Understanding the Short Circuit Principle in Air Leaks and Insulation

I am probably going to end up with a confusing analogy here. You have all had a short circuit with a fuse box right? you have seen the sparks fly when hot wires accidentally get crossed with a ground or neutral wire… The short circuit is basically no resistance to the flow of electrons.

The same principle applies to the flow of conditioned air in your home: no sparks fly- just money flies out of your wallet.    [stextbox id=”info” caption=”Maybe the leaking boat analogy works better for you” bwidth=”2″] If you are on a boat and you have small leaks and a big leak, you would probably understand you had better address the big leak first, or you will get wet… same principle across all natural forces.[/stextbox]

The same principle works in insulation- we will  do the math to illustrate the point for insulation here in a bit, but basically it works for air leakage too. You can spend considerable time sealing areas of potential leaks, but unless you find the big stuff you are making few real gains. You have the equivalent of an air leakage short circuit.   [stextbox id=”info” caption=”Remember seal air leaks before adding insulation” bwidth=”2″] adding a nice layer of fluffy loose fill over everything is conventional wisdom but it will not stop the flow of air leaks. Air leaks are really serious. Air leaks are the source of comfort and health issues. There is math here too, but it is added by using air leakage testing and analysis by software.[/stextbox]

This is a critical issue with trying to track down air leaks by yourself without proper test equipment. We talk about great do it yourself prpjects here, and also talk about not so great… You can work up a healthy sweat and feel you are just busting it, but you are  really just  going bust.  [stextbox id=”info” caption=”This is a great time to mention a professional energy audit” bwidth=”2″]Make sure you look over the info on the process for a professional energy audit. LINK The process and technology will get any engineer smiling. Not an engineer? You will smile because you translate all that energy testing into more comfort, and healthy air while paying less money each month to be cozy.[/stextbox]..

Lets look at some examples now of insulation short circuits.

Lets look at math to cover the concept of lumpy and uneven insulation described in the top article  on insulation. Who knows what happened. But irregular layers of insulation impact the value the R-value well beyond just the average because of the short circuit effect.

We need to do some setup for our math here.

I am not great with subscripts and text editors, so here is an image of the equation I swiped somewhere.U-equals

So let say we have a 2000 square foot ceiling- it is loose fill, but it is not even. OK, I am not doing insulation forensics here, just laying out the basis of a math problem. The story and the physics play out. There are many reasons for uneven insulation- see the top document on insulation, or better yet, get up in your own attic and look for the signs of missing or messed up insulation.

About 10% of the insulation is messed up, and we are going to say this 10% is about 3 inches deep. this means that about 12% is about 15 inches deep. They rest of the roof is pretty much the rolling hills of home. Valley are at least 3″ more shallow than the tops of hills. We figure the roof is intended to be R-30- about 9 inches of loose fill if it all laid flat. The hills are 10.5 inches deep  and the valleys are 7.5 inches deep.

R per  inch = 3.3 our 9 inch average- which we determined by painstaking sampling [joke] – we figure that this attic is intended to be r-30

our R values in this system are

r 3″ = 10 and the area is 200 sq. ft.

r 15″ = 50  and the area is 200 sq. ft.

r 7.5″ =  25 and 800 sq. ft

r10.5  – 35 and 800 sq feet

U ave = ((1/10(200) + 1/50(200) + 1/25(800) + 1/35(800))/2000 =  .039  or
R ave – 1/.039 = 25  we are losing 17% of our insulations effectiveness just by poor management of our insulation. Now this is example is pure totally made up…

BUT the key question is where are the short circuits in your attic and how do we make sure they are being dealt with rather than just some happy go lucky guy pitching loose fill into the ceiling space….

One more math example.

So many attic access doors are nearly uninsulated. We talk about attic access doors here… the problems and even offer a sophisticated DIY solution for your attic. [yes, patting myself on the back again…]

For reference, attic doors are 5 feet long and 2ft. wide. They are basically built on the back of a thin board. We are going to be kind and say that this board provided R = 1 for the 10 square feet of attic space.

Lets look at the math  for  the door.

U ave = ((1 x 10) + (1/30 x 1990))/2000 = ..038

R = 26 –  A poorly insulated access door knocks down the effectiveness of the attic insulation by 13%. Now try mixing in the issues with lumpy and poorly distributed insulation…

So what can we do with this knowledge?

[stextbox id=”info” caption=”Make sure you understand what you are getting” shadow=”false” bwidth=”2″] This would be a good time to mention the opportunities you get to top up on insulation. Lots of companies offer it. But does it really fit for you? lets talk about it here. Would you like 3 new inches of insulation with that new roofing? draft post. [/stextbox]

We need to let this concept color our decisions that contractors ask us to make. What are they proposing?

Have the considered the short circuit impact of… here is an example without math.

You might here lots of this talk about how much energy you can save with new replacement windows. New windows can be 500% or more better insulated than the existing window. But all windows have a hidden partner which will not go away even if  you pay to have the windows replaced. That is the solid wood framing around the window and the structural framing member to fill the span over the window.

These wood members  make a thermal bridge to the interior of the house. And  existing single pane windows- the culprit they are usually comparing to- can have an R value of less than 1.  So R-5 or more for new windows is not the great panacea for energy savings. OK,  I am going to shut up about windows here, because window performance depends so much on placement of the window… to get down to facts and not opinion… BUT I will add one last opinion backed by a whole list of inescapable facts. There are many good reasons to replace windows in your existing house, improving your energy efficiency is not one of them. More on the efficacy or replacing existing windows here. 

What do we do then about short circuits in our insulation?

Glad you asked. 🙂 We need to solve all these critical factors before we move on to conventional thinking about more insulation good. We threat the big problems first lets us build a solid foundation and sound rational for moving on to the next priority.

Which means if you find an effective way to seal and insulate the ceiling access- and this could be a do-t-yourself project– and if you plan well and execute well, it can be a do-it-yourself-once-project. If you do this attic access retrofit, it will be much more effective than paying to have 3 ” of new insulation blown into your attic but leaving the attic access door as it is… We will do the math on that below… NOT

I recommend the services of a  professional energy auditor. Here we talk about what they do, and how they can help you. LINK. It always makes sense to know where you are before you head off in any direction. It also makes sense to have a map of the way there. The right professional can help you find the critical path to your best return on investment- plus help find funds and tax write-offs.

Any questions or comments. Just add them below.


Leave a Reply

Your email address will not be published. Required fields are marked *


This blog is kept spam free by WP-SpamFree.