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

Views: 457

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

Tom White posted a video

Ducts in an Attic

http://www.drenergysaver.com | 1-888-225-6260 In episode 66 of the On the Job video series, Larry Janesky, owner and founder of Dr. Energy Saver, is back to ...
5 hours ago
Stacy Hunt posted events
9 hours ago
Rob Madden, Solar Home Broker posted blog posts
9 hours ago
Rob Madden, Solar Home Broker posted a photo
9 hours ago
Kerry Haglund posted a blog post

Efficient Windows Collaborative Releases Window Selection Tool Mobile App

The Efficient Windows Collaborative (EWC) has released a mobile app based on it web-based Window…See More
10 hours ago
Casey Gesell posted a video

Home Comfort Pays for Itself | Ideas Worth Sharing

http://www.myhomescience.com There are 88 million homes in the U.S. and Canada alone that burn twice as much fuel for heating and electricity for cooling tha...
14 hours ago
Profile IconRob Madden, Solar Home Broker, Alicia Hobbs, Rachel Scott and 2 more joined Home Energy Pros
14 hours ago
Don Fitchett posted a video

HVAC AC Compressors Training

This HVAC training sample is from the HVAC section of the Online Industrial Training Video Library (http://bin95.com/Industrial-Training-Videos/HVAC-Training...
14 hours ago

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service