# [Bldg-sim] exterior convection coefficient for tilted roof

Jeff Haberl jhaberl at tamu.edu
Sat Feb 11 13:55:59 PST 2012

```Leen,

This depends on whether or not the wind is blowing... or not. ASHRAE used to have tabluated values (still in some textbooks) of R = .17 Btu/ft2-hr-F for winter (15 mph wind) and .25 Btu/ft2-hr-F for summer (7 mph wind). Otherwise, for a vertical surface it would be .68 for free convection, probably less for a tilted surface, .9 if horizontal heat flow down.

Also, radiation can be significant if the surface is facing a cloudless sky. I think Kreider's solar textbook has a equation for sky temperature...which is somewhat complicated by cloud cover, etc.

Jeff

8=!  8=)  :=)  8=)  ;=)  8=)  8=(  8=)  8=()  8=)  8=|  8=)  :=')  8=)8=?

Jeff S. Haberl, Ph.D.,P.E., FASHRAE..............jhaberl at tamu.edu

Professor............................................................Office Ph: 979-845-6507

Department of Architecture.............................Lab Ph:979-845-6065

Energy Systems Laboratory.............................FAX: 979-862-2457

Texas A&M University.....................................77843-3581

College Station, Texas, USA, 77843..................URL:www.esl.tamu.edu

8=/  8=)  :=)  8=)  ;=)  8=)  8=()  8=)  :=)  8=)  8=!  8=)  8=? 8=)8=0
________________________________
From: bldg-sim-bounces at lists.onebuilding.org [bldg-sim-bounces at lists.onebuilding.org] on behalf of leen peeters [l.f.r.peeters at gmail.com]
Sent: Saturday, February 11, 2012 1:59 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] exterior convection coefficient for tilted roof

Dear,

can anyone point me to some reference values for exterior convection coefficients for tilted surfaces like a roof?

Thanks

Leen
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