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

Views: 492

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

Laurie DiDonato posted an event
Thumbnail

HERS Rater Training at Online and United Illuminating Company

June 1, 2015 at 9am to June 18, 2015 at 4pm
The Northeast HERS Alliance is offering a NEW Hybrid Rater training course designed to prepare…See More
28 minutes ago
Paul Raymer's blog post was featured

Shades of Gray in Residential Construction Morality

I was called in to perform a last minute duct test for a modular home builder.  He was all in a…See More
1 hour ago
Charlie Hewitt's blog post was featured

Residential Electric Load Disaggregation

“What gets measured gets managed” is a quote often attributed to the venerable management expert…See More
1 hour ago
Paul Haasz joined Evan Mills's group
Thumbnail

Home Energy Ratings

Calculating them; visualizing them; explaining them; selling them .... there is a lot to discuss…See More
20 hours ago
Paul Haasz joined James Sayers's group
Thumbnail

Marketing Energy Efficiency

Sharing ideas, tools and examples of promoting energy efficiency to consumersSee More
20 hours ago
Paul Haasz joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
20 hours ago
Paul Haasz joined Leslie McDowell, BPI's group
Thumbnail

Building Performance Institute (BPI)

BPI is the nation's premier standards development, quality assurance and credentialing organization…See More
20 hours ago
Paul Haasz joined Sean Lintow Sr's group
Thumbnail

Best Practices (Residential)

Best Building, Retrofitting, or even Auditing Practices - what are they, what should change, what…See More
20 hours ago

© 2015   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service