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

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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!

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