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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I don't have a copy of the original BPI charts, but I believe our local utility has adopted the original charts and included some examples of insulation in various states. You can tell which portion of the attached was produced by the utility as they are have the logo attached. The following pages that do not have the logo are the original insulation rating charts that came from BPI.

Insulation Reference Charts.pdf

Open batt or open fiber will wind wash in vary cold weather so the batt says R-30 and with IR its says R-10 test same spot with out wind and is R-30. How do you deal with wind washing?

Check the California Installation Standards with a well thought out and researched listing of old insulation.

Attached HES insulation derating file.

Home Energy Score Assessor Calculator - 19May2016.pdf

I've attached the HES insulation derating page for illustration.

You can find an assessor or train to use their software. It is a simple certification.

homeenergyscore.gov

Home Energy Score Assessor Calculator - 19May2016.pdf

Page 7 & 8 of the old Technical standards have that info. (attached)

Technical Standards for the Building Analyst Professional.pdf

Ah, the table on batt insulation was what I was thinking of but couldn't find. Thanks!

You are welcome. Not sure what that did not make it into the new standards, but I still use it myself, as do many others I know.

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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