residential steel framing U-values?

[pirat]

We're trying to get into residential construcion using steel framing and I've been assigned to seeing that we comply with energy codes. The trouble I'm having is that the code is written with wood framing in mind, and I have to show that our steel construcion is equivalent.

For example, code says the walls must have R-21 wall insulation (based on 2x6 wood framing), but an equivalent wall assembly with a U-value of .060 is an acceptable substitute. I've been able to show comliance for the walls using ASHRAE fundamentals calcualtions and using a simple dos app called EZ-Frame. The floor and ceiling however are a different story. Using ASHRAE or EZ-Frame, I can't get numbers that comply with code listed U-values. Even worse, I can't get numbers to work even if I'm using wood framing values. Thus I'm convinced I'm screwing something up.

Does anybody work with steel frame U-value or residential load calculation that could give me a tip on this? I need to meet Oregon energy code, floor= U-.028, flat ceiling= U-.031. In the floor we have 2 x 12 steel joists .073 thick, in the ceiling we have 1.5 x 8 joists .0536 thick.

Thanks.

 

[KiwiMace]

The normal ASHRAE method you mention is a one-dimensional resistance type calculation that looks at each component of the wall as a series connected resistance to heat flow.

You can average the heat flow over a wall section based on adding a number of these 1-D sections in parallel, factoring the area of each.

The problem with steel framing is in thermal bridging. Thermal bridging makes the problem a 2-D or 3-D heat transfer problem. ASHRAE recommends (Fundamentals Chap 23.4) the hot box method (physical testing) or a more cleverer model.

I've built a hot box once before, and it isn't too much of a task.

 

[pirat]

I'm actually using the parallel flow and isothermal planes methods. ASHRAE Fundamentals 2001 has a steel frame example on P25.9 and 25.10 that I've been using as a guide. It also mentions a parallel path correction factor from ASHRAE Standard 90.1-1989, but I haven't looked into that.

As for building a hot box, I'm not doing a load calculation on the building so I don't actually care what the true U-value is for the assembly. I just need to have calculations to show on the plans so we can get a building permit. There are several steel frame houses around here so I know people are complying with code, but so far I don't see how they are doing it.

 

[KiwiMace]

Well then, it sounds like you are using the right methodology... check your numbers and units are all aligned.

Are you using batt insulation between the steel, or board insulation across the surface? The method varies.

Also affecting the calc for floors and ceilings is heat flow direction. This is all covered in the ASHRAE chapter you mentioned.

 

[DonPhillips]

Try looking up some of your questions in the IECC. It discusses the differences between wood and steel framing. Also, the free Comcheck software available at energycodes.com makes it easy to switch framing members to see the the impact. I cannot recall if 90.1 is an option in the software or not.

Don Phillips

http://worthingtonengineering.com

 

[MintJulep]

There was a great article in ASHRAE Journal a few months ago about how crappy metal stud walls with insulation in the cavities are.