How much insulation do I need? (R-value)

So, if you want to figure out how much insulation your home needs, you want to start with figuring out how much insulation your home already has.

And to measure that, you need to use a standard unit of measurement.  And we call that in the building science world, R-value.

And before we dive into what our value is, let's talk about and remind ourselves what insulation does.  Insulation will resist the flow of heat between two objects, so it slows down heat movement between two objects that are in contact.

Let's say one object is 90 degrees, another object is 70 degrees and they are in contact.  Over time, they are going to equal out (reach equilibrium) in terms of temperature.  Installation slows down heat transfer between the two objects.  So I want you to kind of imagine a box, right?

And half of that box is hotter than the other side of the box. The heat will flow from the area of higher heat concentration to colder heat concentration till things are balanced.

Now, if you add insulation between that hotter area and the cooler area, the insulation will slow down the transfer of heat.  It'll slow down the “temperature equaling out.”

And that's what you want, because when you're heating up your home, you want to slow down the loss of heat.  And in the summertime, when you're keeping it cool, you want to slow down the gain of heat.

Since we’ve reviewed the basics, now we will return our focus to how we measure insulation, which is in unit called R-value.  And R-value is a measurement of how well a material resists conductive heat flow. 

And different materials resist heat transfer differently.

So metal, for example, is a very poor resistor of heat transfer.  It's very good at conducting heat transfer or moving heat.  It's got a very low R-value of approx. R-0.1 per inch. 

Fiberglass, on the other hand, is a relatively effective resistor of heat transfer.  It's got a higher R value of R-3 per inch.  So, theoretically, it takes actually 30 times more time for heat to move through fiberglass insulation than it does through metal.

See the table below for other types of materials and their R-value per inch:

Given material R-value is measured in R-value per inch, the thicker the material, the higher the R-value.  So if you have one inch of fiberglass that has an R-value of R3 per inch, every time you add on another inch of insulation, you increase the total R-value. 

2 inches x R3 per inch = R6.

10 inches x R3 per inch = R30. 

The thicker any given material is, the more effectively it resists heat transfer.

So now we're going to talk about a super important concept, which is weighted R-values.   What does that mean?

Well, in the real world, any given surface often has different levels of R-value in different places.  For example, a wall with a window.  The window may have an R-value of R3 and the wall an R-value of R13.  What is the total R-value of that entire wall surface?  Do you just average each of the areas together?  No, you can’t.  That won’t work because heat travels the path of least resistance. 

Let’s take a look in more detail.  Imagine looking down at an attic floor, and it's got fiberglass insulation, which is R3 per inch of insulation at a level of ten inches deep.  So the total R-value of that insulated area is R30.  Now imagine we remove half of that insulation.

What do you think the combined or weighted R value of this attic floor is?  50% has R-value of R30.  The other 50%, since there is just a piece of drywall there, has an R-value of R1.

Most people think the correct answer is the average of the two of them about R15.  That would be inaccurate.

Believe it or not, the actual answer is…drum roll…R2. Mind blown?

Yes, that's a 93% reduction in insulation effectiveness from the area that is R30.  How is that possible?  It’s because heat travels the path of least resistance.  There is an exponential decrease in the R-value for a given surface in relationship to the area of the lowest R-value of that total surface.

Let's look at another example.  It'll make more and more sense as you look at more and more examples.

So imagine you have an attic that has 96% of the area of R30 and only 4% of the area has R1.  What do you think the total weighted R-value is?

Well, some people would say, oh, well, you're probably losing 4%, maybe it goes down to R25.  In this case, the total areas R value is only R twelve.

That's approximately a 60% loss.  Again, heat travels the path of least resistance.

Now, the formula that we use to get to these calculations can be found here:

We're not going to go off through that in detail today.

But just to ensure you’re 100% clear on how important it is to make sure you have an effective and consistent insulation barrier, let's look at one more example:

Imagine you have an attic floor where 80% of the area is rated at R30 and 20% of the area is R5. 

The total weighted R-value of that area is actually R15.

So it's a 50% loss even with only 20% having an R-value of R5.

So the these examples and this math really proves is that:

 You have to have a consistent thermal barrier, a consistent amount of insulation for it to work well. So when you start to ask the question how much insulation you need, it's really not just important to see how much you need, it's really important to see how well you need it to be installed.

So you may say, okay, I get the point, but my installation is fine.

Why would I be missing a large amount of insulation? 

Again, I want you to imagine you're looking down at your attic floor again, and you forget that you actually have an attic hatch that isn’t insulated.  Attic hatches are almost never insulated, right?  So there you go, now you’re down to 2% of the area at R1. 

Then you have an electrician that comes in.  The electrician installs some recessed can lights, he moves a few bats of insulation, and now you have some more gaps.  Now, you’re down to 4% of the total area at R1. 

The plumber comes in.  The plumber has to run some new pipes down, now you’re down to 5% of the total area at R1. 

The HVAC contractor runs a new duct and removes a batt of insulation.  Now you’re down to 8% of the total area at R1. 

The audio/video guy installs some new speakers and has to cut out a couple of batts of insulation.   Now you’re down to (best case scenario) 10% of the total area at R1. 

You have an attic insulation level that looks like Swiss cheese, and the losses could be anywhere from 50% to 75% or more.

Here's another really common example of when you might lose effectiveness of your attic insulation. 

So your framing in your attic is one and a half inches wide by five and a half inches deep.

If you had two x sixes on your attic floor, the R value of that would be R1 for wood x five and a half inches…so about R 5.5.

And if you add up all those framing components, that may make up 20% of the area. 

So now you have 20% of the area rated at R5.5 and the remainer at R30 before you even started.  

So it's not just important how much insulation you have, it's very, very important how well it is installed.

Okay, so finally, now that Spencer got his nerdy building science lesson in, let's talk about how much you need.

Well, it’s requirements always varied by climate zone, but historically, attic insulation, wall insulation, and floor insulation levels were a lot lower than they are today.

R30 was a typical attic insulation level.

Walls were typically about R11 or R13.

Floors were typically R19 as a standard.

Now values have increased probably 20-30% above just about ten years ago.

And you're going to see a baseline in most cases of R38 in the attic all the way up to R60.

In most cases today you'll see wall insulation starting at R19 and going all the way up to R30.

And floor insulation is now kind of the bar has been risen to levels that are closer to like R30.

So that's kind of a general sense of the insulation levels you're going to want to see in your home.

Okay, so now that you know how much insulation you need, you want to establish how much insulation you already have.

And so we put together a really cool tool for you.  Very simple.

You can use our exiting attic insulation calculator here:
https://www.homeenergyacademy.com/existing-attic-insulation-calculator-public

Just use the drop down to designate what type of insulation you have.

It'll show you the R value per inch of that insulation, and then you can enter the depth of insulation

And then based on the quality of coverage and the percentage of gaps, it will calculate how effective that R-value is and how

So if it's perfect, obviously it's going to be equal to the standard R value times the inches.  But if you have fair or poor insulation, you can see that reduction is very dramatic.

The other thing I would recommend, if you haven’t downloaded it already is our attic insulation checklist and our attic insulation crash course which you can find here:  https://www.homeenergyacademy.com/checklists

Anyways, it's very hard to overestimate the impact that attic installation can make in your home. 

You want to make sure you do it right the first time!

So please do use these tools so that you can be empowered to get your attic installation complete and create a comfy, high performance home.