Site Survey: Area-Weighted Average R-Value

R-Value Weighted Average Worksheet

  Due to the recent plunge into Spring (or more like Summer!) weather, most Residential HVAC Companies in New England have seen a fair amount of early season sales calls for air-conditioning.  This quick changeover tends to spark some very interesting conversations around Load Calculations and Site Survey.  One in particular tends to come up every year: Attic Insulation.  Attic Insulation can be a tough R-Value to identify when out on a quick sales call, so it is important to take notes as to what you see.  This way the Area-Weighted Average R-Value can be calculated prior to entering such details into your software.  The first image is a blank worksheet to calculate this average R-Value, which is always more affected if you have a well insulated attic.  I will walk through (2) examples to examine a properly insulated attic and one with poorly installed insulation.

Example #1

  In my first example, I have a 920 Sq. ft. attic with 12" of blown-in fiberglass - a value of R-30. This value must be discounted for all of the other areas  in that attic without the 12" of insulation. For instance, an uninsulated 2' x 2' access hatch.  This wood has an R-2 value.  The 2"x 6" rafters that have 6" of insulation above them have an effective R-17 (6" of insulation = R-15;Wood = R-2).  As well as the (10) 12" x 12" supply and return registers/grilles, and 24" x 24" hall return I am about to cut-in and turn this ceiling into swiss cheese!  Remember, we insulate our ducts to R-8 outside the building envelope for code (this attic is outside).  When completing this worksheet, I need to figure the areas (in feet) of the devices being deducted from the entire ceiling in order accurately estimate the remaining ceiling area.  In the first example, with perfectly installed R-30 insulation, we reduced the average value of this ceiling to an R-25!  Doesn't sound tremendous, but this can affect your heat loss as well as your heat gain calculations.

Example #2

  The second example uses poorly installed insulation.  Sure, there was still 12" in some locations.  But, did you know that just 2% voids in insulation can reduce that R-30 to only R-15?  No offense to all the homeowners and "DIYers" out there, but this is why we hire licensed contractors - to get what you pay for!  Since the starting value is so low, when weighting out the average and adjusting for the access hatch, rafters, and registers, the value is only reduced to R-14.5.  When entering this into software, it will need to be R-15 anyhow.

  So, finding the R-Value weighted average does not need to be done for every ceiling/attic, only the ones that were well insulated!  If the insulation was not installed professionally, I would take an extra moment to probe around and discount the starting R-Value accordingly.  Just don't take it for granted, see it with your own eyes - this means a trip to the attic on all of those sales calls involving a load calculation!  Otherwise, you are doing the customer and your company a disservice!