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

Views: 606

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

Discussion Forum

Can you trust your Load Calc Software?

Started by Christopher Morin. Last reply by Eric Kjelshus Feb 18, 2014. 1 Reply

Attic insulation vs. Insulated roof deck

Started by Robert DV. Last reply by Robert DV Jul 6, 2012. 3 Replies

Support licensing in 2011

Started by Robert Zockoff. Last reply by Robert Zockoff Jun 20, 2011. 2 Replies

Groups

Latest Activity

Ken Summers replied to Gregory Cisco's discussion Abandoning under-slab HVAC ductwork which fills with groundwater seasonally.
"My largest concern is moisture. I have seen mold forming on interior walls in the heating season…"
37 minutes ago
Seth Romme replied to Sonja Persram's discussion Energy Savings Guarantee for single family dwellings in cold climate??
"Hi Sonja We provide home energy optimization services and provide a 5-year heating and cooling…"
2 hours ago
Lindsay Bachman posted events
4 hours ago
Derrick Koehn's video was featured

How Technology and Innovation Are Making Outdoor Wood Burning Stoves More Efficient and Environmentally Friendly

Introduction Since ancient times, wood burning has been used for heating in some capacity or other. Although, of late, wood stoves are thought of more as a decorative appliance than a heat source, in truth wood heat is still one of the most…
5 hours ago
Efficiency First California's blog post was featured
5 hours ago
Diane Chojnowski's event was featured
Thumbnail

Better Buildings Peer Exchange Call: Home Improvement Catalyst: Keeping the Ball Rolling with Homeowners by Delivering More Value at Eastern Time

September 21, 2017 from 1pm to 2:30pm
Thursday, September 21 1-2:30pm Eastern TimePlease join this Peer Exchange call to…See More
5 hours ago
Diane Chojnowski posted an event
Thumbnail

Better Buildings Peer Exchange Call: Home Improvement Catalyst: Keeping the Ball Rolling with Homeowners by Delivering More Value at Eastern Time

September 21, 2017 from 1pm to 2:30pm
Thursday, September 21 1-2:30pm Eastern TimePlease join this Peer Exchange call to…See More
5 hours ago
Lindsay Bachman is now a member of Home Energy Pros
8 hours ago

Photos

Loading…
  • Add Photos
  • View All

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service