U values for steel building, heat loss

viper · 07-10-2012, 01:41 PM

I am working on some heat loss and gain calcs for a large steel building. I am having trouble finding specific values that I need. The slab will be radiant heated, 6", with 1" EPS or XPS under it. I am using a U value of .19 on that.

The walls will be corrugated 26ga steel over 6" glass batts with vapor barrier and want to add some type of air barrier in there somewhere. All framing is wood. I am using a U value of .05 for that.

I just back calculated the R values for the concrete and foam for the slab.

I have 300sf of double pane windows, 150sf of insulated steel doors, and 200sf of insulated OH doors. Building is 80x150x18 with 3.5x12 pitch. I am coming with with around a 225K btu loss at 65* delta. I would like to verify values with anyone experienced.

I have not looked at heat gain yet but was planning to just adjust this calc and add in my internal loads for now. This is a shop space so I am calculating on a 25*DT for cooling. I think I was around 10T of cooling for heat gain before internal loads.

**beenthere** · 07-10-2012, 06:41 PM

Steel outer shell, with or without an inner shell of some type?

viper · 07-10-2012, 06:47 PM

Technically, 1/3 will have interior sheetrock but for this calc, lets just look at no inner lining other than the vapor barrier on the face of the glass batts.

**beenthere** · 07-10-2012, 07:43 PM

That 1/3 having sheet rock, puts your averaged insulation U at .06

viper · 07-10-2012, 08:20 PM

I just want to ignore the drywall for now and focus on values. I was using .05 for all walls without drywall. What would you use and how did you figure it or find the value?

As well, do you have any data for the floor since that is the largest loss by far?

**beenthere** · 07-10-2012, 08:50 PM

Concrete is R.08 per inch.

A 2X6 is R6.88

Steel shell R.61
inside air barrier R.68
6.5"fiberglass bat R19

Total those R values, including your insulation., R19 for 6.5".
.61+.68+19=20.29
1/20.29=U.0478

Then if you want to be real accurate. You calc how many 2X6s will be in the wall, and convert that to surface area, and figure out the U for that area.

viper · 07-13-2012, 01:51 PM

Thanks! After reviewing my figures, I think I am very close. A few slight tweaks but for a shop, we will be just fine.

I was curious if you can offer any info regarding heat gain calcs? I know most rely on manual J derived software but I find that manual crunching gains a much stronger understanding of the values

By inverting my heat loss calc and using adders for solar gain through windows and such, I think I am close but would like to know more about it.

**beenthere** · 07-13-2012, 04:22 PM

Use Manual N for commercial loads. it has peak load calcs.

Manual J version 8 has a lot more in it for solar gain through windows. Then version 7 did.