Derating / Rating Old Insulation

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!

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  • 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 and I'll send you a copy. 

    • Thanks! Just sent an email. 

  • 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.

  • 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. 

  • 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. 

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