What is R-Value?

If you’re around home building for even a short time, you’ll hear the term R-Value a lot. So what is R-Value, and why is it important? How can you get in trouble if you ONLY focus on R-Value? What is the difference between Nominal and Effective R-Value? We answer these questions below.

Why is R-Value important:
You likely know what it is in the senes that it is referring to insulation. You’ve heard the term and understand that insulation keeps us warm in the winter and cold in the summer, but not all R-Value is the same. There is a difference between effective and nominal R-Value and it definitely depends on how it’s installed!

R-Value is the ability of insulation material to resist heat (R stands for resistance). It’s calculated by getting the temperature difference per unit of heat flux to sustain one unit of heat flux between the warmer and the colder surface of material, under steady state conditions.

So OK, but what does this really mean for you? For most purposes, it’s not really important to understand the calculation of how the R-value was arrived at. What is important is making sure that you understand how the R-value will affect your home or project. The key term to consider here is “steady state conditions”. I don’t know about where you live, but where I live, the conditions are not in a steady state. There is far more going on outside the home than there is inside! 

For example if you have R-22 batt fibreglass inside your basement and that basement floods and your insulation gets wet, is that insulation going to still be R-22? No, it’s not going to retain its R-value (and you may have issues with mold etc. as well). Or perhaps you have R-50 cellulose blown into your attic and it settles over time. Maybe your roof gets wet. Maybe you have insulation on your exterior with an R-10 value but no air barrier, and the wind is whistling at 50km/hr. All of these are examples of where your R-Value will be affected, because the conditions are not in a steady state. We don’t build in steady state conditions. 

Installation is also important! If you squished an R-22 batt down too much, you would also affect the R-Value and it would not perform optimally. Make sure that whoever is installing the insulation knows what they’re doing and ensure that they’re installing it properly.

The point I’m trying to get to here is that it’s about more than just the R-Value of the insulation. It's crucial to understand your building, the building science, and the conditions you’re building in. 

Consider this for a moment: If you had -20 degrees Celsius outside and you have a big down jacket on. Imagine the wind is howling and you turn to face the wind and open your jacket zipper and let that wind in. Would it matter how much down insulation I had in my jacket to keep me warm at this point? No, because insulation is not enough, you’d need to make sure you have that jacket zipped up. You also need to focus on other things like airtightness, for example. Your home works as a system. If you only focus on insulation, that can get you into trouble and seriously impact your results.

The last thing we’ll get into here is Nominal R-value vs. Effective R-value. Nominal R-value is essentially the estimated R-value of what, say, your wall assembly should be when you consider all the materials. Effective R-value is how well it actually performs! 

For example, let’s consider the 2X6 wall with R-22 batts in it, and you build the entire wall with the standard stud-insulation-stud-insulation assembly. The nominal R-value of that wall may be R- 22, however the effective R-value would not be. Why is that? If you were to take all of the wood in that wall and push it to one end, and put the insulation on the other side you would see that the wood probably takes up about 20% of that wall construction. 20% of your wall does not have the same R-value as the insulation. Your wall would have an effective R-Value of approx 17.6, you’re not getting the full R-22. This means that the best way to get a higher effective R-value is to do it from the outside of your home, (the “jacket” so to speak). This would cover up the studs (which can bring other complications re: cladding etc. but more on that in another post) and this is the best way to add insulation and comfort to your home. Keep in mind, there are many other factors that effect comfort, efficiency and the health of a home.

If you’re a builder/contractor, know that it is your responsibility to make sure your customer gets the best possible product for their budget and priorities. As a homeowner, you want to make sure that you bring the right team on, and that your team is working together to get the best possible products so that you can make sure you get a healthy, comfortable, efficient home for you and your family.

While we’re on the topic, check out work with our favourite ROCKWOOL products on the Three Day Cottage series, a Net Zero ready cottage with a nominal R-value of R-30 for the walls and R-64 for the attic!