# What is the procedure for de-rating attic insulation?

| Posted in General Questions on

I am an energy auditor and 90% of the time see attic insulation that looks like the lunar landscape. If blown in insulation levels range from 14″ down to 2″ or drywall at can lights how do I provide an effective R-value to the space? Oh yes uninsulated attic hatches included.

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### Replies

1. GBA Editor
| | #1

Bruce,
I assume you are familiar with RESNET's method for categorizing the quality of insulation installations. There are three grades of installation quality -- Grade I, Grade II, and Grade III. Rating software calculates assembly R-values based on raters' inputs, include the Grade rating for installation quality.

If you're not familiar with the RESNET grading system, see the article on the topic in the January/February 2005 issue of Home Energy magazine. The article is “Insulation Inspections for Home Energy Ratings,” by Bruce Harley.

2. | | #2

Yes I'm familiar with the protocol. However it is gualitative in design by using the grading classification. What does Grade II really mean? If the contractor placed R-30 in the attic and I give it a grade II what is the actual effective R-value, 25? 19? I know software has to convert it to a U-value I need to find out the calculations they use to come up with that number.

3. | | #3

Basically, the calculations involve the simple definition of R being the inverse of U and the basic heat transfer equation: Q=U*Area*dT. What you'd have to do for the attic (or wall or any assembly between in and out) is establish some distribution of insulation that exists as so many square feet at one thickness, so many sqft at another, etc. You add up all those U*Area (or Area/R) values to get heat transfer per degree. Average U is then total U*A divided by total area, and average R is the reciprocal of that.

The result will be only as good as your approximation of areas at a given depth of insulation. If it weren't for the can lights and squeeze between floor and roof deck at the walls, it might almost be faster just to carry a rake to the attic with you and rake the insulation to level.

4. River Hume | | #4

So is this how the RESNET derating calculation is done in general? I can't seem to find any documentation about how the calcs are supposed to work. This doesn't quite seem to fit with the 3 tiers of fixed thresholds for Class I, II, III. I know that for jobs that don't even meet Class III this method is required, but how is one to quantify the impacts of say a Class II derating where there is both up to 2% gaps and up to 10% compression/incomplete fill? The whole point is that Class II is Class II and it is rated as the least common denominator that still meets the bottom boundary condition for that Class rating...

5. Allison A. Bailes III | | #5

Here's what the HERS Standards say:

"303.4.1.4.2 Insulation Assessment: Insulated surfaces categorized as “Grade I” shall be modeled such that the insulation R-value within the cavity is considered at its measured (for loose fill) or labeled value, including other adjustments such as compression, and cavity fill versus continuous, for the insulated surface area (not including framing or other structural materials which shall be accounted for separately). Insulated surfaces categorized as "Grade II" shall be modeled such that there is no insulation R-value for 2% of the insulated surface area and its measured or labeled value, including other adjustments such as compression and cavity fill versus continuous, for the remainder of the insulated surface area (not including framing or other structural materials). Insulated surfaces categorized as "Grade III" shall be modeled such that there is no insulation R-value for 5% of the insulated surface area and its measured or labeled value, including other adjustments such as compression and cavity fill versus continuous, for the remainder of the insulated surface area (not including framing or other structural materials). Other building materials, including framing, sheathing, and air films shall be assigned aged or settled -values according to ASHRAE Fundamentals."

Going back to the original question, here's what the HERS Standards say about assessing the insulation when you have variation:

"303.4.1.4.2.3 Where large areas of insulation that is missing, or has a different R-value from the rest of an assembly exist, these areas shall be modeled with the appropriate R-value and assembly description separately from the rest of the assembly. Insulation R-values may not be averaged according to coverage area. For example, if 50 square feet of a wall area has no cavity fill insulation at all, that 50 square feet shall be recorded as a separate building component with no cavity insulation, but with the existing structural components."