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

Views: 504

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.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Profile IconSheldon White and jerrycarter joined Home Energy Pros
2 minutes ago
Mark posted events
1 hour ago
Jim Gunshinan's video was featured

GoGreen Portland 2014: The Myth of Apathy - Renee Lertzman, Ph.D

http://portland.gogreenconference.net Renee Lertzman, PhD is the Director of Insight for Brand Cool, a marketing agency supporting a sustainable world. An internationally recognized researcher, speaker, author, instructor and thought leader in…
2 hours ago
Jim Gunshinan posted a video

GoGreen Portland 2014: The Myth of Apathy - Renee Lertzman, Ph.D

http://portland.gogreenconference.net Renee Lertzman, PhD is the Director of Insight for Brand Cool, a marketing agency supporting a sustainable world. An internationally recognized researcher, speaker, author, instructor and thought leader in…
2 hours ago
Stacy Hunt posted blog posts
2 hours ago
Colin Genge posted an event

Air Leakage Testing for Home Inspectors at Online

July 7, 2015 from 1pm to 2pm
• What sort of effect will IECC adoption have on the building industry? Home owners? Inspectors? •…See More
5 hours ago
Bijou Lulla posted a blog post

ENERGY EFFICIENCY RETROFITS.....IN NUMBERS WE TRUST

There has been much ado about the recent paper published by E2e; Do Energy Efficiency Investments…See More
20 hours ago
Mariusz Chorazy posted a blog post

Here are a few tips to keep your air conditioning bill low

TAKE GOOD CARE OF YOUR AIR CONDITIONER – Like any major appliance, your AC requires regular…See More
yesterday

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service