"R" Value of LW Concrete on Metal Deck

[WillisV]

Could someone plese provide or point me in the right direction to find the R value for a 3" metal deck with 3 1/4" of lightweight conrete on top (6 1/4" total thickness). Thanks!

 

[KiwiMace]

The R-value for the sandwich can be determined from adding up the individual resistances. Refer ASHRAE fundamentals Chapter 25 (2005). This also has the tabulated values summarised below. You also need convective surface resistances, which will vary according to still or moving air which I leave for for you.

Lightweight concrete varies, but for typical 100lb/ft3 density you can use around 5 BTU.in/h.ft2.degF (or 0.7 W/mK). This ranges from 2 to 9 (0.3 to 1.3) with increasing density.

Steel is generally 26.2 BTU/ft.degF (or 45.3 W/mK).

 

[rdmac]

Be careful. I can't remember for sure but I think Resistances don't just add directly. You may have to add their inverses. It's like parallel vs series resisances in an electrical circuit. I always have to check before I add them up. ie. 1/Rtot = 1/R1 + 1/R2 + 1/R3 etc...

 

[HVACctrl]

R values do add directly. Rtotal=R1+R2+R3 and so on...

I had to refer to a book to remind me though. Its the heat transfer coefficients that add by using inverses.

Ed

http://www.engineerboards.com

 

[KiwiMace]

if there are parallel members of the construct, you get the situation where you might need to add the resistances in parallel too. Like in a typical wood framed wall, you might have series resistances in wall finishes, and parallel resistances in the core insulation and framing.