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

Views: 470

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

Profile IconWes Anderson, Jean-Paul and Bryan joined Home Energy Pros
yesterday
Dennis Heidner commented on Dale Stephens's blog post LED Lighting 24-month update 18,240 hours & counting
"Dale, good write up. For the landscaping bulbs,  if they had been T3 12V wedge bulbs,  I…"
yesterday
Brett Little commented on Tom White's video
Thumbnail

The Future of Housing - And How Airtightness Can Help

"It's very true! So many people and contractors have tons of insulated attics but no air seal…"
yesterday
Chris Leach replied to Richard Beyer's discussion "The Dangers of Using Spray Foam Insulation"
"The problems will surface soon enough . Why don't we take a second look at this over rated…"
yesterday
Don Fitchett commented on Tom White's video
yesterday
tedkidd replied to Bob Blanchette's discussion How does Cycles Per Hour affect real world AFUE?
"Want to spot a simpleton? It's a guy who thinks EE and comfort are disconnected.  Want…"
Thursday
tedkidd commented on Tom White's blog post Clean Energy Works Oregon: Total Home Performance
"Nice post Tom! I really like how he puts that. Leveraging interests so they can optimize…"
Thursday
Kent Mitchell commented on Tom White's blog post Clean Energy Works Oregon: Total Home Performance
"As a contractor in the region - we frequently wonder how/why you can be a non-profit?  Oregon…"
Thursday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service