# Problem with calculation

| Posted in Energy Efficiency and Durability on

So I have wrapped my feeble mind around this for the last time……..

what am I doing wrong???????

I am calculating temp at the inside of my sheathing – but keep getting answers that don’t seem right…..
Zone 6A 8200 HDD – average mean temp over three months is 12.13F, but I am using 2F for my calculations. Inside temp is 70F, RH % 35, Dew Point is 41F.

Here is the wall…..from the outside to the inside

Hardieboard lap siding, 2×4 vent channel (does this have a R-value?), R18 (3″ ISO) or 2 layers of R18 (3″ ISO) for R36, then ice&water, then stripsheathing or plywood, then 3″ stud, then drywall, then latex paint.

Whether I use R 18 or R 36 ISO in my calculation, I get nearly the same temp on the inside of my sheathing……it can’t be right – right?????

Example 1:
delta T = C (comp R) / W (whole assembly R) * ( S (inside temp) – D (outside temp) )
delta T = 6 / 7.24 * (70 – 2)
delta T = .8287 * 68
delta T = 56.35

R6 (1″ ISO) + R1.24 (drywall, plywood, ice&water) = Total assembly R of 7.24.
70 (inside temp) – 2(outside temp) = 68
6(component R)/7.24 (total assembly R) = .828
.828 * 68 (temp change from outside to inside) = 56.35
does 56.35 get subtracted from the inside temp to let me know what the temp is on the inside of the sheathing? 13.65??? this makes more sense if there is only R6 of insulation in the whole wall…

Example 2:
delta T = C (comp R) / W (whole assembly R) * ( S (inside temp) – D (outside temp) )
delta T = 36 / 37.24 * (70 – 2)
delta T = .9667 * 68
delta T = 65.735

R36 (6″ ISO) + R1.24 (drywall, plywood, ice&water) = Total assembly R of 37.24.
70 (inside temp) – 2(outside temp) = 68
36(component R)/37.24 (total assembly R) = .9667
.9667 * 68 (temp change from outside to inside) = 65.735
NOW WHAT??? this is where the math seems right but the calculation seems wrong for the assembly

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### Replies

1. GBA Editor
| | #1

Here is the formula you are looking for:

Temperature of the sheathing = Indoor temperature - (Delta-T * Percentage of the insulation that is on the interior side of the sheathing)

Since 100% of your insulation is on the exterior side of your sheathing, the value of the number inside the parentheses will always be 0. In other words, the interior surface of your sheathing will be at the indoor temperature.

That's simple, isn't it?

Of course, there is a very small R-value attributable to the 3.5-inch empty stud cavity and the drywall and interior air film -- but these can be ignored in this calculation. There is no need for you to worry about condensation or moisture accumulation in your sheathing.

For more information on this topic, see Are Dew-Point Calculations Really Necessary?

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