# U or R of empty stud cavities?

| Posted in General Questions on

Hi gang – I’m looking to do some home-brewed energy modeling to calculate the net present value of some insulation options that I am considering adding to my walls and ceiling. I have a 1955 house with no insulation, and I can find the R-values I get from my drywall, framing, sheathing, siding, etc, here: http://coloradoenergy.org/procorner/stuff/r-values.htm

I’m curious if anyone has a basic guide for how to treat the currently-empty cavities. I suppose I could treat them as contributing zero to the assembly and that would probably be fine, but I would imagine that there is SOME R value, and that it depends on the height, width, depth, and orientation (wall or 4/12 ceiling) of what is essentially a convection cell.

Any mechanical engineers or energy modelers have something dumbed down enough for me to use?

Many thanks,

M

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### Replies

1. Expert Member
| | #1

Empty 2x6 cathedral roof with shingles and drywall on the inside is around R3.

Empty 2x4 stud wall with wood siding over plywood and drywall is around R3.5.

2. | | #2

You can use THERM, but just using the values for the air films + materials will get close enough.

3. | | #3

This is one of those things where most of the time being super accurate isn’t critical. The numbers Akos gave are good. If you need to be accurate, best to measure your heating and cooling loads (thermostat logs, or fuel consumption).

4. | | #4

Precisely, I have my heating load from a fuel use; trying to extrapolate how much my annual heating spend will go down from an insulation project.

Was hoping someone would have a closed form equation or at least a table I could put into my spreadsheet model... @Jon is that what THERM is? And do you have a link? First couple pages of google results were no help.

5. | | #5

https://windows.lbl.gov/software/therm

How well it accounts for convection is unclear to me.

Note that air infiltration will be a big unknown (even with blower door tests).

6. | | #6

Good point on the air infiltration. Thanks for the THERM link, that should keep me busy for a while :)

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