U-factor for post frame building wall?

[NerdelbaumFrinkJr]

I am designing a commercial post frame (pole barn) building in Wisconsin, climate Zone 6a. Wall construction is as follows:
3-ply 2x8 embedded columns at 8 ft o.c., R19 batt insulation, 2x6 wall girts on exterior face, 26 gauge exterior ribbed steel, 4-mil poly vapor barrier, 2x4 girts on inside face, 29 gauge interior liner panels.

I am familiarizing myself with the energy code requirements of these buildings, and working on envelope calculations using COMcheck. I am summing up the R-values of the wall components and determining the U-factor. I understand the ribbed steel and liner panels do not provide any insulation value.

Is it reasonable to consider: 19 (batt insulation) + 0.83 (4-mil poly) + 1.0 (1.5" air space from the 2x4 wall girts on the inside face) = 20.83. Therefore, U = 1/20.83 = 0.048. Is this an acceptable calculation? By comparison, a wood frame wall with studs at 24" o.c. and R19 cavity insulation yields a U-factor of 0.065. Perhaps I should increase to R25 batt insulation, given that the wall cavity is 8".

Any input or feedback is appreciated. Researching online, I have not found much on the topic of energy U-factors for post frame building walls.

-Troy

 

[north star]

@ @ + @ @

Welcome Troy, ...to the Building Codes Forum !
Give it a few days for some other Forum members
to respond.

Initially, ...it appears that you DO indeed need to
increase your thermal insulating.......Either with
Batt type, or spray on, other.

+ + @ + +

 

[TheCommish]

What is the building going to be used for and how and how much heating or cooling energy is going to be used for the building?

 

[tmurray]

Your R-19, is that a blanket installation?

If it is batt with framing, you need to perform a parallel heat flow calculation before the isothermal planes calculation.

 

[Bryant]

Maybe approach it from the angle of a mass wall, composed of layers of materials, using a u factor conversion to derive at an r value. Simple cavity walls only deal with batts, but you could compond addtional layers. Even the liner panels provide some value provide there is no thermal bridging going on.

 

[NerdelbaumFrinkJr]

Thank you for the input. The building occupancy is S-1, unoccupied storage with no toilet facilities currently. There is no HVAC design currently. The client is increasing the wall insulation to R25 and the attic insulation to R50. In my COMcheck model, I am using a 'Other wood framed' wall assembly with a U-factor of 0.055, and the envelope criteria is satisfied (2% above code). I think that U-factor value is conservative, since a post frame wall is simply columns at 8 ft o.c., whereas a stud wall has a large percentage of framing, headers, plates, blocking, etc. that penalize the total R-value of the wall assembly.

I will research the parallel heat flow vs. isothermal planes calculation methods.

Thanks again!

 

[TheCommish]

if there is no HVAC why insulate the space? My recollection of the IEEC is low or no energy buildings do not requie insulation

 

[Mech]

Ditto what TheCommish said. If no space heating or cooling, then why insulate? If the owner will insulate in the future, it may be wise to insulate the building now before the energy code becomes more stringent.

Last I knew, COMCheck used the parallel flow method for U-value calculations, so that is what I use to comply with COMCheck. Parallel flow calculates a higher U-value than Isothermal. I use an average value from the parallel and isothermal methods when calculating heat loss / heat gain.

 

[NerdelbaumFrinkJr]

The owner will add restrooms and possibly have offices in the future, possible some light manufacturing (F-1) too. I think the equipment/materials being stored require a somewhat regulated environment, so he is insulating now.

 

[Bryant]

The owner will add restrooms and possibly have offices in the future, possible some light manufacturing (F-1) too. I think the equipment/materials being stored require a somewhat regulated environment, so he is insulating now.

Tenant shell !

 

[Mr. Inspector]

2015 IECC C503.2 Change in space conditioning. Any nonconditioned
or low-energy space that is altered to become conditioned
space shall be required to be brought into full compliance
with this code.

You can't know what insulation to use unless you are conditioning it before the next code cycle is adopted. Sometimes it changes.