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Problem with calculation

Irishjake| Posted inEnergy Efficiency and Durabilityon

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(整个组件r) *(s(内部临时) - d(外部温度))
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(整个组件r) *(s(内部临时) - d(外部温度))
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
    Martin Holladay||#1

    Brad,
    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?

    当然,有一个很小的R值归因于3.5英寸的空螺柱腔以及干墙和内部空气膜 - 但是在此计算中可以忽略这些。您无需担心护套中的冷凝或水分积累。

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

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