parallel path insulation calculation

OK,  I’ve been asked on several occasions to clarify how to calculate and explain the effect of having an attic door or entry way un-insulated and unsealed on the overall efficacy of the attic as an insulation barrier or upper energy envelope.  This is the easiest way to think about it for me. metaphorically. If you have a bucket of water that is completely water tight- it leaks zero water. Now, If you were to drill 30 tiny little 1/64 inch holes in the bottom of the bucket it would leak the water out as gravity drives the water to the little holes. lets say the bottom of the bucket NOW after drilling represents r38 insulation. now lets say while your water is dripping out of the 30 tiny holes you reconfigure your bit and put a 1 inch bit in the drill and drill one more hole in the bottom of the bucket.

Your water will drain out that 1 inch hole very quickly and your water will take only a fraction of the time to exit the bucket. Lets say this hole represents r-1. Even though the 1 inch diameter hole is significantly smaller in size than the rest of the bottom of the bucket with the 30 holes in it, this leak will be much more responsible for your water loss. This effect is very much akin to how an un-insulated attic door leaks all of your energy up into the attic and out of your home. The problem is significant because the resistance to energy movement is nil. Here is how the calculation works for parallel path where energy loss is much higher because r is so much lower in one area.

Lets say you have a 800 square foot attic that is insulated at r50 that has even a smaller scuttle hole opening 24 x 33. One of those units where the builder just put some drywall over the opening but made it look nice by framing it into the ceiling. so we have a (lets just say 6 sq ft) opening that is insulated at (generously) r-1. So we actually have 794 sq feet at r50 and 6 sq feet at r1. The calculation for the r factor of this entire space is this. Ravg = the inverse of or 1/Uavg

Uvg = ((1/r1) *a1) + (1/r2) * a2)) / entire area

where r 1 is the r factor of the first area and first area is a1
and r2 is the r factor of the second area and second area is a2

so lets fill in the variables. 1/50* 794 sq ft + 1/1 * 6 sq feet
or new total U factor of .062 1/.0262 is r38.

So lets recap. Having only 6 square feet out of 800 completely un-insulated dropped this homeowners entire attic by a factor of r12.

This is a significant leak in the energy envelope that can be addressed easily. IT IS LOW HANGING EFFICIENCY FRUIT and should be the first efficiency investment before high cost low return double pane windows or adding insulation to walls or making the mistake of just dumping a bunch of insulation over existing insulation in the attic without weatherizing. For $200 for a DIY homeowner or $350 or so from a contractor you get a r12 boost OVER YOUR ENTIRE ATTIC and probably save 200-250 cfms in conditioned air. These are very large numbers for such a small investment.
For reference, if you take out an old 2 ft by 5 ft wooden window and replace it with a double pane energy star gas filled window, you are praying to get 30-50 cfms in air exit stoppage. And that window costs you $400-500 bucks. Here you get 5-7 times that for less. If you were to invest in 4-5 windows it would easily cost you a couple grand to get this same CFM reduction. So there we have it.

Allison Bailes, PhD- a fairly well acknowledged building performance pro did a nice video I watched a couple years ago on parallel path. I’ll have to find it and post it sometime.

Oh yeah AND attic zipper solves this problem and with the batt straps it is IRC and IECC compliant because you can add what ever insulation you need to dial in the regulatory requirement. all attic zippers are now shipped with batt straps unless otherwise arranged. You can buy one here..