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

Views: 552

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

Videos

  • Add Videos
  • View All

Twitter

Latest Activity

Jan Green posted a discussion

Retrofitting a 1979 Home to NAHB Silver or Gold - Pending Cert

I'm excited to be the lone speaker for the last in a series of lectures for the Scottsdale Green…See More
18 hours ago
Crosbey Archery liked Crosbey Archery's blog post How to improve the air quality in your home ?
Saturday
Crosbey Archery replied to Don Hynek's discussion Testing "Magic Pak" units?? in the group Multifamily Buildings
"second-floor (and higher) units without humping a ladder all the way around the building is a very…"
Saturday
Colin Genge posted events
Friday
Profile IconJon Turnbaugh, Lights N More and John Kamas joined Home Energy Pros
Friday
Amber Vignieri posted a blog post

Relay Network Targets Multifamily Housing Upgrades Nationwide

Affordable apartment buildings are home to more than 10 million American households, making them a…See More
Friday
Tom White's video was featured

Robby Schwarz on Builder Collaboration

Hear Robby Schwarz, of EnergyLogic discuss the value Owens Corning brings to Thrive Home Builders.
Friday
Crosbey Archery liked Don Hynek's discussion Testing "Magic Pak" units??
Friday

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service