Can anyone tell me how Wrtightsoft treat a a insulated roof deck compared to a insulated flat attic.

Views: 486

Replies to This Discussion

The best way to determine what RSU does with scenarios like this is to do it both ways, then compare the reports to each other. The "component constructions" report (found in the Load section) will show you the gains losses attributed to each portion of your building. You could compare the two to each other here.

The general difference is that different roof types in RSU reference different construction assemblies in ACCA's Manual-J 8th Edition book. These "types" often have different U-values and assumed temperature differences, which means that they have varying HTM's (Heat Transfer Multipliers). For instance, a sloped ceiling with R38 cavity insulation is referencing group 18A, while a flat ceiling under vented attic references group 16B.

You'll also notice that anytime you're specifying a sloped roof surface using the room properties tab, Wrightsoft is making assumptions about the square footage of these surfaces. When you define a ceiling via the "room properties," flat ceilings use the rooms footprint as the sqft, while sloped ceilings assume a "typical" slope (somewhere around a 6/12, I believe). If your roof is steeper than 6/12, you should use the "ceiling tool" to draw a separate ceiling object on top of your room/building. This will allow you actually define the pitch of the roof, thus being more accurate.

Hope that helps!

Here's a recent post on how Manual-J can lead you to undersized ductwork, if you're not careful. I'd love to know what you think about it. Please comment!

Thanks for the information, I'll start running a few models and see what happens.

In addition to Isaac's response below, if you're insulating the roof deck and encapsulating the attic, be sure to model the ductwork as being within building envelope. This will reduce energy losses through the ducts. 

RSS

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Bud Poll replied to randy tolowski's discussion Another unvented roof question
"I've removed caulking that was over 30 years old and it outlasted the materials to which it…"
10 hours ago
Profile IconAmanda Hallock, Kalvis, Tyler Grubbs and 6 more joined Home Energy Pros
13 hours ago
Corbett Lunsford posted videos
14 hours ago
Sean Lintow Sr replied to randy tolowski's discussion Another unvented roof question
15 hours ago
randy tolowski posted discussions
16 hours ago
Todd Collins replied to Tom Strumolo's discussion Ancient (70's) wall foam insulation, now a powder.
"What did you find about the dust?  "
20 hours ago
Susan E. Buchan commented on Shawna Henderson's blog post Drainwater Heat Recovery Units & Water Efficiency
"Makes sense, Shawna. Thanks"
20 hours ago
Shawna Henderson commented on Shawna Henderson's blog post Drainwater Heat Recovery Units & Water Efficiency
"Susan, I'm not sure that insulation on the unit would be of great benefit, as the drainwater…"
20 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service