stating the obvious:

To calculate the volume, you multiply three numbers together: length, depth, and height. If you don't know the exact numbers, you estimate the numbers as close as you can.

1,728 cubic inches = 1 cubic foot

Once you know how many cubic feet you need to fill, you multiply the volume (in cubic feet) by 3.5.

The result is the weight of the cellulose you need in pounds.

Answered by Martin Holladay, GBA Advisor
Posted Jan 14, 2013 1:29 PM ET
Edited Jan 14, 2013 1:40 PM ET.


L X W X H is the obvious formula for the building's joist bays, but it will not give me any accuracy for the 10" Larsen trusses. I can of course do the bulk area calculations per wall but I was hoping for some rule to adjust for the quantity of lumber in this space.

The real problem is the installation process. With bulk volume calculations my disadvantage remains in that I don't know how to determine the quantity necessary for the blow into each Larsen truss space since the cell will flow horizontally from truss to truss.

Remember: Larsen trusses have no cavity walls and in my case no top plates (i.e. they are open into the rafter cavities).

That is why I added my question about IR cameras. Thinking an IR camera might give a visual check on density as we install. But I can see how this might not be so reliable.

Maybe I need guidance on the procedure or order of the blow. I assume it’s the resistance to the blow by the walls of the cavity which gives density.

In my rafters I have five fixed sides in each bay, joining at the eve into open cavity wall spaces (Larsen walls) - which have fixed boundaries on only 3 sides (bottom plate, insulweb and sheathing). What is the best order to blow: ceiling before or after the walls; etc? It there technique to help us get the density in such large and open cavities

Perhaps its thoughtless expectation since there is no way to establish cavity boundaries with Larsen trusses. Therefore the feel of the blow as represented by the resistance to the hose and the back pressure into the blowing machine may be the only means of plumbing the density.

Answered by Oak Orchard
Posted Jan 17, 2013 1:15 PM ET

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