Does anyone have a calculation or guide you follow to de-rate the r-value of existing insulation?

I'm familiar with RESNET grading, but that seems to be for new installs and I can't find what the actual calculation would be - I'm assuming it's built into modeling software, but does anyone know what the actual adjustment factor on the R-value would be if rating something Grade II or Grade III?

I'm interested in this information both for batts and loose-fill. 

Any resources (even tangential) are greatly appreciated!

Tags: correction, derate, derating, factor, insulation, r-value

Views: 470

Reply to This

Replies to This Discussion

I recall BPI had an insulation derating guide for exactly this purpose, but I can't put my hands on it now. Perhaps someone else here can update that document or post a link.

I also thought so, but I can't find anything now. I was also wondering if there was anything in the Saturn Building Shell field guide, but I can't find my copy. 

Home Energy Score has/uses a derating system I think that was developed by LBL. It is an Excel macro that is given to their certified HES raters. I think you might be able to find it somewhere. 

Thanks! 

I 2nd Dav - Need to recreate the wheel! Here it is! 

https://www.dropbox.com/s/j1bxwles5pjx5ja/Home%20Energy%20Score%20A...

Thanks Brett!

Use your IR camera and make your own "R"  If you have a high end IR camera and 40' plus temp - you can get close.   If you know if that one part is  R-15 and next part is 1/2 or R-7.5  its just a % of known.   More time is needed for plan "B"  place/glue a  R-10 foam to needed with temp reading on all 3 spots and its a % of number taken.  the colder it is the better the number.     

Sounds fun! Won't work for all my contractors because they may not have the equipment, but I'll have to try that sometime.

Haley, up here in Vancouver BC we have created Assessed R-Value Guidelines that we have been using in a utility low-income residential program for the last 8+ years. It was developed using the older RESNET & DOE procedures, along with some Canadian NRCan data, and has been signed of by one of our leading building science engineering firms here. Although I'm not sure if it would be considered adequate enough to meet any U.S. standards you might be using, you are more than welcome to use the best and leave the rest of it. I'm not sure how to send it to you via this forum, but you can e-mail me at ken@bcbuildinginfo.com and I'll send you a copy. 

Thanks! Just sent an email. 

Hi Haley, 

the formula for calculating Area-Weighted average R-Value is in the back of the 5th edition Krigger, Residential Energy book page 263.  And if I remember correctly it was also used in the RESNET training.

Here is the formula if you cannot find it.

Calculating wall U-factor and R-values requires considering the surface area of the insulated cavity and also the surface area of framing members.  Wood framed walls have a 0.15 to 0.40 of their surface area in framing lumber which has a higher u-factor and lower R-value than the insulated cavity.  the area-weighted average estimates the U-factor of the whole wall.  The area-weighted R-value is the inverse of the area-weighted U-factor.

U=(A1 x U1) + (A2 x U2)

R= 1/U

U - is area-weighted average U-factor

A1 - is the decimal fraction of wall represented by framing

U1 -  is the U-factor of the framing section

A2 - is the decimal fraction of wall represented by insulation within the cavity

U2 -  is the U-factor of the insulated section

R - is the area-weighted R-value of the wall

Hope this helps you!

Sorry, wasn't asking about area-weighting, just about de-rating the insulation itself. Thank you though!

RSS

Forum

Capillary break retrofit

Started by Paul Raymer in Best Practices. Last reply by Paul Raymer 8 hours ago. 3 Replies

Help with OpenStudio/EnergyPlus

Started by Kerry Haglund in General Forum. Last reply by Kerry Haglund 11 hours ago. 1 Reply

Your Top Energy Efficiency Tips for the Heating Season

Started by rebecca bridges in Best Practices. Last reply by Sarah Alexander yesterday. 9 Replies

Insulating Chilled/Cold Water Lines

Started by Sean Wiens in HVAC. Last reply by Sean Wiens on Monday. 10 Replies

What's the Most Profitable HVAC Job for Your Company?

Started by Wayne Melancon in HVAC. Last reply by bebelyn accessdoors Jan 16. 5 Replies

Looking for a Rater in Sacramento Area

Started by Brian A McFadden in BPI. Last reply by Building Performance Institute Jan 14. 1 Reply

Hydronic System Critique

Started by Sean Wiens in HVAC. Last reply by Sean Wiens Jan 14. 8 Replies

Condensation Quick Fix

Started by Dan Antonioli in General Forum. Last reply by Sean Wiens Jan 13. 12 Replies

Latest Activity

Leslie Jackson commented on Macie Melendez's blog post Re-introducing Myself as the Building Performance Journal’s Managing Editor
"Great to have you back, Macie, and to work beside you this time! Looking forward to the coming…"
4 hours ago
Leslie Jackson liked Macie Melendez's blog post Re-introducing Myself as the Building Performance Journal’s Managing Editor
4 hours ago
Paul Raymer replied to Paul Raymer's discussion Capillary break retrofit
"Thanks for the input."
8 hours ago
Kerry Haglund replied to Kerry Haglund's discussion Help with OpenStudio/EnergyPlus
"I think I may have found the issue with the original residential prototype IDF files. See the last…"
11 hours ago
Jim Gunshinan commented on Macie Melendez's blog post Re-introducing Myself as the Building Performance Journal’s Managing Editor
"We are very glad to have you back on our team Macie. I think anyone reading your post will…"
11 hours ago
Jim Gunshinan liked Macie Melendez's blog post Re-introducing Myself as the Building Performance Journal’s Managing Editor
11 hours ago
Jim Gunshinan liked Building Performance Journal's blog post Dust-Control Strategies for Energy Upgrades
11 hours ago
Robin Henry replied to Robin Henry's discussion Potentially Looking To Buy Insulation Machine in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Thanks for the offers and phone call fellahs. I ended up purchasing a Force2 this past weekend so…"
12 hours ago
Walter Ahlgrim replied to Paul Raymer's discussion Capillary break retrofit
"I think you are confusing terminology. Capillary break retrofit is a impossibility. A sheet…"
12 hours ago
Kerry Haglund posted a discussion

Help with OpenStudio/EnergyPlus

A bit new to the Open/Studio/EnergyPlus world and I am seeking some help. I have downloaded the…See More
12 hours ago
Barbara Smith's video was featured

Project Home Weatherization Day Video 2019

October 30th is National Weatherization Day, so let's show you what weatherization is all about! We are proud to operate the Weatherization Assistance Progra...
13 hours ago
Mark Fetsko's photo was featured

Low-E Coatings: Summer vs. Winter

A depiction of how an insulated glass unit (IGU) performs with Low-E coating on surfaces 2 &…
13 hours ago

© 2020   Created by Building Performance Association   Powered by

Badges  |  Report an Issue  |  Terms of Service